Morphological self-control of a phase-separated polymer during photopolymerization in a liquid-crystalline medium

Morphological self-control of a phase-separated polymer during photopolymerization in a liquid-crystalline medium

Polymer 45 (2004) 6357–6363 www.elsevier.com/locate/polymer Morphological self-control of a phase-separated polymer during photopolymerization in a l...

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Polymer 45 (2004) 6357–6363 www.elsevier.com/locate/polymer

Morphological self-control of a phase-separated polymer during photopolymerization in a liquid-crystalline medium Hideyuki Kihara*, Toshiaki Miura, Ryoichi Kishi, Akira Kaito Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 3058565, Japan Received 26 February 2004; received in revised form 25 June 2004; accepted 9 July 2004 Available online 24 July 2004

Abstract An acrylate monomer having a cyanobiphenyl mesogen (1) was photopolymerized in a liquid-crystalline (LC) ordering field of 4hexyloxybenzoic acid (2). A blend of (1) and (2) (molar ratio: 1:4), containing a photoinitiator, an inhibitor and a crosslinker, was irradiated with UV light at 120 and 137 8C in order to investigate the effect of an LC phase on the resulting polymer. Here, both temperatures are in the nematic temperature range of the blend, however, the former is in the LC temperature range of the polymer, whereas the latter is in the isotropic temperature range. Scanning electron microscopy of the obtained polymers revealed that the polymer prepared at 120 8C consisted of oriented fine fibers, measuring ca. 400 nm in diameter, while that obtained at 137 8C had a fused bead-like morphology. In addition, we investigated the effect of crosslinking on the morphology by comparing the results from the blends with and without a crosslinker. We found that the LC phase of the phase-separated polymer is one of the necessary conditions for the formation of the fine fiber structures. q 2004 Elsevier Ltd. All rights reserved. Keywords: LC polymer; Photopolymerization-induced phase separation; Fiber

1. Introduction In recent years, nanofibers of synthetic polymers have been successfully fabricated by electrospinning [1–4] and block copolymer [5–7] techniques, in the hope of finding useful applications based on their structural characteristics. In addition to these sophisticated methods for the production of such nanofibers, we have shown that photopolymerization of liquid-crystalline (LC) monomers in LC solvents is useful for preparation of polymer fibers with diameters from a few micrometers down to hundreds of nanometers [8,9]. Since an LC monomer is generally miscible with a lowmolar-mass LC solvent, blends composed of them can show an LC phase. However, once the corresponding polymer is generated by photopolymerization of the monomer, it is frequently insoluble in and phase separates from the LC * Corresponding author. Tel.: C81-29-861-6331; fax: C81-29-8616291. E-mail address: [email protected] (H. Kihara). 0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2004.07.013

solvent [10]. By means of this phenomenon, that is, photopolymerization-induced phase separation in (LC monomer/ LC) blends, we obtained fine fibers of LC polymers and networks. It is noteworthy that this technique has the advantage to endowing the fibers with orientation and well-ordered macroscopic structures that are associated with LC textures present at photopolymerization. As templates for photopolymerization, we used a schlieren texture and a focalconic fan texture, which are characteristic of nematic and smectic A liquid crystals, respectively [8]. Recently, by use of a fingerprint and a Grandjean texture of a cholesteric phase as templates, we obtained LC polymer fibers in which helical superstructures were evident [9]. On applying our system for the practical preparation of LC polymer fibers, it may be important to control the orientation of the fibers and construct well-ordered macroscopic structures consisting of the fiber assembly. It is also important to elucidate the mechanism and the factors controlling the formation of fine fiber. We thought that among the several preparative conditions, the types of the

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phases, such as LC phase and isotropic phase, would have a very significant influence on the polymer morphology. Here, we must consider three phases exhibited by the initial blend, the phase-separated polymer and solvent, because the blend consisting of a monomer and an LC solvent shows a single phase before photopolymerization while it separates into the polymer and the solvent after that. Although the morphology of polymers formed in polymer-stabilized liquid crystals (PSLCs) and some of the factors controlling it, have been previously reported [11–14], a clear understanding of the conditions governed by the initial and final phase states is still not available. In the present study, to investigate the effect of the phase exhibited by the phase-separated polymer on its own morphology, we chose an acrylate monomer (1) and a low-molar-mass liquid crystal (2), whose molecular structures are shown in Scheme 1, and prepared a blend from them in a molar ratio such that the isotropization temperatures of (2) and the blend are both higher than that of the phase-separated polymer. Such a blend enabled us to determine two specific photopolymerization temperatures in the LC temperature range of the blend. At one of them (120 8C), the blend and (2) both show LC phases, and the phase-separated polymer also exhibits an LC phase. In contrast, at the higher temperature (137 8C), the blend and (2) show LC phases, however, the phase-separated polymer exhibits an isotropic phase. After photopolymerization under the two conditions, we obtained the polymers and observed their structure, thus enabling us to understand the effect of an LC phase of the obtained polymer on its own morphology. In addition, by comparing the blends with and without a crosslinker (3), as depicted in Scheme 1, we were able to study the effect of crosslinking on the polymer morphology.

by recrystallization from acetone before use. The thermal properties of both substances were described in our previous paper [10]. The diacrylate compound (3), used as a crosslinker, was synthesized according to a procedure outlined previously [16]. Its chemical structure was confirmed by 1HNMR. The compound (3) melts at 80 8C and we could not detect an LC phase. A photoinitiator, 2,2methoxy-2-phenylacetophenone, and an inhibitor, 4-methoxyphenol, were purchased from Tokyo Kasei Kogyo Co., Ltd and Wako Pure Chemical Industries, Ltd, respectively, and were used without further purification.

2. Experimental

2.3. Characterization

2.1. Materials

An Olympus BH2 microscope equipped with a Mettler FP82HT hot stage was used for the optical microscopy. Scanning electron microscopy of the uncoated samples was conducted in a low vacuum mode using a Philips XL30 ESEM-FEG. Differential scanning calorimetry was carried

The monomer (1), having a cyanobiphenyl mesogen, was synthesized according to a method described in the literature [15]. The benzoic acid derivative (2) was purchased from Tokyo Kasei Kogyo Co., Ltd and purified

Scheme 1.

2.2. Blend preparation and photopolymerization Blend preparation and photopolymerization were carried out following a technique similar to one we described previously [8]. The blend contained (1) and (2) in a molar ratio of 1:4. The hydrogen-bonded dimer of (2) was treated as a single molecule. The photoinitiator and the inhibitor were always added to the blend. Their amounts were based on (1) and were 1.0 mol% and 3000 ppm, respectively. In some experiments, we added the crosslinker (3), 2.5 mol% with respect to (1), to the blend. To photopolymerize the monomer (1) at a controlled temperature, the blend was placed between glass slides and irradiated with UV light (1 min; 200 W Hg–Xe lamp; glass fiber lens; 20 mW cmK2) on a hot stage of a temperature controller for microscope observation (Linkam LK-600PH). Immediately after UV irradiation, the samples, sandwiched between the glass plates, were immersed in hexane at 25 8C and the glass plates were carefully separated. The irradiated samples adhering to the plates were extracted with hexane for a few days to remove (2) and unreacted monomer and were then dried in vacuum. The residual polymers remaining on the glass plates were observed by SEM.

Fig. 1. Comparison of the LC temperature ranges of the blend, the component (2), and the solution-polymerized polymer from (1). Abbreviations: IZisotropic, NZnematic, LCZliquid-crystalline, CZcrystalline, GZglassy.

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Fig. 2. Polarizing optical micrograph for a schlieren texture of a nematic phase of the blend taken at 137 8C before UV irradiation.

out using a Seiko Instruments Inc. DSC 6200. Heating and cooling rates were 5 8C minK1. Polarizing infrared spectroscopy was performed using a Digilab FTS 6000ec FTIR spectrophotometer and an infrared microscope, Digilab UMA600 equipped with a wire-grid polarizer.

3. Results and discussion

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The blend showed a nematic phase and the isotropicnematic transition temperature is 140 8C. The compound (2) also exhibited a nematic phase between 95 and 152 8C. The polymer in Fig. 1 (Mn and Mn/Mw were 2.5!105 and 2.4, respectively, from GPC using THF as an eluent and PS as a standard) was prepared from (1) by a conventional solution polymerization in DMF and showed an LC phase between 34 and 128 8C. Since the isotropization temperature of the blend is higher than that of the polymer and lower than that of (2), we can determine two contrasting conditions of photopolymerization in the LC temperature range of the blend. We chose 120 and 137 8C as the representative of the photopolymerization temperature, which are indicated by the arrows in Fig. 1. At 120 8C, not only the blend and (2) but also the solution-polymerized polymer shows an LC phase. In contrast, at 137 8C, the blend and (2) both exhibit the nematic phase, whereas the solution-polymerized polymer shows an isotropic phase. Therefore, comparing the photopolymerization results obtained at 120 and 137 8C, we can understand the effect of LC phase of the obtained polymer on its own morphology formed by photopolymerization-induced phase separation.

In Fig. 1, we compare the LC temperature range of the blend containing the crosslinker with those of its component (2) and the solution-polymerized polymer derived from (1).

Fig. 3. Optical micrographs of the blend irradiated with UV light at (a) 120 and (b) 137 8C, respectively.

Fig. 4. Optical micrographs of the irradiated blend taken at 155 8C (a) under unpolarized light and (b) polarized light. The two parts, irradiated at 120 and 137 8C, respectively, were simultaneously observed.

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Strictly speaking, the solution-polymerized polymer is different from the polymers obtained by photopolymerization at the respective temperatures. However, it is known that photopolymerization of an acrylate group in LC phases proceeds very rapidly, and high conversion and molecular weight are achievable [10,17–19]. Therefore, the LC behavior of the photopolymerized polymers may be similar to that of the solution-polymerized one and we think it is valid to use the solution-polymerized polymer for determination of the photopolymerization temperature in this experiment. A polarizing optical micrograph of the blend taken at 137 8C before UV irradiation is shown in Fig. 2. We could see a schlieren texture characteristic of a nematic phase and recognize that the LC molecules are aligned with their long axes (director) parallel to the glass plates. If the director of a nematic liquid crystal is perpendicular to the glass plates, the LC molecules are in homeotropic alignment, and we observe blackness with a polarizing optical microscope. A schlieren texture could also be seen at 120 8C for the blend. While the schlieren texture was still present, the blend was irradiated with UV light at 120 and 137 8C. We then observed the irradiated samples with an optical microscope while the respective temperatures were maintained constant. Fig. 3 shows optical micrographs of the irradiated samples taken under unpolarized light. Note that the two micrographs were taken at the same magnification. On the surface

of the sample obtained at 120 8C, we could see close striations, resulting from photopolymerization-induced phase separation, and find that the macroscopic pattern reflected the schlieren texture observed before UV irradiation (Fig. 3a). In contrast, while the overall surface pattern of the sample irradiated at 137 8C seemed to be representative of the schlieren texture, the striations were rather coarse compared with those obtained at 120 8C (Fig. 3b). By polarized light optical microscopy, we found more remarkable differences between the phase-separated structures obtained at 120 and 137 8C. Different positions in the same sample of the blend were irradiated with UV light at 120 and 137 8C, respectively, and then the irradiated samples were heated up to 155 8C in order to observe how the mesogenic groups were arranged. At 155 8C, not only the unirradiated blend and the component (2) but also the solution-polymerized polymer from (1) is in the isotropic state. By unpolarized light optical microscopy at 155 8C, the striation patterns were seen in both parts of the sample irradiated at 120 and 137 8C as shown in Fig. 4a. However, as shown by polarized light optical microscopy on the same sample, only the part irradiated at 120 8C exhibited birefringence (Fig. 4b). The birefringence was preserved until the sample was heated up to around 160 8C. This result indicates that the LC order of the mesogens was stabilized by crosslinking for the polymer prepared at 120 8C, while

Fig. 5. SEM images of the phase-separated polymers obtained by photopolymerization of the blend at 120 8C (a) and (b), and 137 8C (c) and (d). The images (a) and (c) were taken at low magnification, while (b) and (d) were taken at high magnification.

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the disordered state of the mesogens was fixed for the polymer prepared at 137 8C. To investigate details of the microscopic morphology, we observed the neat polymers, obtained by extraction of the soluble fractions from the irradiated blends, by scanning electron microscopy. As shown in Fig. 5a and b, the polymer prepared at 120 8C consisted of oriented fine fibers. From the SEM image, the diameter of the fibers was estimated to be ca. 400 nm. In contrast, although orientation seemed to have taken place, the polymer obtained at 137 8C had a morphology like fused beads (Fig. 5c and d). These results indicate that the formation of fine fibers of the phaseseparated polymer can be attributed not to the ordering field of the LC media but to the fact that the phase-separated polymer showed an LC phase. The LC ordering field present in the photopolymerization process is considered to give anisotropy to the phase-separated structures. On the basis of the results so far described, we can conclude that, independently of whether the phase-separated polymer showed an LC phase or an isotropic phase, photopolymerization of the blend containing a crosslinker in the nematic state resulted in the formation of the striation patterns that were macroscopically representative of the schlieren texture. However, the microscopic structures were significantly affected by the phase of the obtained polymer. The phase-separated structure from the LC-phased polymer was finer than that of the isotropic-phased polymer. In a previous paper, we carried out photopolymerization of an (LC monomer/ LC) blend, which was uniaxially aligned by a rubbing technique prior to photoirradiation, and found that the rubbing direction was parallel to the striations formed on the surface of the irradiated blend [10]. Moreover, it is significant in further clarifying the ambiguous relationship between the direction of the LC director of the mesogens in the polymer fibers, obtained by photopolymerization-induced phase separation, and the fiber axis. Therefore, in this study we prepared a polymer film, in which the fibers uniaxially aligned by a rubbing technique, and performed polarizing infrared spectroscopy on the film. Fig. 6 shows the polar plot of the absorbance associated with the CN stretching band (2226 cmK1) of the cyanobiphenyl mesogens in the oriented polymer fibers. As shown in the figure, the band exhibited dichroism and the CN group vibration was predominantly parallel to the fiber axis. This result indicates that the direction of the LC director of the mesogens is consistent with the fiber axis. In order to study the role of crosslinking in the formation of the polymer morphology, we prepared a blend from (1) and (2) in a molar ratio 1:4, which contained the photoinitiator and the inhibitor but not the crosslinker (3). The blend without (3) showed the same thermal property as the blend with (3) and was irradiated with UV light under a nematic phase at 120 and 137 8C. Fig. 7 shows optical micrographs of the blend irradiated at 137 8C and taken just after irradiation. Under polarized light, the phase-separated structure was difficult to recognize (Fig. 7a), while under

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Fig. 6. Polar plots of the CN stretching band of the polymer film consisting of the oriented fibers obtained by photopolymerization of an uniaxially aligned blend.

unpolarized light we could clearly see the droplets of the isotropic-phased polymer dispersed in the matrix of the nematic-phased compound (2) (Fig. 7b). In contrast, for the blend irradiated at 120 8C, we could observe the close striation pattern similar to that obtained in the blend with crosslinking (Fig. 8a and b). However, when this irradiated blend, without crosslinking, was subsequently heated, the

Fig. 7. Optical micrographs of the blend without the crosslinker irradiated with UV light at 137 8C, taken under (a) polarized light and (b) unpolarized light, respectively.

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Fig. 8. Optical micrographs of the blend without the crosslinker, irradiated with UV light at 120 8C, taken at 120 8C (a) under polarized light and (b) under unpolarized light, and taken at 137 8C (c) under polarized light and (d) under unpolarized light, respectively.

striation pattern changed to the sea-island structure at around 130 8C along with the phase transition of the polymer from the LC to the isotropic phase. Fig. 8c and d exhibit the optical micrographs of the blend without crosslinking, which was irradiated at 120 8C and subsequently heated up to 137 8C. The sea part consisted of (2) showing a nematic phase and the island part composed of the polymer exhibiting an isotropic phase, which had a similar structure to that observed in Fig. 7. The blend observed in Fig. 8c and d could not recover the striation pattern but retained the sea-island structure even when the blend was cooled and the polymer part exhibited the LC phase. After photopolymerization of the blend without crosslinking at 120 8C and subsequent extraction of the soluble fraction, we obtained the insoluble polymer and observed its morphology by SEM. As shown in Fig. 9, the polymer without crosslinking had a fibrous morphology. Whereas the fibers are connected from place to place probably due to fusion, the general structure was similar to that seen in the crosslinked polymer. Comparing the results obtained from photopolymerization of the blend without crosslinking at 120 and 137 8C, we infer the following. In the case of photopolymerization at 137 8C, that is, when the phaseseparated polymer shows the isotropic phase, an isothermal transition from some ordered structure to the sea-island

structure in the polymer takes place immediately after photopolymerization. In contrast, in the case of photopolymerization at 120 8C, that is, when the phase-separated polymer exhibits an LC phase, such an isothermal transition is not evident. Therefore, the morphology of the phaseseparated polymer obtained at 137 8C significantly depended on the presence of the crosslinking, whereas the

Fig. 9. SEM image of the phase-separated polymer obtained by photopolymerization of the blend without the crosslinker at 120 8C.

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morphology obtained at 120 8C was not considerably affected by the crosslinking. We found that for the fiber structure of the phaseseparated polymer, the LC phase of the polymer itself is more important than the crosslinking. Among the properties arising from the LC phase, elasticity is thought to be significant because at the LC-isotropic transition, the morphology of the uncrosslinked polymer suddenly changed from the fiber to the droplet structure, which was governed by interfacial tension. Furthermore, whether the polymers were crosslinked or not, they had a fibrous morphology when they were prepared under the LC phase, which suggests that the fiber structures were formed at the early stage of the phase separation and subsequently immobilized by crosslinking. The roles of the phases exhibited by the blend at photopolymerization and by the LC solvent after photopolymerization on the polymer morphology are now under investigation.

4. Conclusions We found that the LC phase of the phase-separated polymer is one of the necessary conditions for the formation of its own fine fiber structure, which was generated by photopolymerization of the blend from the corresponding monomer and the LC medium. We also found that crosslinking was not indispensable for the phase-separated polymer to form fine fiber structures in the LC temperature range, however, the crosslinking had a significant effect on the morphology of the phase-separated polymer prepared at the isotropic temperature range. The phase-separated polymer with crosslinking took on a bead-like morphology when the photopolymerization was carried out in the isotropic phase of the polymer. In contrast, without crosslinking, photopolymerization in the isotropic

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temperature range of the polymer resulted in the macroscopic sea-island structure.

Acknowledgements This study was partially supported by the Nanostructure Polymer Project of NEDO (New Energy and Industrial Technology Development Organization).

References [1] Reneker DH, Yarin AL, Fong H, Koombhongse S. J Appl Phys 2000; 87:4531. [2] Deitzel JM, Kleinmeyer JD, Hirvonen JK, Beck Tan NC. Polymer 2001;42:8163. [3] Bognitzki M, Czado W, Frese T, Schaper A, Hellwig M, Steinhar M, Greiner A, Wendorff JH. Adv Mater 2001;13:70. [4] Gupta P, Wilkes GL. Polymer 2003;44:6365. [5] Liu G, Qiao L, Guo A. Macromolecules 1996;29:5508. [6] Liu G, Yan X, Duncan S. Macromolecules 2002;35:9788. [7] Liu G, Yan X, Duncan S. Macromolecules 2003;36:2049. [8] Kihara H, Miura T, Kishi R, Yoshida T, Shibata M, Yosomiya R. Liq Cryst 2003;30:799. [9] Kihara H, Miura T, Kishi R. Macromol Rapid Commun 2004;25:445. [10] Kihara H, Miura T, Kishi R. Polymer 2002;43:4523. [11] Dierking I, Kosbar LL, Afzali-Ardakani A, Lowe AC, Held GA. Appl Phys Lett 1997;71:2454. [12] Rajaram CV, Hudson SD, Chien LC. Chem Mater 1995;7:2300. [13] Rajaram CV, Hudson SD, Chien LC. Chem Mater 1996;8:2451. [14] Fung YK, Yang DK, Ying S, Chien LC, Zumer S, Doane JW. Liq Cryst 1995;19:797. [15] Shibaev VP, Kostromin SG, Plate NA. Eur Polym J 1982;18:651. [16] Litt MH, Whan WT, Yen KT, Qian XJ. J Polym Sci, Part A: Polym Chem 1993;31:183. [17] Broer DJ, Mol GN, Challa G. Makromol Chem 1989;190:19. [18] Broer DJ, Boven J, Mol GN, Challa G. Makromol Chem 1989;190: 2255. [19] Hikmet RAM, Lub J. Prog Polym Sci 1996;21:1165.