Article pubs.acs.org/Macromolecules
Thermally Reversible Distortion Observed for Triblock Copolymers Comprising Main-Chain Liquid Crystal Polyesters Attached to PhotoCross-Linked Cinnamate Segments at Both Ends Kohei Abe, Maito Koga, Takumi Wakabayashi, Sungmin Kang, Koichi Sakajiri, Junji Watanabe, and Masatoshi Tokita* Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8552, Japan ABSTRACT: ABA triblock B5 Cin copolymers were prepared by attaching photoreactive poly(2-cinnamoyloxyethyl methacrylate) (PCEMA) segments to both ends of main-chain smectic BB-5(3-Me) polyesters. The copolymers with PCEMA volume fraction (φ) ranging from 23% to 50% formed lamellar microdomains, and the BB-5(3-Me) block formed smectic layers parallel to the lamellae. When the B5 Cin melt was drawn into film and annealed at a liquid crystal (LC) temperature of BB-5(3-Me), the lamellae and smectic layers lay perpendicular to the longitudinal direction. The PCEMA block in the monodomain films was cross-linked by UV irradiation leading to 20% conversion. When heated, the cross-linked films with φ > 37% contracted reversibly on the isotropization of the LC segment, coinciding with the reversible decrease in the lamellar spacing (D). The film elongated on the liquid crystallization by a factor which increased from 1.07 to 1.24 with decreasing φ.
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INTRODUCTION Liquid crystal elastomers (LCEs) have attracted much attention because of the coupling between the backbone conformation and the LC orientational order can give rise to spontaneous and reversible elongation along the director axis on the isotropic− LC transition.1 The main-chain LCEs with mesogenic moieties incorporated in the polymer backbone elongate larger than the side-chain LCEs due to the stronger coupling between the chain conformation and the ordering of the mesogens.2−9 Such an anisotropic elongation is particularly pronounced for monodomain LCEs; however, the preparation of monodomain LCEs remains difficult because it requires polymer chains to be cross-linked in a macroscopically oriented state. Another puzzling phenomenon connected to the cross-linking LC polymers is the decreases in the isotropization temperature and enthalpy change.7−11 A cross-linked main-chain smectic BB-5 polyester formed by the incorporation of 2 mol % of 1,3,5benzene tricarboxylic acid exhibited the isotropization temperature (Ti) and enthalpy change (ΔHi) of 182 °C and 2.8 kJ mol−1, respectively, fairly lower than the corresponding values for the BB-5 homopolyester (i.e, Ti = 213 °C and ΔHi = 5.9 kJ mol−1).9 Cross-linking can also alter the type of formed LC phase. When a main-chain BB/PDA-5(3-Me) LC polymer containing photo-cross-linkable p-phenylene diacrylate (PDA) moiety was cross-linked in the monodomain LC state and cooled from the isotropic liquid state, the length recovered by the cross-linked polymer was only 5% of the original length. The formed smectic A (SmA) LC was different from the smectic CA (SmCA) phase formed by the non-cross-linked polymer.8 The © XXXX American Chemical Society
SmCA structure could be recovered on stretching the elastomer in the SmA phase. The difficulties in preparing monodomain LC elastomers may be avoided using rubber−nematic−rubber (RNR) triblock copolymers because, in this case, the cross-links are not included in the LC segment but rather in the rubber (R) segment. The R blocks segregate from the nematic (N) block to link the network of N segments. Although the N block is a main-chain nematic polymer in the model proposed by de Gennes,12 the first RNR copolymer was prepared using a sideon nematic polyacrylate with a narrow molecular-weight distribution in order to form long-range-ordered lamellar microdomains.13 The monodomain triblock copolymer was obtained using a magnetic-field orientation followed by a crosslinking in the R block induced by UV irradiation. The director of the N block was perpendicular to the lamellae, and the film contracted reversibly on the isotropization and elongated by a factor 1.23 on the liquid crystallization. Segregation of crosslinks and nematic LC segments was achieved in the main-chain nematic polymer tethering terphenyl moieties at both ends. The chain-end groups were aggregated to link the nematic networks.14 The fibrous polymer elongated reversibly by a factor of 5, although thermoplastic creep was noticeable in the isotropic phase in which the chain-end aggregates could melt. In this study, we investigated triblock B5 Cin copolymers that comprise a main-chain smectic BB-5(3-Me) polyester Received: July 24, 2015 Revised: October 20, 2015
A
DOI: 10.1021/acs.macromol.5b01646 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Chart 1
where photo-cross-linkable poly(2-cinnamoyloxyethyl methacrylate) (PCEMA) blocks are connected at both ends (Chart 1). BB-5(3-Me) was synthesized by the polycondensation of 4,4′-biphenyl dicarboxylic acid and 3-methyl-1,5-pentanediol and forms a SmCA LC.15−19 We use the main-chain smectic LC polymer as the middle block for the following two reasons. First, copolymers comprising smectic main-chain LC polyesters connected with polymethacrylates at both ends form longrange-ordered lamellar microdomains in a large range of compositions even though the LC segments have polydispersity indices as high as 2.20−23 Second, the lamellae and LC director can be arranged perpendicular to the fiber axis (or the film drawing direction) and perpendicular to the lamellae, respectively. Such lamellae and LC orientations have been characteristically observed for the main-chain smectic triblock copolymers and are similar to the RNR copolymers.12,13 In contrast, a main-chain nematic triblock copolymer formed lamellar microdomains, but both the lamellae and LC director were arranged parallel to the drawing direction.24 Similar to the smectic LC triblock copolymers, the B5 Cin copolymers formed lamellar microdomains, in which the BB5(3-Me) segments formed a SmCA LC arranging the LC director perpendicular to the lamellae. Melt drawing and successive annealing yielded monodomain films with the lamellar normal and the LC director lying along the film drawing direction. The PCEMA block lamellae were crosslinked by UV irradiation up to 20% conversion. Similar to monodomain LCEs, the UV-irradiated monodomain films contracted reversibly along the direction of the LC director on the LC isotropization. This film contraction coincided with the decrease in the lamellar spacing. The film elongated reversibly on the liquid crystallization by a factor ranging from 1.07 to 1.24, which is explained by the ratio of lamellar spacing in the LC phase to that in the isotropic phase.
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Table 1. Characterization of Polymers
a
sample
Mn,LCa
OH-terminated BB-5(3-Me)-146 B5 Cin-146−23 B5 Cin-146−37 B5 Cin-146−46 OH-terminated BB-5(3-Me)-235 B5 Cin-235−23 B5 Cin-235−41 B5 Cin-235−50
14600 14600 14600 14600 23500 23500 23500 23500
Mn,ama
PDIb
4400 8600 12200 7000 16800 23500
2.12 1.77 1.86 1.72 2.17 1.75 1.66 1.69
Determined by 1H NMR spectroscopy. bDetermined by SEC.
with 2-bromoisobutylate bromide produced the telechelic α,ω-di-2bromoisobutyrate BB-5(3-Me) polyester that served as a macroinitiator for the copper(I)-catalyzed atom-transfer radical polymerization of HEMA at 70 °C using anisole as a solvent to yield polydispersed yet unimodal poly(HEMA)-block-BB-5(3-Me)-blockpoly(HEMA) copolymers. To obtain B5 Cin-x-φ copolymers, the corresponding poly(HEMA)-block-BB-5(3-Me)-block-poly(HEMA) copolymer (100 mg) was dissolved in dry pyridine (5.0 mL) and reacted with cinnamoyl chloride (414 mg, 2.48 mmol) at an ambient temperature for 12 h. The product was purified twice by precipitation from a chloroform solution into methanol and once by precipitation into hexane. 1H NMR was used to confirm that the hydroxyl groups in the poly(HEMA) segments were completely reacted with cynnamoyl chloride. Methods. 1H NMR spectra were obtained from a JEOL model AL400 spectrometer at 20 °C. Chemical shifts are reported in ppm downfield from SiMe4 using the solvent’s residual signal as an internal reference. SEC was performed using a JASCO system equipped with polystyrene gel columns with CHCl3 as the eluent after calibration with standard polystyrene. Fourier transform (FT) IR and attenuated total reflectance (ATR) FT IR spectra were measured using a JASCO MFT-200 and a Nicolet AVATR320 equipped with a Spectra-Tech Thunderdome, respectively. Differential scanning calorimetry (DSC) measurements were carried out with a PerkinElmer Pyris 1 DSC at a scanning rate of 2 °C min−1 under the flow of dry nitrogen. Wideangle and small-angle X-ray scattering patterns were measured using Cu Kα radiation with Bruker D8 DISCOVER and NanoSTAR-U, respectively, equipped with Vantec 500 and Vantec 2000 detectors, respectively. The synchrotron radiation (SR) small-angle X-ray scattering (SAXS) measurement was performed at the BL-10C beamline in Photon Factory, Tsukuba, Japan, equipped with a Dectris PILATUS300 K−W detector with a camera length of approximately 2 m. The X-ray wavelength was 0.1488 nm. The temperature dependences of the strain for the B5 Cin films were obtained using a Hitachi TMA7100. The elastomer in the film form with a thickness of 0.1 mm was cut into rectangular strips with lengths of 5 mm (in the elongation direction) and widths of 2 mm and clamped in the tensile testing machine. The sample was heated and cooled at a rate of 2 °C min−1 under a dry nitrogen atmosphere and the film length (L) was measured. The strain was defined as L/Liso, where Liso is the length with the LC block in the isotropic state. Transmission electron microscopy (TEM) was performed using a Hitachi H-7650 Zero A electron microscope operating at 100 kV in bright-field mode. Samples were exposed to RuO4 vapors at ambient temperature to selectively
EXPERIMENTAL SECTION
Materials. Two series of B5 Cin copolymers were prepared using two BB-5(3-Me) polyesters with number-average molecular weights (Mn,LC) of 14600 and 23500. The copolymers are identified as B5 Cinx-φ, where x corresponds to Mn,LC divided by 100 and φ represents the volume percentage of the PCEMA segment. The values of Mn,LC and Mn of amorphous PCEMA segment (Mn,am) were determined by 1 H NMR spectroscopy, and the polydispersity indices (PDIs) of the copolymers were obtained by size-exclusion chromatography (SEC) (Table 1). φ values were calculated from Mn,LC, Mn,am, and the densities of each segment (i.e., BB-5(3-Me): 1.23 g cm−3, PCEMA: 1.23 g cm−3). Synthesis of B5 Cin-x-φ. B5 Cin-x-φ copolymers were obtained by reacting excess cinnamoyl chloride with the poly(2-hydroxyethyl methacrylate) (poly(HEMA)) -block-BB-5(3-Me)-block-poly(HEMA) copolymers that were synthesized as previously reported.20−23,25 Hydroxyl-terminated BB-5(3-Me) polyesters were prepared by the melt transesterification of dimethyl 4,4′-bibenzoate and excess 3methyl-1,5-pentanediol in the presence of the isopropyl titanate catalyst at 240 °C. The reaction of the hydroxyl-terminated polyester B
DOI: 10.1021/acs.macromol.5b01646 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules stain the LC domains and were then microtomed into ultrathin sections.
DSC data for the B5 Cin-x-φ copolymers and corresponding BB-5(3-Me) precursor polyesters are summarized in Table 2. The heating DSC thermogram of the hydroxyl-terminated BB5(3-Me) exhibited a jump in the heat capacity (CP) and an endothermic peak at 30 and 150 °C, respectively. These are attributed to the glass transition and isotropization of the SmCA LC. The B5 Cin-x-φ copolymer thermograms are similar to that of the BB-5(3-Me) polyester although two jumps in CP were observed at 32 and 50 °C. The jumps at the lower and higher temperatures are attributed to the glass transitions of the BB-5(3-Me) and PCEMA segments, respectively. Thus, the DSC data indicate that two types of segments were segregated from each other and exhibited their corresponding thermal properties. The isotropization enthalpy (ΔHi) of the BB-5(3-Me) block ranges from 2.3 to 2.8 kJ mol−1, which is 60%−70% of the value for the hydroxyl-terminated BB-5(3Me) (3.8 kJ mol−1). The isotropization temperature (Ti) is 10 °C lower than that of the corresponding homopolymers. Similar decreases in ΔHi and Ti have been observed for similar LC block copolymers bearing main-chain LC polyester segments.20−25 Morphology and Photo-Cross-Linking of B5 Cin-x-φ Monodomain Films. The BB-5(3-Me) segment was segregated from the PCEMA segment to form a lamellar microdomain. Both the smectic LC and lamellar microdomain could be well oriented by drawing the copolymer melt at 160 °C into 100-μm-thick films and annealing the films for 12 h at 120 °C leading to the formation of the SmCA LC by the LC block. Figure 2 shows the typical wide-angle X-ray scattering
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RESULTS AND DISCUSSION Characterization of B5 Cin-x-φ Copolymers. Figure 1 shows the typical 1H NMR spectra of B5 Cin-x-φ copolymers
Figure 1. 1H NMR spectra measured for (a) B5 Cin-146−46 and (b) the corresponding precursor poly(HEMA)-block-BB-5(3-Me)-blockpoly(HEMA) copolymers in pyridine-d5 at 25 °C.
measured for B5 Cin-146−46 and the precursor copolymer with poly(HEMA) segments. A complete attachment of the cinnamate moiety to the poly(HEMA) segment can be confirmed by the disappearance of the hydroxyl peak, labeled c in the spectrum of the precursor copolymer. The esterification of poly(HEMA) with cinnamoyl chloride caused the down shift of the signals for the protons on the side-chain ethylene (peaks e and f for poly(HEMA) segment at δ = 4.29 and 4.45 ppm, respectively, and peaks i′ and j′ for PCEMA segment at δ ≈ 4.5 ppm). The molecular weight of PCEMA segment may be specified by Mn,LC and the area ratios of the peaks labeled a′ and g′. Similar to the BB-5(3-Me) homopolyester, the BB-5(3-Me) segments in the B5 Cin-x-φ copolymers formed SmCA and isotropic phases in the order of increasing temperature. The
Figure 2. (a) WAXS and (b) SAXS patterns for the B5 Cin-235−41 film. The drawing direction is vertical.
(WAXS) and SAXS patterns of B5 Cin-x-φ measured for the B5 Cin-235−41 film. The smectic layer reflections and scattering maxima ascribed to the stacked lamellae are observed along the film drawing direction in the WAXS and SAXS patterns, respectively, demonstrating that the smectic layers and
Table 2. Thermal Properties of the Polymersa non-UV-irradiated
a
UV-irradiated
sample
Ti/°C
ΔHi/kJ mol−1
Tg/°C
OH-terminated BB-5(3-Me)-146 B5 Cin-146−23 B5 Cin-146−37 B5 Cin-146−46 OH-terminated BB-5(3-Me)-235 B5 Cin-235−23 B5 Cin-235−41 B5 Cin-235−50
150 140 142 143 151 139 139 145
3.75 2.50 2.42 2.33 3.86 2.75 2.82 2.26
28 32, 42 33, 47 33, 49 28 26b 31, 43 32, 53
Ti/°C
ΔHi/kJ mol−1
Tg/°C
142 142 143
2.26 2.16 2.03
32b 31, 49 32, 49
144 147 147
2.78 2.47 2.37
30b 31, 52 31, 54
Determined from DSC thermograms measured at a heating rate of 2 °C min−1. bTg of PCEMA segment was not detected. C
DOI: 10.1021/acs.macromol.5b01646 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules microdomain lamellae stack along the film stretching direction. Thus, both the lamellae and smectic LC can be oriented macroscopically by first drawing the copolymer melt and then annealing at the LC temperature. Such lamellar and smectic LC orientations are a characteristic of ABA triblock copolymers with main-chain smectic LC polyesters as the middle blocks.20−23 The lamellar spacing D determined for the copolymers is listed in Table 3. D depended on Mn,LC and φ although all the B5 Cin-x-φ copolymers maintained the smectic layer spacing of 1.63 nm.
CC bonds in the PCEMA had reacted under UV irradiation for 180 min, and only a negligible increase in the conversion was observed for a longer irradiation time up to 240 min. The same photochemical reaction in the 100-μm-thick monodomain copolymer film was monitored by the ATR FT-IR spectra and measured for both film faces. UV irradiation for 180 min increased the conversion of the lamp-side film up to 20%, comparable to that estimated by FT-IR but higher than that of the opposite face (i.e., 12%). Here, the UV light was irradiated on each film face for 90 min to prepare the cross-linked B5 Cinx-φ films and the photoreaction conversion was assumed to be 20% throughout the films. The cross-linking of the PCEMA segment does not affect the Ti, ΔHi, and glass transition temperature (Tg) of the smectic LC block. The DSC data collected on the second heating run are listed in the right-side columns in Table 2. The values of Ti, ΔHi, and Tg of the cross-linked copolymers are comparable to those of the non-UV-irradiated copolymers. Thus, the crosslinking on the PCEMA block does not disturb the liquid crystal formation in the BB-5(3-Me) block. Reversible Contraction of Photo-Cross-Linked B5 Cinx-φ Monodomain Films on the LC-to-Isotropic Transition. The reversible contraction along the film drawing direction on the LC−isotropic transition of the BB-5(3-Me) block is typical of the photo-cross-linked B5 Cin-x-φ copolymer films. Such reversible contraction was observed in the crosslinked copolymers except for B5 Cin-146−23 although it has not been observed for the non-cross-linked copolymers. The temperature dependence of the film length (L) along the lamellar normal direction is shown in the upper part of Figure 4, where the ratio of the L to the L in the isotropic phase (Liso)
Table 3. Lamellar Spacing of the B5 Cin-x-φ copolymers non-UV irradiated sample B5 B5 B5 B5 B5 B5
Cin-146−23 Cin-146−37 Cin-146−46 Cin-235−23 Cin-235−41 Cin-235−50
UV-irradiated
Da/nm
DLCb/nm
Sb,c
Disod/nm
26.9 26.9 28.0 34.0 34.3 39.5
26.9 26.9 26.5 34.5 34.5 39.5
0.00 0.91 0.91 0.79 0.86 0.80
− 20.8 21.5 − 26.7 33.3
Measured in the LC phase at 25 °C. bMeasured for the sample cooled from 170 to 25 °C. cOrientational order parameter of the lamella normal estimated by the azimuthal intensity distribution of the first-order lamellar scattering maximum. dMeasured in the isotropic phase at 170 °C.
a
The PCEMA block can be photo-cross-linked by UV irradiation. Figure 3 shows the FT-IR spectra of a thin B5
Figure 4. Relative film lengths L/Liso (upper) and DSC thermograms (lower) measured for the cross-linked B5 Cin-235−41 film during first cooling and then heating processes at a rate of 2 °C min−1. Figure 3. IR spectra of the B5 Cin-235−41 copolymer thin films irradiated by UV for different periods are indicated in the figure. The scheme of the photo-cross-linking reaction occurring in the PCEMA block is illustrated in the upper part.
is plotted versus temperature. The measurements were performed by applying a stress of 1.5 kPa to the 100-μmthick film sample, and the temperatures were scanned at a rate of 2 °C min−1. It was found that the cross-linked film contracted upon heating to temperature beyond the Ti of the LC block. The shape change was fully reversible with a variation of 18%. The temperatures at which L/Liso decreased and increased on heating and cooling, respectively, corresponded to the endothermic and exothermic peaks in the heating and cooling DSC thermograms, respectively, as shown in the lower part of Figure 4.
Cin-235−41 film illuminated with UV light (λmax = 365 nm, 50 mW cm−2) for periods indicated in the figure. The CC stretching mode in the PCEMA unit appeared at 1637 cm−1 and decreased in intensity with irradiation time; this is due to the formation of new unconjugated carbonyl groups while the CO stretching band remained essentially unchanged.8,26,27 As calculated from the intensity variation at 1637 cm−1, 20% of D
DOI: 10.1021/acs.macromol.5b01646 Macromolecules XXXX, XXX, XXX−XXX
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film drawing direction was conserved in the entire range of investigated temperatures (see Figure 5, parts c and d). Although the lamellae were conserved, the spacing decreased at T > Ti. Figure 6a shows the SR-SAXS profiles measured for the cross-linked B5 Cin-235−41 film on heating up to 170 °C followed by cooling at a rate of 2 °C min−1. Here, the scattering intensities were obtained from the 2D scattering patterns by averaging the intensity over azimuthal sectors of 10° on each side of the film drawing direction and plotting against the scattering vector q. The as-UV irradiated film showed scattering maxima at q with ratios of 1:2:3:4:5 that were ascribed to lamellar microdomains stacking along the film drawing direction at a spacing (D) of 34.5 nm. The scattering intensity began to decrease at a temperature of 142 °C, and the scattering maxima attributed to lamellae at a smaller D (i.e., 26.7 nm) that began to appear at q with ratio of 1:2. Upon subsequent cooling, the scattering maxima ascribed to the original spacing recovered at 146 °C. Such a reversible decrease in D on the isotropization of the LC segment has been observed for similar ABA LC triblock copolymers and was explained by conformation changes in the LC block on the phase transition.21,22 The reversible contraction of the film length can be connected to the reversible decrease in D. In Figure 6b, D values measured by SR-SAXS for a cross-linked B5 Cin-235−41 film upon heating or cooling at a rate of 2 °C min−1 were plotted versus the temperature. When the two sets of lamellae with different spacings appeared, the series of lamellae exhibiting the stronger scattering were selected. The lamellar spacing at T < Ti was constant at DLC = 34.5 nm and decreased to Diso = 26.7 nm at T > Ti. The comparison of the temperature dependence of D with the temperature dependence of L/Liso allows us to conclude that the film contraction coincides with the decrease in D. Figure 7a shows the values of DLC/Diso and LLC/Liso plotted against φ. For all the copolymers, the LLC/Liso values are smaller than that of DLC/Diso and decreases with increasing φ and independent of Mn,LC. Such a discrepancy between LLC/Liso and DLC/Diso can be resolved considering that the lamellae did not arrange their normal completely along the film drawing direction. The lamellae displayed SAXS maxima
Thus, the reversible film distortion can be connected with the LC−isotropic transition in the BB-5(3-Me) block. WAXS patterns measured for the copolymer films at temperatures above and below Ti indicate that the LC director oriented spontaneously along the film drawing direction on LC formation. While the WAXS patterns of the films at T > Ti show only a ring-shaped halo at a larger scattering angle (Figure 5a), the halo assumes a crescent shape on the equator and the smectic layer reflection appears on the meridian with decreasing temperature below Ti (Figure 5b).
Figure 5. WAXS patterns of the cross-linked B5 Cin-235−41 films (a) heated at 170 °C, and (b) subsequently cooled to 120 °C. The SAXS patterns corresponding to parts a and b are shown in parts c and d, respectively. The film drawing direction is vertical.
The spontaneous LC orientation can be attributed to the conservation of lamellar microdomain at T > Ti. SAXS patterns obtained for the heating of the film at 170 °C and the subsequent cooling to 120 °C include lamellar scatting maxima on the meridian showing that the lamellae stacking along the
Figure 6. (a) SR-SAXS profiles for the cross-linked B5 Cin-235−41 film collected during heating and cooling at a rate of 2 °C min−1. The arrows indicate the positions of scattering maxima. The profiles at an interval of 16 °C are shown by thick lines. (b) Temperature dependences of the lamellar spacing of the cross-linked B5 Cin-235−41 film. The lamellar spacings collected through heating and cooling processes are shown with square and triangle marks, respectively. E
DOI: 10.1021/acs.macromol.5b01646 Macromolecules XXXX, XXX, XXX−XXX
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Figure 7. (a) Variations of L/Liso (closed marks) and D/Diso (open marks) of the cross-linked B5 Cin-146-φ (circle) and B5 Cin-235-φ (triangle) films upon φ. (b) Correlation between L/Liso and D/Diso after considering the tilt of the lamellar normal with respect to the film drawing direction. ⟨cos β⟩ is the cosine of the average tilt angle.
spreading azimuthally. For the lamellae aligned with their individual normal deviating from the film drawing direction by an angle β, the average value of cos2β can be estimated from the intensity distribution I(β) of the lamellar reflections along the azimuthal angle (β) according to the following equation.28 2
⟨cos β⟩ =
∫0
π /2
I(β) cos2 β sin β dβ
∫0
π /2
I(β) sin β dβ
(1)
The values of the orientational order parameter, defined as S = (3⟨cos2 β⟩ − 1)/2, are listed in the fourth column of Table 3. When the lamellae are tilted with respect to the film draw direction by an average angle of β, LLC/Liso is expected to correspond to the value of ⟨cos β⟩DLC/Diso. The expected relationship between these values was confirmed for the crosslinked copolymers as shown in Figure 7b. The variation of LLC/Liso with φ indicates that a cross-linked B5 Cin-x-φ copolymer with a smaller φ can exhibit a larger reversible contraction. However, such an expected contraction was not observed for B5 Cin-146−23 and B5 Cin-235−23. The cross-linked B5 Cin-146−23 film contracted irreversibly on the isotropization of the LC block, and the length did not recover on the LC formation during the cooling. Although the crosslinked B5 Cin-235−23 film exhibited reversible elongation and spontaneous macroscopic LC orientation during the isotropic− LC transition of LC segment, the LLC/Liso was as large as 1.20 and smaller than the value expected from the plots in Figure 7a. In contrast to the other copolymers that exhibited reversible contraction, these cross-linked copolymers at T > Ti displayed SAXS profiles that were not ascribed to any ordered morphology as well as lamellar microdomains. Upon subsequent cooling to 25 °C, the cross-linked B5 Cin-146− 23 film did not recover the orientations of the lamellae and the smectic layers suggesting that the lamellae disordered at T > Ti. On the other hand, while the orientations were recovered in the cross-linked B5 Cin-235−23 film, the degree of orientation decreased significantly (see Figure 8a and 8b). Figure 8c shows that the TEM micrograph obtained for the cross-linked B5 Cin235−23 film quenched from 170 °C includes particles in rows that may be the traces of the lamellae formed at T < Ti. While
Figure 8. (a) WAXS and (b) SAXS patterns of the cross-linked B5 Cin-235−23 film cooled from 170 to 120 °C. The film drawing direction is vertical. (c) TEM micrograph obtained for the sample quenched from 170 °C. The arrow indicates the film longitudinal direction. Scale bar: 500 nm.
the cross-linked B5 Cin-235−23 film reversibly altered the type of microdomain from lamella to sphere on the isotropization of the LC segment, the lamellar orientation was conserved because of the spheres aligning in rows. The copolymer exhibiting a larger spontaneous elongation ratio requires the conservation of lamellar microdomain at T > Ti as well as lower φ. Such a copolymer can be obtained by increasing its molecular weight to prevent the disordering of copolymer lamellae.
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CONCLUSIONS Two series of ABA triblock B5 Cin-x-φ copolymers comprising the B block of main-chain smectic BB-5(3-Me) polyester and A blocks of photoreactive PCEMA segments have been prepared with MLC (= 100x) of 14600 and 23500 and with the volume fraction (φ) of the PCEMA segment ranging from 23% to 50%. All copolymers formed lamellar microdomains where the smectic layers lie parallel to the lamellae. The copolymer melts at T > Ti were drawn into film and annealed at T < Ti to align both the lamellar normal and LC director along the film drawing direction. UV irradiation of the films caused the crossF
DOI: 10.1021/acs.macromol.5b01646 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
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linking of the cinnamate moieties in the PCEMA segment up to 20% conversion without affecting the liquid crystal formation in the LC segment. The cross-linked copolymer film with φ > 37% contracted reversibly on the isotropization of the LC segment. While these copolymers conserved microdomain lamellae on the isotropization, the lamellar spacing decreased from DLC to Diso. During cooling, the lamellar spacing recovered on the LC formation and the smectic layers simultaneously regained their orientation. The rate of change in the film length (LLC/Liso) increased from 1.07 to 1.24 with decreasing φ but was independent of MLC. LLC/Liso values were comparable to the corresponding DLC/Diso values when considering the lamellae tilting with respect to the film drawing direction. However, the decrease of φ down to 23% in the two series of copolymers did not increase LLC/Liso. The cross-linked B5 Cin-235−23 film exhibited similar reversible distortion and spontaneous LC orientation on LC formation; however, the obtained LLC/Liso value of 1.20 was smaller than the expected value. This may be due to the microdomain transformation to spheres in rows at T > Ti. The cross-linked B5 Cin-146−23 film did not exhibit any reversible contraction, and the lamellar microdomain disordered at T > Ti. At present, we are extending our investigations to B5 Cin-x-φ with a higher molecular weight and copolymers with smaller φ values in order to achieve larger and reversible deformation by preserving the lamellar microdomain at T > Ti.
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AUTHOR INFORMATION
Corresponding Author
*(M.T.) E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge Prof. Shinji Ando (Tokyo Institute of Technology) for the use of the ATR FT-IR spectrometer and his helpful discussions on the results. TEM studies were performed by Mr. Jun Koki (Technical Department, Tokyo Institute of Technology), who is gratefully acknowledged. 4,4′dimethyl biphenyldicarboxylate was kindly supplied by Iharanikkei Chemical Industry Co. Ltd. The SR-SAXS measurement has been performed under the approval of the Photon Factory Program Advisory Committee (No. 2013G544).
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REFERENCES
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DOI: 10.1021/acs.macromol.5b01646 Macromolecules XXXX, XXX, XXX−XXX
