Morphological study and photo-addressing in poly(styrene-b-butadiene-b-styrene) block copolymers with azobenzene groups and polystyrene matrix: influence of chemical bonding

1 Morphological study and photo-addressing in poly(styrene-bbutadiene-b-styrene) block copolymers with azobenzene groups and polystyrene matrix: influence of chemical bonding Raquel Fernández, Iñaki Zalakain, José Angel Ramos, Loli Martin, Iñaki Mondragon* ‘Materials + Technologies’ Group, Department of Chemical & Environmental Engineering, Polytechnic School, Universidad País Vasco/Euskal Herriko Unibertsitatea, Plaza Europa 1, 20018 Donostia-San Sebastián, Spain. * Corresponding author. Tel.: +34 943 017 177; fax: +34 943 017 130. E-mail address: [email protected] (I. Mondragon) This is the accepted manuscript of the article that appeared in final form in European Polymer Journal 47 : 1176-1185 (2011), which has been published in final form at https://doi.org/10.1016/j.eurpolymj.2011.01.010. © 2011 Elsevier under CC BY-NCND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) 2 ABSTRACT The main goal of this work was the synthesis of new azo-functionalized block copolymers (BCP) from epoxidized poly(styrene-b-butadiene-b-styrene) modified with azobenzene groups by one-step facile reaction between the epoxy groups and an azoamine. The epoxy/amine reaction was verified by Fourier transform infrared spectroscopy. Additionally, we studied the effect of covalent attachment of the azobenzene moieties by analyzing the morphology and the optical anisotropic response of the resulting azo-containing BCP, with respect to solution mixing of the azobenzene as a guest in the BCP host without chemical bonding. Self-assembly of all modified BCP resulted in phase-separated morphologies on the nanometer scale. Nonetheless, segregation of azobenzene aggregates onto the BCP surface was observed in guest-host systems. In relation to the optical anisotropic behaviour of the resulting materials, two distinct optical responses were observed depending on the existence or not of covalent attachment of the azo-chromophores to the BCP. Keywords: block-copolymer; epoxy; azobenzene; microstructure; birefringence 3 INTRODUCTION The introduction of photo-chromic groups in polymers is very attractive as it offers the opportunity of generating new light-sensitive materials and optical devices [1]. In particular, polymers with azobenzene units or azo-polymers have been widely investigated because of their potential applications in optical recording processes based on the photo-orientation of the azo-chromophores through polarised light induced transcis-trans isomerisation cycles [2-8]. Most studies on this subject have been performed with amorphous and liquid crystalline azo-containing homopolymers and random copolymers. However, recently, the development of block copolymers (BCP) containing azobenzene units has gained importance [9-25]. It is well-known that BCP can form microphase separated nanostructures with cylindrical, lamellar, spherical or bicontinuous morphologies [26-32]. The confinement of photo-responsive units in nanosized block copolymer domains gives these materials unique properties, because of the possibility of altering their self-assembly behaviour, while allowing the incorporation of molecules with optical features. Additionally, the reorientation of nanosized domains themselves, induced by the irradiation of this type of polymers, has also been reported. In particular, Ikeda and co-workers [33] demonstrated a molecular cooperative motion between azobenzene moieties and other photo-inert groups in azocontaining BCP with specifically designed structures. The azo-chromophores became aligned, triggered by the irradiation with a polarized laser beam at 488 nm, and the photo-inert groups were oriented together with the azobenzenes by supramolecular cooperative motions, although they did not absorb the actinic light. The applications related to holographic optical storage are of the most interesting for researchers [34, 35]. To fully exploit the advantages of holography, thick films of tens or hundreds of microns are needed. Nevertheless, it is normally limited to azo- 4 polymer thin films since, due to the optical absorption of azobenzene moieties in the wavelength region of the recording light, thick films of azo-homopolymers cannot be illuminated through the complete film thickness. To decrease the optical absorption, the azobenzene content has to be diluted. This can be achieved by copolymerisation of the azo-chromophore with other monomer that does not absorb at the excitation wavelength. However, it has been verified that random azo-copolymers show a lower photo-induced response with respect to that of the corresponding azo-homopolymers. This fact has been associated with a decrease of interactions among azobenzene moieties, as a consequence of their statistical distribution in the polymeric chain. In an attempt to obtain an azobenzene dilution while keeping those interactions among azobenzenes, BCP with an azo-block having the same composition as the homopolymer and another block that does not absorb light in the photo-excitation region can be used. Depending on molecular weight and composition, block segregation appears in such a way that a microstructure of azobenzene domains in the diluting polymer can be induced. Thus, a decrease of the azobenzene content is achieved while a photo-induced response of the azo-block similar to that of the homopolymer can be expected [36]. Moreover, one of the most important parameters of holographic gratings is the diffraction efficiency (DE). Azo-block copolymers are good candidates to control the DE by surface relief grating enhancement upon microphase separation. Compared with other methods to control the DE, such as mechanical stretch, electrical switch, ..., the microphase separation method has the advantage of being simple and convenient. That is to say, holographic gratings can be inscribed at room temperature and subsequent annealing improves the DE by almost two orders of magnitude [37, 38]. These holographic gratings with enhanced effect might be applied to secure information storage since the information can be easily read out by the thermally induced 5 microphase separation [39]. Based on that knowledge, combining the excellent properties of azo-polymers with microphase separation, azo-block copolymers might find diverse uses in advanced technology as well as newly promising nanotechnology. However, the development of well-defined nanostructures involves the synthesis of BCP with controlled macromolecular architecture, molecular weight distribution, and composition. Several polymerization methods, such as anionic, cationic, free radical and metal-catalyzed polymerizations, have been explored to build azo-containing BCP that meet these requirements [39]. Alternatively, we present here the functionalization via epoxidation of commercial BCP, based on poly(styrene-b-butadiene-b-styrene) (SBS), modified with azobenzene moieties by one-step facile reaction between the epoxy and azo-amine groups. This is a novel and simple way of preparing azo-functionalized block copolymers as well as very versatile considering that the epoxy group is receptive to a wide range of reagents. Therefore, this functional group can react with different kinds of chromophores with electron-donor groups containing hydrogen, such as amines, amides, acids, anhydrides, phenols, ..., giving rise to a large variety of azo-containing BCP, taking into account also the great diversity of commercial available SBS type copolymers. In addition, we investigate the influence of covalent attachment of the azobenzenes by studying the morphology and photo-addressing behaviour of the resulting azo-containing BCP, with respect to solution mixing of the azo-chromophores without chemical bonding as guest-host systems. 6 EXPERIMENTAL Materials An azo-chromophore, 4-(4-nitrophenylazo)aniline (O2N)(C6H4)N=N(C6H4)(NH2), Disperse Orange 3 (DO3), with a melting temperature of 200 ºC, was supplied by Aldrich. Two SBS linear triblock copolymers, C500 and C540, with 30 and 40 wt % of polystyrene (PS), respectively, were kindly supplied by Repsol-YPF. Gel permeation chromatography (GPC) was performed with a PerkinElmer LC-295 chromatograph. The mobile phase was tetrahydrofuran (THF) at flow rate of 1 mL∙min-1. The number-average molar masses (Mn) were 102,000 g∙mol-1 for C500 and 75,000 g∙mol-1 for C540 as calculated using a universal calibration method with polystyrene standards. PS homopolymer was purchased from Polymer Source and had a Mn of 20,800 g∙mol-1 as given by the manufacturer. All materials were used as received without further purification. Synthesis of photo-addressable block copolymers Different epoxidized SBS triblock copolymers, C500epX or C540epX (X being the degree of epoxidation (mol %) with respect to PB double bonds), were obtained by epoxidation of PB-blocks. This reaction was carried out using hydrogen peroxide in the presence of an in situ prepared catalyst system in a water/dichloroethane biphasic mixture, following a procedure described elsewhere [40]. Then, the resulting epoxidized copolymers were reacted with the azo-amine, DO3, in stoichiometric ratios, between epoxy groups and active hydrogens of the amine, at 110 ºC during 24 h under vacuum conditions. Figure 1 shows an schematic illustration of the synthesis procedure followed. In addition, guest-host systems of C500 and C540 containing DO3 (C500DO3 and C540-DO3) and PS (C500-DO3-PS and C540-DO3-PS) were also prepared. 7 All samples have similar amounts of azo-chromophore on the order of 15 (± 2) wt %. Films preparation Films of the azo-block copolymers and the guest-host systems were prepared by spin-coating from 5 wt % solutions in THF:dichloroethane 0.15:0.85 using a P6700 spin-coater from Cookson Electronics. The spinner program was 1000 rpm for 60 s. Residual solvent was removed by evaporation at room temperature. Then, films were annealed at 110 °C during different times, 24 and 48 h, under vacuum. The films thicknesses were determined by ellipsometry using a Semilab Sopra GES-5E ellipsometer from Telstar. Techniques High resolution 1H NMR spectra, recorded in deuterated chloroform solution with a Bruker 300 MHz spectrometer at 25 ºC, were used to determine the degree of epoxidation. 1H NMR chemical shifts were measured with respect to tetramethylsilane (TMS) as internal standard. Differential scanning calorimetry (DSC) was performed using a Mettler Toledo DSC 192 822 differential scanning calorimeter equipped with a sample robot 193 TSO 801 RO. Nitrogen was used as purge gas (10 mL∙min-1). The reaction temperature and the glass transition temperature (Tg), defined as the onset of the change in specific heat, were determined from the thermograms obtained in heating scans at 10 ºC∙min-1. Infrared spectra were taken using a Nicolet Nexus 670 Fourier transform infrared (FTIR) spectrometer equipped with a single horizontal golden gate attenuated total reflectance (ATR) cell. Spectra were recorded using a spectral width ranging from 600 to 4000 cm-1, with 2 cm-1 resolution and an accumulation of 20 scans. 8 The morphology of the samples was studied by atomic force microscopy (AFM). AFM images were obtained with a Nanoscope IIIa scanning probe microscope (Multimode™, Digital Instruments). Tapping mode (TM) in air was employed using an integrated tip/cantilever (125 µm in length with ca 300 kHz resonant frequency). Typical scan rates during recording were 0.7-1 line∙s-1 using a scan head with a maximum range of 16 x 16 µm. Optical storage experiments were carried out at room temperature and under ambient conditions. The experimental setup used was similar to that previously reported [41]. Optical birefringence was induced in films of the azo-block copolymers and the guest-host systems using a linearly polarized argon laser operating at 488 nm (writing beam) with a polarisation angle of 45º with respect to the polarisation direction of a low power He-Ne laser operating at 632.8 nm (reading beam). The power of the writing beam used in the experiments was varied between 6 and 20 mW on a spot of 0.4 mm2 and the change in the transmission of the reading beam, which passed through the sample between two crossed polarisers, was measured with a photodiode. The induced birefringence (Δn) was determined by measuring the reading beam transmission (T = I/I0) according to: Δn = (λ/π d) sin-1 (I/I0)1/2 where λ is the wavelength of the reading beam, d is the film thickness, I is the intensity of the reading beam after the second polariser and I0 is the transmitted intensity of the reading beam between parallel polarisers in absence of anisotropy. RESULTS AND DISCUSSION A brief account of the characteristics of the modified copolymers can be found in Table 1. 1H NMR spectra provided evidence for the existence of reactive epoxy 9 groups in PB-block chains. Figures 2a-d show 1H NMR spectra of the parent copolymers, C500 and C540, and the epoxidized copolymers, C500ep11 and C540ep14. C500 and C540 showed two signals at 5.03 and 5.45 ppm, corresponding to the olefinic protons of 1,2and 1,4-butadiene units. The corresponding epoxidized copolymers showed likewise two new signals attributed to the protons attached to transand cisepoxy groups at 2.70 and 2.96 ppm, respectively. The degree of epoxidation was calculated by the integration of these new signals compared to the sum of the intensities of double bond proton peaks. Firstly, the reaction between the epoxy groups and the azo-amine was investigated. Thermal behaviour of C500ep11-DO3 and C540ep14-DO3 was studied by differential scanning calorimetry. Dynamic DSC scans (not shown here) were utilized to estimate reaction temperatures and Tg values of samples. According to this study, 110 ºC during 24 h was selected as reaction condition for both systems. In order to verify the complete reaction of reactants, isothermal DSC scans (not shown here) were obtained at that experimental condition (110 ºC, 24 h). In a subsequent DSC scan in dynamic mode no residual heat of reaction was seen, proving total reaction. In addition, the synthesis of both azo-block copolymers, C500ep11-DO3 and C540ep14-DO3, was carried out under vacuum to avoid possible oxidation reactions. To corroborate the epoxy groups reaction, FTIR spectra in the mid IR region of the samples were also obtained. In Figures 3a-b the spectra of DO3, C500ep11-DO3 and C500ep11 with and without annealing, at 110 ºC for 24 h under vacuum, are plotted. The main infrared absorption bands of these samples are listed in Table 2. 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Cooperative motion of rigid groups in semicrystalline polymers. Macromolecules 1994; 27:2580-2585. 22 TABLE CAPTIONS Table 1. Characteristics of the block copolymers used. Table 2. FTIR assignments of the relevant absorption bands of the materials used. 23 Sample Epoxidation degree (mol %) PS (wt %) PB (wt %) Epoxidized PB (wt %) AZO (wt %) T g PB (ºC) SBS-DO3 C500-DO3-PS C500ep11-DO3 0 0 11 25 38 25 60 49 50 0 0 10 15 13 15 -80a -80a -55 a Glass transition temperature values for PB/epPB blocks of SBS determined elsewhere [42]. Table 1 Wavenumber (cm-1) Band assignment 3500-3400 3400 3080-3005 2920-2840 1712 NH2 and NH stretching OH stretching C-H stretching of aromatic rings C-H stretching of aliphatic bonds C=O stretching 24 1640 1600 1640-1600 1510 1390 1340 1140 990-910 890 855-835 NH 2 in-plane bending NH in-plane bending C=C stretching NO2 asymmetric stretching N=N stretching NO2 symmetric stretching C-H stretching C=C out-of-plane bending C-O-C asymmetric stretching C-H out-of-plane bending of aromatic rings Table 2 FIGURE CAPTIONS Figure 1. Synthesis of azo-containing block copolymers. Figure 2. 1H NMR spectra of parent copolymers, (a) C500 and (b)C540, and epoxidized copolymers, (c) C500ep11 and (d) C540ep14. Figure 3. FTIR spectra of DO3, C500ep11, and C500ep11 and C500ep11-DO3 after annealing at 110 ºC for 24 h within the range: (a) 3600-800 cm-1 and (b) 1700-800 cm-1. 25 Figure 4. TM-AFM phase image (left) and profile (right) of C500-DO3 after annealing at 110 ºC for 24 h (3 μm × 3 μm). The straight line on the image indicates the position where the profile was measured. Figure 5. TM-AFM phase image (left) and profile (right) of C500-DO3-PS after annealing at 110 ºC for 24 h and 210 ºC for 4 min (3 μm × 3 μm). The straight line on the image indicates the position where the profile was measured. Figure 6. TM-AFM phase images of (a) C500ep11-DO3 and (b) C540ep14-DO3 after annealing at 110 ºC for 24 h, and (c) C500ep11-DO3 and (d) C540ep14-DO3 after annealing at 110 ºC for 48 h (2 μm × 2 μm). Figure 7. Writing-relaxing curves of (a) C500-DO3, (b) C540-DO3, (c) C540-DO3-PS, (d) C500ep11-DO3, and (e) C500ep11-DO3 and (f) C540ep14-DO3 after annealing at 110 ºC for 24 h. The writing beam is on at point A and off at B. 32 Figure 3b 33 Figure 4 34 Figure 5 35 Figure 6a 36 Figure 6b 37 Figure 6c 38 Figure 6d 39 Figure 7a 40 Figure 7b 41 Figure 7c