Template-Directed Synthesis of One-Dimensional Hybrid Nanostructures from Cylindrical Polymer Brushes

Template-Directed Synthesis of OneDimensional Hybrid Nanostructures from Cylindrical Polymer Brushes DISSERTATION zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften (Dr. rer. nat.) an der Bayreuther Graduiertenschule für Mathematik und Naturwissenschaften der Universität Bayreuth vorgelegt von Markus Müllner Geboren in Amberg Bayreuth, 2012 Die vorliegende Arbeit wurde in der Zeit von Februar 2009 bis Januar 2012 in Bayreuth am Lehrstuhl Makromolekulare Chemie II unter Betreuung von Herrn Prof. Dr. Axel H.E. Müller angefertigt. Vollständiger Abdruck der von der Bayreuther Graduiertenschule für Mathematik und Naturwissenschaften der Universität Bayreuth genehmigten Dissertation zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften (Dr. rer. nat.). Dissertation eingereicht am: 02.02.2012 Zulassung durch die Promotionskommission: 14.02.2012 Wissenschaftliches Kolloquium: 07.05.2012 Amtierender Dekan: Prof. Dr. Beate Lohnert Prüfungsausschuss: Prof. Dr. Axel H.E. Müller (Erstgutachter) Prof. Dr. Josef Breu (Zweitgutachter) Prof. Dr. Carlo Unverzagt (Vorsitz) Prof. Dr. Andreas Fery A day without laughter is a day wasted Charlie Chaplin To My Family and Iva Table of Contents Table of Contents Summary ............................................................................................................................. 1 Zusammenfassung.............................................................................................................. 3 Glossary .............................................................................................................................. 7 Chapter 1 – Introduction ................................................................................................ 11 One-Dimensional Hybrid Nanostructures ................................................................... 11 1. Cylindrical Polymer Brushes ..................................................................................... 12 1.1 Cylindrical Polymer Brushes via Grafting Approaches ................................................ 13 1.1.1 Structural Compositions of Cylindrical Polymer Brushes............................................. 17 1.1.2 Core-Shell and Core-Shell-Corona Block Copolymer Brushes ..................................... 18 1.1.3 Statistical, Gradient, Block and Janus-Type Cylindrical Polymer Brushes .................... 20 1.1.4 Branched, Macrocyclic and Multigraft Polymer Brushes ............................................. 23 1.2 Cylindrical Polymer Brushes from Block Copolymers ................................................. 24 1.3 Properties of Cylindrical Polymer Brushes ................................................................. 26 1.3.1 Solution Properties.................................................................................................... 26 1.3.2 Cylindrical Polymer Brushes on Surfaces and in the Bulk ............................................ 28 1.4 Applications of Cylindrical Polymer Brushes.............................................................. 30 2. One-Dimensional Hybrid Organic-Inorganic Nanostructures ....................................... 32 2.1 Template-Directed Approaches Toward One-Dimensional Hybrid Materials ................ 33 2.1.1 Cylindrical Polymer Brushes as Soft Templates for Fabrication of One-Dimensional Hybrid Nanomaterials ............................................................................................... 34 2.1.2 Self-Assembled One-Dimensional Templates from Solution ........................................ 36 2.1.3 Self-Assembled One-Dimensional Templates from Bulk ............................................. 38 2.1.4 Biological and Other One-Dimensional Templates ...................................................... 39 2.2 Porous Membrane-Based Templates .......................................................................... 40 2.3 Electrospinning......................................................................................................... 40 3. Aim of the Thesis ..................................................................................................... 41 References ............................................................................................................... 42 Chapter 2 – Overview of the thesis ................................................................................. 49 2.1 Organo-Silica Hybrid Nanotubes ............................................................................... 50 2.2 Anatase Nanotubes ................................................................................................... 53 2.3 Silica Nanowires and Nanotubes ................................................................................ 56 2.4 Mesostructuring of TiO2 Nanocrystals into One-Dimensional Nanostructures ............... 59 2.5 Individual Contributions to Joint Publications ............................................................. 62 References .................................................................................................... 64 Chapter 3 Water-Soluble Organo-Silica Hybrid Nanotubes Templated by Cylindrical Polymer Brushes .................................................................................................................... 65 Chapter 4 Template-Directed Mild Synthesis of Anatase Nanotubes within Cylindrical Core-ShellCorona Polymer Brushes ........................................................................................... 87 Zusammenfassung 6 Glossary 7 Glossary 1D one-dimensional 1 H-NMR proton nuclear magnetic resonance AAO anodized aluminium oxide AFM atomic force microscopy AMA allyl methacrylate ATRP atom transfer radical polymerization Brij 58 polyoxyethylene(20) cetyl ether CNT carbon nanotubes CPB(s) cylindrical polymer brush(es) CRP controlled radical polymerization Cryo-TEM cryogenic transmission electron microscopy CTAB hexadecyltrimethylammonium bromide CuAAC copper-catalyzed azide-alkyne cycloaddition D distance DCE dichloroethane DLS dynamic light scattering DMAEMA 2-(dimethylamino)ethyl methacrylate DP n number-average degree of polymerization EDX energy dispersive X-ray spectroscopy FWHM full width of half maximum GMA glycidyl methacrylate GPC gel permeation chromatography GTP group transfer polymerization H2SO4 sulfuric acid HCl hydrogen chloride HF hydrogen fluoride HR-TEM high resolution transmission electron microscopy IPEC interpolyelectrolyte complex l m length per monomer unit l p persistence length Lucirin TPO® [diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide] MWCNT multi-walled CNTs MWD molecular weight distribution nBA n-butyl acrylate NMP nitroxide mediated polymerization NP(s) nanoparticle(s) OEGMA oligo(ethylene glycol) methyl ether methacrylate OsO4 osmium tetroxide P2VP poly(2-vinylpyridine) PAA poly(acrlyic acid) Glossary 8 PAMA poly(allyl methacrylate) PAPTS poly(3-acryloylpropyl trimethoxysilane) PB polybutadiene PBIEM poly(2-bromoisobutyryloxyethyl methacrylate) PCEMA poly(2-cinnamoylethyl methacrylate) PCEVE poly(chloroethyl vinyl ether) PCL poly(ε-caprolactone) PDADMAC poly(diallyldimethylammonium)chloride PDMAEMA poly(2-(dimethylamino)ethyl methacrylate) PE polyethylene PEG polyethylene gylcol PFS poly(ferrocenyldimethylsilane) PGMA poly(glycidyl methacrylate) PHEMA poly(hydroxyethyl methacrylate) PI polyisoprene PLA poly(lactic acid) PMDETA N,N,N',N",N''-pentamethyldiethylenetriamine PMETAI poly{[2-(methacryloyloxy)ethyl] trimethylammonium iodide} PMMA poly(methyl methacrylate) PMS 4-(pyrrolmethyl)styrene PnBA poly(n-butyl acrylate) PNIPAM poly(N-isopropylacrylamide) POEGMA poly[oligo (ethylene glycol) methyl ether methacrylate] POSS polyhedral oligomeric silsesquioxane PS polystyrene PSS poly(styrenesulfonate), poly(styrenesulfonic acid) PtBA poly(tert-butyl acrylate) PTEPM poly(3-(triethoxysilyl)propyl methacrylate) PTMS-HEMA poly(2-(trimethylsiloxy)ethyl methacrylate) PVP poly(vinylpyrrolidone) PXRD powder X-ray diffractometry QD quantum dot RAFT reversible addition fragmentation transfer polymerization RI refractive index ROMP ring-opening metathesis polymerization ROP ring-opening poylmerization SAED selected area electron diffraction SDS sodium dodecyl sulphate SDV gel styrenedivinylbenzene gel SEC size exclusion chromatography SEM scanning electron microscopy SiO 1.5 silsesquioxane SiO 2 silicon dioxide, silica Glossary 9 SLS static light scattering Sn(Oct)2 tin(II) 2-ethylhexanoate TALH titanium(IV) bis(ammonium lactate) dihydroxide tBA tert-butyl acrylate TEM transmission electron microscopy TGA thermogravimetric analysis THF tetrahydrofuran Ti(OBu)4 titanium(IV) butoxide TiO2 titanium dioxide, titania TMOS tetramethyl orthosilicate TMS-HEMA 2-(trimethylsilyloxy)ethyl methacrylate TMV tobacco mosaic virus Tween 60 polyoxyethylene sorbitan monostearate Glossary 10 Chapter 1 – Introduction 11 Chapter 1 – Introduction One-Dimensional Hybrid Nanostructures The demand for advanced functional materials with novel properties has led to a continually expanding research area that covers not only chemistry, but also biology, physics and materials sciences. Nanostructured materials, which are materials with structural features of at least one-dimension in the range of 1-100 nm, have become one of the hottest topics in the field of materials science.1 The reason for the increased interest in nanomaterials lies in their unique electrical, optical, magnetic, thermal, mechanical and chemical properties when compared to their bulk parent counterparts.2-4 It is known that the peculiar physical and chemical properties are deeply connected to the morphology and size in nanoscale of the respective material. Especially one-dimensional (1D) nanomaterials, such as nanowires and nanotubes, have attracted immense interest, as these anisotropic nanostructures are expected to play an important role as building blocks, interconnects and functional units in the fabrication of electronic, optoelectronic, electrochemical and electromechanical nanoscale devices. Therefore, it was necessary to develop straightforward syntheses of these nanostructures and alter their composition. The interest and demand for 1D hybrid nanomaterials increased dramatically after their production became much more feasible through various templating techniques and electrospinning. Template-directed or template-assisted production of 1D hybrid nanomaterials became even more facile when polymeric soft templates were used. The large scale production of well-defined polymers and polymeric templates in all kinds of compositions became rather simple due to the many improvements in controlled/living polymerization techniques. The following chapters describe the synthesis of soft 1D templates and their use in the template-directed synthesis of hybrid materials in more detail. This thesis is dealing with the application of soft polymeric templates for the production of 1D hybrid nanostructures. Different types of cylindrical polymer brushes with welldefined structures were prepared through different methods. Further, these 1D soft templates were used for the controlled fabrication of 1D hybrid nanomaterials. Chapter 1 – Introduction 12 1. Cylindrical Polymer Brushes According to Milner, polymer brushes are “long-chain polymer molecules attached by one end to a surface or interface by some means, with a density of attachment points high enough so that the chains are obliged to stretch away from the interface, sometimes much farther than the typical unstretched size of a chain”.5 Although, this definition was originally directed at planar polymer brushes two decades ago, the quintessence still holds true today. However, the term ‘polymer brushes’ refers nowadays to assemblies of polymer chains, which are attached by one end to the surface of a planar (2D), a sphere (3D), a linear polymer chain or a thin polymeric or inorganic rod (1D).6 3D polymer brushes are commonly referred to as spherical polymer brushes (SPBs) whereas 1D polymer brushes are usually given the name ‘molecular brushes’ or are simply called cylindrical polymer brushes (CPBs). By definition, CPBs are organic 1D nanostructures consisting of polymer chains that are densely tethered next to each other. The density of chains of polymer molecules (grafting density) is eventually so high that the chains become crowded and are stretched.7 This stretching of a CPB can be achieved by two major pathways: firstly, the lateral attachment of polymer chains onto a long polymer main chain (backbone) and, secondly, the crosslinking of cylindrical domains in either bulk thin films or cylindrical micelles. Scheme 1-1 shows possible ways to synthesize CPBs. The dense attachment of side chains to a linear polymer backbone can be achieved by three possible pathways: (A) ‘grafting-through’,8-10 (B) ‘grafting-onto’, and (C) ‘grafting-from’ (see Scheme 1-1A). Besides these three methods, where the side chains are covalently linked to the backbone, there are several non-covalent approaches. Non-covalent interactions, such as coordination,11 hydrogen bonding12 and ionic interaction,13,14 have been successfully used to bond surfactants onto linear polymer chains to form brush-like architectures. Many groups have synthesized CPBs through the crosslinking of cylindrical domains in microphase-separated polymer bulk films (see Scheme 1-1C).15-18 Researchers have also found ways to obtain cylindrical brushes from worm-like micelles by preserving their shape through crosslinking of the cylindrical inner domain (see Scheme 1-1B).19-21 Furthermore, crystallization-driven cylindrical polymer micelles were produced by several groups22-25 through crystallizing one block of linear block copolymers into a cylindrical domain. However, the core of these brush-like micelles is not crosslinked and Chapter 1 – Introduction 13 therefore is less stable regarding fracturing compared to the core-crosslinked analogues. However, it is debatable whether the polymer brushes derived from block copolymers are genuine CPBs or rather “brush-like” cylinders compared to the classic CPBs derived from the abovementioned grafting approaches. Regardless, the stretching of lateral polymer chains lead to many new physical phenomena, which opened many new research areas and increased the interest in anisotropic polymeric material dramatically. Scheme 1-1. Different approaches to prepare CPBs: (A) ‘Grafting-through’, ‘grafting-onto’ and ‘graftingfrom’ techniques involving a step-wise build-up of CPBs. (B) Core-crosslinking of cylindrical micelles in solution. (C) Micro-phase separation of block copolymers in bulk into hexagonally packed cylinders and the subsequent crosslinking of the cylindrical domain to produce CPBs after dispersion. 1.1 Cylindrical Polymer Brushes via Grafting Approaches As illustrated in Scheme 1-1A, CPBs can be synthesized by three grafting routes, namely ‘grafting-through’, ‘grafting-onto’ and ‘grafting-from’. One characteristic that all three methods have in common is that polymeric side chains become adhered very closely next to each other and the lateral dimension is relatively small compared to the actual length of the main chain.26 Efforts in gaining increased control over the polymerization kinetics brought out several living/controlled polymerization techniques with which it became Chapter 1 – Introduction 14 feasible to produce defined CPBs with various compositions by the following grafting methods. ‘Grafting-through’ describes the polymerization of macromonomers into polymer brushes. Macromonomers are polymer chains carrying terminal polymerizable groups.27 Since macromonomers have to be produced separately, it is easier to control length and composition of the side chains. In addition, they can be accurately characterized prior to polymerization. Another clear advantage of this method is the grafting density of 100 %, as every repeating unit carries one side chain. Despite the excellent control of the composition and the well-defined grafting density, the ‘grafting-through’ method bears its limitations. As polymer chains tend to coil, the accessibility of the terminal functionality of macromonomers is hampered. Additionally, sterical hindrance and a low concentration of polymerizable groups decrease the propagation of the main chain and hence limit the actual length of the backbone. Polymerizations often show incomplete conversion and consequently make purification tedious.28-30 Conventional radical polymerization allowes the use of a wide range of monomers and reaction conditions; however, the relatively poor control over molecular weight and chain end functionality prevents the preparation of well-defined structures.31 Consequently, many research groups failed to produce poly(macromonomers) with respectable backbone lengths using different kinds of polymerization techniques, such as anionic32-34 and cationic35 polymerization, as well as group transfer polymerization (GTP),29 atom transfer radical polymerization (ATRP),28 and reversible addition fragmentation transfer (RAFT) polymerization.36 However, ringopening metathesis polymerization (ROMP) of norbornene end-functionalized polymers enabled the synthesis of longer polymer backbones. Whereas the so-called ‘Schrock initiator’37 still did not produce long enough main chains,30,38 the use of metallocenecatalyzed ROMP allowed high monomer conversion and hence resulted in polymer brushes with passable backbone length 39,40 and acceptable length and molecular weight distribution.41-44 Next to the significant progress in the ‘grafting-through’ approach, many research groups focused as well on developing ‘grafting-from’ and ‘grafting-onto’ techniques. The ‘grafting-onto’ method involves a polymer main chain that carries functional groups on each monomer unit and end-functional polymer chains. Both backbone and side chain polymers are produced separately and can be characterized prior to the polymer brush formation. The side chains are then grafted onto the backbone by reacting the pendant Chapter 1 – Introduction 15 functional groups of the backbone precursor with end-functionalized polymer chains. A diverse set of techniques has been used so far to produce molecular brushes by the ‘grafting-onto’ approach. Initially, many research groups focused on using living anionic chain ends and quenched them with suitable electrophilic polymer backbones to obtain comb-like polymers or polymer brushes.45-51 These synthetic routes enabled, already back in the early 1980s, the synthesis of many graft copolymers. With the start of the 21st century, so-called ‘click chemistry’ enabled new possibilities regarding the covalent attachment of polymeric side chains onto a precursor backbone. The outrider of today’s ‘click chemistry’ was the azide-alkyne Huisgen cycloaddition,52 where azide and alkyne groups react equimolar to form a triazole ring. Today, there exists a number of improved or different click reactions compared to the classic Huisgen model.53 The most applied version is the copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC).54,55 The diversity, simplicity and efficiency of many click reactions allowed the preparation of polymer backbones with a high grafting density. Biopolymers or polymeric backbones prepared by controlled radical polymerization (CRP) have been equipped with clickable groups and then employed to synthesize CPBs.56-58 Another way to produce molecular brushes is to use non-covalent (secondary) interactions, such as hydrogen bonding,12,59,60 ionic interactions,14,61,62 or coordination bonding.11 Independent on the way side chains are grafted onto a polymer backbone, the ‘graftingonto’ approach has its limitations regarding grafting efficiency. The limitations arise through issues concerning entropy and sterical hindrance. As both the polymer backbone and the yet unattached polymer side chains exist as random coils in solution, the attachment of more and more side chains to the backbone leads to the stretching of the backbone and accordingly to the stretching of the side chains - both are entropically unfavored. The constantly increasing density of the grafted chains also causes difficulties for the diffusion of further chains to the reactive sites due to sterical hindrance. One can overcome these issues by adding a large excess of the to-be-grafted side chains, however, unreacted chains will remain in solution after the reaction and require further purification steps. Another way to increase grafting efficiency is to decrease the lengths of the to-begrafted side chains, as this will reduce sterical hindrance.57 As a result of the limitations faced in the ‘grafting-through’ and ‘grafting-onto’ methods, the ‘grafting-from’ approach became the most utilized method to graft side chains to a polymer backbone. Chapter 1 – Introduction 22 nBA and hence only the modified HEMA units grow side chains.107 Such gradient homopolymer brushes undergo transformation from rod-like into tadpole-like conformations.110 Heterografted copolymer brushes have been synthesized by Neugebauer et al. via the ‘grafting-through’ of macromonomers. The macromonomers had either acrylate or methacrylate groups, which led to reactivity ratios of the macromonomers and therefore to a gradient copolymer brush (see Figure 1-3C).108,109 Figure 1-3. (A) Statistical copolymer brushes were obtained via the ‘grafting-through’ of macromonomers and formation of Janus-type and patchy brushes after quaternization.100 (B) Microphase separation of triblock terpolymers and subsequent crosslinking was used to obtain Janus-type polymer cylinders.18 (C) Copolymerization of macromonomers of different reactivity was undertaken to obtain heterografted gradient polymer brushes.108 (D) A bifunctional polymer backbone was used for the sequential block growth of poly(lactic acid) (PLA) via ROP and polystyrene (PS) via ATRP to obtain block-type CPBs.105 Chapter 1 – Introduction 23 1.1.4 Branched, Macrocyclic and Multigraft Polymer Brushes CPBs have been used as building blocks to construct more complex polymer architectures, such as double-grafted (graft-on-graft) brushes,89,111 cylindrical tubes,112 barbwires113 and flower-like or dumbbell-like structures.104,114 Branched polymer brushes, such as dendridic polymer brushes, have been prepared by several groups via all three grafting approaches (see Figure 1-4C).115-120 Another type of branched brushes are starshaped brushes. Four or five arm stars were successfully synthesized via a coupling of living anionic PS and star-like poly(chloroethyl vinyl ether) (PCEVE) chains.121 By producing a threeor fourarmed star polymer with ATRP initiator groups as monomer units for the arms, it was possible to prepare very uniform threeor fourarmed star molecular brushes (see Figure 1-4A).122 Figure 1-4. (A) Star-shaped molecular brushes obtained when star-shaped precursors were used with an ATRP initiator containing arms, and the respective AFM image. 6,122 (B) The strategy for the synthesis of macrocyclic copolymer brushes using ABC triblock terpolymers and anionic living chain ends. Macrocyclic brushes (see AFM phase image) formed cylindrical tubes.112 (C) Two strategies towards dendronized polymer brushes using either the ‘grafting-onto’ (‘attach to’) route or the ‘grafting-through’ (macromonomer) route. Dedronized polymer brushes can form long cylinders (see AFM image).119 Chapter 1 – Introduction 24 Another special type of polymer brushes are macrocyclic brushes, where the two ends of the brush are connected to each other by a coupling reaction.123 The preparation of large macrocyclic (co)polymer brushes is limited by several factors. Firstly, it is difficult to obtain only α,ω-difunctional high molar mass precursors and, secondly, there is a drastic decrease in the end-to-end ring closing efficiency when the distance between the chain ends becomes too large. Lastly, the separation from non-closed and still-linear contaminants is difficult, as each consists out of comparable molar mass.124 Deffieux et al. developed a strategy to synthesize large polymer macrocycles which are based on an ABC triblock terpolymer.112 The triblock terpolymer has a long central block B, which possesses two short blocks (A and C) on each end. Blocks A and C bear monomer units that react exclusively with each other. The external blocks are then selectively activated under dilute conditions to allow intramolecular coupling between the A and C blocks to form the macrocyclic polymers. Chloroethyl vinyl ether was selected as the monomer for the central block B, because it can be readily derivatized into brushlike polymers by a ‘grafting-onto’ process. The corresponding macrocyclic brushes were decorated with PS or randomly distributed PS and polyisoprene (PI) branches (see Figure 1-4B). In a selective solvent for the PI branches, the macrocyclic brushes selfassemble into cylindrical tubes with a length up to several hundred nanometers.112 1.2 Cylindrical Polymer Brushes from Block Copolymers CPBs or rather brush-like polymer cylinders can also be obtained from diblock copolymers or triblock terpolymers in either solution or bulk. The brush formation in solution can be achieved by producing cylindrical micelles and the subsequent crosslinking of the cylindrical domain.19,125,126 As an example, Liu et al. used the diblock copolymer poly(styrene)-block-poly(2-cinnamoylethyl methacrylate) (PS-PCEMA), where PCEMA formed the cylindrical micellar core and could be crosslinked by UVlight.20 In this case, the PS chains are then the grafts of the PCEMA rod and the whole unit resembles a CPB. Schmalz et al.23 and Winnik et al.,22 as discussed above, used diblock or triblock polymers with a crystallizable block to form CPBs. The crystallizable block formed the cylindrical core upon cooling and therefore preserved the worm-like geometry. Winnik et al. used the diblock copolymer poly(ferrocenyldimethylsilane)- block-poly(isoprene) (PFS-PI) to induce crystallization of PFS into cylinders with PI grafts (see Figure 5B). Chapter 1 – Introduction 25 Alternatively, the same type of cylindrical brush can also be prepared by the crosslinking of cylindrical microdomains of microphase-separated block copolymers in bulk. In this way, CPBs from bulk thin films were achieved by choosing the composition of AB diblock copolymers or ABC triblock terpolymers in a way that the block B would microphase-separate into a cylindrical morphology.15,16,18 For example, it was again Liu et al. who used the photo-crosslinkable polymer PCEMA, but this time they yielded cylinders of PCEMA dispersed in the continuous phase of PS in bulk. The dissolution of the crosslinked cylinders resulted in isolated polymer brushes with crosslinked PCEMA cores and PS grafts (see Figure 1-5A).17 cylindrical micelles microphase-separation dispersion Figure 1-5. (A) PS-PCEMA diblock copolymer can be used for either forming cylindrical micelles in solution or PCEMA cylinders in a PS matrix in bulk. In each case, PCEMA can be crosslinked by UV-light and yield PCEMA cylinders with PS grafts.15,20 (B) PFS-PI diblock copolymers were used to undergo crystallization. Thereby, PFS formed cylinders whereas the PI block produced the grafts.22 Chapter 1 – Introduction 26 1.3 Properties of Cylindrical Polymer Brushes The conformation of CPBs is a result of competing forces between the backbone and the grafts. The densely grafted side chains repel each other, but their ability to move apart is hampered by the backbone, which locally confines the side chains to a cylindrical volume. Consequently, cylindrical brushes may exhibit different conformations on different length scales.127,128 Intensive research has been performed to highlight the unique properties of CPBs. CPBs are structurally more compact when compared to the corresponding linear polymers of the same molecular weight. This compactness derives from a higher density of chain segments. The steric repulsion of densely grafted side chains results in an extended worm-like conformation. The extent of backbone stretching is mostly dependent on the side chain length and the nature of the solvent used. The extended worm-like conformation makes it difficult to characterize CPBs with conventional characterization techniques, such as size exclusion chromatography (SEC), dynamic or static light scattering (DLS and SLS) or viscometry. Characterization becomes even more problematic when the composition of CPBs is heterogeneous. Therefore, several types of scanning probe microscopies have been established and are nowadays frequently used in the characterization of CPBs. The most frequently used type is AFM, as it is a powerful tool and allows the precise imaging of CPBs and the characterization of their molecular weight, size and conformation.82,129-131 There have been many scientific studies on the properties of CPBs in solution, on surfaces and in bulk. 1.3.1 Solution Properties As mentioned above, the cylindrical shape of CPBs derives from the repulsion of side chains that are tethered very densely onto a polymer backbone. In solution, CPBs adopt the conformation of a worm-like object that can be characterized by the length per monomer lm, the brush diameter D, and the persistence length lp. Obviously, these parameters, and hence the cylindrical dimensions, depend on grafting density, side chain length and solvent nature. Many theoretical,61,132-134 simulation135-141 and experimental31,142-146 studies have been performed to learn about side chain effects, solvent effects and main chain conformations. However, opinions and results differ as to how much chain side length is crucial for lp and the overall brush structure. For example, Chapter 1 – Introduction 27 for flexible side chains, it is theoretically predicted that a stiffening of the backbone is not sufficient to cause ordering of CPBs,128 but experiments show hexagonal ordering of cylinders that stiffened with increasing side chain length.147,148 A further property of CPBs is that they can act as liquid crystals and, therefore, form a lyotropic phase when concentrated in solution (see Figure 1-6A).78,149 When a threshold concentration is exceeded, the polymer side chains will interpenetrate and hence show ordering. Threshold concentrations depend strongly on the length of the side chains. Another interesting solution property of CPBs is that they can respond easily to environmental changes and change their morphology accordingly. Their morphology, as well as their flexibility, is mainly directed by solvent quality, which in water depends on salt concentration, surfactants, temperature and pH. It is possible to trigger sharp transitions in the morphology of CPBs. This makes them an interesting material for various applications, such as membranes or sensors. CPBs consisting of thermoresponsive polymers, such as poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) or poly(N-isopropylacrylamide) (PNIPAM) are classical examples where the cylindrical shape will collapse upon heating above a certain temperature.77,150,151 Further, PDMAEMA polymer brushes are responsive to pH151,152 and salt (see Figure 1-6C).152 PDMAEMA loses its responsiveness to pH and temperature upon quaternization due to the permanent charge; however, it stays sensitive to salts. Charged CPBs like poly{[2- (methacryloyloxy)ethyl] trimethylammonium iodide} (PMETAI) can also be used for the formation of interpolyelectrolyte complexes (IPECs) with oppositely charged polyions, such as poly(styrenesulfonate) (PSS).153 Chapter 1 – Introduction 28 salt-responsivepH-responsive b) a) Figure 1-6. (A) [(SiO)1.5-OEGMA]3200 core-shell CPBs with a partially inorganic core are rather stiff in solution (see cryo-TEM) and form (a) isotropic and (b) lyotropic phases upon concentrating.78 (B) Simulated results for the interaction of charged CPBs and surfactants. Depending on the stiffness of the backbone, the main chain can adopt various conformations from stiff cylinders to spheres.140 (C) DLS demonstrates that PDMAEMA brushes show pH-responsiveness and are salt-responsive after quaternization. AFM studies highlight the collapsed spherical structures after the increase of salt concentration of sodium bromide (NaBr).152 The solution properties of heterogeneous brushes, such as core-shell brushes are considerably more complicated. Borisov et al. have reported that the shape of core-shell brushes can be very different depending on the solvent quality for the core and the shell (see Figure 1-2D).97 1.3.2 Cylindrical Polymer Brushes on Surfaces and in the Bulk Similar to the solution properties, where the brushes behave according to the environment and changes therein, polymer brushes tend to behave differently according to the interaction between the individual blocks, the underlying substrate and the surrounding environment.127 Figure 1-7A shows possible morphologies of core-shell CPBs on a substrate. Depending on the strength of the adsorption and the brush architecture, it is Chapter 1 – Introduction 29 possible that molecular brushes undergo association and dissociation during their adsorption on the substrate.154 PnBA brushes associate due to the crystallization of the linear poly(octadecyl methacrylate) chains on both ends of the brush backbone. A more intriguing phenomenon is the scission of polymer brushes on substrates. Long CPBs with long side chains may undergo scission of the backbone upon the adsorption onto a substrate, such as graphite or mica.155 Different incubation times verified the proceeding scission of polymer brushes into sphere-like brush fragments (see Figure 1-7C). The macromolecular destruction originated from side-chain-induced stretching of the polymer backbone to maximize the number of contacts with the substrate. Moreover, CPBs can be used to study the motion and flow of molecules.156,157 It is further possible to order molecular brushes during spreading. Flow-enhanced diffusion of macromolecules resulted in epitaxial alignment of macromolecules, where the orientation was independent of the flow direction.158 In bulk, CPBs behave differently as compared to linear polymer chains. CPBs can be used to produce new materials with unusual mechanical properties. Molecular brush backbones are less entangled in bulk due to the large fraction of densely grafted side chains. This self-disentanglement results in unique viscoelastic properties, which depend on both the length of the main chain (backbone) and the side chains. Transformation of the brush films into crosslinked networks produced a high local mobility and sufficient macroscopic mechanical stability.111,159-161 The resulting class of materials are termed (super-) soft elastomers. Rzayev et al.further reported the phase-separation of block-type PS-PLA polymer brushes analogue to linear block copolymers.105,162 Consequently, it was possible to obtain a cylindrical bulk morphology from CPBs for the first time. After degradation of the cylinder-forming PLA block, a nanoporous polymer network was produced (see Figure 1-7B). The same PS-PLA polymer brush was used for controlled evaporative self-assembly of hierarchically structured bottlebrush block copolymers (see Figure 1-7D).163 In a special evaporation process, a toluene solution of the PS-PLA block copolymer brushes was aligned in gradient stripes with internal lamellar nanodomains. Chapter 1 – Introduction 30 Figure 1-7. (A) Possible morphologies adopted by core-shell CPBs on different substrates.127 (B) Phaseseparated PS-PLA block-type CPBs, which form a nanoporous network after PLA cylinder degradation.162 (C) Scission of long polymer brush backbones with long side chains.155 (D) Hierarchical structuring of block-type CPBs into gradient lines with internal lamellar phase-separation.163 1.4 Applications of Cylindrical Polymer Brushes The many different properties and high functionalities, together with the often straightforward syntheses, render CPBs useful tools for a plethora of applications in all kinds of fields of physics, chemistry, biology and material sciences. The 1D shape and the multiple, concentric and individually separated compartments make CPBs suitable to be used as delivery vehicles or templates. The following sections highlight the versatile applications of CPBs and their use as nanosized building blocks for the fabrication of hierarchically structured 1D materials. Not only can CPBs be used to visualize and demonstrate molecular processes,155,164 but more so, they can be used in medical and biological applications, such as potential drug delivery vehicles or gene transfection. Cell entry is dependent on the shape of the delivery vehicles.165 It is reported that the cell entry of 1D nanomaterials occurs by tip recognition and rotation,166 and that a cylindrical shape is advantageous regarding the retention time Chapter 1 – Introduction 31 in the body.167 Accordingly, Grubbs et al. recently reported the synthesis of drug loaded CPBs, where the drug can be released upon degradation of a linking group (see Figure 18).42,168 Figure 1-8. (A/B) Novel 1D drug delivery vehicles developed via the ‘grafting-through’ of functional and drug-containing bivalent macromonomers.42,168 Aside from the biological applications, CPBs often find use as template materials for the preparation of organic/inorganic hybrid nanomaterials. Among the different structures, core–shell CPBs are of special interest because they can be utilized in the synthesis of cylindrical hybrid nanostructures, such as nanowires and nanotubes, with interesting properties after metallization or other modification. The following section focuses on template-directed syntheses of 1D hybrid nanostructures, in which CPBs play an significant role in the bottom-up processes. Chapter 1 – Introduction 38 characteristics of a living process.24,203 Hence, it was possible to grow block-type cylindrical micelles where only a certain block were hybridized in a controlled way.24 2.1.3 Self-Assembled One-Dimensional Templates from Bulk As illustrated above, block copolymers can be used as soft templates as they can form 1D nanostructures in solution due to their different or incompatible blocks. The incompatibility of blocks is a necessity for the micro-phase separation of block copolymers in thin films. Phase-separated block copolymers have been studied extensively for the fabrication of hybrid and inorganic materials.204-207 Diblock copolymers can phase-separate into a cylindrical morphology, as illustrated in Scheme 11C, and can be used as 1D soft templates after crosslinking of the cylindrical domain. Template CPBs from PB-P2VP were used to produce polyoxometalate nanostructures after loading the P2VP corona with oppositely charged [SiMo12O40]4Keggin ions (see Figure 1-12A).175 Chen et al. used silicon containing and gel-able monomers to synthesize block copolymers where the crosslinked silsesquioxane (SiO1.5) products themselves can be considered as hybrid materials. SiO1.5 nanowires and tubes have been prepared from bulk thin films.208-210 Poly(3-(triethoxysilyl)propyl methacrylate)-blockpoly(2-vinylpyridine) (PTEPM-P2VP) was used for the synthesis of SiO1.5 nanowires with a P2VP shell, which were used in a subsequent step to immobilize gold nanoparticles within the shell.210 SiO1.5 nanotubes were obtained in the case of an ABC triblock terpolymer where the middle block B consisted out of PTEPM.208 Depending on the preparation of the bulk film, block A (PS) could be the outer corona and block C (P2VP) could be in the core, or vice versa (see Figure 1-12B). P2VP was again used for further immobilization of nanoparticles.208 The concept of ABC triblock terpolymers phase separation was also applied by Liu et al. to obtain polymeric nanotubes where the tube-forming block B (PCEMA) was photocrosslinkable .211-215 Depending on the core-forming polymer block, various 1D hybrid nanostructures were synthesized. In the case of a PAA core, Yan et al. produced waterdispersible polymer/Pd/Ni hybrid magnetic nanofibers by sequential filling of the core with Pd and Ni.213 They further reported γ-Fe2O3 hybrid magnetic nanofibers following a similar procedure.211 Chapter 1 – Introduction 39 a) b) a) Figure 1-12. (A) The synthetic strategy for the fabrication of Keggin ion nanostructures, including a SEM image of the hybrid nanofibers.175 (B) The self-assembly of ABC triblock terpolymers that contain a gelable middle block can form (a) nanotubes in bulk thin films. (b) Nanotubes with a P2VP core were filled with gold nanoparticles.208 2.1.4 Biological and Other One-Dimensional Templates Polymeric soft templates need to be shaped into 1D morphology by experimental efforts. However, nature provides already pre-existing 1D nanostructures that can be used for template chemistries. Typical examples of biological nanostructures that consist of building blocks that are aligned one-dimensionally are cellulose,216 collagen,217 DNA218222 and various viruses.223-227 The most famous soft template is the tobacco mosaic virus (TMV), which is very uniform in length and diameter. TMV was, for example, used to synthesize CdS, SiO2 and TiO2 nanowires (see Figure 1-13A).225,227 Next to TMV, DNA often finds use as a template material. Co nanowires were grown on Pd nanoparticles seeded DNA.228 Similar to biological systems, there are other 1D nanostructures that can be applied in template-assisted and template-directed hybridization reactions. Currently existing inorganic, organic and even hybrid 1D objects are useful as templates in their unmodified states. The most frequently used among the many available 1D structures are carbon nanotubes (CNTs). Their straightforward functionalization led to numerous works on Chapter 1 – Introduction 40 polymer-coated CNTs.229-231 These polymer coatings can then again be used for hybridization. For example, gold nanoparticles were attached to poly(diallyldimethylammonium)chloride (PDADMAC) covered CNTs (see Figure 1-13B).232 PDMAEMA-covered multi-walled CNTs (MWCNTs) were able to be used for the synthesis of worm-like silica nanotubes.233 Other 1D objects, such as nickel nanorods, were applied in several strategies to obtain hybrid materials.193,234 Pb(II)/ H 2 S Cd(II)/ H 2 S TEOS Fe(II)/ Fe(III) Gold Colloids Figure 1-13. (A) TMV was used to produce various 1D hybrid material, such as CdS nanowires (see TEM micrograph).225 (B) PDADMAC-covered CNTs were used to align gold nanoparticles into a 1D manner (see TEM micrograph).232 2.2 Porous Membrane-Based Templates The use of porous membranes, such as AAO, track-etched polycarbonate membranes or mesoporous silica, gave rise to simple and straightforward methods for the preparation of 1D nanomaterials. The main advantage of such porous templates with cylindrical pores is the superior uniformity of pore diameters, which can be adjusted during the preparation method. Many 1D materials have been synthesized, including metals,235 oxides,236 semiconductors237 and polymers.235 The process of filling the pores is highly variable and can be easily controlled, which consequently allows the synthesis of very complex nanostructures. However, when compared to the abovementioned soft templating techniques, it is quite difficult to obtain large amounts of materials from such approaches. 2.3 Electrospinning Electrospinning is not a templating technique per se; however, it should be mentioned as it is a highly versatile method for the production of 1D hybrid materials. With electrospinning, it became possible to synthesize ultra-thin nanowires or nanofibers. From many polymer melts or solutions, fibers have been produced via electrospinning processes. By adding inorganic materials or precursors into the melts and mixtures, hybrid nanomaterials of defined thickness have been obtained, either directly or after a Chapter 1 – Introduction 41 subsequent step. Combined with sol-gel chemistry, many polymer-metal oxide hybrid fibers have been synthesized by electrospinning, such as poly(vinylpyrrolidone)/TiO2 (PVP/TiO2) or PVP/ZrO2 hybrids.238 Furthermore, inorganic materials, like ZnO239,240 or CdS241, have been blended into polymeric nanofibers. Greiner et al. used electrospun PLA fibers as templates for the fabrication of TiO2 and Pd tubes 242,243 3. Aim of the Thesis The motivation of this work was to broaden the application range of CPBs as templates for the preparation of novel 1D polymer-inorganic hybrid nanomaterials. It was intended that molecular core-shell or core-shell-corona CPBs, plus CPBs obtained through microphase separation of diblock copolymers, be used for the synthesis of new 1D hybrid materials. Core-shell-corona CPBs containing a gelable shell block were to be synthesized in order to directly incorporate the inorganic part into the polymer brush structure. 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Science 2004, 303, 213-217. (224) Fonoberov, V. A.; Balandin, A. A. Nano Lett. 2005, 5, 1920-1923. (225) Shenton, W.; Douglas, T.; Young, M.; Stubbs, G.; Mann, S. Adv. Mater. 1999, 11, 253-256. (226) Dang, X.; Yi, H.; Ham, M.-H.; Qi, J.; Yun, D. S.; Ladewski, R.; Strano, M. S.; Hammond, P. T.; Belcher, A. M. Nat. Nano. 2011, 6, 377-384. Chapter 2 – Overview of the Thesis 54 Table 2-2. Template core-shell-corona CPBs with different dimensions. Name Compositiona Shell diameterb Template brush 1 [CL14-b-DMAEMA40-b-OEGMA65]7500 23 ± 2 nm Template brush 2 [CL 14 -b-DMAEMA 150 -b-OEGMA 240 ] 7500 39 ± 2 nm a Composition as determined by 1H-NMR. b Shell diameter after TALH infiltration. The loading of the PDMAEMA shell was performed in a water/ethanol mixture by the drop-wise addition of titanium(IV) bis(ammonium lactate) dihydroxide (TALH). TALH is negatively charged and therefore immobilizes only into the PDMAEMA shell compartment, whereas the POEGMA corona cannot complex TALH and remained unloaded. The negatively charged titania precursor complexed exclusively with the amino groups of PDMAEMA, forming concentric TALH nanotubes. Furthermore, TALH shows superior stability in aqueous solution at room temperature, compared to other titania precursors, like titanium(IV) butoxide, which hydrolyze rather rapidly in the presence of water and hence can also deposit in undesired regions, such as the corona. The filling of the template brush with inorganic material was documented by AFM measurements. Filled and hydrolyzed template brushes showed a significant increase in height as compared to the unfilled and pristine CPBs (see Figure 2-3). 50 100 150 200 250 300 350 0 3 6 9 12 15 Height [nm] Width [nm] 60 90 120 150 180 210 240 0.0 0.5 1.0 1.5 Width [nm] Height [nm] Figure 2-3. (A) [CL14-b-DMAEMA40-b-OEGMA65]7500 core-shell-corona CPBs were infiltrated with TALH and produced (B) anatase nanotubes after hydrolysis and condensation. The height of pristine CPBs was 1.5 nm and increased to 15 nm for the anatase nanotubes. Chapter 2 – Overview of the Thesis 55 The filling of the template brushes resulted in very defined TiO2 hybrid nanotubes after hydrolysis and condensation. TALH guaranteed stability upon hydrolysis during the loading of the template at ambient temperatures. Only above 70 °C, TALH hydrolyzed thermally and crystallized into anatase. Cryo-TEM and grey-scale analysis illustrated the tubular morphology of the hybrid nanostructures. TEM micrographs highlighted the uniform thickness of the nanotubes, which could be adjusted by the length of the PDMAEMA side chains (see Figures 2-4 B and C). The amount of TiO2 within the hybrid nanotubes was determined by thermogravimetric analysis (TGA). TGA revealed that longer PDMAEMA side chains increased the loading capacity of the template (see Figure 2-4 F). High resolution TEM (HR-TEM) and powder X-ray diffractometry (PXRD) verified the high crystallinity of the anatase hybrid nanotubes (see Figures 2-4 D and E). Scanning electron microscopy (SEM) highlighted the uniform diameter of the hybrid nanotubes before and after pyrolysis (see Figure 2-5). Figure 2-4. (A) Cryo-TEM and (B) TEM micrographs of anatase nanotubes from template brush 1. (C) TEM micrograph of anatase nanotubes from template brush 2. (D) HR-TEM micrograph of a highly crystalline anatase nanotube. (E) PXRD of anatase nanotubes. (F) TGA of the hybrid nanostructures. Chapter 2 – Overview of the Thesis 56 Figure 2-5. SEM images of hybrid nanotubes from (A) brush template 1 and (B) brush template 2. (C) Calcined hybrid nanostructures from template brush 2. In conclusion, our templating strategy towards crystalline anatase nanotubes has proved to be very effective and versatile in producing well-defined hybrid nanomaterials. The core-shell-corona template brushes provided excellent solubility in various media and prevented crosslinking during hydrolysis and condensation. Highly crystalline 1D TiO2 nanomaterials were obtained, which may serve useful as catalysts or in photovoltaic applications. 2.3 Silica Nanowires and Nanotubes Silica-based materials are attractive materials due to their chemical inertness, corrosion resistance, and mechanical and thermal stability. In this chapter, we used core-shell CPBs as unimolecular soft templates for the synthesis of 1D silica hybrid nanostructures (see Scheme 2-3). Through the combination of anionic polymerization, ROP and ATRP, we produced coreshell CPBs with a degradable core (namely, PCL) and a polyelectrolyte shell (namely, PDMAEMA). The PCL core also acted as a spacer for the initiation sites for ATRP and thereby increased the grafting efficiency of PDMAEMA to 90 % compared to 50-70 % of PBIEM, as reported in literature.1-3 These unimolecular template brushes were then used for the production of pure hybrid silica (see Scheme 2-3 iv) or nanoparticle-doped hybrid silica nanostructures (see Scheme 2-3 vi). By varying the DPn of the backbone and the side chains and consequently the dimensions of the template brush (see AFM images in Figure 2-6 A and D), we adjusted the dimensions of the later silica nanostructure, i.e. the thickness of the core and the shell (see Figure 2-6). Chapter 2 – Overview of the Thesis 57 Scheme 2-3. Schematic illustration of the template build-up achieved by combining multiple polymerization techniques. (i) PHEMA was ‘grafted-from’ via ROP of ε-caprolactone and ATRP of DMAEMA to produce a (ii) core-shell CPB. The template brush was then infiltrated with (iii) silica or (v) metal salts (such as AuCl4or PtCl42-), (vi) prior to silica infiltration into the shell. (iv) Calcination of acid treatment produced hollow silica nanotubes. Figure 2-6. AFM height images of template brushes (A) [CL25-b-DMAEMA76]2700 and (D) [CL14-bDMAEMA43]7500. TEM micrographs of 1D silica hybrid nanostructures from template brushes (B, b) [CL10b-DMAEMA58]2700, (C, c) [CL25-b-DMAEMA76]2700, (E, e) [CL14-b-DMAEMA43]7500 and (F, f) [CL14-bDMAEMA342]7500. Chapter 2 – Overview of the Thesis 58 Consequently, we obtained various 1D silica hybrids for various template brushes (see Table 2-3). Short PDMAEMA side chains gave smooth silica hybrid nanostructures. With increasing length of the PDMAEMA side chains, the hybrid nanostructures became more and more ‘hairy’, as longer side chains tended to form bundles and threads upon silica deposition (see Figure 2-6 E and F). Table 2-3. 1D silica hybrids with different dimensions (in nm) Template compositiona Template lengthb Hybrid lengthc Core diameterc Silica shell diameterc [CL10DMAEMA58]2700 295 ± 20 270 ± 15 5-6 ~25 [CL25DMAEMA76]2700 265 ± 20 235 ± 20 10-12 ~35 [CL14DMAEMA43]7500 1250 ± 200 950 ± 350 6-7 ~25 [CL14DMAEMA342]7500 1250 ± 200 950 ± 350 d 6-7 e ~ 85 f a Determined by 1H-NMR; b measured from AFM images of the template brushes; c measured from TEM micrographs of the as-synthesized silica hybrids; d estimated from TEM micrographs, as it was rather difficult to directly measure the actual length due to the jamming of hairy silica nanostructures; e assumed to be the same dimension as with [CL14DMAEMA43]7500; f taking into account that the core is ~6 nm in diameter. SEM images highlight the network-like structures of the dried hybrid materials (see Figure 2-7). The 1D nanostructures tended to pack closer as the aspect ratio decreased. The material quite resembled the structure of common filter systems. Calcination or treatment with an acid led to the removal of the core and resulted in hollow silica nanostructures. Figure 2-7. SEM images of 1D silica hybrid nanostructures from template brushes (A) [CL10-bDMAEMA58]2700, (B) [CL14-b-DMAEMA43]7500 and (C) [CL14-b-DMAEMA342]7500. We further embedded catalytically active metal nanoparticles (NPs) into the shells of the anisotropic nanomaterials to obtain catalytically active nanomaterials. Figure 2-8 shows TEM micrographs of NP-doped nanostructures. Gold and platinum NPs are visible within the silcia shell. The reduction of 4-nitrophenol to 4-aminophenol by NaBH4 in the presence of the metal NP-doped silica nanomaterials was performed to demonstrate the Chapter 2 – Overview of the Thesis 59 accessibilty and the activity of the catalytically active material. Thus, the incorporation of catalytically active NPs renders very robust carriers for catalysts, which can be easily removed from the system after the reaction. Additionally, the embedment of metallic NPs into silica allows the structures to be used in high temperature applications. This material might also be interesting in the application of catalytically active filter systems. Figure 2-8. TEM micrographs of (A) a silica hybrid doped with Au NPs from template brush [CL14-bDMAEMA342]7500 and (B) a silica hybrid doped with Pt NPs from template brush [CL14-b-DMAEMA43]7500. 2.4 Mesostructuring of TiO2 Nanocrystals into One-Dimensional Nanostructures 1D nanostructures of metal oxides have been under close investigation due to their sizedependent optical and electronical properties, which allow them to be used in catalysis, separation or photovoltaic applications. In this chapter, we demonstrate a highly applicable synthesis concept applied for structuring metal oxides into 1D hybrid nanostructures by a template-directed approach. 1D polyelectrolyte template brushes were produced from a polystyrene-b-poly(allyl methacrylate) (PS-b-PAMA) diblock copolymer, which was obtained by sequential anionic polymerization with narrow molecular weight distribution. Microphase separation of the diblock copolymer in bulk resulted in hexagonally packed PAMA cylinders within a PS matrix. After photocrosslinking of the PAMA cylinders and re-dispersion of the bulk film in THF, cylindrical polymer brushes were obtained. The PS corona was mildly sulfonated in a subsequent step to transform the PS into poly(styrenesulfonic acid) (PSS) and render the corona water-soluble (see Scheme 2-4 ii and iii). Chapter 2 – Overview of the Thesis 60 Scheme 2-4. (i) Diblock copolymer PS-PAMA microphase-separated into hexagonally packed cylinders. (ii) UV-crosslinked PS-PAMA cylindrical polymer brushes are redispersed and (iii) sulfonated into polyelectrolyte brushes with a PSS corona. (iv) Positively charged and pre-synthesized TiO2 nanocrystals are infiltrated into the PSS corona to produce (v) anisotropic and crystalline TiO2 nanowires. The strongly anionic polyelectrolyte brushes (see Figure 2-9 A) were then used as templates for the fabrication of crystalline 1D TiO2 nanostructures by infiltration of oppositely charged TiO2 nanocrystals into the polyelectrolyte corona. The nanocrystals were produced separately prior to the formation of the hybrid. We could adjust the crystallinity of the nanocrystals to either rutile or antase by using different acids for the hydrolysis of the titania precursor. Phase purity of the crystalline colloids was confirmed via PXRD. Their apparent hydrodynamic diameter was either 8 nm (rutile) or 14 nm (anatase), as determined by dynamic light scattering. Figure 2-9. (A) Cryo-TEM micrograph of PSS-PAMA template brushes in water. (B/C) TEM micrographs of as-synthesized 1D rutile nanostructures. SEM micrographs of (D) as-synthesized rutile nanowires and (E) calcined rutile nanowires. Drop-wise addition of a template brush suspension to a particular amount of the respective nanocrystal suspension at 60 °C and pH 1 resulted in highly crystalline hybrid nanostructures (see Figure 2-9). The hybrid nanowires adopted the same crystallinity as the infiltrated nanocrystals, as confirmed by PXRD (see Figures 2-10 A and D). Chapter 2 – Overview of the Thesis 61 Furthermore, HR-TEM highlights the high crystallinity of the hybrid nanostructures (see Figures 2-10 B and E). Figure 2-10. (A and D) PXRD patterns of crystalline TiO2 precursors (red pattern) and as-synthesized TiO2 hybird nanomaterials (black pattern). HR-TEM micrographs and SAED pattern of (B and C) highly crystalline rutile and (E and F) anatase hybrid nanowires. N2 physisorption measurements showed a high surface area of 66 m2·g-1 for the rutile hybrid nanowires. The hybrids formed porous non-woven networks upon drying (see Figure 2-9 D), which leads to the assumption that the nanowires are still quite flexible. TGA supports the assumption, as it confirmed that the rutile hybrid nanomaterial consists of around 50 wt% soft polymeric material. In conclusion, we have developed a mild and generally applicable method to mesostructure metal oxides into 1D hybrid nanostructures. The template-directed synthesis of 1D hybrid nanomaterials via cylindrical polyelectrolyte brushes was demonstrated on titania polymorphs, both rutile and anatase, which were selectively mesostructured into hybrid nanowires. Chapter 2 – Overview of the Thesis 62 2.5 Individual Contributions to Joint Publications The results presented in this thesis were obtained in collaboration with others, and have been published or submitted to publication as indicated below. In the following, the contributions of all the co-authors to the different publications are specified. The asterisk denotes the corresponding author(s). Chapter 3 This work is published in the Journal of the American Chemical Society 132, 1658716592 (2010) under the title: “Water-Soluble Organo-Silica Hybrid Nanotubes Templated by Cylindrical Polymer Brushes” by Markus Müllner, Jiayin Yuan, Stephan Weiß, Andreas Walther, Melanie Förtsch, Markus Drechsler, and Axel H. E. Müller* I conducted all experiments and wrote the publication, except that: • S. Weiß was involved in early experiments during a lab course; • Walther was involved in discussions; • M. Förtsch and M. Drechsler performed the cryo-TEM measurements; and • J. Yuan and A. H. E. Müller were involved in scientific discussions and correcting the publication. Chapter 4 This work will be submitted under the title: “Template-Directed Mild Synthesis of Anatase Hybrid Nanotubes within Cylindrical Core-Shell-Corona Polymer Brushes” by Markus Müllner, Thomas Lunkenbein, Martin Schieder, Nobuyoshi Miyajima, Melanie Förtsch, Josef Breu, Frank Caruso,* and Axel H. E. Müller* I conducted all experiments and wrote the manuscript, except that: • M. Schieder and T. Lunkenbein both performed SEM and PXRD measurements; • N. Miyajima performed the HR-TEM measurements; • M. Förtsch performed the cryo-TEM measurements; and • J. Breu, F. Caruso and A. H. E. Müller were involved in correcting the manuscript. Chapter 2 – Overview of the Thesis 63 Chapter 5 This work has been published in Chemistry of Materials under the title: “Template-Directed Synthesis of Silica Nanowires and Nanotubes from Cylindrical Core-Shell Polymer Brushes” by Markus Müllner, Thomas Lunkenbein, Josef Breu, Frank Caruso, and Axel H. E. Müller* I conducted all experiments and wrote the manuscript, except that: • T. Lunkenbein performed SEM and EDX measurements; and • J. Breu and F. Caruso were involved in correcting the manuscript. • A. H. E. Müller was involved in scientific discussions and correcting the manuscript Chapter 6 This will has been published in Small under the title: “A Facile Polymer Templating Route Toward High Aspect Ratio Crystalline Titania Nanostructures” by Markus Müllner, Thomas Lunkenbein, Nobuyoshi Miyajima, Josef Breu,* and Axel H. E. Müller* This is a joint project between the chairs of AC I and MC II. I conducted all experiments concerning the preparation and the analysis of the polymeric templates. I further assisted in the preparation and the analysis of the nanocrystals and the hybrid materials. I was involved in scientific discussions and wrote the manuscript. T. Lunkenbein performed most of microscopy experiments and the characterization of the hybrid materials. He further developed the synthesis of the nanocrystals and assisted in the characterization of the sulfonated templates. He was involved in discussions and correcting the manuscript. Further: • N. Miyajima performed one of the HR-TEM measurements; and • J. Breu and A. H. E. Müller were involved in scientific discussions and correcting the manuscript. Chapter 3 – Organo-Silica Hybrid Nanotubes 70 The wormlike shape of CPBs has been employed to fabricate inorganic one-dimensional (1D) nanostructures,28 such as γ-Fe2O3,29 CdS,30 CdSe,31 Au,32 and titania33 nanowires. Commonly, in a solution approach, the inorganic precursors have been first localized in the cylindrical core area by selectively interacting with the CPB core block. Through chemical reactions occurring only within the core, the precursors have been converted into corresponding functional inorganic nanomaterials, which were spatially organized by the cylindrical template to adopt a wire-like geometry. The CPB shell, free of interaction with the inorganic moieties, protects the formed inorganic nanowires from agglomeration and solubilizes them in solvents. Solubility in water or organic solvents and biocompatibility of the hybrid nanowires can be achieved by the design of the shell block.33,34 Freestanding, purely inorganic nanowires can be achieved by pyrolytic removal of the polymeric template on a solid substrate. In general, the dimensions of the desired 1D inorganic nanostructure are strictly controlled by the CPBs. For example, the diameter depends on the length of the block in the CPB core, and the length is largely determined by the degree of polymerization of the backbone.34 We recently reported a novel strategy to form hybrid cylinders with an organo-silica core, where the precursor for the inorganic part is a building unit of the core itself.34 Organo-silica hybrid nanowires were produced by using poly[(3-acryloxypropyl) trimethoxysilane] (PAPTS) as the core and poly[oligo(ethylene glycol) methacrylate] (POEGMA) as the corona, followed by hydrolytic condensation of the PAPTS core block to form a crosslinked silsesquioxane structure, which could be pyrolized to form pure silica nanowires.35 Cylindrical or tubular hybrid materials that are not derived from CPBs have been synthesized by using block copolymers as directing agents.18,36 So far, only core-shell structured CPBs with diblock copolymer side chains have been chosen as synthetic 1D templates. Herein, we demonstrate the first time that core-shellcorona structured CPBs with triblock terpolymer side chains are employed as an in-situ template for the construction of organo-silica hybrid nanotubes, which are soluble in various solvents. Firstly, block terpolymer side-chains of poly(tert-butyl acrylate)-blockPAPTS-block-POEGMA were grown from a poly(2-(2-bromoisobutyryloxy)ethyl methacrylate) (PBIEM) polyinitiator backbone via ATRP. They were then used as a unimolecular cylindrical template for the in-situ fabrication of water-soluble organo-silica hybrid nanotubes via condensation of the PAPTS shell block. The formed tubular Chapter 3 – Organo-Silica Hybrid Nanotubes 71 structures were characterized by transmission electron microscopy (TEM), cryogenic TEM (cryo-TEM) and atomic force microsopy (AFM). Soft tubular nanostructures have also been prepared from small surfactants37,38 amphiphilic block copolymers39-43 or multicomponent copolymer cylindrical brushes.44,45 However, most of these conventional tubular structures are only dynamically stable and can collapse upon a tiny perturbation in the external environment such as a solvent, temperature, concentration, or pH change. In addition, the size and size distribution of assembled structures are usually hard to control. In contrast, due to the living / controlled polymerization techniques employed in the preparation of CPBs, the obtained hybrid tubular structures are uniform in diameter and length. They are stable and tolerant to variations in their environment because the shape and structure of each nanotube is covalently locked. Experimental Section Materials. All chemicals were of analytical grade and used as received without further purification, except that (3-acryloxypropyl)trimethoxysilane (APTS) (95%, ABCR) was freshly distilled, and tert-butyl acrylate (tBA) (98%, Aldrich) and oligo(ethylene glycol) methacrylate (OEGMA) (98%, Aldrich) were filtered through a basic alumina column shortly before each polymerization. Preparation of core-shell-corona CPB [tBA75-APTS115-OEGMA150]3200. The poly(macroinitiator) backbone poly(2-(2-bromoisobutyryloxy)ethyl methacrylate) (PBIEM) was prepared by anionic polymerization of 2-(trimethysilyloxy)ethyl methacrylate, acidic cleavage of the trimethylsilyl groups, and an esterification reaction to attach the ATRP initiating sites onto each repeating unit as detailed earlier.13 The degree of polymerization (DP) of the PBIEM polyinitiator backbone is 3200, and its polydispersity index, determined by gel permeation chromatography (GPC), is 1.14. The synthesis of a PtBA homopolymer CPB in anisole was detailed in our previous paper.13 The initiating efficiency of the PBIEM poly(macroinitiatior) backbone towards tBA was determined as 0.65 by cleaving the PtBA side chains and determining their molecular weight by GPC. The ATRP of APTS for the shell block and OEGMA for the corona block was conducted exclusively in benzene in order to suppress the hydrolysis and condensation of the trimethoxysilyl groups in the PAPTS shell block.34 Typically, in a flask equipped with a Chapter 3 – Organo-Silica Hybrid Nanotubes 72 septum, CuBr, the poly(macroinitiator), and the monomer (APTS or OEGMA) were added in benzene. The mixture was degassed and stirred until complete dissolution of the poly(macroinitiator) and then heated to 110 °C (in the case of APTS) or 80 °C (in the case of OEGMA). Finally, the degassed ligand, N,N,N',N",N''-pentamethyldiethylenetriamine (PMDETA), was injected to start the polymerization and an initial sample was taken for 1H-NMR measurement. The polymerization was monitored by withdrawing samples for 1H-NMR measurements. When a desired conversion was achieved, the reaction was quenched by cooling the reaction mixture to room temperature and exposing it to air. The reaction mixture was purified by filtration through a basic alumina column, and by ultrafiltration using benzene as the eluent under nitrogen atmosphere. Preparation of [(tBA)75-b-(SiO1.5)115-b-(OEGMA)150]3200 hybrid organo-silica nanotubes. 400 mg of [tBA75-APTS115-OEGMA150]3200 core-shell-corona CPBs in 200 ml 1,4-dioxane was mixed with 20 ml of a 25% aqueous solution of ammonia. The reaction mixture was kept under constant stirring at room temperature for 5 days to complete the condensation of the trimethoxysilyl groups. The ammonia was largely removed by rotational evaporation at 30° C and the resulting solution was purified by dialysis against dioxane. Characterization Methods. Gel Permeation Chromatography (GPC) in THF was conducted at an elution rate of 1 mL/min using PSS SDVgel columns (300 X 8mm, 5 µm): 105, 104, 103, and 102 Å and RI and UV (λ =254 nm) detection. Poly(tert-butyl acrylate) calibration curve was used to calibrate the columns, and toluene was used as an internal standard. Atomic force microscopy (AFM) images were recorded on a Digital Instruments Dimension 3100 microscope operated in tapping mode. The samples were prepared by dip-coating from dilute solutions (0.02 g/L) of the polymer brush or hybrid nanotubes solution in dioxane or benzene onto a clean silicon wafer or freshly cleaved mica to form a monomolecular film. Transmission electron microscopy (TEM) images were taken on a Zeiss EM EF-TEM instrument operated at 200 kV. A 5µL droplet of a dilute solution (0.05 g/L) in dioxane or benzene was dropped onto a copper grid (200 mesh) coated with carbon film, followed by blotting the liquid and drying at room temperature for a short time. Chapter 3 – Organo-Silica Hybrid Nanotubes 73 Cryogenic transmission electron microscopy (cryo-TEM) was conducted by dropping the aqueous dilute solution (0.1 g/L) on a hydrophilized lacey TEM grid, where most of the liquid was removed with blotting paper, leaving a thin film stretched over the grid holes. The specimens were shock frozen by rapid immersion into liquid ethane and cooled to approximately 90 K by liquid nitrogen in a temperature-controlled freezing unit (Zeiss Cryobox, Zeiss NTS GmbH, Oberkochen, Germany). After the specimens were frozen, the remaining ethane was removed using blotting paper. The specimen was inserted into a cryo-transfer holder (CT3500, Gatan, München, Germany) and transferred to a Zeiss EM922 EF-TEM instrument operated at 200 kV. Cryo-TEM samples from organic solvents, such as THF, were shock frozen in liquid nitrogen, respectively. Proton nuclear magnetic resonance (1H-NMR) spectra were recorded to determine the monomer conversion on a Bruker AC-300 spectrometer at room temperature in CDCl3. Results and Discussion ATRP was employed to graft PtBA-b-PAPTS-b-POEGMA block terpolymer side chains from a PBIEM polyinitiator backbone, along which 3200 ATRP initiating sites were tethered onto each repeating unit.13 As shown in the general synthetic route in Scheme 31, three monomers - namely tert-butyl acrylate (tBA), (3-acryloxypropyl)trimethoxysilane (APTS) and oligo(ethylene glycol) methacrylate (OEGMA) - were sequentially polymerized in anisole or benzene using CuBr / PMDETA as the catalytic system. Finally, the PAPTS shell block of the obtained core-shell-corona structured CPBs was condensed into a silsesquioxane network in the shell. Chapter 3 – Organo-Silica Hybrid Nanotubes 74 Scheme 3-1. Synthetic route to obtain water-soluble organo-silica hybrid nanotubes templated by coreshell-corona structured CPBs. (A) ATRP polyinitiator backbone (PBIEM) with DP ~ 3200; (B) core-shellcorona structured CPB [tBA75-b-APTSx-b-OEGMAy]3200; and (C) water-soluble organo-silica hybrid nanotubes [tBA75-b-(SiO1.5)x-b-OEGMAy]3200. To confirm the successful introduction of each block into the side chains, 1H-NMR spectra were recorded at each block growth step. When tBA was polymerized from the PBIEM polyinitiator backbone, the 1H-NMR peaks of PBIEM in Figure 3-1A completely vanished due to their rather low content (< 3%). Instead, the homopolymer CPB [tBA75]3200 (Figure 3-1B) showed a characteristic sharp peak at 1.44 ppm, assigned to the protons in the tert-butyl groups. The PtBA homopolymer CPBs were then used as the poly(macroinitiator) for the growth of the PAPTS shell. Figure 3-1C shows the 1H-NMR spectrum of the diblock copolymer CPBs [tBA75-b-APTS50]3200. Besides the peak at 1.44 ppm, another intensive peak appears at ~3.5 ppm, indicating the appearance of trimethoxysilyl groups corresponding to the successful growth of the PAPTS block. In the same manner, the block copolymer CPBs [tBA75-b-APTS50]3200 was used as poly(macroinitiator) for the ATRP of OEGMA. The intensity of the peak at 3.5 ppm (Figure 3-1D/E) is enhanced due to the overlapping of the ethylene proton signals of the oligo(ethylene glycol) moieties and those of the trimethoxysilyl groups. Chapter 3 – Organo-Silica Hybrid Nanotubes 75 Figure 3-1. 1H-NMR spectra of: (A) PBIEM polyinitiator backbone, (B) [tBA75]3200 CPB, (C) [tBA75-bAPTS50]3200 CPB, (D) [tBA75-b-APTS50-b-OEGMA30]3200 CPB, and (E) [tBA75-b-APTS50-b-OEGMA300]3200 CPB. All samples were measured in CDCl3. We found the length of the POEGMA block to be very crucial to the success of the synthetic strategy. A short POEGMA corona (DP = 30, 1H-NMR in Figure 3-1D) resulted in an insufficient screening, leading to intermolecular coupling and resulting in large agglomerates that are unstable in solution. Therefore, various terpolymer brushes with a rather long POEGMA corona were synthesized via the “grafting from” approach. After the dialysis of [tBA75-b-APTSx-b-OEGMAy]3200 from benzene to dioxane, the condensation of the PAPTS shell was carried out by aqueous ammonia. The trimethoxysilyl groups were condensed into a crosslinked silsesquioxane shell. The crosslinked products, [tBA75-b-(SiO1.5)x-b-OEGMAy]3200 organo-silica hybrid nanotubes, are stable in various solvents, like non-polar benzene and toluene, as well as polar methanol and water. Table 3-1 summarizes the synthesized organo-silica hybrid nanotubes and their dimensions in aqueous solution. Chapter 3 – Organo-Silica Hybrid Nanotubes 76 Table 3-1. Organo-silica hybrid nanotubes with different dimensionsa (in nm) Nanotube compositionb Lengthc Tube diameterc Shell thicknessc,d tBA75-b-(SiO1.5)50-b-OEGMA300 460 ± 120 18 ± 2 ~4 tBA 75 -b-(SiO 1.5 ) 115 -b-OEGMA 150 330 ± 70 27 ± 3 ~10 tBA 75 -b-(SiO 1.5 ) 170 -b-OEGMA 400 285 ± 55 33 ± 3 ~14 a Polymethacrylate backbone with 3200 repeating units. b Nanotube composition after crosslinking. c Length as evaluated from cryo-TEM measurements, d taking into account that the PtBA core is always around 8 ± 1 nm. Molecular visualization via atomic force microscopy (AFM) on mica or silicon wafer has been proven to be a powerful characterization method to verify the successful synthesis and the morphological changes of CPBs.10,34 Figures 3-2A-I are the AFM images of intermediate and final product CPBs at each synthetic step. Figure 3-2A shows a densely packed monolayer of [tBA75]3200 CPBs with uniform diameter and narrow length distribution. A statistical measurement determines that their average length is 285 ± 74 nm. The cross-section analysis of a single flattened PtBA CPB (Figure 3-2B) shows a height in its center of 1.7 nm (Figure 3-2C). It is reported that the repulsion among the dense side chains increases with the side chains length and monomer bulkiness.1,46,47 Here, by extending the side chains by grafting PAPTS as the shell block, the repulsion between the side chains increases as expected. Figures 3-2D and 3-2E show the AFM images of the diblock copolymer CPBs [tBA75-b-APTS115]3200. The average length is measured to be 375 ± 50 nm, 30% longer than that of the [tBA75]3200 CPBs. The crosssection analysis of the individual CPBs (Figure 3-2F) reveals an increase in the height up to 4.5 nm, ca. 200 % higher than that of [tBA75]3200. In the absence of a corona block, during the condensation step, the diblock copolymer [tBA75-b-APTS115]3200 CPBs undergo both intramolecular and intermolecular crosslinking, which precipitates the CPBs out of solution. Thus a corona block is required to screen the intermolecular coupling before the condensation step and acts as a protective layer. Therefore, a POEGMA block with a DP of 150 was grafted (Figure 3-2G-I) to obtain the final [tBA75-b-APTS115-bOEGMA150]3200 CPB. The average length slightly increased to 400 ± 50 nm. The crosssection analysis revealed a further increase in height to 8.2 nm. The widths (Figure 3-2 C/F/I) also increased with each polymerization step. However, the worm-like structures appear broader in AFM than in TEM or cryo-TEM measurements due to their spreading Chapter 3 – Organo-Silica Hybrid Nanotubes 77 on the silicon wafer surface. It is possible to visualize the POEGMA corona with AFM (Figure 3-2G/H), however the values for the width of above 150 nm can only derive from the worms being spread out onto the silicon wafer surface. Figure 3-2. Tapping-mode AFM height images (overview and close view) and the corresponding height cross-section analysis of [tBA75]3200 (A-C), [tBA75-b-APTS115]3200 (D-F), [tBA75-b-APTS115-bOEGMA150]3200 (G-I), and [tBA75-b-(SiO1,5)115-b-OEGMA150]3200 (J-L). Z-ranges are 5 (A), 8 (B), 9 (D), 10 (E), 15 (G), 20 (H/J), and 25 nm (K), respectively. The scale-bars correspond to 500 nm (A/D/G/J) and 100 nm (B/E/H/K), respectively. Chapter 3 – Organo-Silica Hybrid Nanotubes 78 As mentioned, the crosslinked product, [(tBA)75-b-(SiO1.5)115-b-(OEGMA)150]3200 organosilica hybrid nanotubes, are stable in various solvents and water. AFM images of the hybrid organo-silica nanotubes are shown in Figure 3-2J/K. The cylindrical morphology was maintained during the complete synthetic route, and actually shaped the silsesquioxane network into a tubular structure. Interestingly, the average length of the crosslinked [tBA75-b-(SiO1,5)115-b-OEGMA150]3200 shrinks from 400 ± 50 to 300 ± 60 nm; meanwhile, their height increases further to 13.2 nm (Figure 3-2L), ~60% higher than the precursors (8.2 nm). The longitudinal size contraction and horizontal size expansion result from the intramolecular crosslinking of the side chains. Since more chemical bonds are generated among the side chains in the condensation process, the repulsion force among the side chains is largely compensated. At the same time, both the PtBA core and the hybrid silica shell were chemically locked in the CPB center and could not spread over the surface, which enhances the height in the CPB center. Treatment with hydrogen fluoride in THF opens the silsesquioxane network again. Due to the missing crosslinks, the backbone is then able to stretch again. In the case of [tBA75-b-(SiO1.5)170-bOEGMA400]3200, the average length of the backbone increased from 285 nm to above 400 nm (see Supporting Information 3-S3). As AFM measurements only depict the surface morphology, the intrinsic structure of these hybrid nanotubes was revealed by TEM and cryo-TEM measurements. With TEM characterization, nanotubes appear lighter in the center than at the wall, similar to carbon nanotubes. However, for the hybrid organo-silica nanotubes synthesized here, the core is not empty, but filled partially with PtBA polymer. Since the polymer has a weak contrast compared to inorganic or hybrid materials, a tubular structure is thus still expected. In the dry state, the hybrid nanotubes in normal TEM measurements show worm-like morphology (Figure 3-3), indicating that the cylindrical templates work efficiently for the present synthetic strategy. The nanotubes enlarged in Figure 3-3B/C/E appear lighter in the core, as expected. The diameters of the core and of the shell of [tBA75-b-APTS115-bOEGMA150]3200 in the dry state are 13-17 nm and 33-37 nm. That gives a wall thickness of ~10 nm. For the [tBA75-b-APTS170-b-OEGMA400]3200 nanotubes, the diameter of the core stays around 14-17 nm, where as the total diameter (core and shell) increases to around 45-54 nm. This results in a shell thickness of approximately 15.5 to 18.5 nm (in dry state). Chapter 3 – Organo-Silica Hybrid Nanotubes 79 Figure 3-3. TEM characterization of organo-silica nanotubes in THF: non-stained TEM images of [tBA75b-APTS115-b-OEGMA150]3200 (A/B/C), and [tBA75-b-APTS170-b-OEGMA)400]3200 (D/E); (B/C/E) are closeups of non-stained hybrid nanotubes. The scale-bars are 100 nm (B/C/E) and 200 nm (A/D), respectively. Figure 3-4. Cryo-TEM images of (A) non-crosslinked nanotubes [tBA75-b-APTS170-b-OEGMA400]3200 in THF; (B) non-crosslinked nanotubes [tBA75-b-APTS115-b-OEGMA150]3200 in water; (C) hybrid nanotube [tBA75-b-(SiO1.5)50-b-OEGMA300]3200 in water; (D) hybrid nanotubes [tBA75-b-(SiO1.5)115-b-OEGMA150]3200 in water; (E) hybrid nanotubes [tBA75-b-(SiO1.5)170-b-OEGMA400]3200 in water; and (F) a single hybrid nanotube in aqueous solution (the insert is a gray scale analysis of the area shown in image F). The scalebars represent 200 nm (A-E) and 20 nm (F), respectively. As shown above, TEM investigations clearly confirmed the tubular structures. However, the weak contrast of the non-crosslinked organo-silica nanotubes in cryo-TEM measurements in THF made it difficult but possible to depict the tubular structures Chapter 3 – Organo-Silica Hybrid Nanotubes 86 500 nm 1µm AB Figure 3-S3. TEM images of HF treated [tBA75-b-(SiO1.5)170-b-OEGMA400]3200 Chapter 4 – Anatase Hybrid Nanotubes 87 Chapter 4 Template-Directed Mild Synthesis of Anatase Hybrid Nanotubes within Cylindrical Core-Shell-Corona Polymer Brushes The results of this chapter will be submitted as: “Template-Directed Mild Synthesis of Anatase Hybrid Nanotubes within Cylindrical Core-Shell-Corona Polymer Brushes” by Markus Müllner, Thomas Lunkenbein, Martin Schieder, Nobuyoshi Miyajima, Melanie Förtsch, Josef Breu, Frank Caruso,* and Axel H. E. Müller* Chapter 4 – Anatase Hybrid Nanotubes 88 Chapter 4 – Anatase Hybrid Nanotubes 89 Abstract: We demonstrate the synthesis of uniform one-dimensional (1D) titania nanostructures using core-shell-corona cylindrical polymer brushes (CPBs) as soft templates. The CPBs consist of a polymethacrylate backbone with densely grafted poly(εcaprolactone) (PCL) in the core, poly(2-(dimethlamino)ethyl methacrylate) (PDMAEMA) in the cationic shell and poly(oligo(ethylene glycol) methyl ether methacrylate) (POEGMA) as the corona. The weak polyelectrolyte shell complexed an oppositely charged titania precursor, namely titanium(IV) bis(ammonium lactate) dihydroxide (TALH), and then acted as a nanoreactor for the hydrolysis and condensation of TALH, resulting in TiO2. The POEGMA shell provides solubility in aqueous and organic solvents. The hybrid titania nanotubes containing crystalline anatase nanoparticles were characterized by atomic force microscopy (AFM), transmission electron microscopy (TEM) and scanning electrion microscopy (SEM). The phase purity of the crystalline nanostructures was verified by powder X-ray diffractometry (PXRD). Chapter 4 – Anatase Hybrid Nanotubes 90 Chapter 4 – Anatase Hybrid Nanotubes 91 Introduction One-dimensional (1D) nanostructures have been intensively studied in recent years.1-4 The ability to precisely produce nanometer-sized materials opens new possibilities in modern science and technology. Their unique sizeand shape-dependent properties and their continually expanding application in various research areas have dramatically increased the interest in anisotropic nanostructures, such as rods, wires and tubes.5-8 Many difficulties associated with the synthesis of 1D nanostructures have been overcome, and it is now possible to precisely fine-tune the dimensions of these nanostructures, as well as control their morphology, phase purity and chemical composition.9 To date, several strategies have been developed to fabricate organic, hybrid and inorganic 1D nanostructures.4 They can be synthesized from either vapor, liquid or solid phases by using multiple methods, and using two fundamental steps: nucleation and growth.5, 10-16 Xia et al. highlighted several strategies for “bottom-up” methods as key factors for the fabrication of homogenous 1D inorganic nanostructures.2 The use of capping agents (such as surfactants)17-19 or the self-assembly of 0D nanostructures20, 21 are examples of the promising pathways toward anisotropic nanomaterials. Another elegant route toward 1D nanostructures is the direct use of 1D templates, including organic systems. Cylindrical polymer brushes (CPBs),4, 22 carbon nanotubes,23, 24 self-assembled block copolymers25-27 and biological superstructures28-30 (viruses or DNA) are examples of templates with preexisting asymmetric shapes. The main challenges in using template-directed approaches arise with the synthesis and design of the cylindrical template rather than the fabrication of the hybrid material. A promising and uniform 1D template is unimolecular CPBs. These are molecular brushes carrying linear side chains densely grafted from a backbone. They can be synthesized by using “grafting-from”, “grafting-onto” and “graftingthrough” strategies.31 The dense packing of side chains along the polymer backbone causes stretching of the backbone and stiffening of the entire polymer brush. Core-shellcorona structured CPBs – i.e., polymer brushes carrying ABC triblock terpolymers as side chains – have proven to be interesting building blocks and templates. Rzayev and coworkers recently showed molecular transport through polymeric nanotubes prepared from core-shell-corona CPBs.32 Through the incompatibility of each side chain block, the polymer brush can be divided into different 1D interior domains, with lengths up to several hundred nanometers. Such Chapter 4 – Anatase Hybrid Nanotubes 92 structures act as ideal 1D nanoreactors for the synthesis of anisotropic hybrid and inorganic nanostructures. There are a number of studies on using unimolecular soft templates such as CPBs for the fabrication of well-defined anisotropic nanomaterials. Core-shell CPBs have been used for the fabrication of TiO2,33 CdS,34 CdSe35 and SiO236 hybrid nanowires, as well as core-shell-corona CPBs for the fabrication of silica hybrid nanotubes.37 All of these nanomaterials have only one compartment of the template, for example the shell or the core, that hosts guest molecules, such as salts or inorganic precursors. These guest molecules can be either loaded into the compartment or already covalently bonded inside the compartment. The loading of inorganic material can be performed via two ways, the in situ generation of inorganic material within the template35, 38 or the loading of presynthesized nanoscopic materials into a template compartment.25, 38 Such nanostructuring of inorganic materials has attracted considerable interest, as the resulting materials often exhibit high surface areas and small sizes of the inorganic nanoparticles, which provide them with unique optical, electrical and catalytic properties.39-42 Nanomaterials of titania (TiO2) are of particular interest, as it is possible to control their physical and chemical characteristics through synthesis pathways. Consequently, there exists a large variety of applications of TiO2 nanomaterials in the fields of gas sensing, dielectric ceramics, catalysts, photovoltaic solar cells and pigments.42-47 In our previous work, we synthesized TiO2 nanowires from a bis-hydrophilic core-shell polymer brush [HEMA85-OEGMA200]3200 and titanium tetra(n-butoxide), Ti(OBu)4.33 In that study, Ti(OBu)4 was immobilized into the poly(2-hydroxyethyl methacrylate) (PHEMA) compartment through transalcoholysis and a subsequent hydrolysis step led to uniform TiO2 nanowires. However, alkoxy-based titania precursors are very labile to hydrolysis, especially in aqueous solution. Mostly they only form amorphous TiO2 and are then converted into crystalline TiO2 through additional steps like heat treatment.33, 48, 49 Although the loading was performed in dioxane, it nevertheless had the side effect that titania nanoparticles were not only complexed in the PHEMA core but also in the shielding POEGMA corona. Herein, we infiltrated a negatively charged inorganic titania precursor to coordinate exclusively into one of the template brush compartments. We used a core-shell-corona CPB, consisting of a PHEMA backbone with poly(ε-caprolactone) (PCL) as the core, poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) as the polycationic shell, and Chapter 4 – Anatase Hybrid Nanotubes 93 poly(oligo(ethylene gylcol) methyl ether methacrylate) (POEGMA) as the solublizing corona, as a template for the fabrication of anatase nanotubes. The applied polymerization techniques gave excellent control over the synthesis of the template brushes and hence allowed precise adjustment of the diameter of the TiO2 nanotubes. Using ring-opening polymerization of ε-caprolactone (CL) led to a high grafting efficiency of side chains because the PCL chains have low steric requirements. In addition, they can be degraded by ester hydrolysis. Atom transfer radical polymerization (ATRP) allowed the homogenous and sequential tailoring of the shell and the corona. The use of a charged TiO2 precursor, titanium(IV) bis(ammonium lactate) dihydroxide (TALH), had two key advantages compared to our previous work. First, TALH guaranteed stability in respect to spontaneous hydrolysis during the loading of the template at ambient temperatures. TALH has already been used to produce titania coatings on silica gels,50 layered gold nanoparticles51 and polymeric substrates,52 and its controlled hydrolysis and condensation at elevated temperatures and different pH values is well studied and documented. Second, the precise and exclusive coordination of TALH into the PDMAEMA shell compartment allowed the fabrication of homogenous TiO2 nanomaterials. No undesired TiO2 was found in the corona or in solution. Chapter 4 – Anatase Hybrid Nanotubes 94 Scheme 4-1. Schematic illustration of the template prepared from PHEMA by the combination of ROP and ATRP to form (i) core-shell-corona CPBs (ii). Complexation of the titanium salt TALH into the CPBs (iii) and its hydrolysis and condensation to form soluble anatase nanotubes (iv). Experimental Section Materials All chemicals were of analytical grade and used as received without further purification, except for 2-(dimethylamino)ethyl methacrylate (Aldrich, 98%) and oligo(ethylene glycol) methyl ether methacrylate (Aldrich, 98%, Mn ~ 300 g·mol-1), which were passed through a silica column prior to polymerization. Preparation of the polymer brush [PCL-b-PDMAEMA-b-POEGMA]x. The template was synthesized through the combination of anionic, ring opening and ATRP. The stepwise buildup from a polyinitiator backbone to a core-shell cylindrical polymer brush [CL14-DMAEMA40-OEGMA65]7500 is explained in detail below. Chapter 4 – Anatase Hybrid Nanotubes 95 Synthesis of HEMA7500 backbone. The polyinitiator backbone PHEMA was obtained through deprotection of poly(2-(trimethylsiloxy)ethyl methacrylate) (PTMS-HEMA). The PTMS-HEMA backbone was synthesized as previously reported by Mori et al.53 The molecular weight was determined by static light scattering (SLS). The number-average degree of polymerization (DPn) and the polydispersity index (PDI) are 7500 and 1.14, respectively. The deprotection was performed with acetic acid in methanol. Synthesis of the cylindrical polymer brush [CL14]7500. PHEMA (80 mg, 0.62 mmol) was dissolved in CL (6 mL, 54.2 mmol) and water traces were distilled off in the presence of benzene. Afterward, the mixture was degassed by bubbling argon for 30 min. The ringopening polymerization (ROP) of CL was catalyzed by the addition of tin(II) 2ethylhexanoate (1.5 mg, 3.70 µmol) at 125 °C. The polymerization was allowed to proceed for 11 h, until the mixture became very viscous. The polymerization was quenched by cooling and exposing to air and then diluted with THF and precipitated into cold cyclohexane. The conversion was determined after purification by 1H-NMR by comparing the polymeric CH2-signal at 4.1 ppm and the terminal CH2-signal at 3.65 ppm. The PCL homopolymer brush [CL14]7500 was precipitated twice into a cold water/methanol mixture (10/90 v/v) and then freeze-dried from dioxane. [CL14]7500 was then reacted with a 1.5-fold molar excess of 2-bromoisobutyryl bromide and a 2-fold molar excess of triethylamine (TEA) in dry THF to functionalize the PCL brush with ATRP initiating groups. The reaction mixture was stirred for 24 h at room temperature, and then the functionalized polymer was concentrated by solvent evaporation, precipitated in a cold water/methanol mixture (80/20 v/v) and freeze-dried from dioxane. Synthesis of the cylindrical core-shell polymer brush [CL14-DMAEMA40]7500. [CL14]7500 (8.0 µmol) was dissolved in 7 mL of anisole and deoxygenated for 10 min in a screw-cap flask sealed with a septum. Then, 0.01 mmol of CuCl was added and argon was bubbled through the mixture for 20 min. Meanwhile, 5 mmol of destabilized DMAEMA, 0.01 mmol of PMDETA and 1 mL of anisole were degassed. The polymerization was started after adding the DMAEMA/PMDETA mixture to the reaction flask at 50 °C. The polymerization was monitored via 1H-NMR and quenched at the desired conversion by cooling it and exposing it to air. The polymer solution was passed through a short silica gel column before it was precipitated into cold cyclohexane. The precipitate was immediately dissolved in ethanol before a second precipitation. The polymer was dissolved in anisole and the excess ethanol was removed by reduced pressure. Chapter 4 – Anatase Hybrid Nanotubes 102 Figure 4-2. AFM height images of (A) template brush 1 and (B) hydrolyzed TALH infiltrated into hybrid nanotubes, on mica. The cross-sections of the corresponding AFM height images can be found underneath the images. The z-values are (A) 6 nm and (B) 40 nm. Figure 4-3 shows TEM micrographs of the TALH-loaded polymer brush 1 (A, B) and the thermally hydrolyzed and condensed analogues (C, D). In all micrographs, there was no excess TALH/titania nanoparticles visible in the background and the brush templates were homogeneously loaded with TALH/TiO2. The diameter of the TALH loaded nanotubes was around 28 ± 2 nm. The diameter decreased slightly to 23 ± 2 nm after hydrolysis and condensation. However, the shape and length of the nanotubes remained Chapter 4 – Anatase Hybrid Nanotubes 103 unchanged. The PCL core of the nanotubes appeared lighter as compared to the TALH/titania containing shell, due to the much lower contrast used (see grey-scale analysis in Figure 4-3B and D). The PCL core remained unchanged during hydrolysis and had a diameter of around 7 nm. The POEGMA corona of the polymer brush (65 and 150 monomer units, respectively) still surrounded the nanotubes and prevented crosslinking between the individual nanotubes. Incidentally, some brushes appear to have merged, e.g. in Figure 4-3D. However, these are drying artifacts. Figure 4-3. (A, B) TEM micrographs of TALH loaded core-shell-corona polymer brushes of template brush 1 and (C, D) their hydrolyzed crystalline analogues. (E) Cryo-TEM micrograph of TALH-loaded template brush 1 in water. (F) TEM micrograph of TALH-loaded and hydrolyzed template brush 2. The insets in B, D, E and F show the grey-scale analysis of cross-sections through the respective nanotube. Chapter 4 – Anatase Hybrid Nanotubes 104 The POEGMA corona solubilized and stabilized the TiO2 nanotubes in both organic and aqueous solutions. Cryogenic TEM (cryo-TEM) images highlighted the good dispersibility of TALH nanotubes in water (see Figure 4-3E). The PCL core was again distinguishable due to the lower contrast (see magnification and grey-scale analysis in Figure 4-3E). The POEGMA corona was not visible in the TEM images. The loading into and subsequent hydrolysis of TALH within the brush 2 led to an increase in thickness to about 39 ± 2 nm due to the larger PDMAEMA shell compartment (see Figure 4-3F). Several groups have studied the hydrolysis of TALH and demonstrated that it can crystallize to give the anatase polymorph of TiO2.52 It is well known that TALH rapidly undergoes hydrolysis at temperatures above 70 °C.52, 56 It was also shown that the thermal hydrolysis of TALH proceeds smoothly upon step-wise heating.57 Therefore, we commenced hydrolysis of the TALH-loaded polymer brushes at 60 °C and continued stepwise heating toward 80 °C within 3 hours (raising temperature by 6-7 °C every 30 min). The nanotubes were then refluxed (95 °C) over night in ethanol to ensure complete hydrolysis. Powder X-ray diffractometry (PXRD) of the air-dried TiO2 nanotubes confirmed the anatase crystal structure (see Figure 4-4B). Peak broadening revealed that the crystalline nanoparticles were quite small. Thus, as might be expected, the hybrid brushes do not represent single crystals of µm dimension but the inorganic walls are rather polycrystalline, composed of many small, tightly aggregated crystallites. Evaluating particle sizes applying the Scherrer formula gave diameters of 3-4 nm which is in good agreement with HR-TEM observations (Figure 4-4A). Moreover, as the crystal lattices are clearly visible, HR-TEM verified that the nanotubes, produced upon hydrolysis, were highly crystalline. Chapter 4 – Anatase Hybrid Nanotubes 105 Figure 4-4. (A) HR-TEM micrograph of an anatase nanotube from template brush 1. The magnification clearly reveals crystalline areas within the nanotube. (B) Powder XRD pattern of hydrolyzed TALH nanotubes. The ticks on the x-axis in (B) indicate the expected positions of the anatase reflexes . Both template brushes 1 and 2 were designed to have an equal weight content of PDMAEMA of around 25 wt% compared to the overall Mn. This made it easier to compare the amount of inorganic material that was incorporated into the PDMAEMA shell. Thermogravimetric analysis (TGA) determined the weight content of TiO2 in the template brushes with 40 repeating units of DMAEMA to be 39 wt%, whereas the template brushes with 150 repeating units of DMAEMA were able to embed 47 wt% of TiO2 (Figure 4-5). TGA thereby revealed that longer PDMAEMA chains, as expected, were able to load slightly more TALH into the shell. We assume that with increasing length of polymer brush side chains, the mobility and space of these chains increases as well. Consequently, there is more room for the incorporation of material, which accordingly increases the loading capacity. Chapter 4 – Anatase Hybrid Nanotubes 106 Figure 4-5. TGA of the anatase hybrid nanotubes from template brush 1 (black solid line) and template brush 2 (red dashed line). Figure 4-6. (A/B) TEM micrographs of calcined anatase nanotubes. (B) The white arrows indicate the tubular structure after calcinations. Chapter 4 – Anatase Hybrid Nanotubes 107 Calcination of the anatase hybrid nanomaterial in air resulted in the removale of the template brush which thus resulted in pure anatase nanotubes (Figure 4-6). Additionally, the crystalline nanomaterials were deposited on silicon wafers and investigated with scanning electron microscopy (SEM; Figure 4-7). SEM highlighted, again, the uniformity in thickness of the anatase nanotubes. Highly concentrated dispersions of anatase hybrid nanotubes formed non-woven networks of hybrid nanotubes upon (freeze-)drying (Figure 4-7A–D). Those networks were preserved after calcination in air at 650 °C. Figures 4-7E and 4-7F underlined the high porosity of inorganic nanomaterials obtained by this method. TEM and SEM both revealed that the worm-like structures retained their shape after calcination (see Supporting Information 4-S2). The surface area of the dried hybrid material was 16 m2·g-1, as determined by Brunauer-Emmett-Teller analysis of N2physisorption isotherms. Chapter 4 – Anatase Hybrid Nanotubes 108 Figure 4-7. SEM images of (A, B) as-prepared anatase hybrid nanotubes from template brush 1 (dried from solution), (C, D) freeze-dried anatase hybrid nanotubes from template brush 2 at different magnifications, and (E, F) freeze-dried anatase hybrid nanotubes from template brush 2 after calcination in an air atmosphere. Chapter 4 – Anatase Hybrid Nanotubes 109 Conclusions Our template-directed approach to hybrid formation allows the synthesis of highly crystalline 1D TiO2 nanotubes in a mild process at relatively low temperatures (70 °C). The molecular core-shell-corona brushes with a polycationic shell served as ideal 1D nanoreactors for the infiltration of negatively charged molecular titania precursors and guaranteed a homogeneous filling in one dimension. The hydroand solvophilic corona provides sufficient solubility in various media and prevents crosslinking during hydrolysis and condensation. With this versatile route toward highly crystalline anisotropic TiO2 nanostructures, it is possible to vary the length and diameter of the hybrids by adjusting the backbone and length of the side chains. The excellent dispersibility in various media makes the 1D anatase hybrids interesting for producing TiO2 films or networks (Figure 4-7D). Those films should be porous (similar to Figure 4S2D) after heat treatment. Furthermore, the non-woven mesostructure of the hybrid materials is retained even after calcination. Accordingly, highly crystalline TiO2 nanomaterials were obtained, which may serve as catalysts, battery materials or in photovoltaic applications. Acknowledgments. This work was funded by the Collaborative Research Center (SFB) 840 within project A7 and the Australian Research Council under the Discovery Project Scheme. The authors thank Marietta Böhm and Lena Geiling for performing SEC and N2 physisorption measurements, respectively. We further thank Prof. Andreas Fery (Physical Chemistry II, Universität Bayreuth) for giving us access to the AFM. M. Müllner thanks BayEFG for a scholarship. T. Lunkenbein acknowledges the fellowship provided by the international graduate school of the ENB “Structures, Reactivity and Properties of Metal Oxides”. Supporting Information Available. NMR of PCL brush (4-S1) and TEM and SEM images of calcined anatase nanostructures (4-S2). This material is available free of charge via the Internet at http://pubs.acs.org. 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(57) Mayya, K. S.; Gittins, D. I.; Dibaj, A. M.; Caruso, F. Nano Lett. 2001, 1, (12), 727-730. Chapter 5 – Silica Nanowires and Nanotubes 118 Chapter 5 – Silica Nanowires and Nanotubes 119 Introduction The use of hollow inorganic nanostructures as potential nanoscale reactors has been studied extensively.1-9 Interior cavities can host reactants or catalysts and shield them from the outer environment. The most studied hollow nanostructures are hollow spheres. However, one-dimensional (1D) inorganic nanostructures, such as tubes, rods and wires, have gained considerable interest due to their high aspect ratio and their potential application in electronic, optical and mechanical devices.10-12 The unique sizeand shapedependent properties of 1D nanomaterials and their continually expanding applications in various research areas have dramatically increased the interest in anisotropic nanostructures.13-15 1D inorganic nanostructures often find use as sensors or in catalysis.12,16,17 In early research, the exploration of 1D nanomaterials was hampered as the synthesis and control of a distinct geometry at extremely small sizes were difficult.18 Many 1D nanostructures are built up via templating processes where a suitable template predetermines the shape and size of the hybrid material. It is generally accepted that template-directed syntheses provide a simple, high-through-put, and cost-effective procedure which allows the straightforward production of hybrid material, often in only one step.12 In general, the applied templates are referred to as either hard or soft. Whereas hard templates are mostly from anodized aluminuim oxide (AAO), soft templates can vary from simple surfactant micelles to more complex templates, such as peptides, carbon nanotubes, viruses, or cylindrical polymer brushes (CPBs).16,19 CPBs are molecular brushes carrying linear side chains densely grafted from a backbone.20 The dense packing of side chains along the polymer backbone leads to a streching of the backbone and a stiffening of the entire brush. Various types of CPBs with different structures and chemical compositions have been reported.20,21 Core-shell or core-shell-corona structured CPBs (that is, polymer brushes carrying dior triblock copolymers as side chains) have proven to be interesting building blocks in template chemistry. Through the incompatibility of the side chain blocks with each other, the polymer brush can be divided into two or three different concentric compartments, which can be used as unimolecular templates or nanoreactors for the synthesis of 1D organic, hybrid or purely inorganic nanostructures. Core-shell(-corona) CPBs with a degradable core have been used to form uniform tubular nanostructures.22-24 In addition, a polyelectrolyte core or shell of coreshell CPBs was used to immobilize metal precursors, such as Cd2+, Fe2+/Fe3+, AuCl4-, Chapter 5 – Silica Nanowires and Nanotubes 120 PtCl6and Ti4+ ions, and convert the precursors into the corresponding nanoparticles in the core or the shell.16,25-31 Moreover, CPBs have been used as building blocks for the fabrication of organo-silica hybrid nanotubes and nanowires, where the silica precursor was covalently attached to the template brush.32,33 In the case of silica nanostructures, many of the abovementioned synthesis routes have been explored.34-39 Synthetic routes toward silica nanostructures mainly depend on both soft and hard anisotropic templates and involve multiple steps including the introduction of silica.39 Due to difficulties in obtaining sacrificial 1D templates of high quality and large quantity, the wide use of silica nanostructures, especially nanotubes, has been greatly limited. More specifically, the precise control of the size and aspect ratio, scale-up, and cost minimization during synthesis are a general issue. As a result, a facile synthesis of well-defined and sizetunable silica nanowires and nanotubes on a large scale is highly desirable to fully explore their practical applications. Anisotropic silica-based materials, in particular, are attractive materials due to their chemical inertness, corrosion resistance, and mechanical and thermal stability. Herein, we use core-shell CPBs, consisting of poly(ε-caprolactone) (PCL) as a core and poly[2-(dimethylamino) ethyl methacrylate] (PDMAEMA) as a polycationic shell, as a template for the fabrication of silica nanowires and nanotubes (Scheme 5-1). The applied polymerization techniques permitted excellent control over the synthesis of the template brushes and allowed precise adjustment of the aspect ratio and morphology of the 1D silica nanostructures. Anionic ring-opening polymerization (ROP) of ε-caprolactone (CL) increased the grafting efficiency and allowed the removal of the core-forming block. The precise production of silica nanomaterials with different lengths and different core and shell diameters was achieved by loading the amine-containing compartment with a silica precursor, namely tetramethyl orthosilicate (TMOS), and the subsequent hydrolysis and condensation. TMOS has already been used for the synthesis of various silica nanostructures.40-42 Acid treatment or calcination of the PCL-filled nanowires led to hollow silica nanotubes. Furthermore, we loaded the polyelectrolyte shell with metal ions (e.g. PtCl42or AuCl4-) and embedded the corresponding Pt or Au nanoparticles into the silica shell, giving catalytically active silica nanomaterials. Chapter 5 – Silica Nanowires and Nanotubes 121 Scheme 5-1. Synthesis of Template CPBs and their Use in the Template-Directed Synthesis of Silica Hybrid Nanostructures Experimental Section Materials. All chemicals were of analytical grade and used as received without further purification, except for 2-(dimethylamino)ethyl methacrylate (Aldrich, 98%), which was passed through a silica column prior to polymerization. Preparation of the Soft Template Polymer Brush [CLnDMAEMAp]m. The template was synthesized through the combination of anionic, ring-opening and atom transfer radical polymerization. Table 5-1 provides an overview of the synthesized polymers and polymer brushes. The stepwise build-up from a polyinitiator backbone to a core-shell CPB is explained in detail below using [CL14DMAEMA43]7500 as an example. Chapter 5 – Silica Nanowires and Nanotubes 122 Table 5-1. Overview of the Synthesized Polymer Backbones and Polymer Brushes Polymer composition Mn [106g·mol-1] Mw/Mnc PHEMA 2700 a 0.35a 1.05 PHEMA 7500 a 0.98a 1.14 [CL 10 ] 2700 b 3.08b 1.13 [CL 25 ] 2700 b 7.70b 1.25 [CL 14 ] 7500 b 12.0b 1.40 [CL 10 DMAEMA 58 ] 2700 b 26.0b - [CL 25 DMAEMA 76 ] 2700 b 37.4b - [CL 14 DMAEMA 43 ] 7500 b 59.0b - [CL 14 DMAEMA 342 ] 7500 b 382b - a Molecular weight as measured by SLS; b A determined by 1H-NMR; c A determined by SEC in DMAc. The SEC traces of the PCL brushes can be found in the Supporting Information (Fig. S1). The polyinitiator backbone poly(2-hydroxyethyl methacrylate) (PHEMA) was obtained through the deprotection of poly(2-(trimethylsiloxy)ethyl methacrylate) (PTMS-HEMA). The backbone was synthesized as reported by Mori et al..43 Two PTMS-HEMA homopolymers with different length were synthesized. The number-average degree of polymerization (DPn) and the polydispersity index (PDI) were 7500 and 1.14 or 2700 and 1.05, respectively. The deprotection of the TMS group was performed with acetic acid in methanol. PHEMA (80 mg, 0.62 mmol) was dissolved in ε-caprolactone (6 mL, 54.2 mmol) and water traces were removed via distillation in the presence of benzene. The mixture was then degassed by bubbling argon for 30 min. The ROP of CL was catalyzed via the addition of tin(II)-ethylhexanoate (1.5 mg, 3.70 µmol) at 125 °C. The polymerization was allowed to proceed for 11 h until the mixture became very viscous. The polymerization was quenched with MeOH, exposed to air and diluted with THF. The conversion was determined after purification with 1H-NMR by comparing the polymeric CH2-signal at 4.1 ppm and the terminal CH2-signal at 3.65 ppm. The PCL homopolymer brush [CL14]7500 was precipitated twice in a cold water/methanol mixture (10/90 v/v) and then freeze-dried from dioxane. [CL14]7500 was then reacted with a 1.5 fold molar excess of 2-bromoisobutyryl bromide and a 2 fold molar excess of triethylamine (TEA) in dry THF to functionalize the PCL brush with ATRP initiating groups. The reaction mixture was stirred for 24 h at room temperature, and then the functionalized polymer was concentrated by solvent evaporation, precipitated in a cold water/methanol mixture (80/20 Chapter 5 – Silica Nanowires and Nanotubes 123 v/v) and freeze-dried from dioxane. [CL14]7500 (0.008 mmol) was dissolved in 7 mL anisole and deoxygenated for 10 min in a screw-cap flask sealed with a septum. 0.01 mmol CuCl was then added and argon was continued to be bubbled through the mixture for 20 min. Meanwhile, 5 mmol of destabilized DMAEMA, 0.01 mmol of N,N,N′,N′′,N′′- pentamethyldiethylenetriamine (PMDETA) and 1 mL of anisole were degassed as well. The polymerization started after adding the DMAEMA/PMDETA mixture to the reaction flask at 50 °C. The polymerization was monitored via 1H-NMR and quenched at the desired conversion by cooling it and exposing it to air. The polymer solution was passed through a silica gel column before it was precipitated into cold cyclohexane. The precipitate was immediately dissolved in ethanol and precipitated a second time. The weight content of the polymer brushes dissolved in ethanol was determined before the solution was dialyzed to water. Quaternization of [CLnDMAEMAp]m. An excess of methyliodide (MeI) was added dropwise to the core-shell polymer brushes in water. The solution became turbid immediately and was allowed to stir for another 24 h. The viscosity increased significantly during that time. Unreacted MeI was removed under reduced pressure. Preparation of Silica Hybrid Nanowires. 100µL of tetramethyl orthosilicate (TMOS) was added drop-wise to 2 mL of the cylindrical polymer template in water (0.25 g·L-1) at 15 °C under vigorous stirring. The mixture was allowed to stir for 20 min before it was diluted with 6 mL of ethanol. The mixture was then centrifuged at 12 500 rcf for 1 min and washed with ethanol and water, aided by ultrasound. Preparation of Hollow Silica Nanotubes. The freeze-dried silica hybrids (1 mg) were either mixed with 2M HCl and stirred at 50 °C for three days or calcined as a dry powder in air atmosphere at 650 °C with a heating rate of 10 K·min-1. Preparation of the Platinum-Doped Silica Hybrid Nanowires. 100 µL of an aqueous solution of potassium tetrachloroplatinate (24 mmol·L-1) was added drop-wise to 10 mL of the template brush solution (0.25 g·L-1) under stirring. After stirring for 1 h, 2 mL of the solution was mixed at 15 °C with TMOS as described above. After washing the platinum-doped hybrid silica nanowires, they were treated with 100 µL of freshly prepared NaBH4 (1 g·L-1) solution and mixed for 1 h using ultrasound, before the NaBH4 was washed off using centrifugation. Chapter 5 – Silica Nanowires and Nanotubes 124 Preparation of the Gold-Doped Silica Hybrid Nanowires. 100 µL of an aqueous solution of chloroauric acid (HAuCl4) (0.3 wt%) was added drop-wise to 10 mL of the template brush solution (0.25 g·L-1) under stirring. After stirring for 1 h, 2 mL of the solution was mixed at 15 °C with TMOS as described above. After washing the gold-doped hybrid silica nanowires, they were treated with 100 µL of freshly prepared NaBH4 (1 g·L-1) solution and mixed for 1 h using ultrasound, before the NaBH4 was washed off again. Reduction of 4-Nitrophenol Catalyzed by Nanoparticle-Doped Silica Hybrid Nanowires. 0.5 mL of NaBH4 solution (60 mmol·L-1) was added to 2.5 mL of 4-nitrophenol solution (0.12 mmol·L-1) that was contained in a glass cuvette. Then, 0.5 mL of either platinum or gold nanoparticle-doped hybrid nanowires solution (nanowire concentration 0.001 g·L-1) was added. Immediately after the addition of the composite particles, ultraviolet (UV) spectra of the sample were taken continuously in the range of 250-500 nm. The rate constants of the reactions were determined by measuring the change in intensity of the peak at 399 nm with time. Characterization Methods. Size Exclusion Chromatography (SEC). SEC in N, Ndimethylacetamide (DMAc) with 0.05M lithium bromide was conducted at an elution rate of 0.7 mL·min-1 using polyester copolymer network (GRAM) columns (300 × 8 mm, 7 µm): 103 and 102 Å and RI and UV (λ = 260 nm) detection. A poly(methyl methacrylate) (PMMA) calibration curve was used to calibrate the columns. Atomic Force Microscopy (AFM). AFM images were recorded on a Digital Instruments Dimension 3100 microscope operated in tapping mode. The samples were prepared by dip-coating of freshly cleaved mica into a solution of the polymer brush solution diluted in ethanol or water to form a monomolecular film. Transmission Electron Microscopy (TEM). Bright field TEM was performed using a Zeiss CEM 902 electron microscope operated at 80 kV. A droplet of a solution of the polymer brush solution (0.05 g·L-1) in water or ethanol was dropped onto a copper grid (200 mesh) coated with carbon film, followed by blotting the liquid and drying at room temperature for a short time. Cryogenic Transmission Electron Microscopy (cryo-TEM). Cryo-TEM was conducted by dropping the aqueous dilute solution (0.1 g·L-1) on a hydrophilized lacey TEM grid, where most of the liquid was removed with blotting paper, leaving a thin film stretched over the grid holes. The specimens were shock frozen by rapid immersion into liquid Chapter 5 – Silica Nanowires and Nanotubes 125 ethane and cooled to approximately 90 K by liquid nitrogen in a temperature controlled freezing unit (Zeiss Cryobox, Zeiss NTS GmbH, Oberkochen, Germany). After the specimens were frozen, the remaining ethane was removed using blotting paper. The specimen was inserted into a cryo-transfer holder (CT3500, Gatan, München, Germany) and transferred to a Zeiss EM922 EF-TEM instrument operated at 200 kV. Scanning Electron Microscopy (SEM) and Energy-Dispersive X-Ray (EDX). SEM and EDX analysis was performed using a Zeiss Model 1530 Gemini instrument equipped with a field-emission cathode with a lateral resolution of ∼ 2 nm. The samples were measured on silica wafer and sputtered with platinum for 1 min. In the case of the EDX investigated samples, the samples were centrifuged, dried, not sputtered and directly measured on the plain stud without silica wafer. Proton Nuclear Magnetic Resonance. 1H-NMR spectra were recorded to determine the monomer conversion on a Bruker AC-300 spectrometer at room temperature in CDCl3. N2-physisorption. N2-physisorption was conducted at 77 K on a Quantachrome Autosorb 1 instrument.Prior to the measurements, the samples were degassed at 403 K for 24 h. The hybrid nanotubes were calcined in a tube furnace in air atmosphere from 30°C to 650 °C (heating rate 10 K min-1). p/p0 values between 0.01 and 0.06 were taken to determine the specific surface areas. The recommendations of Rouquerol et al. regarding the BET equation were followed.44 Results and Discussion Template Synthesis and Characterization. Core-shell CPBs (Scheme 5-1, ii) were used as a template for the fabrication of various 1D silica nanostructures. The polymer brushes were synthesized via the “grafting-from” approach, where side chains are grown from a polyinitiator backbone. Poly(hydroxyethyl methacrylate) (PHEMA) was chosen for the backbone, as it can be used to initiate the anionic ROP of εCL. PHEMA was produced via anionic polymerization of TMS-HEMA and a subsequent deprotection step with acetic acid. To obtain different lengths of the later silica hybrids, we synthesized two PHEMA backbones with different molecular weights. The weight-average molecular weight (Mw) for both backbones was determined by static light scattering (SLS; data not shown). Dividing Mw by the respective PDI, obtained from SEC, and the molecular weight of HEMA, resulted in the number-average degree of polymerization (DPn) of each Chapter 5 – Silica Nanowires and Nanotubes 126 polymer backbone, namely 2700 (PHEMA2700) and 7500 (PHEMA7500), respectively. PHEMA was then used for the ROP of CL in bulk. Due to the rather high molecular weights of the polyol backbones, it was complicated to dry the polyinitiator completely and, accordingly, it was difficult to exclude all traces of the water during the polymerization. The presence of water is problematic since it can also act as an initiator for the ROP of CL, leading to non-grafted poly(caprolactone) (PCL). The ROP was catalyzed by tin(II)-ethylhexanoate and is known to have a very high grafting efficiency of above 90%.45 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 chemical shift (ppm) h h i i j j f f aa b b c c g g dioxane d d‘ d d‘ e e 3.70 3.65 Figure 5-1. 1H-NMR spectra in CDCl3 of (A) the [CL25]2700 homopolymer brush, (B) the end-group modified PCL brush and (C) [CL25DMAEMA76]2700. Before attaching an ATRP initator, the as-synthesized PCL polymer brushes were purified from the homo-PCL by selective precipitation in a THF/cyclohexane mixture. A 1H-NMR spectrum of the purified PCL homopolymer brushes can be seen in Figure 5-1A. To determine the DPn of PCL, we compared the ratio of the terminal CH2-OH-group (d) at 3.65 ppm to the corresponding polymeric CH2-group (a) at 4.1 ppm. The ratio equals the DPPCL. Upon esterification, the terminal CH2-group shifts completely lowfield and can be found at 4.25 ppm (d’). This is a clear indication of successful esterification. In Chapter 5 – Silica Nanowires and Nanotubes 127 addition, a new peak (e) at 1.9 ppm appears, which originates from the two methyl groups of the 2-bromoisobutyrate group (Figure 5-1B). In a final step, 2-(dimethylamino)ethyl methacrylate (DMAEMA) was grafted from the PCL brush to give water-soluble polyelectrolyte CPBs [CLnDMAEMAp]m. PDMAEMA was chosen as it is partially charged in water at pH 7. It is also known that cationic polymers can promote localized silica deposition.40 The conversion of DMAEMA was confirmed by 1H-NMR. Figure 51C shows the corresponding spectrum of [CL25DMAEMA76]2700 in CDCl3. To determine the efficiency of the grafting of PDMAEMA, the PCL part of the polymer brushes was degraded in 2 M HCl over three days. The turbid brush solution cleared already after several hours, indicating the successful cleavage of the PDMAEMA side chains. The SEC traces (in DMAc, with PDMAEMA calibration) of the cleaved PDMAEMA had a very similar molecular weight compared to the one obtained from 1H-NMR (see Supporting Information, Figure 5-S2). This confirms that the grafting from PCL brushes has ≥ 90% efficiency.45 The increase in grafting density, compared to the grafting from a poly(2bromoisobutyryloxyethyl methacrylate) (PBIEM) polyinitator (ca. 50-70%),25,33 is explained by the reduced sterical hindrance of PCL brushes. Grafting from very close to the backbone increases sterical hindrance and hence decreases the grafting efficiency. PCL side chains act as a spacer and therefore increase the grafting efficiency. Several polymers with different dimensions, with respect to length as well as core and shell diameter, were synthesized and used for the deposition of silica. Four polymer brush compositions are highlighted in Table 5-2. Table 5-2. 1D Silica Hybrids with Different Dimensions (in nm) Template compositiona Template lengthb Hybrid lengthc Core diameterc Silica shell diameterc [CL 10 DMAEMA 58 ] 2700 295 ± 20 270 ± 15 5-6 ~25 [CL 25 DMAEMA 76 ] 2700 265 ± 20 235 ± 20 10-12 ~35 [CL 14 DMAEMA 43 ] 7500 1250 ± 200 950 ± 350 6-7 ~25 [CL 14 DMAEMA 342 ] 7500 1250 ± 200 950 ± 350d 6-7e ~ 85f a Determined by 1H-NMR; b measured from AFM images of the template brushes; c measured from TEM micrographs of the as-synthesized silica hybrids; d estimated from TEM micrographs, as it was rather difficult to directly measure the actual length due to the jamming of hairy silica nanostructures; e assumed to be the same dimension as with [CL14DMAEMA43]7500; f taking into account that the core is ~6 nm in diameter. Chapter 5 – Silica Nanowires and Nanotubes 134 However, we assume that the core was degraded equally quickly, as in the degradation of the pristine PCL-b-PDMAEMA brush and hence resulted in hollow silica nanotubes. TEM and SEM measurements verify the unchanged structure of the nanomaterials after acid treatment (see Figure 5-S8). Figure 5-5. TEM micrographs of calcined silica nanotubes templated from (A-C) [CL14DMAEMA43]7500 and (D-F) [CL25DMAEMA76]2700. (A, D) Agglomerates of silica nanotubes after calcination. (B, C, E, F) Separated silica nanotubes after sonication. Metal-Containing Brushes as Catalysts. To demonstrate the potential of the templated silica nanowires in catalysis applications, we loaded the PDMAEMA shell with metal salts prior to the incorporation of silica. Thus, the addition of TMOS embedded the metal ions (e.g. [AuCl4]- or [PtCl4]2-) within the silica shell. Excess salt was removed by ultracentrifugation. The addition of NaBH4 as a reducing agent led to the formation of gold or platinum nanoparticles (NPs). The TEM micrographs in Figure 5-6 confirm the successful incorporation of the nanoparticles and clearly verify the location of the nanoparticles within the silica shell. The size of the nanoparticles was measured via TEM. The average diameter of the platinum and gold nanoparticles is ~ 1.6 nm and ~ 3.5 nm, respectively. Energy-dispersive X-ray measurements confirmed the presence of nanoparticles and showed a gold content of 3 wt% with respect to silicon. For platinum, the content was 4 wt% (see Supporting Information 5-S8). The reduction of 4-nitrophenol to 4-aminophenol by NaBH4 in the presence of the NPdoped silica nanowires was performed to check their accessibility and catalytic activity. This reaction has been used widely and has become a model reaction for testing the catalytic activity of noble metal nanoparticles.30,48,49 As shown in the Supporting Chapter 5 – Silica Nanowires and Nanotubes 135 Information (Figure 5-S8), the strong UV absorption of 4-nitrophenate ions at 399 nm decreased gradually with time after the addition of nanoparticle-containing hybrid nanowires. Simultaneously, a new peak appeared at 300 nm, which was due to the product 4-aminophenol.50,51 Figure 5-6. TEM micrographs of NP-doped silica hybrid nanowires. Silica hybrid nanowires were filled with (A, C, E) Au NPs, or (B, D, F) Pt NPs. Magnification of the nanowires shows that they are filled with NPs. The nanowires were templated from [CL14DMAEMA342]7500 in the case of Au NPs and from [CL14DMAEMA43]7500 in the case of Pt NPs. Chapter 5 – Silica Nanowires and Nanotubes 136 Conclusions The 1D soft template build-up for the fabrication of silica nanowires was realized via the combination of several polymerization techniques that allowed fine-tuning of the dimensions and morphology of the hybrid nanostructures. CPBs proved to be excellent soft templates, as their production is straighforward and cost-effective. Silica deposition into the template shell formed 1D silica hybrid nanowires in a mild procedure in water at ambient temperature. With this versatile route towards anisotropic silica nanostructures, it is possible to not only vary length and diameter, but also to vary the surface morphology by adjusting the shell length. Calcination or treatment with an acid led to the removal of the core and resulted in silica nanotubes with high microporous volumes and high specific surface areas. These materials might be interesting as filter or storage systems. Furthermore, the incorporation of catalytically active nanoparticles was facile and yielded robust catalysts, which can be easily removed from the system after the reaction. Additionally, the incorporation of metallic NPs into silica allows the structures to be used in high temperature applications.52 These materials might also be interesting in the application of catalytically active filter systems. Associated content Supporting Information. SEC traces of PCL brushes (5-S1) and of cleaved PDMAEMA side chains (5-S2); AFM height analysis of template brushes (5-S3); TEM micrographs of silica hybrids from quaternized templates (5-S4), N2-physisorption measurements and pore volume distributions (5-S5), silica hybrids from core-shell-corona templates (5-S6); silica nanotubes via acid treatment (5-S7); EDX spectra of platinum and gold nanoparticle-doped hybrids (5-S8), and catalytic activity of nanoparticle-doped hybrids (5-S9). This material is available free of charge via the Internet at http://pubs.acs.org.” Acknowledgments This work was supported by the Deutsche Forschungsgemeinschaft (DFG) within the Collaborative Research Center (SFB) 840 and the Australian Research Council. The authors thank Marietta Böhm and Melanie Förtsch for performing SEC and cryo-TEM measurements, respectively. M. M. acknowledges BayEFG for a scholarship and BayNAT for a fellowship. T. 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(50) Hayakawa, K.; Yoshimura, T.; Esumi, K. Langmuir 2003, 19, 5517-5521. (51) Esumi, K.; Isono, R.; Yoshimura, T. Langmuir 2003, 20, 237-243. (52) Joo, S. H.; Park, J. Y.; Tsung, C.-K.; Yamada, Y.; Yang, P.; Somorjai, G. A. Nat. Mater. 2009, 8, 126-131. Chapter 5 – Silica Nanowires and Nanotubes 140 Supporting Information 5-S1. SEC Traces of PCL Homopolymer Brushes 13 14 15 16 17 18 19 20 Elutione Volume (mL) [CL 14 ] 7500 [CL 25 ] 2700 [CL 10 ] 2700 Figure 5-S1. SEC traces in DMAc of the PCL homopolymer brushes. Chapter 5 – Silica Nanowires and Nanotubes 141 5-S2. SEC Trace of Cleaved PDMAEMA Grafts Cleaving of the PDMAEMA side chains of the template brush via acidic ester hydrolysis resulted in a linear homopolymer PDMAEMA with number-average molecular weight (Mn) of 54 x103 g·mol-1and a polydispersity index (PDI) of 1.18 (determined via SEC in DMAc and PDMAEMA calibration). According to the conversion measured by 1H-NMR, the Mn was determined to be 49.5 x103 g·mol-1. This led to a grafting efficiency of DMAEMA of 91-92%. 16 18 20 22 24 26 28 Elution Volume (mL) Figure 5-S2. SEC trace in DMAc of PDMAEMA350 after cleaving from [CL14DMAEMA350]7500 under acidic conditions. SEC was calibrated with PDMAEMA standards. Chapter 5 – Silica Nanowires and Nanotubes 142 5-S3. Height Analysis of AFM Images Cross-sections of the various template brushes listed in Table 5-2 and height profiles of the respective polymer brush (see Figure 5-S3A-D). As already discussed in the manuscript, the length of the polymer brushes is firstly dependent on the degree of polymerization of the backbone. If the backbone of different brushes is identical (see in Figure 5-S3A and B; Figure 5-S3C and D), then the length is dependent on the brush composition. The height in AFM increases when the side chain length increases, as more polymeric material is deposited onto the substrate (compare Figure 5-S3C and D). Despite having a similar overall side chain length, the height in AFM may vary dramatically, as can be seen in Figure 5-S3A and B. This phenomenon is attributed to the different size of the PCL core. As PCL is assumed to prevent direct contact to mica, it will force the core to be completely shielded by PDMAEMA. Due to that fact, the brush with a larger PCL compartment shows an increase in height. In addition, this increase is also based on the segment density of the polymer brush. Due to the very high grafting efficiency, the polymer core is surrounded by a relatively compact PDMAEMA shell, which adds dramatically to the height in AFM (compare cartoons in Figure 5-S3). Chapter 5 – Silica Nanowires and Nanotubes 143 Figure 5-S3. AFM height images of various template brushes and their corresponding height profile (crosssection): (A) [CL10DMAEMA58]2700, (B) [CL25DMAEMA76]2700, (C) [CL14DMAEMA43]7500 and (D) [CL14DMAEMA342]7500. The z-values are: (A, C) 2 nm, (B) 15 nm and (D) 10 nm.