Periodic Surface Undulation in Cholesteric Liquid Crystal Elastomers

Dec 12, 2016 - ABSTRACT: Cholesteric liquid crystal elastomer (CLCE) films, whose helical axis is parallel to a direction in the film plane, exhibit a...
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Periodic Surface Undulation in Cholesteric Liquid Crystal Elastomers Hama Nagai,† Xiaobin Liang,‡ Yukihiro Nishikawa,† Ken Nakajima,‡ and Kenji Urayama*,† †

Department of Macromolecular Science and Engineering, Kyoto Institute of Technology, Sakyo-ku, Kyoto 606-8585, Japan School of Materials and Chemical Technology, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan

‡

ABSTRACT: Cholesteric liquid crystal elastomer (CLCE) films, whose helical axis is parallel to a direction in the film plane, exhibit a periodic surface undulation with a period corresponding to a half-pitch (about 100 μm) of the helical director configuration. Temperature variation drives not only a finite macroscopic uniaxial deformation along the helical axis but also a marked variation in the surface undulation. AFM results demonstrate that the surface undulation varies thermoreversibly like a stationary wave with an amplitude of about 600 nm. The top and bottom positions at 40 °C become the bottom and top ones, respectively, at 130 °C, while the undulation disappears near 100 °C. Microspectroscopy reveals the periodic spatial variation in optical birefringence, which reflects the quasi-helical superstructure of local directors in the CLCE films. The combined analysis of the results of AFM and microspectroscopy indicates that the thermally induced variation in the surface undulation becomes largest at positions with the most vertical and planar alignment in the quasi-helical director configuration.

I. INTRODUCTION Liquid crystal elastomers (LCEs) combine the high elasticity of rubbers with the orientation properties of liquid crystals. The coupling of rubber elasticity and liquid crystallinity results in a strong correlation between macroscopic deformation and the molecular orientation.1−5 This feature enables us to actuate the LCEs by imposing external fields which alter the liquid crystal orientation. In fact, finite actuation of LCEs has been demonstrated via temperature variation,6−8 electric9−11 and magnetic fields,12 and irradiation with light.13,14 Among the LCEs with several types of orientation order, cholesteric LCEs (CLCEs) have attracted considerable interest as photonic rubbers. The periodic helical configuration of the director in cholesteric liquid crystals (CLCs) leads to a selective Bragg reflection of incident light with a characteristic wavelength that is proportional to the helical pitch.15 Since the helical pitch is sensitive to temperature and electric fields, the CLCs are considered a promising material for color reflectors, reflection displays, and tunable lasers.16−18 The CLCEs possess both the optical properties of CLCs and the mechanical properties of rubber and need no mechanical support for a stable helical configuration of LCs. The helical pitch of the CLCEs is strongly correlated with the macroscopic dimension along the helical axis. Temperature variation19−21 and electric fields22 simultaneously drive a shift of the selective reflection band and dimensional changes. The imposition of mechanical strain also shifts the selective reflection band.23−27 Thus, CLCEs have a variety of potential applications in rubbery optical sensors and devices. CLCE films are classified into two types depending on the relationship between the helical axis and the film surface, i.e., whether the helical axis is normal or parallel to the film surface. Films oriented normal or parallel to the film surface are © XXXX American Chemical Society

designated here as N-CLCE and P-CLCE, respectively. To the best of our knowledge, earlier experimental studies of CLCEs exclusively employed N-CLCEs, although one theoretical study investigated the tensile behavior of P-CLCEs.28 The absence of earlier experimental studies of P-CLCEs is primarily due to difficulty in the fabrication of specimens. The corresponding helical director configuration for low molecular weight CLCs can be obtained in a special geometry, i.e., in cells with homeotropic boundary conditions.29 In principle, the fabrication of P-CLCEs requires the cross-linking reaction for the reactive chiral LCs to be confined in this geometry. Broer et al. fabricated CLC polymer networks with the same director configuration as P-CLCEs; however, their networks were glassy rather than elastomeric due to their high concentrations of cross-links. They employed rigid CLC polymer networks for the photoresponsive surface coating of substrates.30 Some experimental studies19−21 showed that N-CLCEs exhibited simple uniaxial elongation or contraction along the direction of thickness (helical axis) under temperature variation. What is the thermal deformation of the P-CLCE films like? One expects the uniaxial deformation along the helical axis to be similar to that of N-CLCE films, i.e., elongation or contraction in the direction parallel to the film surface. The present study reveals that the surface of the P-CLCE films exhibits periodic undulation with a period corresponding to a half-pitch of the helical director configuration. Further, the surface undulation depends markedly on temperature. Temperature variation drives not only macroscopic uniaxial deformation along the helical axis of the films Received: August 4, 2016 Revised: November 22, 2016

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Macromolecules but also a remarkable change in the undulation of the film surface. We also characterize the local director configuration by microspectroscopy and shed light on the correlation between the distribution of the local director and the surface undulation. The results of the present study reveal important and fundamental aspects of the stimulus-response properties of CLCEs that originate from the periodic helical director configuration. This feature also extends the range of potential applications of CLCEs to thermally responsive rubbery surfaces.

were washed away from the gels. The fully swollen gels were gradually deswollen via stepwise increases in the methanol (poor solvent for the gels) content of the surrounding solvent. The CLCE specimens were obtained by drying the deswollen gel films. CLCE specimens with different helical pitches designated as P-CLCE-A and P-CLCE-B were fabricated by varying the S-811 concentration of the mixtures. The characteristics of the two specimens are listed in Table 1. Detailed

Table 1. Characteristics of Specimens

II. EXPERIMENTAL SECTION

specimen

A-6OCB/6OCB/S-811/HDDA (by molar ratio)

TCh‑I (°C)

thicknessa (μm)

The CLCE specimens were prepared using the “chiral imprint” method31,32 with mixtures of reactive achiral mesogens and nonreactive chiral dopants. The details of this method are described elsewhere.33,34 The CLCE specimens were prepared via photopolymerization of achiral monoacrylate mesogen (A-6OCB) and 1,6-hexanediol diacrylate (HDDA) in the presence of optimized amounts of a nonreactive chiral dopant (S-811) and a miscible nonreactive nematic solvent (6OCB) using IRGACURE 784 as a photoinitiator. The chemical structures of A-6OCB, 6OCB, HDDA, and S-811 are given in Figure 1. The reactant mixtures are loaded into a glass cell whose

P-CLCE-A P-CLCE-B

100/100/0.17/10 100/100/0.26/10

115 125

37 20

a

At 25 °C.

characterizations of the macroscopic deformation and surface roughness were conducted on P-CLCE-A. The textures of the specimens and thermally driven macroscopic deformations were observed using a polarizing optical microscope (POM, Nikon LV100POL) with a hot stage (Linkam TMS94). A digital optical microscope (Hyrox KH-8700) was used to observe the film surfaces. These optical microscope observations were conducted in the unconstrained geometry. The specimens were placed on a glass substrate whose surface was coated with highly viscous silicone oil (a nonsolvent for the specimens) so that the specimens could undergo thermal deformation without external mechanical constraints. The surface profiles of the specimens were characterized via atomic force microscopy (AFM, Bruker MultiMode AFM with a NanoScope V controller) in tapping mode. The AFM was equipped with a specimen heating/cooling system. Tapping-mode topographic images were obtained using a silicon cantilever with a nominal spring constant of 40 N/m (MPP-11100, Bruker probes). Moderate tapping forces corresponding to set-point amplitude ratios of about 0.80 were used. In AFM experiments, the films were placed on the glass substrates without lubricants, but the mechanical friction between the specimens and substrates had no significant effect on the surface modulation of interest, as will be demonstrated later. The microspectroscopy of the specimens was conducted using a hyperspectrum camera (NH-7, EVA Japan). The camera can acquire the spectrum of each pixel over a 2D field of view. Transmission spectra in the range of ca. 400−800 nm were obtained under crossed nicols conditions. It should be noted that even this camera cannot obtain the spectra at the extinction position and that the camera itself has an intrinsic birefringence due its internal prism optics. In these experiments, we carefully chose the orientation of the specimens to avoid the extinction positions and also the orientation of the analyzer in the same direction to the prism of the camera.

Figure 1. Chemical structures of A-6OCB, 6OCB, S-811, and HDDA. surfaces were coated with a polyimide layer to induce vertical alignment (Figure 2). In this cell, a helical structure was formed with the

III. RESULTS AND DISCUSSION Figure 3 shows images from the polarized optical microscope (POM) for the two CLCE films. Each specimen exhibits a fingerprint texture which is similar to that of conventional low molecular mass CLCs confined in cells with homeotropic anchoring conditions.29 The fingerprint textures confirm the director configuration with the helical axis (x-direction) lying in the film plane. In homeotropic anchoring conditions without any special treatments, the fingerprint texture is typically random, but the texture observed here is considerably regular. The aligned line structure results from the orientation treatment of the polyimide layer by unidirectional gentle rubbing (y-direction) during fabrication of the films. The period of the line texture corresponds to half of the period of the helical director configuration (pH).29 The values of pH for P-CLCE-A and P-CLCE-B are about 200 or 120 μm, respectively. The value of pH becomes shorter as the concentration of the chiral dopant in the feed increases, which agrees with the familiar

Figure 2. Cell geometry for sample fabrication. helical axis lying in the plane of the substrates, but without further special care, the helical axis had no preferred direction in the substrate plane. The polyimide layers were uniaxially rubbed in order to induce the global orientation of the helical axis. The reactant mixtures in the cell were annealed for 15 h at 40 °C. The formation of a cholesteric texture whose helical axis was globally perpendicular to the rubbing direction was confirmed using a characteristic fingerprint pattern via a polarizing optical microscope, the details of which will be described later. Photopolymerization was carried out by irradiating the cell with light that had a central wavelength of 526 nm for 30 min at 40 °C. After polymerization, the glass cells were immersed in dichloromethane, and the gel films were detached from the glass substrates using swelling pressure. The surrounding dichloromethane was renewed several times, and the swelling was equilibrated. In this swelling procedure, the nonreactive chiral dopant and unreacted materials B

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about 6% along the helical axis is induced in the temperature range examined. This macroscopic thermal deformation results from a change in orientational order of the underlying nematic structure. Similar thermal variation-induced uniaxial deformation was also observed along the helical axis for N-CLCEs with their helical axes along the direction of film thickness, i.e., film thickening or thinning during heating or cooling, respectively.19−21 Figure 5 displays the surface profiles of P-CLCE-A at various T, observed using the digital microscope. In the

Figure 3. POM images of P-CLCE specimens. The period of the line textures in P-CLCE-A and P-CLCE-B is about 100 and 60 mm, respectively.

trend for pH in mixtures of low molecular mass CLCs and chiral dopants.15 The different colors of the transmission images at 40 and 100 °C reflect the difference in the optical retardation of the helical superstructures at the two temperatures, the details of which will be discussed later. The CLCE films exhibit finite degrees of macroscopic thermal deformation. Figure 4 illustrates the macroscopic dimen-

Figure 5. Surface profile of P-CLCE-A at 40, 100, and 130 °C. The cholesteric−isotropic transition temperature is 115 °C.

low-temperature LC state at 40 °C, the film surface exhibits periodic undulation with a height of about 1 μm and a pitch of ca. 100 μm. The pitch of the surface undulation matches the period of the fingerprint texture, i.e., a half-period of the helical director configuration. Interestingly, the surface undulation depends significantly on T. At ca. 100 °C, the undulation disappears and the surface becomes almost flat. Further heating again drives the periodic surface undulation, but with the top and bottom reversed relative to the undulation pattern at 40 °C. The surface profiles at various T for P-CLCE-A were investigated in more detail using AFM measurements. So that accurate comparisons could be made, the same 120 μm × 35 μm area of each specimen (Figure 6a) was scanned at each T. The length of the scanned area corresponds to ca. 60% of pH. Figure 6b shows the AFM images at various T. Figure 6c displays the position dependence of height variation (Δh) in the central line along the x-axis in the scanned area. The features of the T-responsive undulation in Figure 5 are also confirmed in the results of AFM: The top and bottom of the surface undulation at 40 °C become the bottom and top at 130 °C, respectively, while the surface is almost flat at 100 °C. It is also seen that the height variation is thermally reversible. Furthermore, the surface undulation has a sinusoidal shape, and it varies with T like as stationary wave with an amplitude of ca. 600 nm and a period of ca. 100 μm: The positions of the peaks and nodes are almost unchanged in the entire T range. For experimental convenience, the AFM measurements were conducted while the films were fixed on the glass substrates, rather than in the lubricant-driven unconstrained geometry using the

Figure 4. Macroscopic dimension of P-CLCE-A along each axis as a function of temperature. The x-, y-, and z-axis corresponds to the helical axis, rubbing axis, and thickness direction, respectively. The dimension along each axis is reduced by that at 120 °C in the isotropic state. The open and closed symbols denote the data in cooling and heating processes, respectively.

sions of P-CLCE-A as a function of temperature (T). The measurements were conducted in the range of 40 °C < T < 120 °C. The glass transition and cholesteric−isotropic transition (Tg and TCh‑I, respectively) temperatures are about 30 and 115 °C, respectively. In the figure, the dimension along each axis is reduced by the value at 120 °C in the isotropic state. The reduced thickness (λz) was calculated from the data for λx and λy by assuming conservation of volume (λxλyλz = 1). The effect of thermal volumetric expansion on λ was negligible here because the corresponding LCE films with polydomain random alignment showed no appreciable degree of dimensional change under temperature variation of interest. During heating in the LC state of T < TCh‑I, λx increases while λy and λz decrease in similar ways. Heating drives uniaxial elongation along the helical axis (x-direction). No appreciable difference in deformation is observed between heating and cooling. Uniaxial strain of C

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microspectroscopy. Instead, two different areas of the same film with similar textures and features, as revealed via POM imaging, were employed for these two measurements. This allowed us to perform a combined analysis with sufficient accuracy. Figure 7a shows the transmission spectrum at position X, under

Figure 6. (a) POM image of P-CLCE-A and the scanned area by AFM. (b) AFM images at various temperatures. (c) Position dependence of height variation in the central line along the x-axis in the scanned area.

lubricants between them. The undulation period observed (ca. 100 μm) is similar to that in the digitial microscope observations (Figure 5) as well as the period of the line texture in the POM observations (Figure 3), both of which were conducted in the unconstrained geometry. This suggests that the constraint imposed by the glass substrates in the AFM measurements have no significant influence on the surface undulation, probably because the film is significantly thicker than the undulation amplitude. No appreciable shift in the undulation nodes is observed via AFM across the entire T range, although the films undergo a thermally driven uniaxial strain of about 6% along the helical axis (Figure 4). The possibility of a constraint effect on one surface of the substrate in the AFM measurements cannot be ruled out, but the detection of the small contribution of thermal deformation to the undulation pitch might be precluded by the inherent roughness of film surface which is recognized in the AFM profile. Significantly, the surface undulation depends considerably on T in the LC state of T < TCh‑I, while it is independent of T in the isotropic state of T > TCh‑I (TCh‑I = 115 °C). No thermal effect on the surface undulation in the isotropic state is confirmed by the agreement of the data at 130 and 150 °C. These results clearly indicate that the T-induced variation in surface undulation is driven by a change in the underlying nematic structure. The spatial distribution of local orientation in the regular fingerprint patterns is investigated in more detail via microspectroscopy. Because of experimental limitations, the same areas of the films could not be imaged by both AFM and

Figure 7. (a) Transmission spectrum under crossed nicols condition at the position X for P-CLCE-A. The fitting of I* = sin2(πR/λ0) (depicted by the black line) on the basis of the locations of multiple peaks results in R = 5475 nm. (b) Position dependence of optical birefringence (Δn) along the line in (a) at various temperatures. (c) POM images of P-CLCE-A at 40 °C with different angles between the line texture and crossed polarizers.

crossed nicols conditions, at 40 °C. A finite birefringence results in the undulation of the intensity (I*). The locations of multiple peaks in the spectrum enable us to evaluate the retardation (R) by fitting the relation I*(λ0)/I*max = sin2(πR/λ0), where λ0 represents wavelength. The fitted result is depicted by the solid black line in the figure, which results in R = 5475 nm. The same procedure was used at multiple positions with an interval of 1.8 μm along a horizontal line with a length of 120 μm, as shown in Figure 7a. Figure 7b shows the dependence of the optical birefringence (Δn) on the position along the line (Figure 7a) at 40, 80, and 100 °C. The birefringence Δn at each position is calculated D

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Macromolecules using the relation Δn = R/h, where h is the thickness. A constant value of h = 37 μm was employed in this calculation because the effects of the surface undulation (Δh ≈ 1.5 μm) and thermal expansion (less than 5%) on h are sufficiently small. The position dependence of Δn shows an undulation, which reflects the quasi-helical superstructure of the local directors. When T increases, the values of Δn decrease, while the minimum and maximum Δn positions, located at x ≈ 20 and 70 μm, respectively, remain almost unchanged. This indicates that heating and cooling cause an increase or decrease in the degree of local orientation order, respectively. In contrast, the global quasi-helical director configuration is maintained in the range of T < TCh‑I. The position dependences of Δh and Δn are compared in Figure 8 based on data from Figures 6c and 7b to elucidate the

with the perfect homeotropic alignment should remain dark independently of the angles between the line texture and crossed polarizers, but no corresponding portions were observed in the films, which is shown in Figure 7c. The maximum values of Δn (Δnmax ≈ 0.15) at 40 °C are close to the value of Δnpl (≈ 0.16) measured in separate experiments for LCE films with globally planar alignments at 40 °C. The agreement between Δnmax and Δnpl shows that the CLCE film has well-developed planar portions. The director configuration of the CLCE films examined has a quasi-helical superstructure without highly homeotropically aligned regions. This is shown schematically in Figure 9b. The disorder is expected to originate from the present geometry, where the film thickness (and the cell gap in the film fabrication) is smaller than the helical pitch. For conventional CLCs, the geometry of h < pH often causes finite deformation of the helical superstructure via elastic interaction with the bounding substrate plates, resulting in a significant deviation from the ideal helical director configuration.29 It is difficult to explain exactly how the present geometry causes the disorder observed, and the experiments using the CLCE films prepared in the geometry of pH > h will help the understanding of this issue. The corresponding experiments will be made in our future work. Figure 10 illustrates the schematics of the surface undulation and the underlying nematic structure of the CLCE films

Figure 8. Comparison of the position dependence of Δh and Δn at 40 °C using the data in Figures 6c and 7b. The two different areas but with the similar texture in the POM images are selected for comparison.

correlation between the surface undulation and the spatial distribution of local nematic order. The comparison is made at 40 °C with the largest Δh amplitude in the range of T < TCh‑I. This comparison obviously demonstrates that the antinodes of the sinusoidal undulation of Δh are assigned to the positions with the most vertical and planar alignments. The minimum values of Δn (Δnmin) in the films, however, are finite. This is in contrast to the expectation of Δnmin ≈ 0 for ideal helical director configurations with perfectly homeotropic portions (Figure 9a). The finite values of Δnmin indicate that

Figure 10. Schematics of the correlation between the thermally induced variation in surface undulation and the local orientation of the quasi-helical superstructure in P-CLCE films.

estimated from experimental results. The flat specimens are originally prepared in the LC state, and thus the films become flat at a temperature (Tflat) in the range of T < TCh‑I. The temperature Tflat (ca. 100 °C), however, is different from that (41 °C) where the cross-linking reaction was performed because the as-prepared flat gel films are subject to a ca. 50% volume reduction during the deswelling process. The volume reduction (i.e., an increase in mesogenic network concentration) also causes a variation in the nematic order of local directors, resulting in a finite surface undulation. The qualitatively similar disagreements in the values of Tflat in the dry and swollen states were also observed for the LCE ribbons with hybrid alignment showing bending deformation35 and those with twist alignment forming helical shapes.33,34 The theoretical simulation studies demonstrate that the removal of solvent

Figure 9. Schematics of (a) the ideal helical superstructure of local directors and (b) the conjectured superstrucutre for P-CLCE-A with a finite nonzero value of Δnmin.

the most vertical alignment in the films deviates from the perfect homeotropic alignment. The POM images of the portions E

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Macromolecules from the LC gels with such nonuniform mesogen alignments leads to a finite change in shape.35,36 The global helical director configuration is fixed during the cross-linking reaction. Cooling or heating from Tflat causes an increase or decrease in the local nematic order (S) from S(Tflat). Since an increase in S from S(Tflat) (i.e., cooling from Tflat) drives elongation along the local director, elongation in homeotropically aligned regions occurs in the thickness (z) direction. In contrast, in the planar aligned regions, elongation takes place in the y-direction (rubbing direction), and a contraction occurs in the z-direction due to conservation of volume. These local distortions result in the surface undulation whose top and bottom are located at the most vertically and planar aligned regions, respectively. A decrease in S (i.e., heating from Tflat) from S(Tflat) also drives the surface undulation, but with the top and bottom positions reversed as a result of the contraction along the local director, through the same mechanism. The mechanism is also explained as follows. Upon reduction in S by heating from 40 °C, the homeotropic portions, which are initially bulging out, shrink whereas the planar portions expand. This cancel out the initial undulation at Tflat and can even result in opposite effects in the region of T > Tflat. The thermally induced variation in surface undulation in the CLCE films not only demonstrates the coupling between the macroscopic shape and local directors with the helical superstructure but also gives rise to potential applications as an elastomer with thermally controlled surface roughness. As in the case of light-induced undulation of fingerprint patterns in LC coatings on top of substrates,30 the CLCE films are expected to significantly vary their friction with or adhesion force to solid surfaces in response to temperature change as a result of significant changes in the areas of the contact surfaces.

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ACKNOWLEDGMENTS

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REFERENCES

The authors thank Dr. Jun Yoshioka in Riken for his helpful suggestions on specimen fabrication. This work was partly supported by a Grant-in-Aid for Challenging Exploratory Research (Grant No. 16K14080) from the Japan Society of Promotion Science, and the Eno Science Foundation.

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IV. CONCLUSIONS CLCE films with their helical axes along a direction in the film plane exhibited not only macroscopic deformation but also a significant change in periodic surface undulation in response to temperature variation, which results from the helical superstructure of the director. Cooling and heating drive macroscopic film contraction and elongation, respectively, along the helical axis. The film surface has a sinusoidal undulation with a period corresponding to a half-pitch of the quasi-helical director configuration. The surface undulation varies with temperature like a stationary wave with an amplitude of about 600 nm. The top and bottom of the undulation in the low-temperature LC state become inverted in the high-temperature isotropic state, while the surface becomes flat at a temperature in the LC state. Microspectroscopy reveals that the antinodes of the surface undulation correspond to the positions with the most vertically and planar aligned regions in the global helical director configuration, although the most vertical alignment in the films deviates appreciably from the perfect homeotropic alignment.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (K.U.). ORCID

Ken Nakajima: 0000-0001-7495-0445 Kenji Urayama: 0000-0002-2823-6344 Notes

The authors declare no competing financial interest. F

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DOI: 10.1021/acs.macromol.6b01688 Macromolecules XXXX, XXX, XXX−XXX