Letter Cite This: ACS Macro Lett. 2018, 7, 353−357
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Multitemperature Memory Actuation of a Liquid Crystal Polymer Network over a Broad Nematic−Isotropic Phase Transition Induced by Large Strain Rong Yang†,‡ and Yue Zhao*,‡ †
Jiangsu Key Laboratory of Environmentally Friendly Polymeric Materials, School of Materials Science and Engineering, Jiangsu Collaborative Innovation Center of Photovolatic Science and Engineering, Changzhou University, Changzhou, 213164, People’s Republic of China ‡ Département de chimie, Université de Sherbrooke, Sherbrooke, Québec J1K 2R1, Canada S Supporting Information *
ABSTRACT: The shape change of a polymer actuator based on liquid crystal network (LCN) generally occurs over a relatively sharp LC-isotropic phase transition. Reported herein is the discovery of an unusual phenomenon and the enabled actuation control for LCN. The smectic phase of a LCN with mesogenic moieties on the chain backbone can be suppressed by high elongation of the specimen, which gives rise to a broad nematic−isotropic phase transition. Consequently, the actuation force and related shape of the actuator can be activated to a given degree by easily varying the temperature over a wide range (35 K for LCN prepared with 500% strain) to adjust the proportion of the order− disorder phase transition. This reversible multitemperature memory actuation can translate into many stable and interconvertible shapes with one single LCN actuator.
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ross-linked liquid crystal polymers, referred to as liquid crystal networks (LCNs) or elastomers (LCEs), depending on the glass transition temperature (Tg), have been extensively studied over the past decade or so and are emerging as a promising system of polymer actuators.1−23 To fulfill their potential for applications, easy processing, ability of complex shape morphing, and control of versatile shape transformations under stimulation are among the important issues to address. In a previous work, the appealing attributes of a designed liquid crystal polymer as LCN actuator were unveiled.24 In essence, this polymer (chemical structure in Figure 1A) on heating displays a smectic phase between Tg ∼ 23 °C up to 49 °C, followed by a narrow nematic phase before the nematic− isotropic phase transition at Tni ∼ 58 °C. It can be easily stretched in LC phases to yield a monodomain of uniaxial orientation of the biphenyl mesogens on the chain backbone; the deformation is mostly plastic with little relaxation of the elongated specimen after removal of external stress; and the cynnamyl units in the structure allows photo-cross-linking to be applied nonuniformly to pattern the actuation domains and thus generate a differential force leading to complex shape morphing over the nematic−isotropic transition (characterizations of unstretched liquid crystal polymer before and after uniform photo-cross-linking are shown in Figures S1−S3). Herein, we report the discovery of yet another intriguing property of this LCN, namely, mechanically induced smecticnematic phase transition and the multitemperature memory actuation over a broad nematic−isotropic phase transition region appeared as a result of the mechanically suppressed smectic ordering. This finding is of both fundamental interest © XXXX American Chemical Society
Figure 1. (A) Chemical structure and phase transition temperatures (in °C) of the liquid crystal polymer. (B) Contractile force vs temperature for LCN actuators of different initial sample elongation degrees (200%, 300%, and 500% strain for LCN200, LCN300, and LCN500, respectively) under constant strain upon heating to the isotropic phase and subsequent cooling to room temperature.
Received: February 1, 2018 Accepted: February 26, 2018
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DOI: 10.1021/acsmacrolett.8b00089 ACS Macro Lett. 2018, 7, 353−357
Letter
ACS Macro Letters
shown in Figure 2. While the unstrained LCP sample displays two ring-shaped reflections, with the inner one attributed to the
and practical implications for actuation control of LCNs, which, to our knowledge, has not been reported previously. Extensive plastic deformation of this liquid crystal polymer, as large as 1000% strain, can readily be obtained by stretching in the nematic phase (e.g., 51 °C). Interestingly, when uniformly photo-cross-linked, the initial strain seems to affect little the extent of the contraction in the isotropic state and the extension in the LC phase of the LCN actuator along the initial stretching direction (essentially 40−50% variation in length). However, it is easy to picture that for two specimens subjected to, say, 200% and 500% strain, respectively, they may have a similar degree of uniaxial orientation of the mesogens along the stretching direction, the actual extent of their chain extension or organization should be different, which would impact somehow the actuation behavior. To get some insight into the possible effect, we carried out the isostrain tests for LCN samples with different initial strains (200%, 300%, and 500%) by monitoring the contractile force developed in the samples held to the constant strain on heating and cooling cycle. As can be seen from the results in Figure 1B, on the one hand, similar features can be noticed for all samples. On heating, the contractile force is basically absent at lower temperatures, then starts to rise quickly when the order−disorder phase transition takes place, and finally changes slowly above ∼70 °C toward a plateau as the transition into the isotropic state is completed. On subsequent cooling, the measurable force drops to zero when the extensional force accompanying the reformation of the monodomain cancels the contractile force. The apparent hysteresis loop comes from the supercooling effect of the transformation between the nematic and isotropic phases. On the other hand, however, the effect of initial strain on the evolution of the contractile force is obvious. First, the actuator prepared from a larger elongation generates a greater actuation force upon entering the isotropic phase, indicating relaxation of more extended polymer chains and concomitant release of a larger amount of strain energy stored in the stretched sample. Second, the initial strain shows influence on the actuation temperature: the contractile force rises at a lower temperature with increasing the initial strain. Indeed, for the LCN with the highest elongation of 500%, the apparent actuation temperature is lowered by about 15 °C with respect to the LCN with 200% initial strain, and the temperature range over which the contractile force develops appears to be broadened as well (Movie S1). This observation is intriguing. The actuation, that is, contraction of the sample, requires chain conformational change from extended (ellipsoidal) to relaxed state (random coil) that occurs during an order−disorder phase transition (nematic−isotropic or smectic-isotropic); whereas an effective actuation due to smectic-nematic phase transition (only losing layering of mesogens) is unlikely. Therefore, the result in Figure 1B implies that nematic−isotropic phase transition starts at a much lower temperature for the actuator with 500% initial elongation. What happens in the specimen then? To find an answer to the above question, we used X-ray diffraction (XRD) to investigate the structural change caused by sample elongation. The measurements were carried out on the liquid crystal polymer right after stretching to 200%, 300%, and 500% elongation at 51 °C followed by cooling (denoted as LCP200, LCP300, and LCP500, respectively) and on their corresponding actuators prepared by uniform photo-crosslinking and heating to the isotropic phase (80 °C) for equilibrium before cooling to room temperature (denoted as LCN200, LCN300, and LCN500, respectively). The results are
Figure 2. 1D and 2D X-ray diffraction patterns recorded at room temperature for (A) the liquid crystal polymer (LCP) stretched to different strains (200%, 300%, and 500%), and (B) the corresponding actuators (LCN) after thermal equilibrium in the isotropic phase followed by cooling. (C) Schematic illustration of the structural change of LCP during stretching in LC phase.
smectic layer ordering and the outer one to the mesogens within a layer (Figure S2), after stretching to 200%, the inner ring-shaped reflection turns to a spot near the meridian (along the stretching direction), and the outer ring-shaped reflection becomes an arc-shaped reflection near the equator. This result indicates uniaxial orientation of the mesogenic groups as well as the normal of smectic layer along the stretching direction. This stretching induced orientation basically remains in the sample with 300% strain, but notable changes are visible. Both the sharp smectic layer reflection and the diffuse nematic reflection appear at smaller diffraction angles (2θ shift from 2.5 to 2.3° and from 19.4 to 17.6°, respectively), indicating an increase in the spacing between the smectic layers and an increase in the average distance between the oriented mesogens (including other chain segments). On further stretching to 500%, the inner spot reflection disappears and the outer arc-shaped reflection becomes a halo, implying an oriented nematic phase and the suppression of the smectic phase in this highly stretched sample. The absence of smectic ordering was unequivocally confirmed by the azimuthal diffraction profile in the 2θ = 1.7−3.5° region (Figure S4). For the LCN samples (Figure 2B), after photo-cross-linking, chain relaxation in the isotropic phase (contraction of the actuator) and subsequent recovery of the monodomain on cooling to room temperature 354
DOI: 10.1021/acsmacrolett.8b00089 ACS Macro Lett. 2018, 7, 353−357
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ACS Macro Letters (extension of the actuator), they all show persistent uniaxial LC orientation with the nematic ordering reflection at 2θ = 19.4°. However, the stretching-induced suppression of the smectic ordering is retained in the LCN actuator, as can be seen from the reduced intensity of the smectic layer reflection in the LCN with 300% strain and the hardly discernible reflection in the actuator with 500% strain. In other words, in the LCN actuator prepared with large strain, a mechanically induced smecticnematic phase transition takes place. On the basis of the XRD results, the proposed mechanism leading to the suppression of the smectic phase in the LCP subjected to stretching in the LC phase is schematically depicted in Figure 2C. Before stretching, LCP in polydomain shows a regular chain-folding and packing with a bent flexible spacer and the pendent phenyl groups aligned in parallel with the mesogens.25,26 On 200% stretching, the LCP undergoes a polydomain-monodomain transition while having most chains still folded. Upon further stretching to 300%, folded chains become unfolded; while smectic layers remain, the spacing dilates slightly. Also, the pendent phenyl group on flexible spacer is now positioned perpendicular to the chain backbone and expands the distance between the mesogens. These changes account for the observed shifts of the diffraction peaks from LCP200 to LCP300. At a large strain like 500%, more chain extension and unfolding occurs, which is accompanied by chain sliding and relaxation due to the weak physical cross-linking of chain entanglements. While the uniaxial orientation persists, the positional order of the smectic phase is disrupted. Understandably, this disruption becomes increasingly severe with increasing the stretching degree and eventually completes the smectic−nematic phase transition under a large strain. Photo-cross-linking the LCP at a given strain basically fixes the state of LC orientation and organization in the stretched sample. Although chain relaxation and folding can occur upon stress release with the LCN in the isotropic state, once cooled to the LC phase, the uniaxial LC orientation is recovered with the reduced or suppressed smectic ordering retained. As pointed out above, the actuator prepared with high strain (LCN500 in Figure 1) appears to develop an actuation force over a broad temperature range. The suppression of the smectic phase in the actuator thus implies a broad nematic−isotropic phase transition. This is likely considering the effect of chain cross-linking on the ordering and organization of the mesogens, and the fact that the strain-induced smectic to nematic order transition is a gradual process governed by the actual chain conformation resulting from extension, sliding, folding or unfolding, which may not be homogeneous across the LCN. The broadening of the nematic−isotropic phase transition as a result of the large strain-suppressed smectic phase in the LCN actuator is indeed observable from the differential scanning calorimetry (DSC) measurements (Figure S5). This feature enables more control of the LCN actuation, because it means that the occurrence of the nematic−isotropic phase transition, thus the actuation, can be reversibly spread over a wide range of temperatures with varying actuation strength. To further confirm this feature, we carried out an isostrain test on both LCN200 and LCN500 subjected to a stepwise temperature increase and decrease cycle, the results are shown in Figure 3. For LCN200, the actuator was given a 10 °C stepwise change in temperature and held at a given temperature for 10 min. The contractile force appears at about 60 °C and shows stepwise increase up to 80 °C on heating and drops similarly on cooling.
Figure 3. Change in contractile force of LCN200 (A) and LCN500 (B) under constant strain while being subjected to a stepwise temperature increase (second heating run) and decrease (first cooling run).
In the case of LCN500, the actuator was subjected to a 5 °C stepwise temperature change and held at each temperature for 5 min. The contractile force shows up at 45 °C and displays stepwise increase and decrease on the respective heating and cooling. This result indicates clearly that the mechanically suppressed smectic phase endows a broad nematic−isotropic phase transition to the LCN actuator so that the actuation force determined by the extent or proportion of the nematic− isotropic phase transition can be reversibly controlled by adjusting the temperature over a wide range (up to about 35 K). Generally, stretch-aligned LCN actuators have a narrow LCisotropic phase transition temperature range (a few to about 10 K). Basically, they switch between two shapes associated with the ordered and disordered state, respectively, while the intermediate shapes determined by partial phase transition cannot be exploited in practice. The practical implication of the large strain-induced smectic-nematic phase transition is the capability for the LCN actuator to display the reversible multitemperature memory actuation over a wide temperature range. The photos in Figure 4 show a comparison of actuation between LCN300 and LCN500. Here, the actuators were programmed to undergo a flat to roll shape change upon the LC-isotropic phase transition (one side cross-linking). On heating to various temperatures, the shape change for LCN300 mainly takes place between 55 and 65 °C, while the process starts at 45 °C for LCN500 due to its wider phase transition. It should be emphasized that each of the shapes of LCN500 at a given temperature is stable, and that the shapes at any two temperatures are interconvertible on both heating and cooling (see Movie S2 for another example). This appealing actuation feature of our LCN contrasts with the multishape memory effect discovered with Nafion (a perfluorosulfonic acid 355
DOI: 10.1021/acsmacrolett.8b00089 ACS Macro Lett. 2018, 7, 353−357
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ACS Macro Letters
and “closure” are reversible over the temperature range of 40− 60 °C (Movies S3, S4, and S5). In conclusion, we have reported the discovery of stretching induced smectic-nematic phase transition in LCN actuators prepared with a large strain used to obtain the monodomain of uniaxial LC orientation. The results suggest that the mechanically suppressed smectic ordering in the used LCP gives rise to a broad nematic−isotropic phase transition that endows the LCN with an easily usable multitemperature memory actuation effect. With a temperature-controllable extent of the nematic−isotropic phase transition that determines the actuation force and related shape, one single LCN actuator can adopt many stable and interconvertible shapes over a wide temperature range (35 K). This property offers enhanced control of actuation under stimulation for LCNs toward applications.
Figure 4. Photos comparing the multitemperature memory actuation behavior of the actuators prepared with 300% strain (LCN300) and 500% strain (LCN500). Specimen dimension: 25 × 2 × 0.08 mm3.
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ionomer),27 with which any shape change due to entropydriven chain relaxation is not reversible. It also differs from the temperature memory actuation known for actuators made with a few semicrystalline polymers such as EVA (random copolymer of ethylene and vinyl acetate),28,29 for which the shape change is reversible upon partial crystal melting but becomes irreversible if heated into the isotropic state. The LCN actuator has no similar restraints. It should be mentioned that broad nematic−isotropic phase transition and use for multitemperature memory are known for surface-aligned LCNs,14,30 for which the phase transition broadening is caused by complex spatial and hierarchical patterns of LC directors obtained using alignment surfaces. Before conclusion, we demonstrate the use of the multitemperature memory actuation of our LCN to mimic a kind of reversible shape change found in nature. The “bloom” motion of flowers is sensitive to the climatic and environmental conditions, especially to temperature, sunlight, and humidity. While most flowers bloom in the day, some tropical flowers bloom in the night for prevention from high temperature and intense sunlight. Water Lily (nymphaeaceae) is a family of flowering plants that lives as rhizomatous aquatic herbs in temperate and tropical climates around the world. By hybridization, they can either bloom in the day (hardy and day-blooming tropical water lily) or bloom in the night (nightblooming tropical water lily). Making use of the varying and stable shape at a given temperature of the actuator LCN500, and the ability of the LCN actuator to perform the same shape change either on heating or cooling thanks to its easy processing as previously reported,24 we fabricated two flowershaped actuators. As shown in Figure 5, one actuator exhibits the “blooming” and “closure” in response to temperature increase/decrease, while the other does the same in response to temperature decrease/increase. In both cases, the “booming”
ASSOCIATED CONTENT
S Supporting Information *
and Movie Captions (PDF) Movies S1−S5 (MPEG4) The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.8b00089. Materials, preparation of LCN actuators, and characterizations, Figures S1−S5 (PDF). Movie S1: Comparison of two polymer actuators prepared using specimens stretched to 300% and 500% (LCN300 and LCN500), respectively; they both roll upon heating in a temperature-controlled chamber, but the one with 500% strain proceeds faster due to a lower actuation temperature and greater actuation force (MPG). Movie S2: Right-handed helix to left-handed helix upon temperature switch from 45 to 50 to 55 and 60 °C in a water bath, showing the multitemperature memory actuation of LCN500 (MPG). Movie S3: Artificial flower: “closure” upon heating inside an oven and “blooming” upon cooling outside the oven (MPG). Movie S4: Artificial flower: “closure” upon cooling and “blooming” upon heating on a hot plate set to low temperature (MPG). Movie S5: Artificial flower: “closure” upon cooling and “blooming” upon heating on a hot plate set to high temperature (MPG).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Rong Yang: 0000-0002-0193-6949 Yue Zhao: 0000-0001-5544-5697 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Y.Z. acknowledges financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC) and le Fonds de recherche du Québec: Nature et technologies (FRQNT). R.Y. thanks financial support of overseas training from Natural Science Foundation of Jiangsu Province (BK20150257) and Top-notch Academic Programs Project of
Figure 5. Photos of two thermosensitive flower-shaped actuators that undergo reversible “blooming” and “closure” in response to temperature change of either heating−cooling or cooling−heating, mimicking Water Lily flowers that bloom in the day or in the night. 356
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(21) Liu, L.; Liu, M. H.; Deng, L. L.; Lin, B. P.; Yang, H. NearInfrared Chromophore Functionalized Soft Actuator with Ultrafast Photoresponsive Speed and Superior Mechanical Property. J. Am. Chem. Soc. 2017, 139, 11333−11336. (22) Yang, Y.; Pei, Z.; Li, Z.; Wei, Y.; Ji, Y. Making and Remaking Dynamic 3D Structures by Shining Light on Flat Liquid Crystalline Vitrimer Films without a Mold. J. Am. Chem. Soc. 2016, 138 (7), 2118−2121. (23) Ube, T.; Kawasaki, K.; Ikeda, T. Photomobile Liquid-Crystalline Elastomers with Rearrangeable Networks. Adv. Mater. 2016, 28 (37), 8212−8217. (24) Yang, R.; Zhao, Y. Non-uniform Optical Inscription of Actuation Domains in Liquid Crystal Polymer of Uniaxial Orientation: An Approach to Complex and Programmable Shape Changes. Angew. Chem., Int. Ed. 2017, 56, 14202−14206. (25) Yang, R.; Chen, L.; Ruan, C.; Zhong, H.-Y.; Wang, Y.-Z. Chain folding in main-chain liquid crystalline polyesters: from π−π stacking toward shape memory. J. Mater. Chem. C 2014, 2 (30), 6155−6164. (26) Yang, R.; Ding, L.; Chen, W.; Chen, L.; Zhang, X.; Li, J. Chain Folding in Main-Chain Liquid Crystalline Polyester with Strong π−π Interaction: An Efficient β-Nucleating Agent for Isotactic Polypropylene. Macromolecules 2017, 50 (4), 1610−1617. (27) Xie, T. Tunable polymer multi-shape memory effect. Nature 2010, 464 (7286), 267−270. (28) Kratz, K.; Madbouly, S. A.; Wagermaier, W.; Lendlein, A. Temperature-Memory Polymer Networks with Crystallizable Controlling Units. Adv. Mater. 2011, 23, 4058−4062. (29) Behl, M.; Kratz, K.; Noechel, U.; Sauter, T.; Lendlein, A. Temperature-memory polymer actuators. Proc. Natl. Acad. Sci. U. S. A. 2013, 110 (31), 12555−12559. (30) Yoon, H.-H.; Kim, D.-Y.; Jeong, K.-U.; Ahn, S.-k. Surface Aligned Main-Chain Liquid Crystalline Elastomers: Tailored Properties by the Choice of Amine Chain Extenders. Macromolecules 2018, 51 (3), 1141−1149.
Jiangsu Higher Education. D.F. is acknowledged for assisting the XRD measurements. Y.Z. is a member of the FRQNTfunded Centre québécois sur les matériaux fonctionnels (CQMF).
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DOI: 10.1021/acsmacrolett.8b00089 ACS Macro Lett. 2018, 7, 353−357
