LETTERS PUBLISHED ONLINE: 1 DECEMBER 2013 | DOI: 10.1038/NMAT3812
Mouldable liquid-crystalline elastomer actuators with exchangeable covalent bonds Zhiqiang Pei1† , Yang Yang1† , Qiaomei Chen1 , Eugene M. Terentjev2 *, Yen Wei1 and Yan Ji1 * Liquid-crystal elastomers (LCEs) are a class of actively moving polymers with remarkable practical potential for converting external stimuli into mechanical actuation1 . However, real-world applications of LCEs are lacking because macroscopic orientation of liquid-crystal order, which is required for reversible actuations2,3 , is hard to achieve in practice. Here we show that the processing bottleneck of LCEs can be overcome by introducing exchangeable links in place of permanent network crosslinks, a concept previously demonstrated for vitrimers4,5 . Liquid-crystal elastomers with exchangeable links (xLCEs) are mouldable, allow for easy processing and alignment, and can be subsequently altered through remoulding with different stress patterns, thus opening the way to practical xLCE actuators and artificial muscles. Surprisingly, instead of external-stress relaxation through the creep of non-liquid-crystal transient networks with exchangeable links6,7 , xLCEs develop strong liquid-crystal alignment as an alternative mechanism of mechanical relaxation. The invention of the two-step crosslinking method of permanent alignment was the watershed moment in the field of LCEs, allowing large-size monodomain materials capable of thermal or photoinduced actuation to be prepared by many laboratories around the world8,9 . Other methods (alignment by surface anchoring or by external fields10–12 ) have always been available. The recently developed methods for processing LCEs by inkjet printing, soft lithography, microfluidics13–15 and so on are also based on those principles3 . However, there is a fundamental problem with all known alignment techniques: one has to align the liquid crystal before the crosslinking. The two-step crosslinking, in particular, has an inherent contradiction: in order to apply a good aligning stress at the second step, the gel has to be well crosslinked initially—which increases disorder in the liquid-crystal state later on; it also cannot produce non-trivial shapes. Aligning by external field or boundary conditions can work only across thin films on scales below 20–30 µm. There has been much hope in the use of thermoplastic LCEs, as they rapidly self-assemble on cooling and thus are capable of aligning and shaping16,17 . However, it turns out that at high temperatures of actuation, the mechanical stability of thermoplastic elastomers is not sufficient to preserve the material shape and alignment over many actuation cycles. These technical issues are the main reason for the present lack of practical applications of LCEs in spite of their remarkable potential. Exchangeable links were recently shown to be able to release the external stress applied to the thermoset polymer networks4,7 . It is important to distinguish this principle from the relaxation of physically bonded thermoplastic elastomers: here the total number
1 The
of covalent bonds, within and between chains, remains constant at all times, with only their topology changing. Following this new concept, we used the exchangeable links to form elastomeric liquid-crystalline networks, which we call exchangeable LCEs, or xLCEs. We chose the vitrimers in ref. 5 as a demonstration of our strategy. Usually, a transesterification reaction occurs between an ester and a hydroxyl group, generating a new ester and a new hydroxyl group (Fig. 1a). Owing to the fast breaking and reforming of ester bonds at a temperature above the activation barrier, which is set by the catalyst, vitrimer networks could behave like a viscoelastic melt above the topology-freezing transition temperature Tv (using a superficial analogy with the process of vitrification of glass). At temperatures below Tv , exchange reactions become slow exponentially, and the network acts as a classical covalently bonded thermoset. As illustrated in Fig. 1b, we synthesized the xLCE network through the reaction between an epoxy-terminated biphenyl mesogen and decanedioic acid, in 1:1 molar ratio. These molecular groups have already been used to form an ordinary LCE network18 ; however, we show here that with the added catalyst (triazabicyclodecene6 , Fig. 1b) the crosslink bonds become rapidly exchangeable and the network becomes malleable at high temperatures (T > Tv ). Consistent with ref. 18, we obtain a smectic-A LCE, with a glass transition Tg ≈ 55 ◦ C and the isotropic transition Ti ≈ 100 ◦ C on cooling, as measured by differential scanning calorimetry. Swelling in trichlorobenzene verified that it remains a fully covalently crosslinked elastomer at all temperatures. According to dilatometry4 (Supplementary Fig. 5), the Tv of this xLCE is about 160 ◦ C at 3 K min−1 heating rate, so there are wide temperature ranges between the three characteristic points Tv > Ti > Tg . As expected, the designed xLCEs can be reshaped or reprocessed at T > Tv . Two pieces of elastomer film could be joined together by simple compression moulding at 180 ◦ C (Fig. 1c). At this temperature, the overlap traces are still discernible after 20 min of moulding, but after 40 min the overlap mark is no longer visible. Not only xLCE samples are mouldable, but also materials with different chemical compositions could be attached together and bonded covalently. For instance, Fig. 1c shows an xLCE film bonded with a non-liquid-crystal epoxy elastomer obtained from the reaction of the diglycidyl ether of bisphenol A and diacid. Forming such fully bonded bi- or multi-layers of elastomers with actuating capacity is a route to many applications involving bending and surface wrinkling. Reversible transesterification also allows us to pattern LCE surfaces, as shown in an example of micropatterning achieved at 240 ◦ C in 6 min that remains on reheating to T > Tv without load (Fig. 1c).
Key Laboratory of Bioorganic Phosphorus Chemistry and Chemical Biology, Department of Chemistry, Tsinghua University, Beijing 100084, China,
2 Cavendish Laboratory, University of Cambridge, Cambridge CB3 OHE, UK. † These authors contributed equally to this work. *e-mail: [email protected];
[email protected] 36
NATURE MATERIALS | VOL 13 | JANUARY 2014 | www.nature.com/naturematerials
NATURE MATERIALS DOI: 10.1038/NMAT3812 a
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Figure 2 | Shape-memory effects in a non-aligned polydomain xLCE. a, One-way shape memory of a micropatterned xLCE, showing the embossing and the recovery of an additional pattern. b, Triple shape memory of an xLCE using the polydomain smectic phase (below Ti ) to fix the shape B, and the glass transition Tg to fix the shape C, and their sequential recovery on heating. c, Quantitative illustration of the triple shape memory in the example of linear extensional stress and strain: the equilibrium sample (shape A) is stretched at T > Ti and then cooled (the additional large extension below Ti is the equilibrium actuation of the stress-aligned xLCE, an effect unrelated to the triple shape memory). The shape B is fixed by the polydomain smectic order below Ti . An additional deformation into shape C is then fixed by cooling the sample below Tg . Shape B0 is recovered when the sample is relaxed by heating back into the LCE phase, and the original shape A0 is recovered by heating back into the isotropic phase. NATURE MATERIALS | VOL 13 | JANUARY 2014 | www.nature.com/naturematerials
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At temperatures below Tv , xLCEs maintain mechanical integrity and behave as traditional, permanently crosslinked LCE networks. We shall discuss the alignment process below, but first explore the properties of a non-aligned polydomain xLCE. Owing to the smectic-A layer constraints19,20 , as well as the glass transition, Tg , which is 50 ◦ C below Ti , these epoxy xLCEs have excellent and diverse shape-memory effects21,22 . For instance, Fig. 2a shows the embossing of an additional pattern (the upper right corner) on top of the micropatterned xLCE at 130 ◦ C, which was fixed when the temperature dropped below Ti . Reheating above Ti deletes the new pattern and recovers the original one. The triple shape-memory effect is illustrated in Fig. 2b, and a corresponding quantitative investigation of this effect by dynamic mechanical analysis is shown in Fig. 2c. At 130 ◦ C, the sample with a flat shape (A) was deformed into a new shape (B), which was fixed by cooling down to 75 ◦ C (smectic layer constraints). It was then further deformed at this temperature into a new shape (C), which was then fixed by cooling below Tg (glassy constraints). On heating, the sample first recovered the shape B (above Tg ) and then the original shape A (above Ti ). This polydomain xLCE is also capable of reversible shape change under constant stress21 . The stress-induced polydomain– monodomain transition is well known23,24 . Figure 3a demonstrates the diffuse stress plateau where the main alignment takes place, and Fig. 3b illustrates the equilibrium (reversible) nature of the shape change with temperature under increasing load (and the corresponding induced alignment). If we cycle the temperature above and below the isotropic transition Ti , the mechanical state 38
of a stress-aligned xLCE will respond as a usual monodomain LCE actuator8,11 . Figure 3c,d demonstrates cycles of large and very stable elongation/contraction under constant load monitored by dynamic-mechanical analysis (DMA). In Fig. 3c one can see a detailed comparison of an original polydomain xLCE with the sample that has been brought above Tv and remoulded into another strip. In Fig. 3d we illustrate the thermal stability (the complete lack of creep) of xLCE. In stark contrast, a non-liquid-crystal epoxy elastomer with exchangeable links under the same stress shows a noticeable creep (plastic flow) at an elevated temperature, as indeed any transient network should6,7 . In contrast, unexpectedly, we find a very different behaviour in the xLCE, with a strong induced liquid-crystal alignment even at a high temperature, with no plastic deformation at all. Following the fundamental principles of forming the uniform aligned monodomain LCE networks2,24 , and using the exchangeable nature of links in our xLCE, we apply a uniaxial load to a polydomain sample and increase the temperature. Figure 4a,b illustrates the process of annealing under stress, showing the key stages of thermal treatment and loading. We assume that the induced orientational order (at T > Tv under load; Fig. 4b) leads to a large increase in the sample natural length (to L1 in Fig. 4a) and is the reason for stress relaxation and the lack of creep (Supplementary Information); this is confirmed by birefringence measurements (Supplementary Fig. 7). However, our main interest is in forming (and reforming, if necessary) a well-aligned, freestanding monodomain xLCE, which is the base material for thermal and photomechanical actuators. NATURE MATERIALS | VOL 13 | JANUARY 2014 | www.nature.com/naturematerials
NATURE MATERIALS DOI: 10.1038/NMAT3812
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Figure 4 | The scheme of xLCE processing leading to an equilibrium monodomain alignment, and its reversal. a, The original polydomain sample of length L0 (heated to above Tv , starting the fast transesterification) is put under uniaxial stress and its length increases to L1 . We find the sample to be highly birefringent in this state, in spite of the very high temperature. We see no further creep while the sample is kept under load, remaining at length L1 until we lower the temperature to bring it to the liquid-crystal state. At this point the well-aligned xLCE spontaneously elongates to a length L2 . Removing the load does not change this length L2 , as it is fixed by the smectic phase (shape C in Fig. 2c). Bringing the aligned xLCE into the isotropic phase (above Ti ) returns the length back to the original L0 . However, the material retains the memory of alignment and on cooling it spontaneously returns to a monodomain liquid-crystal state with the natural length Lm . Lm is less than L2 as it is the natural equilibrium shape of the aligned xLCE with no internal stress frozen in. However, if the sample is heated above Tv , the fast transesterification deletes the memory of alignment and the sample returns to the original polydomain liquid crystal of length L0 . b, We suggest that, in the isotropic phase under load, the induced orientational order is established, which causes an increase of the underlying natural length and the relaxation of stress, a process that replaces the creep of non-liquid-crystal transient networks. c, X-ray image of the natural monodomain xLCE shows a well-aligned smectic-A order with a high orientational (nematic) order parameter Q ≈ 0.86.
Such exotic behaviour of LCEs with exchangeable links offers an unprecedented opportunity to prepare monodomain xLCE actuators of any desired shape, as long as they can be moulded properly. Importantly, and in contrast with common two-step crosslinking alignment procedures in ordinary LCEs (ref. 2), our process is stable and robust in the sense that it does not rely on precise timing or the magnitude of the applied load. All monodomain phases obtained have their crosslinks established in a defect-free, high-temperature aligned phase—that is, of ‘isotropic genesis’ (in terminology of refs 25,26)—and that means of much ‘better quality’27 . Figure 5a demonstrates that the spontaneous change of length of a uniaxially aligned strip of xLCE (at no or very small load) reaches 35%, a value higher than in usual aligned smectic LCEs (refs 28,29). On increasing the applied load, the magnitude of actuation stroke increases significantly. Reversible bending is also possible (Supplementary Fig. 10), when the two sides of the xLCE film are aligned differently, for example by keeping one side above Tv and the other below Tv during loading. Moreover, we can easily remould an existing sample into a new shape or state of order. As a demonstration, we moulded the xLCE film into a pin- or dowel-shape actuator (Fig. 5c). This configuration is very important for practical applications, especially in tactile (haptic) display technology including dynamic Braille displays30 , when the actuation has to deliver a linear pushing force action (as opposed to a much more common and easily achieved low-force bending). NATURE MATERIALS | VOL 13 | JANUARY 2014 | www.nature.com/naturematerials
Figure 5d shows a smaller, free-standing dome-shaped actuating sample of an initial height of 0.5 mm: as the temperature goes up, the height of the dome decreases until the sample returns to its original flat shape above Ti ; on cooling back to the liquid-crystal phase the dome reappears in its original shape. The main point of this Letter is to illustrate how exchangeable links (using transesterification in our case, but clearly not uniquely7 ) enable the processability of LCE networks, leading to a completely new and robust method of monodomain alignment and of shaping of the resulting elastomers. This should end the dependence of LCE actuator applications on the tricky and very unreliable process of alignment, which has been the limiting factor in the field for almost two decades13 . Our process allows moulding of xLCE shapes of any dimension, not relying on surface alignment at microscopic length scales31 . There can be many exciting further developments using these ideas. For instance, we have worked with a smectic-A phase merely because of the ease of synthesis and to obtain a triple shape-memory material—but a main-chain nematic liquid-crystal network with similar exchangeable links would achieve a much higher amplitude of actuation. The whole arsenal of LCE knowledge can now be applied; for example, doping the elastomer network with photoresponsive dyes32,33 or carbon nanotubes34 will make a photo-actuator in exactly the same way. We hope this will finally open the way to practical applications of LCE actuators and artificial muscles. 39
NATURE MATERIALS DOI: 10.1038/NMAT3812
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Methods A stoichiometric amount of diglycidyl ether of 4,40 -dihydroxybiphenyl and sebacic acid were mixed at 180 ◦ C. A triazabicyclodecene catalyst (5 mol% to the COOH groups) was introduced and stirred manually until homogeneous. The mixture was cooled to room temperature and sandwiched between two glass slides covered with polytetrafluoroethylene tape. The clamped glass slides enclosing the reacting mixture were left at 180 ◦ C for at least 4 h. For the matching non-liquid-crystal network, dihydroxybiphenyl was replaced by diglycidyl ether of bisphenol A. After full characterization of the network using infrared spectroscopy, differential calorimetry, thermal gravimetry and swelling in trichlorobenzene, the thermo-mechanical measurements were carried out on a TA-Q800 DMA apparatus in the film-tension geometry under a controlled load, measuring the changes in the film length with temperature. These measurements are first done to identify Tv in the networks that include the catalyst and thus are capable of transesterification, then to monitor the multi-stage shape-memory response across the two transition temperatures, Ti and Tg , and finally to characterize the thermal actuation response of polydomain xLCE under load, and of aligned monodomain xLCE in equilibrium. The important procedure of making a monodomain film follows the following steps. An initial polydomain film of length L0 was heated to 160 ◦ C (just above Tv ) in a heating mantle and annealed for 3 min. Uniaxial stress was then applied either by a home-made stretching device until the strain (creep of the slowly transesterifying network) reached 75%. Then the film was cooled down to room temperature while still stretched. After this, the total length of an aligned film becomes about two times the original length owing to the further spontaneous elongation of the aligned xLCE. The sample was then reheated to 115 ◦ C (T > Ti ) without load to remove any internal mechanical constraints due to the smectic layers: the original length L0 is recovered. However, the thus prepared film ’remembers’ its uniaxial stretching direction and the monodomain xLCE is recovered, spontaneously and reversibly elongating on cooling below Ti to the equilibrium length Lm (about 40% greater than L0 ). When the sample was instead heated to 170 ◦ C (T > Tv ), the memory of alignment is lost and the sample reverts back to its polydomain configuration when subsequently cooled. Moulding of xLCE into complex dome-like shapes was done using a home-made metal punch and die mould. The procedure follows the steps of monodomain preparation: preheating the mould parts and the xLCE film at 160 ◦ C for 3 min, and placing the film on top of the pins of the punch. The sample was covered with the die part, and gently pressed so that the pins were inserted into the complementary holes; after 30 s, the whole set was cooled to room temperature. We then took off the die mould and peeled the shaped xLCE sample out of the pin mould. 40
Received 8 June 2013; accepted 15 October 2013; published online 1 December 2013
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Acknowledgements This research was supported by the National Science Foundation of China (nos 21274075 and 51203086) and the National 973 Project (no. 2011CB935700).
Author contributions Y.J. and E.M.T. developed the concept, Y.J. and Y.W. arranged the funding and infrastructure for the project, Z.P., Y.Y. and Y.J. performed the experiments, Q.C. participated in the synthesis of xLCEs, and Y.J., E.M.T. and Y.W. contributed to writing the paper.
Additional information Supplementary information is available in the online version of the paper. Reprints and permissions information is available online at www.nature.com/reprints. Correspondence and requests for materials should be addressed to E.M.T. or Y.J.
Competing financial interests The authors declare no competing financial interests.
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Mouldable liquid-crystalline elastomer actuators with exchangeable covalent bonds Zhiqiang Pei1† , Yang Yang1† , Qiaomei Chen1 , Eugene M. Terentjev2 *, Yen Wei1 and Yan Ji1 * Liquid-crystal elastomers (LCEs) are a class of actively moving polymers with remarkable practical potential for converting external stimuli into mechanical actuation1 . However, real-world applications of LCEs are lacking because macroscopic orientation of liquid-crystal order, which is required for reversible actuations2,3 , is hard to achieve in practice. Here we show that the processing bottleneck of LCEs can be overcome by introducing exchangeable links in place of permanent network crosslinks, a concept previously demonstrated for vitrimers4,5 . Liquid-crystal elastomers with exchangeable links (xLCEs) are mouldable, allow for easy processing and alignment, and can be subsequently altered through remoulding with different stress patterns, thus opening the way to practical xLCE actuators and artificial muscles. Surprisingly, instead of external-stress relaxation through the creep of non-liquid-crystal transient networks with exchangeable links6,7 , xLCEs develop strong liquid-crystal alignment as an alternative mechanism of mechanical relaxation. The invention of the two-step crosslinking method of permanent alignment was the watershed moment in the field of LCEs, allowing large-size monodomain materials capable of thermal or photoinduced actuation to be prepared by many laboratories around the world8,9 . Other methods (alignment by surface anchoring or by external fields10–12 ) have always been available. The recently developed methods for processing LCEs by inkjet printing, soft lithography, microfluidics13–15 and so on are also based on those principles3 . However, there is a fundamental problem with all known alignment techniques: one has to align the liquid crystal before the crosslinking. The two-step crosslinking, in particular, has an inherent contradiction: in order to apply a good aligning stress at the second step, the gel has to be well crosslinked initially—which increases disorder in the liquid-crystal state later on; it also cannot produce non-trivial shapes. Aligning by external field or boundary conditions can work only across thin films on scales below 20–30 µm. There has been much hope in the use of thermoplastic LCEs, as they rapidly self-assemble on cooling and thus are capable of aligning and shaping16,17 . However, it turns out that at high temperatures of actuation, the mechanical stability of thermoplastic elastomers is not sufficient to preserve the material shape and alignment over many actuation cycles. These technical issues are the main reason for the present lack of practical applications of LCEs in spite of their remarkable potential. Exchangeable links were recently shown to be able to release the external stress applied to the thermoset polymer networks4,7 . It is important to distinguish this principle from the relaxation of physically bonded thermoplastic elastomers: here the total number
1 The
of covalent bonds, within and between chains, remains constant at all times, with only their topology changing. Following this new concept, we used the exchangeable links to form elastomeric liquid-crystalline networks, which we call exchangeable LCEs, or xLCEs. We chose the vitrimers in ref. 5 as a demonstration of our strategy. Usually, a transesterification reaction occurs between an ester and a hydroxyl group, generating a new ester and a new hydroxyl group (Fig. 1a). Owing to the fast breaking and reforming of ester bonds at a temperature above the activation barrier, which is set by the catalyst, vitrimer networks could behave like a viscoelastic melt above the topology-freezing transition temperature Tv (using a superficial analogy with the process of vitrification of glass). At temperatures below Tv , exchange reactions become slow exponentially, and the network acts as a classical covalently bonded thermoset. As illustrated in Fig. 1b, we synthesized the xLCE network through the reaction between an epoxy-terminated biphenyl mesogen and decanedioic acid, in 1:1 molar ratio. These molecular groups have already been used to form an ordinary LCE network18 ; however, we show here that with the added catalyst (triazabicyclodecene6 , Fig. 1b) the crosslink bonds become rapidly exchangeable and the network becomes malleable at high temperatures (T > Tv ). Consistent with ref. 18, we obtain a smectic-A LCE, with a glass transition Tg ≈ 55 ◦ C and the isotropic transition Ti ≈ 100 ◦ C on cooling, as measured by differential scanning calorimetry. Swelling in trichlorobenzene verified that it remains a fully covalently crosslinked elastomer at all temperatures. According to dilatometry4 (Supplementary Fig. 5), the Tv of this xLCE is about 160 ◦ C at 3 K min−1 heating rate, so there are wide temperature ranges between the three characteristic points Tv > Ti > Tg . As expected, the designed xLCEs can be reshaped or reprocessed at T > Tv . Two pieces of elastomer film could be joined together by simple compression moulding at 180 ◦ C (Fig. 1c). At this temperature, the overlap traces are still discernible after 20 min of moulding, but after 40 min the overlap mark is no longer visible. Not only xLCE samples are mouldable, but also materials with different chemical compositions could be attached together and bonded covalently. For instance, Fig. 1c shows an xLCE film bonded with a non-liquid-crystal epoxy elastomer obtained from the reaction of the diglycidyl ether of bisphenol A and diacid. Forming such fully bonded bi- or multi-layers of elastomers with actuating capacity is a route to many applications involving bending and surface wrinkling. Reversible transesterification also allows us to pattern LCE surfaces, as shown in an example of micropatterning achieved at 240 ◦ C in 6 min that remains on reheating to T > Tv without load (Fig. 1c).
Key Laboratory of Bioorganic Phosphorus Chemistry and Chemical Biology, Department of Chemistry, Tsinghua University, Beijing 100084, China,
2 Cavendish Laboratory, University of Cambridge, Cambridge CB3 OHE, UK. † These authors contributed equally to this work. *e-mail: [email protected];
[email protected] 36
NATURE MATERIALS | VOL 13 | JANUARY 2014 | www.nature.com/naturematerials
NATURE MATERIALS DOI: 10.1038/NMAT3812 a
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Figure 2 | Shape-memory effects in a non-aligned polydomain xLCE. a, One-way shape memory of a micropatterned xLCE, showing the embossing and the recovery of an additional pattern. b, Triple shape memory of an xLCE using the polydomain smectic phase (below Ti ) to fix the shape B, and the glass transition Tg to fix the shape C, and their sequential recovery on heating. c, Quantitative illustration of the triple shape memory in the example of linear extensional stress and strain: the equilibrium sample (shape A) is stretched at T > Ti and then cooled (the additional large extension below Ti is the equilibrium actuation of the stress-aligned xLCE, an effect unrelated to the triple shape memory). The shape B is fixed by the polydomain smectic order below Ti . An additional deformation into shape C is then fixed by cooling the sample below Tg . Shape B0 is recovered when the sample is relaxed by heating back into the LCE phase, and the original shape A0 is recovered by heating back into the isotropic phase. NATURE MATERIALS | VOL 13 | JANUARY 2014 | www.nature.com/naturematerials
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At temperatures below Tv , xLCEs maintain mechanical integrity and behave as traditional, permanently crosslinked LCE networks. We shall discuss the alignment process below, but first explore the properties of a non-aligned polydomain xLCE. Owing to the smectic-A layer constraints19,20 , as well as the glass transition, Tg , which is 50 ◦ C below Ti , these epoxy xLCEs have excellent and diverse shape-memory effects21,22 . For instance, Fig. 2a shows the embossing of an additional pattern (the upper right corner) on top of the micropatterned xLCE at 130 ◦ C, which was fixed when the temperature dropped below Ti . Reheating above Ti deletes the new pattern and recovers the original one. The triple shape-memory effect is illustrated in Fig. 2b, and a corresponding quantitative investigation of this effect by dynamic mechanical analysis is shown in Fig. 2c. At 130 ◦ C, the sample with a flat shape (A) was deformed into a new shape (B), which was fixed by cooling down to 75 ◦ C (smectic layer constraints). It was then further deformed at this temperature into a new shape (C), which was then fixed by cooling below Tg (glassy constraints). On heating, the sample first recovered the shape B (above Tg ) and then the original shape A (above Ti ). This polydomain xLCE is also capable of reversible shape change under constant stress21 . The stress-induced polydomain– monodomain transition is well known23,24 . Figure 3a demonstrates the diffuse stress plateau where the main alignment takes place, and Fig. 3b illustrates the equilibrium (reversible) nature of the shape change with temperature under increasing load (and the corresponding induced alignment). If we cycle the temperature above and below the isotropic transition Ti , the mechanical state 38
of a stress-aligned xLCE will respond as a usual monodomain LCE actuator8,11 . Figure 3c,d demonstrates cycles of large and very stable elongation/contraction under constant load monitored by dynamic-mechanical analysis (DMA). In Fig. 3c one can see a detailed comparison of an original polydomain xLCE with the sample that has been brought above Tv and remoulded into another strip. In Fig. 3d we illustrate the thermal stability (the complete lack of creep) of xLCE. In stark contrast, a non-liquid-crystal epoxy elastomer with exchangeable links under the same stress shows a noticeable creep (plastic flow) at an elevated temperature, as indeed any transient network should6,7 . In contrast, unexpectedly, we find a very different behaviour in the xLCE, with a strong induced liquid-crystal alignment even at a high temperature, with no plastic deformation at all. Following the fundamental principles of forming the uniform aligned monodomain LCE networks2,24 , and using the exchangeable nature of links in our xLCE, we apply a uniaxial load to a polydomain sample and increase the temperature. Figure 4a,b illustrates the process of annealing under stress, showing the key stages of thermal treatment and loading. We assume that the induced orientational order (at T > Tv under load; Fig. 4b) leads to a large increase in the sample natural length (to L1 in Fig. 4a) and is the reason for stress relaxation and the lack of creep (Supplementary Information); this is confirmed by birefringence measurements (Supplementary Fig. 7). However, our main interest is in forming (and reforming, if necessary) a well-aligned, freestanding monodomain xLCE, which is the base material for thermal and photomechanical actuators. NATURE MATERIALS | VOL 13 | JANUARY 2014 | www.nature.com/naturematerials
NATURE MATERIALS DOI: 10.1038/NMAT3812
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Figure 4 | The scheme of xLCE processing leading to an equilibrium monodomain alignment, and its reversal. a, The original polydomain sample of length L0 (heated to above Tv , starting the fast transesterification) is put under uniaxial stress and its length increases to L1 . We find the sample to be highly birefringent in this state, in spite of the very high temperature. We see no further creep while the sample is kept under load, remaining at length L1 until we lower the temperature to bring it to the liquid-crystal state. At this point the well-aligned xLCE spontaneously elongates to a length L2 . Removing the load does not change this length L2 , as it is fixed by the smectic phase (shape C in Fig. 2c). Bringing the aligned xLCE into the isotropic phase (above Ti ) returns the length back to the original L0 . However, the material retains the memory of alignment and on cooling it spontaneously returns to a monodomain liquid-crystal state with the natural length Lm . Lm is less than L2 as it is the natural equilibrium shape of the aligned xLCE with no internal stress frozen in. However, if the sample is heated above Tv , the fast transesterification deletes the memory of alignment and the sample returns to the original polydomain liquid crystal of length L0 . b, We suggest that, in the isotropic phase under load, the induced orientational order is established, which causes an increase of the underlying natural length and the relaxation of stress, a process that replaces the creep of non-liquid-crystal transient networks. c, X-ray image of the natural monodomain xLCE shows a well-aligned smectic-A order with a high orientational (nematic) order parameter Q ≈ 0.86.
Such exotic behaviour of LCEs with exchangeable links offers an unprecedented opportunity to prepare monodomain xLCE actuators of any desired shape, as long as they can be moulded properly. Importantly, and in contrast with common two-step crosslinking alignment procedures in ordinary LCEs (ref. 2), our process is stable and robust in the sense that it does not rely on precise timing or the magnitude of the applied load. All monodomain phases obtained have their crosslinks established in a defect-free, high-temperature aligned phase—that is, of ‘isotropic genesis’ (in terminology of refs 25,26)—and that means of much ‘better quality’27 . Figure 5a demonstrates that the spontaneous change of length of a uniaxially aligned strip of xLCE (at no or very small load) reaches 35%, a value higher than in usual aligned smectic LCEs (refs 28,29). On increasing the applied load, the magnitude of actuation stroke increases significantly. Reversible bending is also possible (Supplementary Fig. 10), when the two sides of the xLCE film are aligned differently, for example by keeping one side above Tv and the other below Tv during loading. Moreover, we can easily remould an existing sample into a new shape or state of order. As a demonstration, we moulded the xLCE film into a pin- or dowel-shape actuator (Fig. 5c). This configuration is very important for practical applications, especially in tactile (haptic) display technology including dynamic Braille displays30 , when the actuation has to deliver a linear pushing force action (as opposed to a much more common and easily achieved low-force bending). NATURE MATERIALS | VOL 13 | JANUARY 2014 | www.nature.com/naturematerials
Figure 5d shows a smaller, free-standing dome-shaped actuating sample of an initial height of 0.5 mm: as the temperature goes up, the height of the dome decreases until the sample returns to its original flat shape above Ti ; on cooling back to the liquid-crystal phase the dome reappears in its original shape. The main point of this Letter is to illustrate how exchangeable links (using transesterification in our case, but clearly not uniquely7 ) enable the processability of LCE networks, leading to a completely new and robust method of monodomain alignment and of shaping of the resulting elastomers. This should end the dependence of LCE actuator applications on the tricky and very unreliable process of alignment, which has been the limiting factor in the field for almost two decades13 . Our process allows moulding of xLCE shapes of any dimension, not relying on surface alignment at microscopic length scales31 . There can be many exciting further developments using these ideas. For instance, we have worked with a smectic-A phase merely because of the ease of synthesis and to obtain a triple shape-memory material—but a main-chain nematic liquid-crystal network with similar exchangeable links would achieve a much higher amplitude of actuation. The whole arsenal of LCE knowledge can now be applied; for example, doping the elastomer network with photoresponsive dyes32,33 or carbon nanotubes34 will make a photo-actuator in exactly the same way. We hope this will finally open the way to practical applications of LCE actuators and artificial muscles. 39
NATURE MATERIALS DOI: 10.1038/NMAT3812
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Methods A stoichiometric amount of diglycidyl ether of 4,40 -dihydroxybiphenyl and sebacic acid were mixed at 180 ◦ C. A triazabicyclodecene catalyst (5 mol% to the COOH groups) was introduced and stirred manually until homogeneous. The mixture was cooled to room temperature and sandwiched between two glass slides covered with polytetrafluoroethylene tape. The clamped glass slides enclosing the reacting mixture were left at 180 ◦ C for at least 4 h. For the matching non-liquid-crystal network, dihydroxybiphenyl was replaced by diglycidyl ether of bisphenol A. After full characterization of the network using infrared spectroscopy, differential calorimetry, thermal gravimetry and swelling in trichlorobenzene, the thermo-mechanical measurements were carried out on a TA-Q800 DMA apparatus in the film-tension geometry under a controlled load, measuring the changes in the film length with temperature. These measurements are first done to identify Tv in the networks that include the catalyst and thus are capable of transesterification, then to monitor the multi-stage shape-memory response across the two transition temperatures, Ti and Tg , and finally to characterize the thermal actuation response of polydomain xLCE under load, and of aligned monodomain xLCE in equilibrium. The important procedure of making a monodomain film follows the following steps. An initial polydomain film of length L0 was heated to 160 ◦ C (just above Tv ) in a heating mantle and annealed for 3 min. Uniaxial stress was then applied either by a home-made stretching device until the strain (creep of the slowly transesterifying network) reached 75%. Then the film was cooled down to room temperature while still stretched. After this, the total length of an aligned film becomes about two times the original length owing to the further spontaneous elongation of the aligned xLCE. The sample was then reheated to 115 ◦ C (T > Ti ) without load to remove any internal mechanical constraints due to the smectic layers: the original length L0 is recovered. However, the thus prepared film ’remembers’ its uniaxial stretching direction and the monodomain xLCE is recovered, spontaneously and reversibly elongating on cooling below Ti to the equilibrium length Lm (about 40% greater than L0 ). When the sample was instead heated to 170 ◦ C (T > Tv ), the memory of alignment is lost and the sample reverts back to its polydomain configuration when subsequently cooled. Moulding of xLCE into complex dome-like shapes was done using a home-made metal punch and die mould. The procedure follows the steps of monodomain preparation: preheating the mould parts and the xLCE film at 160 ◦ C for 3 min, and placing the film on top of the pins of the punch. The sample was covered with the die part, and gently pressed so that the pins were inserted into the complementary holes; after 30 s, the whole set was cooled to room temperature. We then took off the die mould and peeled the shaped xLCE sample out of the pin mould. 40
Received 8 June 2013; accepted 15 October 2013; published online 1 December 2013
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Acknowledgements This research was supported by the National Science Foundation of China (nos 21274075 and 51203086) and the National 973 Project (no. 2011CB935700).
Author contributions Y.J. and E.M.T. developed the concept, Y.J. and Y.W. arranged the funding and infrastructure for the project, Z.P., Y.Y. and Y.J. performed the experiments, Q.C. participated in the synthesis of xLCEs, and Y.J., E.M.T. and Y.W. contributed to writing the paper.
Additional information Supplementary information is available in the online version of the paper. Reprints and permissions information is available online at www.nature.com/reprints. Correspondence and requests for materials should be addressed to E.M.T. or Y.J.
Competing financial interests The authors declare no competing financial interests.
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