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Liquid Crystalline Gels. 4. Water- and Stress-Induced Mesophase Transition Kanji Yamaoka,† Tatsuo Kaneko,† Jian Ping Gong,†,‡ and Yoshihito Osada*,† Graduate School of Science, Hokkaido University, and Presto, JST, Sapporo 060-0810, Japan Received April 8, 2003. In Final Form: July 24, 2003 A stretching test of the hydrogel with a smectic A (SmA) type of liquid crystalline structure, poly(11cyanobiphenyloxyundecyl acrylate-co-acrylic acid), poly(11CBA-co-AA), was carried out in terms of various water contents. It was found that Young’s modulus of the hydrogel substantially decreased with increasing the swelling degree, q, but suddenly jumped up at q ) 1.13. Wide- and small-angle X-ray diffraction studies revealed that the hydrogel underwent a mesophase transition from the SmA to smectic I (SmI) phase between these q values. When the samples were further stretched, the backward transition from the SmI to SmA phase occurred accompanied by “necking”. The possible mechanism of the tension-induced mesophase transition is proposed.
Introduction 1,2
In the previous papers, we reported that copolymers consisting of a hydrophobic monomer with a mesogenic moiety, 11-(4′-cyanobiphenyloxy)undecyl acrylate (11CBA), and a hydrophilic monomer, acrylic acid (AA), showed the smectic A (SmA) phase in which side chains form bilayers aligning perpendicularly to the main chain axis. This structure was formed only when the molar fraction of 11CBA, F ) [11CBA]/([11CBA] + [AA]), which was determined by the 1H NMR spectrum, was 0.26 and higher. The extent of organization and layer spacing of the smectic structure increased with an increase in the composition of amorphous AA units. These copolymers did not dissolve in water due to the hydrophobic nature of 11CBA but swelled to form hydrogels, keeping the liquid crystalline ordering (liquid crystalline gel; LCG). The LCG was the hydrated copolymers physically cross-linked by the hydrophobic junctions with the liquid crystalline state where the mesogens can sensitively respond to environmental changes. In this sense, LCGs are different from conventional gels. As long as the swelling degree, q, which was defined as the weight ratio of wet gel to dry sample, was less than 1.11, LCGs of F ) 0.29 and 0.37 maintained the SmA structure. However, when the water content exceeded q ) 1.13, LCGs formed a smectic I (SmI) phase in which the side chains are tilted to the normal direction of the main chain backbone and take a pseudohexagonal arrangement.2 Such a structural change of LCGs by water incorporation may cause substantial changes in their mechanical properties, as is the case of the cross-linked copolymer hydrogel consisting of stearyl acrylate and AA.3 In this paper, mechanical properties of the poly(11CBAco-AA) gel of F ) 0.37 are described in terms of water content and the mesophase transition. A discontinuous jump in Young’s modulus is observed when SmA transforms to SmI by introducing water. When the gel sample of q higher than 1.08 is stretched, the “necking phenom* Corresponding author. Telephone: +81-11-706-2768. Fax: +81-11-706-2635. E-mail:
[email protected]. † Hokkaido University. ‡ Presto, JST. (1) Kaneko, T.; Yamaoka, K.; Gong, J. P.; Osada, Y. Macromolecules 2000, 33, 412. (2) Kaneko, T.; Yamaoka, K.; Gong, J. P.; Osada, Y. Macromolecules 2000, 33, 4422. (3) Miyazaki, T.; Kaneko, T.; Gong, J. P.; Osada, Y.; Demura, M.; Suzuki, M. Langmuir 2002, 18, 965.
enon” occurs accompanied by a backward transition from SmI to SmA. Experimental Section Syntheses. 11-(4′-Cyanobiphenyloxy)undecyl acrylate (11CBA)4,5 and poly(11-(4′-cyanobiphenyloxy)undecyl acrylateco-acrylic acid), poly(11CBA-co-AA), were prepared by the procedures described in the preceding paper.1 The number- and weight-average molecular weight of poly(11CBA-co-AA) were 3000 and 3600, respectively, which are not high compared with those of common polymers but are of the same order as those of poly(11CBA) in the literature.4,5 The mole fraction of the 11CBA unit, F, which is defined as [11CBA]/([11CBA] + [AA]) was determined from 1H NMR spectra of poly(11CBA-co-AA)’s according to the method described in the preceding paper.1 Measurements. The stress-strain measurements of the fibrous samples were made by stretching at a rate of 1 mm min-1 using a tensile tester (Orientec Corp. RTC-1150A). The fibrous samples were prepared by spinning from the liquid crystalline melt (90 °C) at a rate of about 5 cm s-1 in the dry state. In estimation of the molar mechanical energy, the integrated area below the stress-strain curve up to the maximum was multiplied by the averaged molecular weight of the repeating units. Wideangle X-ray diffraction patterns were taken with a flat-plate camera mounted to a Shimazu X-ray generator XD-610 emitting Ni-filtered Cu KR radiation at 40 kV and 40 mA in transmission geometry. The distance from the sample to the film was determined by calibration with silicone powder. Small-angle X-ray diffraction patterns were recorded on a Rigaku X-ray diffractometer (RINT-2000) at 40 kV and 200 mA in transmission geometry. A 2θ scanning speed of 1° min-1 with a sampling interval of 0.01° was used. The degree of swelling, q, was defined as the weight ratio of a swollen gel to the dried one. q was regulated by adding a required amount of water into the dry copolymer, and then the wet sample was sealed with the poly(vinylidene chloride) film for more than 7 days to establish an equilibrium.
Results and Discussion The mechanism of the gel formation of poly(11CBAco-AA) in water was considered due to local segregation of hydrophobic 11CBA side chains that were dispersed in the hydrophilic matrix of AA units.2 The effects of water on the mechanical properties of the copolymer were investigated. Figure 1 shows the results of the stress(4) Shibaev, V. P.; Kostromin, S. G.; Plate, N. A. Eur. Polym. J. 1982, 18, 651. (5) Kostromin, S. G.; Sinitzyn, V. V.; Talroze, R. V.; Shibaev, V. P.; Plate, N. A. Makromol. Chem., Rapid Commun. 1982, 3, 809.
10.1021/la0346088 CCC: $25.00 © 2003 American Chemical Society Published on Web 08/26/2003
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Figure 1. Stress-strain curves of poly(11CBA-co-AA) of F ) 0.37 with various water contents (inset: in the initial stretching stage). Temperature: 25 °C.
Figure 2. Change in Young’s modulus, E, of poly(11CBA-coAA) gel as a function of q.
strain measurements of the fibrous samples with various q values. The samples with q ) 1.00 and q ) 1.03 showed straight lines up to a stress of 3.5 × 105 Pa, while the samples with q ) 1.08 and higher exhibited “necking” phenomena after showing local maxima, and flowed like viscous liquids after further stretching. The initial step of the stress-strain curve of these samples can be more precisely seen in Figure 1 (inset). From the slope of these curves Young’s modulus, E, was calculated and plotted as a function of q, as shown in Figure 2. It can be seen that E decreased significantly with increasing q until q ) 1.11 but suddenly jumped up to the value equivalent to that of the dry sample at q ) 1.13. Since this q value well coincides with the mesophase transition from SmA to a more organized phase of SmI,2 the increase in E is obviously attributed to the change in the structural transformation of the gel. The present study reveals that transformation from lower to higher ordering of the mesophase structure enhances the E value of the hydrogels. If the q values of LCGs are transformed into the number of water molecules per 1 unit of 11CBA, q ) 1.11 and q ) 1.13 refer to 3.4 and 4.0 molecules, respectively. This finding indicates that a very small difference in the number of water molecules in LCGs affects the mesomorphism and E. The poly(11CBA-co-AA) gel of F ) 0.29 also showed an E increase with the SmA-SmI transition. It should be noted that a similar water-induced organization has been reported in chemically cross-linked poly(stearyl acrylateco-AA) (poly(SA-co-AA)) hydrogels. In this case, the water incorporation gave the chain backbone mobility and, at the same time, stretched the SA side chains to take a β-zigzag conformation and to align in the normal direction to the chain backbone, and increased the E value.3 To confirm this speculation, short range (d1) and long range (d2) structures of the hydrogel of F ) 0.37 were
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Figure 3. WAXD (a) and SAXD (b) patterns of poly(11CBAco-AA) of F ) 0.37 under no tensile stress and under a tensile stress corresponding to λ ) 1.2. q ) 1.18.
investigated by wide-angle X-ray diffraction (WAXD) and small-angle X-ray diffraction (SAXD) measurements. We previously reported2 that the hydrogels with q ) 1.08 and 1.11 (SmA) showed a halo on the meridian line (longitudinal axis of the fiber) in the WAXD photograph and a distinct SAXD peak at 2θ ) 1.45-1.50° (θ: diffraction angle) corresponding to the spacing of 5.0-5.1 nm.2 When the sample of q ) 1.08 was stretched as much as the “necking” occurred, the SAXD peak disappeared. This suggests that the smectic layer is broken and transformed to the amorphous or to the nematic state. On the other hand, as-prepared hydrogels with q ) 1.13, 1.16, and 1.18 before stretching showed six sharp diffractions lying symmetrically to the meridian line and a distinct SAXD peak. These are characteristic of the SmI structure,6 and the WAXD pattern of the hydrogel with q ) 1.18 is shown in Figure 3a as an example. The sample maintained this pattern until being stretched to the strain λ (value of stretched sample length divided by initial sample length) of 1.14 (below the stress maximum). However, when the sample was stretched more than λ ) 1.15 (after the stress maximum) until the “necking” occurred, six sharp diffractions disappeared and two broad diffractions characteristic of SmA appeared on the meridian line. Since the diffraction of the smectic ordering in the SAXD pattern was still observed at θ ) 1.5° (5.0 nm) (Figure 3b), one can confirm the sample showed a transition from the SmI to SmA phase. A similar transition was also confirmed in the samples of q ) 1.13 and q ) 1.16 by WAXD and SAXD studies. The SmI-SmA transition induced by tensile stress is schematically illustrated in Figure 4. We can guess that AA and 11CBA units may be randomly sequenced as reported previously.1 It is considered that the AA units in the main chain backbone may take the coiled state due to the strong segregation nature of mesogenic side chains and may dimerize by the hydrogen bonding.7 The AA dimerization may support the organization of side chains in the SmI phase. When the hydrogel is stretched, a relatively low tension might be applied to the coiled main chain, while the structure of mesogenic domains stays unchanged. When the main chains are further stretched, the hydrogen bonding is broken, presumably under the support of the hydration, and then 11CBA mesogens show an increased interval (6) Demus, D.; Goodby, J.; Gray, G. W.; Spiess, H.-W.; Vill, V. Handbook of Liquid Crystals Vol.1 Fundamentals; Wiley-VCH: Weinheim, 1998. (7) Yamaoka, K.; Kaneko, T.; Gong, J. P.; Osada, Y. Macromolecules 2001, 34, 1470.
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Figure 4. Schematic illustration of the tension-induced SmISmA transformation in poly(11CBA-co-AA) of F ) 0.37.
and an alignment change from the tilted to normal direction against the chain backbone (SmI to SmA phase). An increase in bilayer spacing of side chains accompanying this transition might be compensated by the “thinning effect” of the main chain backbone described above. The structural transition accompanying the “necking” phenomenon is widely known in some crystalline polymers such as a polyethylene fiber. In these cases, however, they are accompanied by an increase in both the crystalline degree and E.8 It is generally accepted that the stressstrain curve from λ ) 0 to the λ at which the necking occurs corresponds to the energy required to align the flexible domains for the crystallization. If we consider our necking phenomena in the same manner, that is, if “necking” is used to induce the transition from SmI to SmA, we obtain 17.2, 13.4, and 43.4 kJ mol-1 for the hydrogels of q ) 1.13, q ) 1.16, and q ) 1.18, respectively, which are much smaller than those for the case of crystalline polymers. (8) Marchall, I.; Thompson, A. B. Proc. R. Soc. 1954, A221, 541.
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It should also be noted that a SmI-SmA transition occurred if the sample of q ) 1.18 was heated to 40 °C, which was confirmed by X-ray diffraction studies and differential scanning calorimetry. Compared with the thermotropic transition, the described stress-induced mesophase transition is reported only a few times,9 although the stress-induced crystal transition has been widely investigated.10-14 While the liquid crystalline molecules dissipate the stress easily due to their flowing nature, the structural control of the polymer networks by mechanical treatment is effectively made.15-17 Similarly, the observed tension-induced mesophase transition might be associated with the network structure where the tensile stress applied is effectively transferred over the whole polymeric chain. Acknowledgment. This research was partly supported by a Grant-in-Aid for the Special Promoted Research Project “Construction of Biomimetic Moving Systems Using Polymer Gels” from the MEXT, Japan, and by a Grant-in-Aid for The Inamori Foundation. LA0346088
(9) Pujolle-Robic, C.; Noirez, L. Nature 2001, 409, 167. (10) Boye, C. A.; Overton, J. R. Bull. Am. Phys. Soc. 1974, 19, 352. (11) Yokouchi, M.; Sakakibara, Y.; Chatani, Y.; Tadokoro, H.; Tanaka, T.; Yoda, K. Macromolecules 1976, 9, 266. (12) Tashiro, K.; Nakai, K.; Kobayashi, M.; Tadokoro, H. Macromolecules 1980, 13, 137. (13) Kaneko, T.; Imamura, K.; Watanabe, J. Macromolecules 1997, 30, 4244. (14) Ichikawa, Y.; Washiyama, J.; Moteki, Y.; Noguchi, K.; Okuyama, K. Polym. J. 1995, 27, 1230. (15) Brodowsky, H. M.; Boehnke, U. C.; Kremer, F.; Gebhard, E.; Zentel, R. Langmuir 1999, 15, 274. (16) Lehmann, W.; Skupin, H.; Tolksdorf, C.; Gebhard, E.; Zentel, R.; Kruger, P.; Losche, M.; Kremer, F. Nature 2001, 410, 447. (17) Assfalg, N.; Finkelmann, H. Macromol. Chem. Phys. 2001, 202, 794.
