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Shape-responsive liquid crystal elastomer bilayers† Cite this: Soft Matter, 2014, 10, 1411
Aditya Agrawal, TaeHyun Yun, Stacy L. Pesek, Walter G. Chapman and Rafael Verduzco* Monodomain liquid crystal elastomers (LCEs) are shape-responsive materials, but shape changes are typically limited to simple uniaxial extensions or contractions. Here, we demonstrate that complex surface patterns and shape changes, including patterned wrinkles, helical twisting, and reversible folding, can be achieved in LCE–polystyrene (PS) bilayers. LCE–PS bilayer shape changes are achieved in response to simple temperature changes and can be controlled through various material parameters
Received 15th June 2013 Accepted 15th November 2013
including overall aspect ratio and LCE and polystyrene film thicknesses. Deposition of a patterned PS film on top of an LCE enables the preparation of an elastomer that reversibly twists and a folding leaf-like
DOI: 10.1039/c3sm51654g
elastomer, which opens and closes in response to temperature changes. The phenomena are captured
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through finite element simulations, in quantitative agreement with experiments.
Introduction Natural systems that can fold or change shape in response to environmental changes are well-known, and synthetic materials that behave similarly are desirable for applications including optics,1,2 three dimensional (3D) biological scaffolds,3–5 particle alignment,6 responsive surfaces7,8 and controlled release and encapsulation of drugs, particles and cells,9–11 among other applications. Synthetic self-folding materials have been demonstrated through the use of homogeneous bilayers,12–14 comprised of one active and one passive layer. Folding is triggered by a shape-transition in the active layer, which leads to folding due to restricted motion by the passive layer. Previous studies explored pH-responsive,15 thermoresponsive,16–18 solvent responsive19,20 and electrically sensitive active layers,21 and patterning of an active layer on passive layers provides more complex shape changes.19,20 Solvent swelling is typically used since it can provide large, reversible volume changes.22 In this study, we demonstrate versatile self-folding behaviour in elastomeric bilayers in response to temperature changes. The phenomenon is based on monodomain nematic liquid crystal elastomers (LCEs), which have a uniformly aligned nematic director and undergo spontaneous uniaxial shape changes with variations in temperature. This enables the preparation of reversible self-folding elastomeric materials, and with patterning of a passive layer on the active LCE, more complex, 3D shape changes can be achieved. While shape-memory
polymers enable complex 3D shape changes, LCEs offer the advantage of full reversibility and signicant shape and texture changes near room temperature. Self-folding lms have also been demonstrated in photolithography patterned polymermultilayer lms driven by residual stresses present,4,23 and LCEs offer the possibility to control the magnitude of these stresses through the temperature-dependent shape of the LCE. Previous work with monodomain LCEs have demonstrated uniaxial extensions and contractions with temperature changes, and work with liquid crystal lms has led to more complex deformations24–32 achieved by 3D patterning of the liquid crystal director in planar, twisted nematic, homeotropic, and splay geometries. Here, we demonstrate a straightforward approach using monodomain LCE coated with a thin PS top lm, which we refer to as an LCE–PS bilayer. In recent work, we studied reversible wrinkling and folding in LCE–PS bilayers.33 Here we show that this relatively simple system enables a rich response beyond reversible folding through variation of several system parameters: the thickness ratio of PS to LCE, the overall aspect ratio of the bilayer, the relative stiffness of the top lm and LCE, bilayer preparation temperature, and patterning of the top passive lm. Finite element method (FEM) simulations can predict the self-folding behaviour of LCE bilayers and are in quantitative agreement with experimental observations (Fig. 1).
Chemical and Biomolecular Engineering, Rice University, Houston, Texas, USA. E-mail: [email protected] † Electronic supplementary information (ESI) available: Synthesis and sample preparation procedures, calculation and data on the role of lm stiffness, videos showing shape changes in LCE–PS bilayers, and snapshots of FEM simulations.49 See DOI: 10.1039/c3sm51654g
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Fig. 1
Schematic for wrinkling and folding in LCE–PS bilayer.
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Result and discussions Monodomain LCEs are prepared using the two-step crosslinking method34 (see ESI for more details, Fig. S1†). The resulting monodomain LCEs spontaneously change shape as a function of temperature,35,36 and the shape change is fully reversible on both heating and cooling over several cycles, as shown in Fig. 2. LCE bilayers are prepared by a lm-transfer technique (see ESI, Fig. S2†). In the self-folding materials studied below, lm transfer is carried out near room temperature. Residual stresses are present aer lm transfer due to unintended strains and deformations during deposition. Earlier investigations have shown that residual stresses can cause debonding37,38 and cracks in the lm.39,40 To reduce the effect of residual stresses, all LCE–PS bilayer samples are annealed for 2 hours above the Tg of PS before measurement. The PS lm thickness was measured by spectroscopic ellipsometry (Accurion, GmbH, Germany) for thin PS lms and by UV-Vis for thicker lms (thickness > 1 mm).41 FEM simulations were performed using COMSOL Multiphysics 4.2 simulation package. The LCE substrate and thin polymer lm are assumed to be linear elastic solids with no slipping at the bilayer interface. To reproduce the experimental conditions, the temperature of the bottom surface of the LCE is increased from 298 K (Tref) to 349 K (the nematic-to-isotropic transition TNI) in the simulations. All other surfaces are open to ambient conditions, resulting in a small temperature gradient across the sample. The thermal conductivity of poly(dimethylsiloxane) is used in the calculations, and the LCE thickness is 500 mm in the simulations. LCEs were modelled as having different thermal expansion coefficients in different directions relative to the nematic director: 1 DLx ax ¼ 1 TNI Tref Lx
Fig. 2 Change in the LCE dimensions parallel (Lk) and perpendicular (Lt) to the nematic director for several heating and cooling cycles (bottom). Lk and Lt are normalized with respect to their values at 30 C. Length changes of monodomain LCEs were measured using DMAQ800 at a heating and cooling rate of 5 C min1.
1412 | Soft Matter, 2014, 10, 1411–1415
Paper
ay ¼
1 TNI Tref
az ¼
1 TNI Tref
DLy 1 Ly
DLz 1 Lz
where DLx, DLy and DLz represents the change in dimensions of the LCE as temperature increases from Tref to TNI. Dimension changes along the length and width (DLx and DLy) of LCEs were measured by temperature-controlled optical microscopy using a Zeiss microscope equipped with THMS 600 hot plate, and the value of DLz is calculated assuming volume conservation.42 We used a value of 7 105 K1 for the thermal expansion coefficient of PS.43 As shown in Fig. 3, the primary direction of folding or wrinkling is set by the temperature at which PS is deposited. When PS lm transfer is carried out at temperatures below room temperature, 5–6 C, the LCE order parameter decreases on heating back to room temperature and the sample contracts along the director. Conversely, when lm transfer is carried out at elevated temperatures, the nematic order parameter increases on cooling down to room temperature, resulting in a uniaxial extension of the LCE along the director and wrinkles parallel to the director. The temperature of lm transfer also determines the primary folding direction for samples described below. By varying PS lm thickness, we can induce folding in PS–LCE bilayers, as shown in Fig. 4. At low lm thicknesses (10 mm), as shown in Fig. 4. Videos showing the unfolding of an LCE–PS bilayer and snapshots of FEM simulations of LCE–PS bilayer folding are provided in the ESI.† The aspect ratio of the LCE bilayer is also important for the shape response, (Fig. 6) as has been reported for other selffolding materials.48 For bilayer aspect ratios Lk/Lt greater than 1, the bilayer folds along the nematic director, with increasing curvature for higher aspect ratios. For aspect ratios less than one, the lm folds primarily along an axis perpendicular to the nematic director but also along the director, resulting in a saddle-like shape, as shown in Fig. 6. This reects the presence of two orthogonal stresses in the LCE bilayers: tensile stress
Fig. 7 (Top) Schematics for patterned LCE–PS and LCE–Au bilayers
Comparison of simulations and experimental results for the curvature values of the deformed LCE–PS bilayers as a function of PS film thickness. Points correspond to experimental measurements and the line shows FEM simulation predictions. The LCE thickness is 0.48 mm in the experiments and 0.5 mm in the simulations. LCE bilayer aspect ratio is 3. Snapshots of FEM simulations are shown. Fig. 5
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for achieving controlled shape changes. The schematics show an initially flat LCE sample with PS or Au film deposited on a portion of the LCE. Shaded (purple) regions indicate where PS or Au is deposited. (Middle) FEM simulation predictions of the resulting shape-change for patterned LCE bilayers. (Bottom) Experimental examples of LCE–PS and LCE–Au bilayers with 3D shape changes.
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along the director and compressive stress perpendicular to director. More complex 3D shape changes can be achieved by patterning the PS lm on top of the LCE. Depositing a patterned top lm constrains the shape-change in a particular direction, allowing for curling in opposite directions or twisting with controlled chirality. As shown in Fig. 7, relatively simple surface patterns can give controlled curling and twisting. An ‘S-shape’ is achieved by coating half of the elastomer on opposite sides with a thin PS or Au lm. A twisted elastomer is achieved by depositing a lm of PS diagonally across the LCE, and a 4-arm folding elastomer is created by coating each arm on either the top or bottom surface with a PS lm. Note that the LCE is a monodomain, and the nematic director is aligned parallel to the long axis of two arms and perpendicular to the long axis of two arms. These patterned LCEs were tested experimentally and in FEM simulations. Experiments were carried out with both LCE– gold and LCE–PS bilayers. The former were straightforward to pattern using a Denton Desk V Sputter system and covering a desired portion of LCE during deposition. Prior to depositing gold on LCE, a very thin layer of chromium (5 nm) was deposited to ensure good adhesion of the metal lm. The thickness of the gold lm deposited was 80 nm. FEM simulations demonstrate that curling to form an Sshape, twisting, and closing of a 4-armed LCE structure can be achieved through patterned LCE–PS and LCE–Au bilayers. For each sample, the nal curvature is affected by the thickness ratio and aspect ratio of the bilayers. As shown in Fig. 7, controlled curling and twisting was achieved experimentally in patterned LCE bilayers. Four-armed curling was achieved in both LCE–gold (LCE–Au) and LCE–PS bilayers. The latter gave a larger degree of curling on heating and a better match to FEM simulations, likely due to the large modulus of gold. Although FEM simulations and measurements indicate the top lm modulus has a relatively modest effect, very stiff lms can suppress shape changes (see ESI†). A video demonstrating the folding of the four-armed elastomer leaf is provided in the ESI.† As-prepared LCE bilayer samples are at at room temperature and folded at elevated temperature, but the temperature dependence of the folding can be easily reversed in LCE–PS
Paper
bilayer samples through thermal or solvent annealing, as shown in Fig. 8. Films heated above the TNI initially fold or wrinkle, but on annealing above the PS glass-transition temperature Tg (105 C), folding or wrinkling is erased due to relaxation of the PS lm. On cooling back to room temperature, the PS lm becomes glassy and the LCE deforms below the TNI resulting in a folded sample at room temperature. The temperature dependence below TNI of the resulting curling is opposite that of the initially prepared sample, and the nal deformation is xed at room temperature. To recover the initial response – at at room temperature and folded at elevated temperature – the same sample can be solvent annealed at room temperature. Solvent annealing (dichloromethane) swells the PS lm, resulting in relaxation of the bilayer and recovery of the undeformed bilayer sample. Subsequent heating results in folding or wrinkling as in the initially prepared sample. Thus, LCEbilayers enable fully reversible folding and a tunable temperature dependence of the shape.
Conclusions We demonstrated a straightforward method for achieving 3D temperature dependent shape changes in LCE bilayers. Homogeneous LCE–PS bilayers spontaneously curl at elevated temperature, with a curvature dependent on the thickness ratio of the PS and LCE layers and the overall sample aspect ratio. Patterned samples show more complex 3D shape changes, including controlled curling and twisting. As-prepared samples are undeformed at room temperature and folded at elevated temperatures, but the temperature dependence can be reversed by thermal annealing above the Tg of the PS top lm. Thus, the shape-response of LCE–PS bilayers is fully reversible and easily controlled through patterning, annealing, and variation of material parameters.
Acknowledgements This project was supported by the Johns S. Dunn Foundation Collaborative Research Award Program administered by the Gulf Coast Consortia. Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund for partial support of this research (grant 52345-DNI7). We acknowledge support from Louis and Peaches Owen.
Notes and references
Fig. 8 Behavior of LCE–PS bilayers at temperatures below Tg and after annealing above Tg.
1414 | Soft Matter, 2014, 10, 1411–1415
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Soft Matter, 2014, 10, 1411–1415 | 1415
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Shape-responsive liquid crystal elastomer bilayers† Cite this: Soft Matter, 2014, 10, 1411
Aditya Agrawal, TaeHyun Yun, Stacy L. Pesek, Walter G. Chapman and Rafael Verduzco* Monodomain liquid crystal elastomers (LCEs) are shape-responsive materials, but shape changes are typically limited to simple uniaxial extensions or contractions. Here, we demonstrate that complex surface patterns and shape changes, including patterned wrinkles, helical twisting, and reversible folding, can be achieved in LCE–polystyrene (PS) bilayers. LCE–PS bilayer shape changes are achieved in response to simple temperature changes and can be controlled through various material parameters
Received 15th June 2013 Accepted 15th November 2013
including overall aspect ratio and LCE and polystyrene film thicknesses. Deposition of a patterned PS film on top of an LCE enables the preparation of an elastomer that reversibly twists and a folding leaf-like
DOI: 10.1039/c3sm51654g
elastomer, which opens and closes in response to temperature changes. The phenomena are captured
www.rsc.org/softmatter
through finite element simulations, in quantitative agreement with experiments.
Introduction Natural systems that can fold or change shape in response to environmental changes are well-known, and synthetic materials that behave similarly are desirable for applications including optics,1,2 three dimensional (3D) biological scaffolds,3–5 particle alignment,6 responsive surfaces7,8 and controlled release and encapsulation of drugs, particles and cells,9–11 among other applications. Synthetic self-folding materials have been demonstrated through the use of homogeneous bilayers,12–14 comprised of one active and one passive layer. Folding is triggered by a shape-transition in the active layer, which leads to folding due to restricted motion by the passive layer. Previous studies explored pH-responsive,15 thermoresponsive,16–18 solvent responsive19,20 and electrically sensitive active layers,21 and patterning of an active layer on passive layers provides more complex shape changes.19,20 Solvent swelling is typically used since it can provide large, reversible volume changes.22 In this study, we demonstrate versatile self-folding behaviour in elastomeric bilayers in response to temperature changes. The phenomenon is based on monodomain nematic liquid crystal elastomers (LCEs), which have a uniformly aligned nematic director and undergo spontaneous uniaxial shape changes with variations in temperature. This enables the preparation of reversible self-folding elastomeric materials, and with patterning of a passive layer on the active LCE, more complex, 3D shape changes can be achieved. While shape-memory
polymers enable complex 3D shape changes, LCEs offer the advantage of full reversibility and signicant shape and texture changes near room temperature. Self-folding lms have also been demonstrated in photolithography patterned polymermultilayer lms driven by residual stresses present,4,23 and LCEs offer the possibility to control the magnitude of these stresses through the temperature-dependent shape of the LCE. Previous work with monodomain LCEs have demonstrated uniaxial extensions and contractions with temperature changes, and work with liquid crystal lms has led to more complex deformations24–32 achieved by 3D patterning of the liquid crystal director in planar, twisted nematic, homeotropic, and splay geometries. Here, we demonstrate a straightforward approach using monodomain LCE coated with a thin PS top lm, which we refer to as an LCE–PS bilayer. In recent work, we studied reversible wrinkling and folding in LCE–PS bilayers.33 Here we show that this relatively simple system enables a rich response beyond reversible folding through variation of several system parameters: the thickness ratio of PS to LCE, the overall aspect ratio of the bilayer, the relative stiffness of the top lm and LCE, bilayer preparation temperature, and patterning of the top passive lm. Finite element method (FEM) simulations can predict the self-folding behaviour of LCE bilayers and are in quantitative agreement with experimental observations (Fig. 1).
Chemical and Biomolecular Engineering, Rice University, Houston, Texas, USA. E-mail: [email protected] † Electronic supplementary information (ESI) available: Synthesis and sample preparation procedures, calculation and data on the role of lm stiffness, videos showing shape changes in LCE–PS bilayers, and snapshots of FEM simulations.49 See DOI: 10.1039/c3sm51654g
This journal is © The Royal Society of Chemistry 2014
Fig. 1
Schematic for wrinkling and folding in LCE–PS bilayer.
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Result and discussions Monodomain LCEs are prepared using the two-step crosslinking method34 (see ESI for more details, Fig. S1†). The resulting monodomain LCEs spontaneously change shape as a function of temperature,35,36 and the shape change is fully reversible on both heating and cooling over several cycles, as shown in Fig. 2. LCE bilayers are prepared by a lm-transfer technique (see ESI, Fig. S2†). In the self-folding materials studied below, lm transfer is carried out near room temperature. Residual stresses are present aer lm transfer due to unintended strains and deformations during deposition. Earlier investigations have shown that residual stresses can cause debonding37,38 and cracks in the lm.39,40 To reduce the effect of residual stresses, all LCE–PS bilayer samples are annealed for 2 hours above the Tg of PS before measurement. The PS lm thickness was measured by spectroscopic ellipsometry (Accurion, GmbH, Germany) for thin PS lms and by UV-Vis for thicker lms (thickness > 1 mm).41 FEM simulations were performed using COMSOL Multiphysics 4.2 simulation package. The LCE substrate and thin polymer lm are assumed to be linear elastic solids with no slipping at the bilayer interface. To reproduce the experimental conditions, the temperature of the bottom surface of the LCE is increased from 298 K (Tref) to 349 K (the nematic-to-isotropic transition TNI) in the simulations. All other surfaces are open to ambient conditions, resulting in a small temperature gradient across the sample. The thermal conductivity of poly(dimethylsiloxane) is used in the calculations, and the LCE thickness is 500 mm in the simulations. LCEs were modelled as having different thermal expansion coefficients in different directions relative to the nematic director: 1 DLx ax ¼ 1 TNI Tref Lx
Fig. 2 Change in the LCE dimensions parallel (Lk) and perpendicular (Lt) to the nematic director for several heating and cooling cycles (bottom). Lk and Lt are normalized with respect to their values at 30 C. Length changes of monodomain LCEs were measured using DMAQ800 at a heating and cooling rate of 5 C min1.
1412 | Soft Matter, 2014, 10, 1411–1415
Paper
ay ¼
1 TNI Tref
az ¼
1 TNI Tref
DLy 1 Ly
DLz 1 Lz
where DLx, DLy and DLz represents the change in dimensions of the LCE as temperature increases from Tref to TNI. Dimension changes along the length and width (DLx and DLy) of LCEs were measured by temperature-controlled optical microscopy using a Zeiss microscope equipped with THMS 600 hot plate, and the value of DLz is calculated assuming volume conservation.42 We used a value of 7 105 K1 for the thermal expansion coefficient of PS.43 As shown in Fig. 3, the primary direction of folding or wrinkling is set by the temperature at which PS is deposited. When PS lm transfer is carried out at temperatures below room temperature, 5–6 C, the LCE order parameter decreases on heating back to room temperature and the sample contracts along the director. Conversely, when lm transfer is carried out at elevated temperatures, the nematic order parameter increases on cooling down to room temperature, resulting in a uniaxial extension of the LCE along the director and wrinkles parallel to the director. The temperature of lm transfer also determines the primary folding direction for samples described below. By varying PS lm thickness, we can induce folding in PS–LCE bilayers, as shown in Fig. 4. At low lm thicknesses (10 mm), as shown in Fig. 4. Videos showing the unfolding of an LCE–PS bilayer and snapshots of FEM simulations of LCE–PS bilayer folding are provided in the ESI.† The aspect ratio of the LCE bilayer is also important for the shape response, (Fig. 6) as has been reported for other selffolding materials.48 For bilayer aspect ratios Lk/Lt greater than 1, the bilayer folds along the nematic director, with increasing curvature for higher aspect ratios. For aspect ratios less than one, the lm folds primarily along an axis perpendicular to the nematic director but also along the director, resulting in a saddle-like shape, as shown in Fig. 6. This reects the presence of two orthogonal stresses in the LCE bilayers: tensile stress
Fig. 7 (Top) Schematics for patterned LCE–PS and LCE–Au bilayers
Comparison of simulations and experimental results for the curvature values of the deformed LCE–PS bilayers as a function of PS film thickness. Points correspond to experimental measurements and the line shows FEM simulation predictions. The LCE thickness is 0.48 mm in the experiments and 0.5 mm in the simulations. LCE bilayer aspect ratio is 3. Snapshots of FEM simulations are shown. Fig. 5
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for achieving controlled shape changes. The schematics show an initially flat LCE sample with PS or Au film deposited on a portion of the LCE. Shaded (purple) regions indicate where PS or Au is deposited. (Middle) FEM simulation predictions of the resulting shape-change for patterned LCE bilayers. (Bottom) Experimental examples of LCE–PS and LCE–Au bilayers with 3D shape changes.
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along the director and compressive stress perpendicular to director. More complex 3D shape changes can be achieved by patterning the PS lm on top of the LCE. Depositing a patterned top lm constrains the shape-change in a particular direction, allowing for curling in opposite directions or twisting with controlled chirality. As shown in Fig. 7, relatively simple surface patterns can give controlled curling and twisting. An ‘S-shape’ is achieved by coating half of the elastomer on opposite sides with a thin PS or Au lm. A twisted elastomer is achieved by depositing a lm of PS diagonally across the LCE, and a 4-arm folding elastomer is created by coating each arm on either the top or bottom surface with a PS lm. Note that the LCE is a monodomain, and the nematic director is aligned parallel to the long axis of two arms and perpendicular to the long axis of two arms. These patterned LCEs were tested experimentally and in FEM simulations. Experiments were carried out with both LCE– gold and LCE–PS bilayers. The former were straightforward to pattern using a Denton Desk V Sputter system and covering a desired portion of LCE during deposition. Prior to depositing gold on LCE, a very thin layer of chromium (5 nm) was deposited to ensure good adhesion of the metal lm. The thickness of the gold lm deposited was 80 nm. FEM simulations demonstrate that curling to form an Sshape, twisting, and closing of a 4-armed LCE structure can be achieved through patterned LCE–PS and LCE–Au bilayers. For each sample, the nal curvature is affected by the thickness ratio and aspect ratio of the bilayers. As shown in Fig. 7, controlled curling and twisting was achieved experimentally in patterned LCE bilayers. Four-armed curling was achieved in both LCE–gold (LCE–Au) and LCE–PS bilayers. The latter gave a larger degree of curling on heating and a better match to FEM simulations, likely due to the large modulus of gold. Although FEM simulations and measurements indicate the top lm modulus has a relatively modest effect, very stiff lms can suppress shape changes (see ESI†). A video demonstrating the folding of the four-armed elastomer leaf is provided in the ESI.† As-prepared LCE bilayer samples are at at room temperature and folded at elevated temperature, but the temperature dependence of the folding can be easily reversed in LCE–PS
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bilayer samples through thermal or solvent annealing, as shown in Fig. 8. Films heated above the TNI initially fold or wrinkle, but on annealing above the PS glass-transition temperature Tg (105 C), folding or wrinkling is erased due to relaxation of the PS lm. On cooling back to room temperature, the PS lm becomes glassy and the LCE deforms below the TNI resulting in a folded sample at room temperature. The temperature dependence below TNI of the resulting curling is opposite that of the initially prepared sample, and the nal deformation is xed at room temperature. To recover the initial response – at at room temperature and folded at elevated temperature – the same sample can be solvent annealed at room temperature. Solvent annealing (dichloromethane) swells the PS lm, resulting in relaxation of the bilayer and recovery of the undeformed bilayer sample. Subsequent heating results in folding or wrinkling as in the initially prepared sample. Thus, LCEbilayers enable fully reversible folding and a tunable temperature dependence of the shape.
Conclusions We demonstrated a straightforward method for achieving 3D temperature dependent shape changes in LCE bilayers. Homogeneous LCE–PS bilayers spontaneously curl at elevated temperature, with a curvature dependent on the thickness ratio of the PS and LCE layers and the overall sample aspect ratio. Patterned samples show more complex 3D shape changes, including controlled curling and twisting. As-prepared samples are undeformed at room temperature and folded at elevated temperatures, but the temperature dependence can be reversed by thermal annealing above the Tg of the PS top lm. Thus, the shape-response of LCE–PS bilayers is fully reversible and easily controlled through patterning, annealing, and variation of material parameters.
Acknowledgements This project was supported by the Johns S. Dunn Foundation Collaborative Research Award Program administered by the Gulf Coast Consortia. Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund for partial support of this research (grant 52345-DNI7). We acknowledge support from Louis and Peaches Owen.
Notes and references
Fig. 8 Behavior of LCE–PS bilayers at temperatures below Tg and after annealing above Tg.
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