A water-free route to porous materials via cryoextraction and supercritical drying
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Departamento de Ingeniería Química y Tecnología del Medio Ambiente
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UNIVERSIDAD DE VALLADOLID ESCUELA DE INGENIERIAS INDUSTRIALES Máster en Ingeniería Química A water-free route to porous materials via cryoextraction and supercritical drying Autor: Castedo Hernández, Cecilia María José Cocero Alonso Technische Universität Hamburg-Harburg Valladolid, Abril 2021.
TFM REALIZADO EN PROGRAMA DE INTERCAMBIO TÍTULO: A water-free route to porous materials via cryoextraction and supercritical drying ALUMNO: Cecilia Castedo Hernández FECHA: 26 de Marzo de 2021 CENTRO: Institute of Thermal Separation Processes (V8) UNIVERSIDAD: Technische Universität Hamburg-Harburg TUTOR: Jun.-Prof. Pavel Gurikov
Resumen En el trabajo se ha desarrollado una nueva ruta, exenta de agua, para la preparación de materiales porosos a partir de polímeros que no pueden ser utilizados en la ruta convencional de producción de aerogeles. La nueva ruta está compuesta por cuatro etapas: disolución del polímero en DMSO, posteriormente congelación a -28°C, extracción del disolvente con etanol a baja temperaturas -28°C y secado supercrítico mediante CO2 a 120 bar y 50 °C. Los materiales obtenidos se caracterizan mediante su superficie específica via medición BET y la contracción volumétrica. Entre todos los polímeros estudiados poliacrilonitrilo (PAN) es seleccionado para su estudio en profundidad. Materiales creados con PAN tienen una superficie específica de 119 y 107 m2g-1 para concentraciones de 5 y 7,5 % en peso respectivamente. Su combinación con PVA y agar, así como la adición de urea provoca materiales de peores características, menor superficie específica y peores propiedades físicas. Palabras clave Aerogel, ruta de preparación, polímero, secado supercrítico, cryoextracción.
Hamburg University of Technology Institute of Thermal Separation Processes Master’s Thesis A water-free route to porous materials via cryoextraction and supercritical drying by Cecilia Castedo Hernández Matriculation number: 497130 1st Examiner: Jun.-Prof. Dr. Pavel Gurikov 2nd Examiner: Prof. Dr.-Ing. Irina Smirnova Supervisor: Jun.-Prof. Dr. Pavel Gurikov 05.10.2020 – 26.03.2021
Eidesstattliche Erklärung Hiermit erkläre ich, Cecilia Castedo Hernández, geboren am 29.11.1996 in Spain, dass diese Diplomarbeit zur Vorlage beim Prüfungsamt der Technischen Universität Hamburg-Harburg von mir in selbständiger Arbeit, unter Verwendung der angegebenen Quellen angefertigt wurde. Hamburg, den DATUM Unterschrift
Einverständnis Erklärung Ich gestatte der Technischen Universität Hamburg-Harburg die Diplomarbeit unter dem Gesichtspunkt des Urheberrechts für Zwecke in Forschung und Lehre nichtausschließlich und kostenfrei zu nutzen und zu vervielfältigen. Die Gestattung schließt die Weitergabe der Arbeit oder hiervon gefertigter Kopie zu wissenschaftlichen Zwecken an interessierte Dritte, die nicht der TUHH angehören müssen, ein. Im Falle einer eigenen Verwertung der Diplomarbeit werde ich mich zuvor mit der TUHH in Verbindung setzen, damit diese die Arbeit zur Wahrung eventueller eigener Rechte und der Rechte ihrer sonst beteiligten Angehörigen (z.B. (Mit-) Erfindungsanteile des Betreuers oder anderer schutzrechtsfähiger Ergebnisse, die nicht oder nicht allein von mir stammen; Vermeidung der Weitergabe unrichtiger Ergebnisse unter Namensnennung der TUHH) prüfen kann. Ich bin bereit, eigene patentoder sonstige schutzrechtsfähige Erkenntnisse der TUHH auf Wunsch gegen Zahlung einer angemessenen Vergütung zu übertragen bzw. an mir erteilten Schutzrechten gegen Zahlung einer angemessenen Lizenzgebühr ausschließliche oder nichtausschließliche Nut-zungsrechte einzuräumen. Hamburg, den DATUM Unterschrift
Abstract In this study a novel process combining freezing, cryoextraction and supercritical drying for producing porous materials is developed. The process consists of three main steps: first of all, freezing a polymer/DMSO solution at −28 °C to obtain a monolith. Second step is solvent exchange with EtOH at −28 °C to extract DMSO crystals, obtaining an alcogel at the end of this process. The last part of the process is drying the alcogel with scCO2at 120 bar and 50 °C for 3 h. The method has been used for the production of polyacrylonitrile (PAN) aerogels, it is demonstrated that for this material a high quality aerogel is obtained at 5 wt %volumetric shrinkage 36.8 ±6.1 %, porosity is 93.5 ±0.6 %and 119 ±18 m2g−1 of specific surface area (SSA). In addition to PAN, other polymers such as poly(vinyl alcohol) (PVA) and agar-agar and their combination with PAN are tested. Pure PVA aerogels show the higgest value of SSA at 7.5 wt %of 107 ±15 m2g−1. Both combinations result in a aerogel with worse quality than pure PAN. In particular, porosity and linear shrinkage experimented by PAN/PVA aerogels have a exponential relation with PVA concentration. In addition to standard conditions, liquid nitrogen at −196 °C is used as a freezing agent obtaining aerogels with lower SSA that lack physical integrity. However, whereas in conventional freezing at −28 °C directional freezing is observed, more pronounced with the decrease in concentration, for aerogels frozen at −196 °C non-hierarchical pore structures are found. Generally speaking, it has been observed that lower concentrations leave more space for DMSO crystals to grow due to looser arrangement of polymer chains. At the same time, pore walls become thinner making the network weaker leading to higher shrinkage.
iv List of Symbols Latin symbols Symbol Unit Meaning Vshrinkage - Volumen shrinkage lshrinkage - Linear shrinkage SSA m2g-1 Specific surface area Tm °C Melting point Tb °C Boiling point TFR °C Process freezing temperature Vpore cc/g Pore volume Greek symbols Symbol Meaning εPorosity xiMolar fraction
v List of Abbreviations Abbreviation Meaning 3D Three dimensional LMW Low molecular weight SEM Scanning electron microscopy BET Brauauer-Emmett-Teller
LIST OF FIGURES xii Figure 4.3: Comparison of linear shrinkage in solvent exchange and scCO2 drying of PAN aerogels, scCO2drying EtOH exchange. . 49 Figure 4.4: PAN aerogel 7.5 wt%with radial heterogeneity. (a) After solvent exchange (b) after supercritical drying. . . . . . . . . . . . . . . 50 Figure 4.5: PAN aerogel 5 wt%.......................... 51 Figure 4.6: SEM images of PAN 2.5 wt%aerogel. .............. 51 Figure 4.7: SEM images of PAN 5 wt%aerogel. ............... 53 Figure 4.8: PAN aerogel in syringe: (a) 0.625 wt%(b) 5 wt%......... 54 Figure 4.9: Comparison of specific surface area, density and volumetric shrinkage for different moulds. . . . . . . . . . . . . . . . . . . 55 Figure 4.10: Comparison of specific surface area, porosity and volumetric shrinkage for different grams 5g 10g. . . . . . . . . . . . . 57 Figure 4.11: Comparison of specific surface area, porosity and volumetric shrinkage of PAN aerogels different freezing temperature - 28°C -196°C............................ 60 Figure 4.12: SEM images of PAN 5 wt%aerogel frozen at -196 °C. . . . . . 62 Figure 4.13: Comparison of specific surface area, porosity and volumetric shrinkage of PAN aerogels 5 wt%7.5 wt%with the concentrationofurea........................... 64 Figure 4.14: Comparison of specific surface area, density and volumetric shrinkage for PAN and PVA. . . . . . . . . . . . . . . . . . 66 Figure 4.15: Effect of ratio PAN/PVA of aerogels 5 wt %on specific surface area, porosity and linear shrinkage. . . . . . . . . . . . . . . . . 68 Figure 4.16: PAN/Agar-agar(50/50%) 7.5 wt%aerogel (a) top side (b) bottomside................................ 69
LIST OF FIGURES xiii Figure 4.17: Comparison of specific surface area, porosity and volumetric shrinkage of aerogels PAN hybrid PAN/Agar (50/50%). . 70
xiv List of Tables Table 2.1: Solvents in which polymers are soluble . . . . . . . . . . . . . 28 Table 3.1: List of chemicals used in this study. . . . . . . . . . . . . . . . . 33 Table 4.1: Polymers soluble in DMSO and TBA/Water. . . . . . . . . . . 44 Table 4.2: Results of properties of different polymers’ aerogels. . . . . . . 45 Table 4.3: PAN aerogel properties at different concentrations. . . . . . . 47 Table 4.4: Results of PAN aerogels in different moulds. . . . . . . . . . . 56 Table 4.5: Results of PAN aerogels with change is sample size. . . . . . . 58 Table 4.6: Results of PAN aerogels frozen with liquid nitrogen. . . . . . . 59 Table 4.7: Results of properties of hybrid PAN/urea aerogels at different compositions. ............................ 63 Table 4.8: Results of properties of pure PAN and PVA aerogels at differentcompositions........................... 65 Table 4.9: Results of properties of hybrid PAN/PVA aerogels at different ratios at concentration 5 wt%.................... 67 Table 4.10: Results of properties of pure PAN and hybrid 50%PAN 50% agar-agar aerogels at different polymer concentration. . . . . . 69 Table 7.1: Skeletal densities of polymers. . . . . . . . . . . . . . . . . . . 76 Table 7.2: Density of mixtures of EtOH and DMSO. Calibration data. . . 77 Table 7.3: Density of mixtures of EtOH and DMSO aqueous solution of TBA at 20%density. Calibration data. . . . . . . . . . . . . . . 77
1 1. Introduction and research purpose 1.1 Introduction Gels are semisolid materials made at least of two constituents, that posses elastic characteristics. The constituents are usually a polymer and solvent, the polymer forms a 3D network structure able to trap the solvent. Gels are characterised for their limited swelling degree. Polymer gels are observed in human bodies, the cornea, vitreous and connective tissues [1, 2, 3]. Gels can be classified in different types of gels based on different properties. Among all of them, aerogels are known to be a highly porous material, with low density 0.003–0.15 kg m−3 and relatively high specific surface area 300–1500 m2g−1. These unique properties make aerogels exceptional materials for a wide range of applications such as insulating materials, catalysts, thickening agents, water repellents, gas filters [4, 5]. Currently research has been done focused on different biopolymer aerogels for their potential use in the biomedical industry such as cardiovascular implantable devices, tissue engineering substrates, drug delivery systems [5, 6]. 1.2 Research purposes Production of aerogels with the conventional route have some limitations related to the material’s nature and the use of water as a solvent. For example, some materials are complicated and in some cases impossible to turn into a wet network in the gelation step. When water is used as a solvent it is required a solvent exchange
Research purposes 2 step with an antisolvent compatible with CO2at supercritical conditions to assure minimal collapse of the network. As a result, binary mixture of EtOH and water is formed which has a azeotrope at 96%, thus, recycling of the mixture is very complex[7]. The aim of this work is to further study the route established by Ching Ma [8] that overcomes these limitations for different polymers and optimization of process parameters.
3 2. Fundamentals and State of the art In this chapter the current research and fundamentals of polymer gels and specific polymers are presented. Afterwards the current state of the art is discussed. 2.1 Polymeric gels Polymeric gels are defined as soft and/or solid-like systems made of one polymer or more than one polymer [9]. These systems usually consist of the polymer and the solvent, the interaction with each other create a three-dimensional (3D) network able to trap a solvent [10]. A wide variety of polymers, such as gelatins, collagen, alginates, polysaccharides, cellulose are available in the nature [11]. Their properties, biodegradability and biodiversity, make these systems have multiple applications. For these reason, in the past years, they have gained interest among researchers around the world [3]. The mechanism by which polymeric gels are form can be described by the percolation model. In this model, chemical bonds between monomeric subunits are created randomly in order to create independent clusters. The continuous paths in which they grow result in a discontinuous change of the system properties [12]. When a certain amount of bond is reached, the individual agglomerates bond into a single agglomerate, Figure 2.1, and the system changes from sol to gel.
Polymeric gels 4 Figure 2.1: Simplified percolation model for sol phase and gel phase, reprinted from [12]. 2.1.1 Properties of polymeric gels Depending on different factors, such as polymers, nature of cross-linking, size, polymeric matrix types, polymeric gels have a different set of properties. One of the important properties is swelling, the capacity of the solvent molecules to be absorbed and confined in the polymer network. During this process gel-gel interactions are restored via the gel-solvent interactions [13]. 2.1.2 Classification of polymeric gels Classification of polymer gels can be made based on different characteristics (solvent in the 3D network, polymers that form the gel and formation method of polymer network). In this case, the classification will be by the nature of the chemical linkage. Polymeric gels can be classified into three different categories, physical, chemical and entanglement network gels [13]. Physical gels are formed by the aggregation caused by hydrophobic, electrostatic,
Polymeric gels 5 van der Waals or hydrogen bond interactions. The cross-linking of these gels is temporary which causes some gels to be thermoreversible being able to be liquefied or dissolved, some conditions that influence this instability are temperature, pH and ionic strength. This behaviour depends in every physical gel based on the material types and characters [3]. One example is aqueous solutions of agar and gelatin that become gels if the temperature is lowered and it will become solution again if the temperature is increased [14]. Different methods for the cross-linking are: hydrogen bonding, ionic bonding, coordination bonding, helix formation, hydrophobic bonding [14]. Physical gels can be further classified into "strong" physical gels and "weak" physical gels [15]. The difference between these two is that strong gels are solid in the large deformations and weak gels are structured fluids and flow as the liquid at larger deformations [16]. Chemical gels (also known as covalently cross-linked gels) are prepared through different methods, for example: addition polymerization of oligomeric multi-functional precursors, vulcanization of high-molecular weight linear polymeric chains, end linking of the reactive chains with branching units, etc [16]. Due to the nature of network’s junctions, covalent bonding between the macromolecules and the crosslinkers, these gels can only be damaged via de bond rupturing or by thermal degradation of polymer [17]. Due to their covalent bonding they have a permanent structure for swelling and shrinking in a dynamic reversible way and are less susceptible to external variables [12]. Contrary to physical gels, they generally do not dissolve. The difference between physical and chemical gels is shown in Figure 2.2
Gels classified by continuous phase 6 Figure 2.2: Schematic representation of (a) a physical gel formed by association of macromolecular chains and (b) a chemical gel formed through polymerization and crosslinking reaction, reprinted from [18]. Entanglement network are formed via topological interactions of polymeric chains either in the melt or in the solutions, when the concentration and molecular weight of the entanglement network is higher than the critical molecular mass for the entanglement [16]. These behaviour is observed in polymeric gels that contain polymer(s) of high molecular weight, specially elastomers.These gels are usually dissolved in a suitable solvent to produce dilute polymer solutions [16]. 2.2 Gels classified by continuous phase According to the definition of gels there are multiple examples of them. Gels that are explained on the following pages could present different classification depending on the properties mention in section 2.1.2.
Gels classified by continuous phase 7 2.2.1 Hydrogels Hydrogels are three-dimensional cross-linked polymer networks which can absorb and retain large amount of water [19], mostly made of hydrophilic polymer(s) [20]. The classification of hydrogels, Figure 2.3 [21, 22, 23]: • Based on source: Natural, synthetic or hybrid. • Based on the preparation method: Homopolymeric hydrogels derived from only one specie of monomer, copolymeric hydrogels with tho or more different monomers and multipolymer interpenetrating polymeric hydrogel (IPN) made out of two independent polymers that are in a network form. • Based on configuration: Amorphous, semicrystalline and crystalline. • Based on the type of cross-linking (see Section 2.1.2): chemical or physical hydrogels. • Based on physical appearance: It is influenced by the production proccess and they can be matrix, microspheres and films. • According to network electrical charge: nonionic, cationic and anionic. • Based on their response to chemical, biochemical or physical stimuli. Hydrogels Source Preparation Configuration Chemical Ionic charge Cross-linking Response Homopolymeric Copolymeric Interpenetrating Natural Synthetic Hybrid Non-ionic Cationic Anionic Physical Chemical Amorphous Crystalline Semicrystalline PhysicalBiochemical pH Glucose Oxident Antigens Enzymes Ligands Temperature Pressure Light Electric field Magnetic field Configuration Response Chemical Figure 2.3: Classification of hydrogels based on the different properties.
Gels classified by continuous phase 14 (EtOH), acetone (Ac), dimethyl sulfoxide (DMSO), ethylene glycol, 2-propanol are used. Among this solvents ethanol is preferred due to its low vapour pressure and non-toxicity [38]. Multistep solvent exchange with EtOH increasing the concentration of the solvent each step results in lower shrinkage [39]. Not only solvents have to be miscible with CO2but must have a small miscibility gap in order to operate at reasonable mild conditions to preserve aerogel backbone. Gurikov et al. [7] studied the effect of different anti-solvents in shrinkage during solvent exchange and scCO2 drying for alginate and guar galactomannan gels. One significant result show that solubility parameters and hydrogen bonds expressed with the Hansen solubility parameters, have a relation with shrinkage. In conclusion, the selection of the solvent is a crucial step and has to be done carefully taking into account different factors: miscibility of solvent with scCO2and properties of the solvent that could have an impact in the final aerogel, i.e. toxicity of solvent residuals. 2.2.5 Cryogels Cryogels have gained a lot of attention in the last years in the medical fields for their potential application as tissue scaffolds. The term ‘cryogel’ comes from ‘cryo’ that means cold or ice and the already mention ‘gel’. The scientific definition for these gels is not fixed, so multiple definitions are used to three different groups of substances [40]: (1) gelatinous precipitate formed during cryoprecipitation - blood plasma treatment upon cooling at 4 °C; (2) polymeric gels produced by the sol–gel method followed by freeze drying; (3) synthetic and natural polymers produced in a frozen solvent, usually water. In this process ice crystals are formed and used as a
Gels classified by continuous phase 15 porogen, and removed by thawing instead of freeze-drying. Contrary to conventional gels, cryogels are heterophase systems where the solvent is inside the pores and bound to the polymer network [41]. These pores have a size of 1–100 µm and they are surrounded by thin walls of a highly concentrated polymer. The route followed for their preparation is shown in Figure 2.6. Preparation of porous material via thermally induced phase separation (TIPS) has been studied for the preparation of membranes [42, 43]. This method is based on the phenomenon that the solvent quality usually decreases when the temperature is decreased. After demixing is induced, the solvent is removed by extraction, evaporation or freeze drying [44]. Transition from polymer solution to porous structure has an important role in the final properties of the material. In the phase diagram of a binary polymer system three regions can be distinguished inside of the binodal curve determined by the spinodal curve, Figure 2.7. Depending on the concentration, liquid-liquid demixing proceeds through different mechanism. Inside the region located between the binodal and spinodal metastable compositions are obtained. Only processes with the same concentration as the critical point or processes with high freezing rate avoid demixing in the metastable region [45]. Figure 2.6: Scheme of formation of macroporous gels, reprinted from [41].
Gels classified by continuous phase 16 Figure 2.7: A schematic representation of a binary phase diagram of a polymer solution showing a liquid-liquid demixing gap, reprinted from [44]. These gels can be made with any monomer that could be polymerized by radical polymerization in aqueous solution. Pore size can be controlled with the temperature and also the nature and concentration of the monomer. Temperature has an effect in the final structure and the reaction rate. Lower temperature produces more ice nucleation sites but smaller, faster freezing and smaller non-liquid phase. If temperature is lower than the system’s eutectic point, cryogels cannot be formed as no liquid phase is present [40].
Examples for polymer gels 17 2.3 Examples for polymer gels Polymer and biopolymer aerogels have gained a lot of interest in the past years, approximately 17.8%of the literature studies. These polymer aerogels present very attractive applications and properties in different fields such as energy and biomedicine [46]. Production of hybrid aerogels from synthetic and natural based polymers have been studied [47]. Production of aerogels derived from synthetic polymer has mainly be done for polyureas, polyurethanes, polynorbornene, polydicyclopentadiene all of them in a monolithic shape [48]. Following there is a brief explanation of various polymers with current research on gel formation and their potential applications. 2.3.1 Polyacrylonitrile (PAN) Polyacrylonitrile (PAN) is a derivative of polyethylene that has a nitrile (CN) group in the unit structure, Figure 2.8 [49], widely prepared by using nitrle polymerization [50]. It has strong intermolecular bonds, resulting in good barrier properties [51]. PAN has high thermal stability i.e, a high melting point. It is difficult to melt due to the cyclization of the nitrile group. It also shows high mechanical strength, dielectric constant and high dipole moment (3.9D) that causes strong polarity. Hence, it is capable to form stable fibers and nanofibers by electrospinning, spin casting, gel spinning, and dry-jet wet spinning [52]. Research on gel materials that contain PAN has been made over the past years for a wide variety of applications in different fields. For example, in the develop of smart materials Umemoto et al. [53] produced pH-sensitive hydrogel fibres from polyac-
Examples for polymer gels 18 Figure 2.8: Molecular structure of PAN, reprinted from [49]. rylonitrile (PAN) textile fibers; in the medical field Ramseyer et al. [54] developed an injectable PAN-based hydrogel powder for the treatment of urinary incontinence; Mao et al. [55] developed the solvent-sensitive artificial muscle by hydrolyzed acrylonitrile-g-cellulose fiber; Yu et al. [56] studied the pH response of hydrolyzed PAN-blen-gelatin hydrogel fibers. Some authors have developed material for bone tissue engineering with electrospinning of PAN [57, 58]. Bhuiyan et al. [59] developed a PAN-silica aerogel nanofibre fabric with resistance against radiant heat and liquid penetration. 2.3.2 Poly(vinyl alcohol) (PVA) Poly(vinyl alcohol) (PVA) is a simple hydrophilic biodegradable polymer containing a single hydroxyl group per monomer polymerized, Figure 2.9 from vinyl acetate followed by hydrolysis. PVA is commonly used as a hydrogel or as a sponge and can be chemically cross-linked using small-molecule bifunctional cross-linking agents like flutaraldehyde or throuh gamma irradiation or can undergo repeating freeze/thaw cycles to form physical cross-links between polymer chains [60]. PVA has a unique combination of properties such as solubility in water, film orientation characteristics for the polarizer of a liquid crystal display, adhesive ability to a number of substrates, low toxicity, biodegradability and biocompatibility. These
Examples for polymer gels 19 Figure 2.9: Molecular structure of PVA, reprinted from [61]. properties make PVA a very interesting material in chemical, material and biomedical fields [62]. 2.3.3 Amide pectin Pectins are structural polysaccharides of vegetable origin. The polymer backbone is based on (1→4)-linked α-D-galacturonate residues interrupted by insertion of (1→2)-linked rhamnosyl residues where the neutral sugar side chains are attached [63]. Pectin has been used as gelling agent, stabilizers, emulsifiers, and bioactive components in the food, pharmaceutical and cosmetic industry. However one negative property that impacts in their use is that it tends to form lumps when it is dispersed in water [64]. One of the solutions for this problem is to enhace the functional attributes of pectin by the amidation of pectin. The amidation is obtained by the action of ammonia on the ester groups of pectin under alkaline conditions [65]. Figure 2.10: Molecular structure of pectin and amidated pectin with ethanolamine, reprinted from [66].
Examples for polymer gels 20 2.3.4 Carboxymethyl Cellulose Sodium salt Carboxymethyl cellulose (CMC) is cellulose ether made by reaction of alkali cellulose with an alkyl halide [67], Figure 2.11. It is produced by treatment of cellulose with aqueous sodium hydroxide solution and subsequent reaction with monochloroacetic acid or its sodium salt [68]. Purified CMC is a white to off-white, nontoxic, odorless, biodegradable powder,which can be dissolved in hot or cold water [69]. CMC is used in numerous industries, cosmetic and pharmaceutical, textile, paper, ceramic, and food industry [70]. Figure 2.11: Molecular structure of CMC sodium salt, reprinted from [71]. 2.3.5 Inulin Inulin is a non-starch polysaccharide and can be found in a large number of plants chicory, Jerusalem artichoke, onion, garlic, barley, rye [72]. It is composed of β-Dfructofuranosyl units linked 2→1. Some properties of inulin are, solubility in water at 40–80 °C with a concentration lower than 50%. Thermoreversible gels are formed when the concentration is higher than 25%and there is a cooling of the solution. The rheological properties of inulin gels make their use as a fat replacer in the food industry very common [73]. In addition to its rheological properties, there are studies that show their health benefits
Examples for polymer gels 21 and their food application fibre enrichment, as a prebiotic and as a sugar replacer [74]. They are also used in feed and pet food [75]. Figure 2.12: Molecular structure of inulin, reprinted from [71]. 2.3.6 Gum arabic It is a tree gum exudate that it is called also Acacia gum, there are two species of acacias that the regulation acknowledge as the trees for the extraction gum, Acacia senegal and Acacia seyal. Both gums consist of polysaccharides and contain a small ammount of nitrogenous material that cannot be removed . The difference between the two gums is that A. seyal gum has lower rhamnose, glucuronic acid content and lower proportion of nitrogenous material [76]. Gum arabic is a highly heterogeneous material that consists of highly branched polysaccharides species. It can be divided into three fraction, a 70%–90%polysaccharide molecules, 10%of high molecular weight molecules that have a hydro-
Examples for polymer gels 22 phobic protein which provides the functionality of the gum, and the last fraction only up to 1%is a glycoprotein [77]. Gum arabic is mainly used in the food industry in different applications, in confections to reduce or prevent sucrose crystallization and to avoid the accumulation of fatty components on the surface. It is also used in beverages, bakery and dairy. Not only its uses are limited to food industry but to the textile, ceramics and pharmaceutical industry [78]. It is used as a suspending agent, emulsifier, adhesive and binder. 2.3.7 Agar-agar Agar is a polymer composed of two fractions: neutral agarose and anionic agaropectin. It is a linear chain of 3-O-substituted β-d-galactopyranosyl units joined by (1→4) linkages to 3,6-anhydro-α-l-galactopyranosyl units, its molecular structure can be seen in Figure 2.13. They are extracts from agarophyte members of the Rohodophyta. The pattern of substituting groups depends on the species, environmental factors and physiological [79]. Figure 2.13: Molecular structure of agar-agar, reprinted from [80]. Agar is soluble in boiling water but insoluble at lower temperature, it forms gels at 30–40 °C. Gelation mechanism is based on the aggregation of helixes and subsequent phase separation. Properties of the gel depend on agarose content degree
Summary and Background of Study 23 of sulfonation, generally with higher content and lower degree better and stronger gels are formed [81]. Applications of agar based in the gelling power, high hysteresis and perfect gel reversibility. Mainly it is used in the food industry and for biotechnological applications [82]. Research has been conducted in the field of aerogel production [83, 84, 85]. 2.4 Summary and Background of Study 2.4.1 Porous materials via cryoextraction As mentioned in section 2.3, biopolymer aerogels have gained a lot of interest in the past years, these aerogels are mainly prepared with the routed explained in section 2.2.4.1. Therefore, a gel from an aqueous solution must be created in the first step. Cardea et al. [86] successfully developed a production path for chitosan aerogels that combines cryogels and aerogels, in which there is a step of solvent exchange and low temperatures followed by scCO2drying. The formation of the polymer network is done by thermally induce phase separation at −20 °C, this gelation step is responsible for the final aerogel structure. The results Cardea et al. obtained show that there is a maximum concentration of chitosan for the formation of hydrogels at low temperatures for which the solution falls into the gelation region, this region size increases with the decreasing of temperatures [87]. In addition, it is demonstrated that chitosan hydrogels that undergo through a solvent exchange at −20 °C preserve their gel structure as opposed to solvent exchange at room temperature [86]. At this temperature gel remains in
Summary and Background of Study 30 p/p0 V(1−p/p0)=1 VmC+p/p0(C−1) Vmc(2.1) V is the volume of the adsorbate, Vmis the volume of the amount of adsorbate required to shape a monolayer, and Cis the equilibrium constant used in the Langmuir isotherm improved by the vapour pressure of the adsorbate. Parameter C is exponentially related to the first-layer adsorption energy. The plot of Equation 2.1 in the form of p/[V(p0−p)]versus p/p0should give a straight line with slope s= (C−1)/VmCand intercept i=1/VmC. The validity of this theory for measurement of specific surface area is limited to a certain range of relative pressure, Figure 2.17. Brunauer et al. found that the range in which the isotherm gave linear BET plots was p/p0=0.05 −0.35. Later research has shown that this range is too wide, and some deviations from linearity start at p/p0∼0.25 [110]. This validity pressure range is because of the initial assumptions made in the theory [111]: 1. Adsorption occurs only on well defined sites of the surface with one per molecule. 2. A molecule can act as a single adsorption site for a molecule of the upper layer. 3. The uppermost molecule layer is in equilibrium with the gas phase, i.e, the same adsorption and desorption rates. 4. The desorption is kinetically limited and is homogeneous, the same heat of adsorption for a given molecular layer, layers greater than the first layer have a heat of adsorption equal to the heat of liquefaction. 5. At saturation the layer number tend to infinity, equivalent to the sample being
Summary and Background of Study 31 surrounded by a liquid layer. Figure 2.17: Curve (A) adsorption isotherms of nitrogen at 77 K on non-porous silicas and aluminas. Curve (B) adsorption isotherm calculated from equation. Reprinted from [109]. 2.4.3.2 Scanning electron microscopy (SEM) Scanning electron microscopy is a technique widely used to analyze the morphology of a material. It is widely used due to the high resolution that can be obtained, of the order of 1–5 nm and the very high magnification range (10–500000 times). This technique produces images of a sample by scanning it with a beam of electrons. The interactions produced between the material and the electrons can be divided into
Summary and Background of Study 32 two different categories: elastic and inelastic interactions. Elastic scattering result from the deflection of the electron by the atomic nucleus or outer shell electrocs of the sample, electrons that are elastically scattered are called backscattered electrons (BSE) and yield a useful signal for imaging the sample. Inelastic scattering occurs through a variety of interactions between the incident electrons and the electrones and atoms of the sample, during this interaction specimen atoms are ionized and generate secondary electrones (SE). Both signals are primarily used to form and image but they are not the only signals produced during the process, other signals are, characteristic X-rays, Auger electrons, and cathodoluminiscence (CL) [112]. These signals are collected by various detectors in the chamber, these detectors can be for SE, BSE signal detection and/or X-ray spectrometers. Modern SEMs devices have detector for these three signals [113]. Features analyzed by SEM are specimen shape, within this category some of its applications are examining porosity (size, distribution), measuring particles and microstructural feature sizes and distribution; quantification of chemical composition measured by BSE imaging or X-ray spectroscopy; surface crystallography carried out by electron backscatter diffraction [113].
33 3. Materials and methods 3.1 Materials Chemicals used in this study have been listed in Table 3.1 Table 3.1: List of chemicals used in this study. Chemical Supplier Purity Alginic acid ammonium salt Carl ROTH Xanthan (E-415) Carl ROTH Gummi arabicum Carl ROTH CMC soldium salt (low viscosity) SIGMA Aldrich CMC sodium salt (medium viscosity) SIGMA Aldrich Phytagel SIGMA Aldrich Guar Flour Carl ROTH Agar - Agar Carl ROTH ι-Carrageenan Gelcarin GP 379 NF κ-Carrageenan Gelcarin GP 911 NF λ-Carrageenan Viscani GP 109 NF Polyacrilonitrile (PAN) Pektin Amid Herbstreith & Fox KG Polyvinyl alcohol (PVA) SIGMA Aldrich MKCK8710 Inulin Orafti GR Dimethly sulfoxide Merk ≥99% Ethanol Carl Roth ≥99.5% tert-Butanol Carl Roth ≥99%
Preparation of aerogels 34 3.2 Preparation of aerogels In this section it is explained the innovative route followed for the preparation of aerogels. It was established a standardized route with PAN dissolved in DMSO. Some conditions were modified to study their effect on the final aerogel and reach optimal properties. These conditions were, moulds used for the preparation of the frozen sample, total weight of the sample and concentration of PAN (0.625–10 wt %). The procedure followed for the preparation of the monoliths consisted in three different steps. First, dissolution of the polymers in the solvent, second, solvent exchange at −28 °C and third, scCO2drying. The scheme of the preparation route is shown in Figure 3.1 Dissolution scCO2 drying EtOH solvent exchange DMSO PAN 70 °C Freezing-28 °C -28 °C 50 °C 120 bar for 3h Aerogel Figure 3.1: Preparation route for PAN aerogels.
Preparation of aerogels 35 3.2.1 Dissolution of polymers Solvent was placed in a beaker covered with a watch glass to avoid evaporation, heated at 70 °C with constant stirring. When temperature was reached polymer was added in the exact quantity and kept the stirring at 70 °C until the polymer was completely dissolved. At that point, the solution was placed at room temperature until it cooled down and afterwards it was poured in an aluminium mould and placed inside of the freezer overnight at −28 °C. 3.2.2 Cryoextraction with ethanol To carry out the extraction of DMSO, samples were carefully removed from the mould and placed in plastic containers filled with 100 ml ethanol at −28 °C and placed back in the freezer for 2 to 3 days. EtOH was replaced twice a day with a time difference of 5–6 h. The objective of solvent exchange was to obtain an alcogel of EtOH and no DMSO. To check the concentration of DMSO in the samples a linear correlation between the density and the concentration was made. A calibration curve for DMSO/EtOH [8] and solution of TBA in water (20 wt %)/EtOH were used. Densities were measured by the density meter (DMA 4500 M, Anton Paar, Germany). Calibration curves were plotted, density versus concentration, Appendix 7.1. After 3 days, 15 ml of the solution in the containers was taken out and placed at room temperature. Afterwards, concentration of EtOH was monitored with the density meter and calibration curves. If the concentration was ≤99 wt %cryoextraction was extended.
Preparation of aerogels 36 3.2.3 Supercritical CO2drying To obtain final aerogels from alcogels supercritical drying was used. Once the alcogels were free of DMSO, they were measured, weighted and placed inside of teabags. Teabags were placed into the preheated 250 ml autoclave, Figure 3.2. After that, the autoclave was sealed and pressurized to 120 bar and 50 °C for 3 hours. During the process, scCO2mass flow rate was approximately 35 g/min. After 3 hours autoclave was depressurized and samples were taken out, weighted and measured. Each sample was placed in a sealed container and stored at room temperature. Figure 3.2: 250 ml autoclave for scCO2drying. Volume shrinkage and linear shrinkage were calculated with Equation 3.1 and 3.2. Vshrinkage =V0−V1 V0 ·100%(3.1) where V0is volume after DMSO extraction and V1is volume after scCO2drying. Lshrinkage =D0−D1 D0 ·100%(3.2)
Preparation of aerogels 37 where D0is volume before and D1is volume after the process. It was calculated for both cryoextraction and scCO2drying. Porosity is calculated with Equation 3.3 ε=1−ρbulk ρskeletal (3.3) where ρbulk is the density of the monolith calculated with its volume and grams, and ρskeletal is different for each polymer and are listed in Table 7.1. When there are more than one polymer, the weighted average is used in ρskeletal. In addition, urea was added at different concentrations to modify the properties of the aerogel. First of all, the solution of PAN at different concentration (5 and 7.5 wt %) was prepared following the method already described. Solution was let to cool down to 50 °C and urea was added at different concentrations 1, 2 and 4 wt %, this concentration relates to the already prepared 5 and 7.5 wt %solution, under constant stirring. Once urea was dissolved, it was transferred to the freezing mould to froze. Afterwards the standard route was followed. Furthermore, PVA and agar-agar where combined with PAN at different concentrations. Total polymer concentration was 5 and 7.5 wt %and the polymer ratio was 50%for agar-agar and 25, 50, 75%for PVA. It was prepared by dissolving the exact amount of PAN in DMSO at 70 °C followed by the addition of the other polymer and its dissolution under constant stirring at 70 °C. After this point the standard route was followed.
Screening of process variables 38 3.3 Screening of process variables In this section it is explained all the variables studied and the method used in order to achieve an aerogel with better properties. 3.3.1 Solubility of polymers in DMSO and TBA Solubility was analysed visually at room temperature with the following procedure. Into a closed container 19 g of DMSO were poured. Solubility at 70 °C were test for 5 wt %if polymer was not soluble at this concentration, concentration was lowered to 1 wt %and the procedure was repeated. Polymers that were not soluble in DMSO with a concentration of 1 wt %were tested again but changing the solvent to a 20 wt %aqueous solution of tert-butanol (TBA). Same procedure was followed but the concentrations were 3 wt %and 1.5 wt %. Polymers that could be dissolved were further prepared as described in section 3.2. 3.3.2 Freezing moulds Different freezing moulds were analyzed. All the moulds tested are shown in Figure 3.3. In addition to these moulds, cell culture plate size 6 wells were also used. The route followed after freezing is the one described in section 3.2. p Candle moulds were preconditioned flattening the surface before pouring any solution. Polymers PAN, PVA and pectin amid were tested in syringe moulds. Every polymer listed in Table 3.1 was tested in aluminium plates. Candle and hard moulds were only tested for PVA and PAN.
Screening of process variables 39 Figure 3.3: Moulds tested for the preparation of aerogels: (a) Candle mould before preconditioning (b) Candle mould (c) Hard mould (d) Aluminium plate (f) Plastic syringe. Figure 3.4: Cell culture plate size 6 wells. 3.3.3 Thickness of monolith Variation of the thickness was studied through the quantity of sample that was poured into the mould, samples of 5 g and 10 g were compared for concentrations of PAN in the range of (2.5 – 10 wt %).
PAN aerogels 46 analysis (2–50 nm, this explanation is based on the high porosity of all the aerogels (>85%). One of the reasons could be owing to the natural property of different materials. Some biopolymers have higher intermolecular affinity (i.e. affinity between the molecules). They tend to form thicker walls during the freezing stage, leaving relatively large vacancies for DMSO crystals to grow. At the end, they form porous structures with high porosity but low SSA. m 2/ g Figure 4.1: Comparison of specific surface area. solvent DMSO solvent TBA and porosity. Individual concentration is expressed in the corespondent bar. Among all of the polymers tested, due to high specific surface area, relatively low shrinkage and performance, PAN is selected to be further studied. 4.2 PAN aerogels After the initial study of different polymers, polyacrilonitrile (PAN) aerogels are studied at different concentrations (0.625, 1.25, 2.5, 5, 7.5 and 10 wt %). The aim of this part is to optimize certain parameters of the process to achieve aerogels with
PAN aerogels 47 the best quality. It is studied the effect that freezing mould, thickness of the monolith and freezing temperature have in the quality of PAN aerogels. Results of PAN aerogels at every concentration are shown in Table 4.3. PAN aerogels at concentrations 1.25 wt %and lower experience a relative high shrinkage, a complete loss of their initial shape and SSA impossible to measure or extremely low. This behaviour is noticeable during solvent exchange where the gel becomes more delicate and difficult to handle in each change of solvent. Additionally, concentrations higher than 7.5 wt %present high viscosity which causes the removal of air bubbles inside of the solution before freezing almost impossible. Consequently, PAN aerogels outside of the concentration range 2.5–7.5 wt %were not studied in further experiments. Table 4.3: PAN aerogel properties at different concentrations. Concentration Vshrinkage Porosity SSA (wt %) (%) (%) (m2g-1) 0.625 1.25 44.3 98.0 ±0.3 40 ±56 2.5 41.8 ±8.1 96.9 ±0.4 96 ±36 5 36.8 ±6.1 93.5 ±0.6 119 ±18 7.5 41.8 ±1.3 90.8 ±0.5 91 ±14 10 34.2 ±0.3 87.8 ±0.4 42 ±5 Volumetric shrinkage, porosity and specific surface area are shown in average±standard deviation. All of the experiments have been repeated at least three times. In Figure 4.2 there is a comparison of PAN aerogels properties at different concentrations. Porosity decreases with the increase of concentration, this trend is also existent in volumetric shrinkage. However, there is an anomaly at 7.5 wt %where the volumetric shrinkage is higher than aerogels at 5 wt %. On one hand, low con-
PAN aerogels 48 centrations allow more DMSO crystals to grow but pore walls are thinner which leads to higher volumetric shrinkage and on the other hand, higher polymer concentrations tend to form thicker pore walls, consequently, DMSO crystals are less numerous and bigger in size. The contribution of each phenomenon to specific surface area and pore size is still to be determined. From Figure 4.3 linear shrinkage of PAN aerogels can be analysed. In concordance with what many authors have pointed out [7, 37, 35, 114] shrinkage of aerogels is caused by both solvent exchange and supercritical drying. However, in contrast to volumetric shrinkage, total linear shrinkage increases with the increase of the concentration. This could be explained by a higher shrinkage in the thickness at lower concentrations. m 2/ g Figure 4.2: PAN aerogel properties at different concentrations. The observed trend is opposite in each process, linear shrinkage experimented in
PAN aerogels 49 solvent exchange decreases with the concentration, whereas in scCO2drying increases. One of the possible reasons could be the direction in which solvent exchange is carried out, and the position of the monolith inside of the plastic containers. Furthermore, Takeshita et al. [115] proposes a mechanism that explains aerogel shrinkage during scCO2drying based on the low affinity between the polymer and CO2that produce coagulation between chains. These coagulated chains interact with each other by hydrogen bonds aggravating heterogeneous pore structure. They also point out that higher flows of CO2lead to higher shrinkage due to the formation mechanism. scCO 2 Figure 4.3: Comparison of linear shrinkage in solvent exchange and scCO2drying of PAN aerogels, scCO2drying EtOH exchange. Moreover, aerogels have a convex surface, the outside of the monolith is thicker than the center, caused by the shrinkage during the process, Figure 4.5. Another feature visible in some samples is radial heterogeneity which is caused by two opposite phenomena. During freezing, small DMSO crystals grow and at the same time low thermal conductivity and heat capacity establish a limit of mass available of cooling that leads to radial heterogeneity. This effect has also been observed by Gutiérrez
PAN aerogels 50 et al. [116] in the preparation of PVA scaffolds at different freezing rates. As a conclusion, PAN aerogels have a maximum specific surface area of 119 ± 18 m2g−1 at 5 wt %. For concentrations lower than 2.5 wt %or higher than 10 wt % results are not good enough to explore them further. High standard error in volumetric shrinkage and SSA might be caused by human error introduced while measuring dimensions and the presence of macropores along with differences in pore structure due to directional freezing and small cooling rate that produces an anisotropic aerogel, Figure 4.6 and 4.7. In addition, another effect already mentioned by Aubert et al. [117] takes place, due to the design of the process polymer solution does not freeze instantly, consequently there is a bottom layer that is in contact with the cold surface that freezes first that creates an insulation effect with the solution on top leading to different pore structure between the bottom surface and the pore surface. Figure 4.4: PAN aerogel 7.5 wt%with radial heterogeneity. (a) After solvent exchange (b) after supercritical drying. As shown in Figure 4.6, 2.5 wt %aerogels present a hierarchical pore structure consisting in aligned rows, visible channels, interconnected through pores. Irregular shaped pores and their size distribution is wide, approximately they are in the range
PAN aerogels 51 of 200 nm. Furthermore, it is also perceptible a defined longitudinal shape of the pores. This phenomena supports the theory of directional freezing presented in section 2.4.1. Some possible reasons are low freezing rate, freezing from the bottom of the sample, unequal freezing due to differences of conductivity between the surface of the freezer, freezing mould and air. Figure 4.5: PAN aerogel 5 wt%. Figure 4.6: SEM images of PAN 2.5 wt%aerogel.
PAN aerogels 52 As shown in Figure 4.7 (b) and (e), 5 wt %aerogels present a heterogeneous morphology and wide range of pore size. Whereas in some parts pores are about 500 nm in size, in others 100 nm. Similarly to 2.5 wt%aerogels directional freezing is also present at this concentration. However there is not only one direction but multiple, Figure 4.7 (f) . This might be caused by crystallization heat and irregularities during freezing, possible hot spots and cold spots that create an uneven freezing surface. From Figure 4.7 (d), it can be seen that pore structure it is more uniform when it is closer to the skin of the aerogel. According to SEM images pore structure is tighter and more regular than 2.5 wt % aerogels. Less PAN content allows DMSO crystals to grow freely in a looser structure where direction in pores is more accentuated. 4.2.1 Effects of the freezing mould Selection of the mould between aluminium dishes, candle moulds, syringes, cell culture plate and hard moulds was done to see the effect it has on cryoextraction, final properties and also to make the shape and size of aerogel as reproducible as possible. It is expected that the freezing rate of the gels in moulds of aluminium is higher than those of plastic due to its thermal conductivity. The highest the freezing rate the better for the creation of numerous smaller size pores [118]. In regards of freezing, whilst samples produced in disposable aluminium dishes and syringes are easily removed from the mould due to non constant diameter and non-stiff bottom material in the aluminium dishes and syringe’s shape. Samples in the rest of the moulds require to begin solvent exchange with the gel in the mould,
PAN aerogels 53 Figure 4.7: SEM images of PAN 5 wt%aerogel.
PAN aerogels 54 due to shrinkage observed during this step after one exchange gels can be removed. Moreover, samples in culture plates are very difficult to remove and have to be for a longer period in EtOH because of material’s rigidity and even in some cases they can’t be taken out due to minimum shrinkage. This aspect leads to nonuniform solvent exchange that might affect the final properties of the gel. When samples are frozen in syringes, irregularities in its shape appear due to volume contraction while freezing and the direction of heat transfer. They present a hole in the center of the cylinder that during drying makes the samples at low concentrations collapse and lose its cylindrical shape, Figure 4.8 (a). These gels are fragile and difficult to handle during the process. Therefore, the use of syringes as freezing moulds is rejected. Figure 4.8: PAN aerogel in syringe: (a) 0.625 wt%(b) 5 wt%. The results of the different moulds for PAN aerogels are shown in Table 4.4. Some moulds were tested only once because of bad results. From Figure 4.9, freezing mould have an effect in specific surface area. When comparing candle and hard moulds, the later show a better result at every concentration, with a maximum of 120 m2g−1 at 5 wt %. One relevant result is that for each mould there is a maximum specific surface area at different concentrations of the polymer. This could be attributed to the different dimensions of the moulds. Thus, different
PAN aerogels 55 freezing rate for the same amount of sample. In addition, taking into account not only measurable properties but also physical integrity during the process, ethanol consumption and reproducibility, the mould selected to achieve consistent results is the hard mould. Unlike candle moulds this one does not need any preconditioning, also the diameter of the mould remains constant and the surface is completely smooth after using it due to the material’s stiffness. Overall it is the most convenient and it retrieves consistent results. However, in the case of polymers that show no shrinkage during solvent exchange, disposable aluminium dish are recommended to be used. m 2/ g Figure 4.9: Comparison of specific surface area, density and volumetric shrinkage for different moulds.
PAN based aerogels 62 Figure 4.12: SEM images of PAN 5 wt%aerogel frozen at -196 °C. 4.3 PAN based aerogels In this section addition of urea to PAN and multiples combinations of PVA and agar-agar with PAN for the preparation of aerogels are studied.
PAN based aerogels 63 4.3.1 PAN/urea aerogels The size of pore walls created by the polymer in the freezing stage have an effect on the size of DMSO crystals. This can be controlled by the reduction of hydrogen bonds between molecules with the addition of urea. Results for the addition of urea to PAN aerogels in 1, 2 and 4 wt %. Table 4.7: Results of properties of hybrid PAN/urea aerogels at different compositions. Material Concentration Vshrinkage Porosity SSA (wt %) (%) (%) (m2g-1) PAN 5 36.8 ±6.1 93.5 ±0.6 119 ±18 Urea/PAN 1/5 33.4 ±3.8 95.2 ±0.1 52 ±8 2/5 32.3 ±2.8 94.5 ±0.5 40±23 4/5 31.5 ±4.6 95.2 ±0.1 15 ±15 PAN 7.5 41.8 ±1.3 90.8 ±0.5 91 ±15 Urea/PAN 1/7.5 38.0 ±2.4 90.6 ±0.5 94 ±18 2/7.5 31.3 ±7.4 90.9 ±0.6 23±32 4/7.5 21.1 ±5.5 92.6 ±0.2 Volumetric shrinkage, porosity and specific surface area are shown in average±standard deviation. All of the experiments have been repeated at least three times. From Figure 4.13, specific surface area decreases with the increase of urea. The expected improvement due to the reduction of hydrogen bonds is non-existent. One possible reason could be that hydrogen bonds in PAN aerogels do not play a big role in the polymer network. Another reason could be that the crystallization of urea itself leads the creation of macropores, therefore, the increase in porosity with the addition of urea.
PAN based aerogels 64 In terms of porosity and volumetric shrinkage, the addition of urea reduces volumetric shrinkage at both concentrations. Increasing the content of urea increases porosity and decreases volumetric shrinkage. However, the influence at 7.5 wt %is higher and volumetric shrinkage becomes constant at a value of ∼31%. m 2/ g Figure 4.13: Comparison of specific surface area, porosity and volumetric shrinkage of PAN aerogels 5 wt%7.5 wt%with the concentration of urea. 4.3.2 PAN/PVA aerogels Blends of PAN and PVA have been studied in literature to combine both properties, mechanical and hydrophobicity from PAN and the potential biomedical and pharmaceutical applications of PVA due to the presence of OH groups and the possibility
PAN based aerogels 65 of hydrogen bond formation with other chemicals. When they are mixed together, interactions with each other are expected to be through interchain hydrogen bonding [120]. Results of pure PAN and PVA aerogels are shown in Table 4.8, comparison of properties at different concentrations is in Figure 4.14. Table 4.8: Results of properties of pure PAN and PVA aerogels at different compositions. Concentration Material Vshrinkage Porosity SSA (wt %) (%) (%) (m2g-1) 2.5 PAN 41.8 ±8.1 96.9 ±0.4 96 ±36 PVA 53.6 ±2.9 92.3 ±1.6 43 ±13 5.0 PAN 36.8 ±6.1 93.5 ±0.6 119 ±18 PVA 53.3 ±5.1 90.1 ±2.1 81 ±38 7.5 PAN 41.8 ±1.3 90.8 ±0.5 91 ±15 PVA 47.7 ±5.6 90.6 ±3.9 107 ±15 10.0 PAN 34.2 ±0.3 87.8 ±0.4 42 ±5 PVA 40.1 ±5.2 85.8 ±1.1 86 ±32 Volumetric shrinkage, porosity and specific surface area are shown in average±standard deviation. All of the experiments have been repeated at least three times. From Figure 4.14, specific surface area is maximum for different concentration depending on the polymer, for PAN is 5 wt %and for PVA 7.5 wt %. This difference is caused by the interaction between polymer and DMSO and the nature of polymer that leads to different pore structure and variation in pore walls. Moreover, volumetric shrinkage is higher and porosity lower for PVA aerogels at every concentration. One possible explanation is that pore walls in this case are thinner and hence collapse of the network is more severe, higher than 50%in some cases.
PAN based aerogels 66 m 2/ g Figure 4.14: Comparison of specific surface area, density and volumetric shrinkage for PAN and PVA. PAN was combined with PVA in solutions of 5 wt %and 7.5 wt %polymer content to see what are the effects of different PVA/PAN ratio in the properties of the aerogel. The results obtained are shown in Table 4.9. In Figure 4.15 a comparison of different PVA/PAN aerogels can be seen. All of the aerogels have a concentration of 5 wt %and differ on the proportion of both polymers, being 0 and 100%entirely PAN and PVA respectively. Porosity decreases with the increase of PVA content in a non-linear form, the opposite trend is observed in linear shrinkage, increasing PVA content results in higher shrinkage. Unlike porosity and linear shrinkage, specific surface area do not present any linear
PAN based aerogels 67 trend versus PVA content, particularly for 25, 50 and 75%there is a slight increase. Overall, addition of PVA decreases specific surface area at every PVA concentration in comparison to aerogels made exclusively from PAN or PVA. These results may be related to the network created by the interaction of both polymers and their interaction with DMSO. He et al. [121] reports that compatibility of PAN/PVA blends calculated through Schneier’s theory is complete for concentrations lower than 39% and 87%of PVA content. Outside of this range it is possible that blends have irregular properties what might lead to different freezing temperature and consequently wide pore size distribution. Table 4.9: Results of properties of hybrid PAN/PVA aerogels at different ratios at concentration 5 wt%. PVA content lshrinkage Porosity SSA (%) (%) (%) (m2g-1) 0 14.4 ±1.2 93.5 ±0.6 119 ±18 25 14.4 ±0.8 93.2 ±0.5 53 ±20 50 16.0 ±1.7 92.8 ±0.3 55 ±17 75 17.8 ±1.9 92.2 ±0.6 61 ±30 100 22.4 ±3.1 90.2 ±0.8 81 ±38 Volumetric shrinkage, porosity and specific surface area are shown in average±standard deviation. All of the experiments have been repeated at least three times. In addition, aerogels with 25%PVA content have an specific surface area drastically low although porosity is very similar to pure PAN, 93 ±1 and 94 ±1 m2g−1. The arrangement of the polymer network concedes DMSO crystals to be bigger in size that lead to pores size outside of the range of BET measurement.
PAN based aerogels 68 m 2/ g Figure 4.15: Effect of ratio PAN/PVA of aerogels 5 wt %on specific surface area, porosity and linear shrinkage. 4.3.3 PAN/agar-agar aerogels PAN was combined with agar-agar in order to study the effect of aerogel preparation with two polymers. The results of a solution PAN/agar-agar 50/50%are shown in Table 4.10. In terms of visual aspect and integrity, throughout the process PAN/agar-agar aerogels are fragile and softer, this condition is specially noticeable after drying where their physical integrity is compromised and some outer parts break into smaller pieces, Figure 4.16. Results of specific surface area, porosity and volumetric shrinkage are shown in Table 4.10
PAN based aerogels 69 Figure 4.16: PAN/Agar-agar(50/50%) 7.5 wt%aerogel (a) top side (b) bottom side. Table 4.10: Results of properties of pure PAN and hybrid 50%PAN 50%agar-agar aerogels at different polymer concentration. Concentration Material Vshrinkage Porosity SSA (wt %) (%) (%) (m2g-1) 5.0 PAN 36.8 ±6.1 93.5 ±0.6 119 ±18 PAN/Agar (50/50%) 38.1 ±2.5 92.4 ±0.3 6 ±10 7.5 PAN 41.8 ±1.3 90.8 ±0.5 91 ±15 PAN/Agar (50/50%) 35.1 ±2.5 88.7 ±0.4 6 ±10 Volumetric shrinkage, porosity and specific surface area are shown in average±standard deviation. All of the experiments have been repeated at least three times. From Figure 4.17 specific surface area lowers drastically. One possible reason could be that the high viscosity of agar-agar in DMSO at those concentrations, leads to large vacancies for the DMSO crystal to grow, hence a prevalence of macropores over mesopores. This explanation is also concordant to porosity values of aerogels at both concentrations similar to pure PAN aerogels. Volumetric shrinkage might suggest that the pore walls formed in PAN/agar solution are thicker than those of only PAN.
PAN based aerogels 70 m 2/ g Figure 4.17: Comparison of specific surface area, porosity and volumetric shrinkage of aerogels PAN hybrid PAN/Agar (50/50%).
71 5. Summary The aim of this work is to develop and optimize a process for the production of aerogels from precursors (polymers) which usually cannot be used in the conventional aerogel production process due to the lack of the gelling ability. The novel process proceeds via freezing of the precursors’ solution in DMSO followed by a cryoextraction step with EtOH and finally by scCO2drying of the wet network. A polymer solution in DMSO is first frozen at −28 °C. Afterwards, frozen monolith undergo a cryoextraction with EtOH at −28 °C. Finally, wet networks are dried in a scCO2process at 120 bar and 50 °C for 3 h, resulting in a porous material (aerogel). Textural properties of the obtained porous materials are characterised by measuring the specific surface area (SSA), porosity (ε) and shrinkage (volume reduction throughout the processing). Generally speaking, it is desired to obtain a high specific surface area (over 100 m2g−1), along with a high porosity and possibly low shrinkage. Our results show that the polymer concentration in the solution has a strong effect on these properties. Initial screening is conducted with a series of polymers, phytagel, PAN, PVA, κcarrageenan, guar flour, pectin, λ-carrageenan, gum arabic, alginic acid, CMC sodium salt (both low and medium viscosity). Among all the polymers, PAN is studied in a more depth due to promising results in comparison to the rest. To obtain PAN aerogels the polymer concentration is varied between 0.625 and 10 wt %. Concentrations lower than 1.25 wt %results in materials with a high volu-
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