Programmable Complex Shape Changing of Polysiloxane Main-Chain Liquid-Crystalline Elastomers

1. Introduction

Liquid crystalline elastomers (LCEs), a class of outstanding stimuli-responsive materials, are liquid crystal (LC) moieties incorporated polymers with lightly crosslinked networks, in which the LC moieties with predesigned orientations are covalently bonded on side of polymer backbones (side-chain LCEs) or directly linked into to the polymer backbones (main-chain LCEs) [1,2,3,4,5,6]. The synergy of self-organization property of LC systems and polymeric network elasticity endows the LCEs with the function of behaving reversible, large and anisotropic dimensional change upon the application of external energy stimuli [1,2,3,4,5,6], and thus makes them to be appealing candidate in diverse application fields including soft actuators [7,8,9,10,11,12,13,14,15,16], soft robotics [17,18,19,20,21,22,23,24], artificial muscles [25,26,27,28,29,30,31,32], and biomedical devices [33,34,35,36,37,38,39,40,41,42], etc.

The shape changes of LCEs are governed by a coupling between the LC orientations and anisotropic polymer networks, and rely on a reversible nematic-isotropic transition associated with LC moieties. By being heated above the nematic-isotropic transition temperature (T_ni), the alignment order of LC moieties is disrupted, and the LCE transfers from nematic LC phase into isotropic state and drives the macroscopic shape change. Upon the cooling below T_ni, the alignment order and LC phase are recovered, and the LCE reverts to its initial shape [1]. The spatial distribution of LC molecule alignment is responsible for the shape-shifting behavior of LCEs. Uniaxial alignment can only lead to simple shape morphing modes such as in-plane contraction/elongation or bending/unbending [1,2,3] which cannot meet the development requirement towards versatile 3D shape morphing modes.

More diverse shape morphings in LCE materials can be achieved by imprinting complex LC orientations into LCE networks. A variety of in situ LC molecule alignment techniques, such as surface treatment [43,44,45,46,47], microfluidic process [48,49,50], photoalignment [6,51,52,53], and the use of magnetic field [54,55,56] or electric field [1], have been employed to achieve the spatial control of LC alignment which can bring about complex shape morphings. Recently, direct ink writing 3D printing has been introduced to fabricate LCEs with programmed spatial LC orientations induced by shear stress during filament-extrusion, and the resulting 3D shape changing performances [8,57,58,59,60,61]. These strategies have their respective benefits, but often face the problems of complication and application limitation, such as tedious orientation technology, monotonous geometries and sophisticated set-ups, etc, for reason that they generally require determining the LC orientation profiles for producing the desired shape changes. In recent years, dynamic covalent bonds have been introduced into LCEs to prepare covalent adaptable LCE networks which enable the programmability of LC orientations. When a LCE is subjected to mechanical load, an applied stimulus, such as heating or UV irradiation, can trigger an exchange reaction in which the covalent bonds undergo cleavage and rearrangement. After removal of the stimulus, the LCE retains the mechanically programmed alignments of LC moieties in the network [17,62,63,64,65,66,67,68]. The approach of incorporating dynamic covalent bonds into LCEs has shown to be robust and facile for the fabrication of LCE materials with complex shapes and programmable shape morphing behaviors. However, external stimuli and catalysts or initiators are often needed to drive the bond exchange reaction, and thus the materials’ performances may be prone to deteriorate after they have been used repeatedly.

Historically, Finkelmann et al, first developed mechanical alignment to prepare uniaxially aligned monodomain LCEs [69]. Their way was based on two-step crosslinking combining with a mechanical alignment, and has been commonly adopted in the preparation of many uniaxially aligned LCE materials. In our previous work, we first achieved direct programmable complex 2D shape changing performance in a classical side-chain polyhydrosiloxane LCE by using this way [70]. The reason for such performance should be due to the two-way memory between the first network structure and the second network structure formed in the two-step crosslinking process. The two-way network memory induced two-way shape memory. During the thermal-triggered phase transition between LC phase and isotropic state, the LCE could reversibly change between the initial shape formed in the first crosslinking step and any another shape programmed by mechanical process and fixed by the second crosslinking step [70]. This demonstrated performance was like shape memory polymers (SMPs), whose stimuli-responsive shape changes can be readily achieved by utilizing the “mechanical programming processes” [71]. A temporary shape can be directly imprinted in a SMP under an external stress and elevated temperature, and remained after quenching. Upon heating, the SMP recovers to its initial shape and the programmed shape transformation is completed. From then on, researchers have achieved “SMP-liked” programmable complex shape changes in some other types of LCEs, such as thiol-acrylate main-chain LCEs and alkylamine-acrylate main-chain LCEs, etc, by using the methods based on two-step crosslinking combining with a mechanical alignment [21,72,73,74]. In their methods, photo-crosslinking was employed as the second step crosslinking to lock the LC orientations in LCE matrices and the mechanically programmed shapes in order to impart reversible and programmable shape morphing behavior. However, light-initiated polymerization reactions may be not very convenient in constructing complex 3D structures due to the shallow penetration depth and self-shadowing effect of the lights.

In this work, we report an achievement of direct programming complex reversible shape morphings in polysiloxane main-chain LCEs by using an approach based on two-step crosslinking combining with a mechanical alignment. Main-chain LCEs can exhibit much higher mechanical and strain actuation properties compared to side-chain LCEs due to direct coupling of the LC moieties to polymer backbone chains [1], and thus provided more sophisticated 3D programmable shape morphing behaviors compared to our first side-chain LCEs [70]. In the first step crosslinking, a partially crosslinked polydomain LCE was generated through the hydrosilylation polyaddition and crosslinking reaction driven by ultrasonics sonochemistry process [75]. Then via stretching, pressing, stamping or embossing, it was mechanically programmed to be another 1D, 2D or 3D-shaped LCE. Lastly, the LC alignment distribution and the programmed shape of the material was locked by the second step thermal crosslinking. Reversible transformation of the LCEs between the initial shapes and the programmed shapes were successfully demonstrated when exposed to heat, resulted in direct mechanically programmed shape morphings. This facile and versatile strategy of realizing programmable shape morphing performance utilized the property of two-way network memory, necessitates no foresight of the required profile of LC orientation and is not limited to the fabricated shapes of LCE materials, and thus can be potential to facilitate the design and application of LCE materials in sophisticated smart devices and multi-functional structures where arbitrary and easily programmed shape morphings are needed.

2. Experiment Section

2.1. Material Preparations

The mesogens (LC units), which were “4-(but-3-en-1-yloxy)phenyl 4-(but-3-en-1-yloxy)benzoate (LC44)” and “4-(hex-5-en-1-yloxy)phenyl 4-(hex-5-en-1-yloxy)benzoate (LC66)”, were synthesized as our previous reported work [76]. The chain extender, cyclic siloxane crosslinker and commercial platinum catalyst, which were respectively “1, 1, 3, 3-teramethydisiloxane (TMDSO)”, “2, 4, 6, 8, 10-pentamethyl-1, 3, 5, 7, 9, 2, 4, 6, 8, 10-pentaoxapentasilecane (PMPOPS)”, and “dichloro (1,5-cyclooctadiene) platinum (II) (Pt(COD)Cl₂)”, were purchased from Aldrich (St Louis, USA). The catalyst solution was prepared as our previous reported work [75].

The precursor reactant solutions were prepared by dissolving mesogenic monomer “LC44” and “LC66”, chain extender “TMDSO” and crosslinker “PMPOPS” in toluene solvent. Table 1 lists the used reactant amounts, the volumes of toluene solvent and catalyst solution for the preparation of polysiloxane main-chain LCEs with 1D, 2D or 3D programmed shapes. The preparation processes of the LCEs are described in “S-I”, “S-II” and “S-III” of the ESI†, and illustrated in Scheme S1 to Scheme S6 of the ESI†.

2.2. Characterization Methods

A polarizing optical microscope (POM, SMZ 1500, Nikon Instruments Co.) was used to determine the mesogenic alignment distributions in LCE matrices by measuring the transmittance of a probe light through a couple of crossed polarizer and analyzer with a LCE sample being set between them.

3. Results and Discussion

The network of polysiloxane main-chain LCE was synthesized via a sol-gel processed hydrosilation [1,75], as illustrated in Scheme 1. The precursor reactants included the monomers of mesogen “LC44” and “LC66”, chain extender “EDDET” and crosslinker “PMPOPS”. The terminal vinyl bonds of mesogenic molecules reacted with the Si-H bonds of chain extender molecules and crosslinker molecules under the action of platinic acid catalyst for hydrosilation polyaddition polymerization and crosslinking reaction to generate the LCE network. In the first step crosslinking, a partly crosslinked polydomain network, in which the mesogens were randomly oriented on average [1], was generated. Then an external force was applied to reshape it and establish a spatial alignment distribution of mesogens. After the second step crosslinking, the network topology with the mesogen alignments and the mechanically programmed material shape were permanently fixed. The preparation procedures of our LCEs with 1D, 2D or 3D programmed shapes were based on above principle, as described in Experiment Section, and “S-I”, “S-II” and “S-III” of the ESI†.

The two fabricated LCEs with 1D programmed shapes were in rectangular strip and round rod shape respectively, as expounded in “S-I” of the ESI†. Figure 1a shows a POM observation of the strip-shaped LCE. The LCE exhibited brightest transmittance when its stretching direction was at an angle of ±45° with the polarizer or analyzer (Figure 1(a-I, a-IV)), while extinctive transmittance was found when its stretching direction was parallel or perpendicular to the polarizer or analyzer (Figure 1(a-II, a-III)), thus revealed that the mesogens were axially aligned along its stretching direction [1], as illustrated by Figure 1b. The thermal-actuation behavior of strip-shaped LCE was examined on a program-controlled hot stage, as shown in the photo-images of Figure 1c. Upon heating to 80 ℃, it contracted along the length direction, which was the alignment direction, and expanded along the width direction. Upon cooling to room temperature, it returned to the initial length and width. This reversible thermal-induced deformation was caused by the reversible phase transition between the alignment structure and isotropic state upon the heating/cooling process [1,2,3,4,5,6]. When the LCE network transitioned from LC phase to isotropic state, it contracted along the alignment direction and expanded along the directions which were perpendicular to the alignment [1]. We found that the resulted dimensions in length and width of the rectangular strip-shaped LCE, under being heated above the T_ni ( about 62 °C [75] ), were basically consistent with those of the partly crosslinked polydomain LCE with rectangular strip shape after the first step crosslinking, as expounded in “S-I” and illustrated in Scheme S1 of the ESI†. This revealed that the LCE demonstrated two-way shape memory between the initial shape formed after the first step crosslinking and the mechanically programmed shape which was fixed by the second step crosslinking. This two-way shape memory should be induced by the two-way network memory between the first network structure and the second network structure generated in the two-step crosslinking process [70].

A transversely cut slice of the rod-shaped LCE was used for POM measurement. As illustrated in Figure 2(a-I), the coordinate locations marked as “1” to “9” were selected to be observed by POM. Coordinate locations “1” to “8” were near to the circumference edge, while coordinate location “9” was in center area. Coordinate locations “1” and “5” together located in a line which was parallel to the analyzer. Coordinate locations “3” and “7” together located in a line which was parallel to the polarizer. The line connecting with coordinate locations “2” and “6”, and the line connecting with coordinate locations “4” and “8” were all at ±45° angle relative to the analyzer or polarizer. The POM images are shown in Figure 2(a-II), the coordinate locations “1”, “3”, “5”, “7” and “9” exhibited dark transmittance, while the coordinate locations “2”, “4”, “6” and “8” exhibited bright transmittance. The transmittance effects of these coordinate locations were constant after the cut slice was rotated by any angles. POM measurement evidenced that in the rod-shaped LCE, the mesogens were aligned along the radial directions in the area near to the circumference edge, but the center area was in disordered polydomain structure, as illustrated by Figure 2(a-III). The photo-images shown in Figure 2b exhibited the thermal-actuation behavior of rod-shaped LCE on a program-controlled hot stage. Upon heating to 80 ℃, it elongated along the axial direction, and became a little thinner along the radial direction. Upon cooling to room temperature, it returned to initial axial and radial dimensions. This reversible shape changing mode upon the heating/cooling process also resulted from the alignment distribution. We found that the resulting dimensions in axial height and radial diameter of the rod-shaped LCE, under being heated to 80 ℃, were also basically consistent with those of the partly crosslinked polydomain LCE with rod shape after the first step crosslinking, as expounded in “S-I” and illustrated in Scheme S2 of the ESI†. This revealed that the rod-shaped LCE also demonstrated two-way shape memory between the initial shape formed after the first step crosslinking and the mechanically programmed shape which was fixed by the second step crosslinking.

In order to qualify the two-way shape memory ability of our polysiloxane main-chain LCE materials, several parameters of actuation fixity for the 1D programmed LCEs were defined through leveraging the definition of actuation fixity parameters of SMP [71]. The defined actuation fixity parameters are cure fixity, “f_cure”, heating deformation fixity, “f_heat”, and cooling deformation fixity, “f_cool”, as expounded in in “S-IV” of the ESI†. The measured actuation fixities of the strip-shaped LCE and rod-shaped LCE are listed in Table 2. It was indicated by Table 2 that actuation fixities were superior. Moreover, the measured values of “f_heat” and “f_cool” of 1D programmed LCEs were basically constant after several tens of repeated heating/cooling actuation. Thus, our polysiloxane main-chain LCE materials should be capable of effectively performing programmable complex shape morphings.

The three fabricated LCEs with 2D programmed shapes were in regular triangle shape, four-pointed star and six-pointed star respectively, as expounded in “S-II” of the ESI†. The partly crosslinked polydomain LCE with disc shape, which was synthesized in first step crosslinking, was mechanically programmed to be an other shape of regular triangle, four-pointed star or six-pointed star, and then the programmed shape was fixed by the second step crosslinking, as shown in Scheme S3. As illustrated in Figure 3(a-I), for a placed regular triangle-shaped LCE with one sideline being parallel to the analyzer, the coordinate locations marked as “1” to “11” were selected for POM observation. Coordinate location “1” located in the bisector of top angle, which was parallel to the polarizer, and near to the top angle. Coordinate locations “2” to “10” were respectively near to one sideline. Among them, coordinate locations “2”, “3”, “7”, “8”, “9” and “10” were respectively near to one angle, while coordinate locations “4”, “5” and “6” were respectively near to the middle of one sideline. Coordinate location “11” located in center area of the regular triangle. The POM images are shown in Figure 3(a-II), the coordinate locations “1”, “6”, “8”, “10” and “11” exhibited dark transmittance, while the coordinate locations “2”, “3”, “4”, “5”, “7” and “9” exhibited bright transmittance. The transmittance effects of these coordinate locations were periodically consistent by cyclical rotating the sample with an interval angle of 120°. The alignment distribution in regular triangle-shaped LCE evidenced by POM measurement is shown in Figure 3(a-III). In every angle, the alignments were symmetrically distributed with respect to the angular bisector. The alignments in the areas close to the angular bisectors were essentially parallel to them. The alignments in the middle part between any two angles were basically parallel to the sideline connecting the two angles. The center area of regular triangle-shaped LCE was in disordered polydomain structure. As illustrated in Figure 3(b-I), for a placed four-pointed star-shaped LCE with the two diagonals being at ±45° angle relative to the analyzer or polarizer, the coordinate locations marked as “1” to “9” were selected for POM observation. Coordinate locations “1”, “3”, “5” and “7” were respectively near to one star tip. Coordinate locations “2”, “4”, “6” and “8” respectively located in one middle area between two adjacent star-blades. Coordinate location “9” located in center area of the four-pointed star. POM images are presented in Figure 3(b-II), the coordinate locations “1”, “3”, “5” and “7” exhibited bright transmittance, while the coordinate locations “2”, “4”, “6”, “8” and “9” exhibited dark transmittance. The transmittance effects of coordinate locations “1” to “8” were periodically consistent by cyclical rotating the sample with an interval angle of 90°. Coordinate location “9” kept dark transmittance at any rotation angle of the sample. The alignment distribution in four-pointed star-shaped LCE evidenced by POM measurement is shown in Figure 3(b-III). In every star-blade, the alignments were symmetrically distributed with respect to the bisector. The alignments in the areas close to the bisectors were essentially parallel to them. The alignments in the middle area between every two adjacent star-blades were approximately parallel to the line connecting the two blade tips. The center area of four-pointed star-shaped LCE was in disordered polydomain structure. As illustrated in Figure 3(c-I), for a placed six-pointed star-shaped LCE with one diagonal being parallel to the polarizer, the coordinate locations marked as “1” to “13” were selected for POM observation. Coordinate locations “1”, “3”, “5”, “7”, “9” and “11” were respectively near to one star tip. Coordinate locations “2”, “4”, “6”, “8”, “10” and “12” respectively located in one middle area between two adjacent star-blades. Coordinate location “13” located in center area of the six-pointed star. POM images presented in Figure 3(c-II) indicate that the coordinate locations “1”, “4”, “7”, “10” and “13” exhibited dark transmittance, while the coordinate locations “2”, “3”, “5”, “6”, “8”, “9”, “11” and “12” exhibited bright transmittance. The transmittance effects of these coordinate locations were periodically consistent by cyclical rotating the sample with an interval angle of 60°. The alignment distribution in six-pointed star-shaped LCE evidenced by POM measurement is shown in Figure 3(c-III). In every star-blade, the alignments were symmetrically distributed with respect to the bisector. The alignments in the areas close to the bisectors were essentially parallel to them. The alignments in the middle area between every two adjacent star-blades were approximately parallel to the line connecting the two blade tips. The center area of six-pointed star-shaped LCE was in disordered polydomain structure.

The thermal-actuation behaviors of the regular triangle shaped LCE, four-pointed star shaped LCE and six-pointed star shaped LCE were examined on a program-controlled hot stage. As shown in Figure 4, Moive S1, S2 and S3 of the ESI†, upon heating to 80 ℃, the regular triangle shaped LCE, four-pointed star shaped LCE and six-pointed star shaped LCE all changed to be disc shape, whose shape and diameter were consistent with the partly crosslinked polydomain LCEs synthesized in first step crosslinking. Upon cooling to room temperature, they resumed their respective shapes and dimensions. These LCEs with 2D programmed shapes all demonstrated well two-way shape memory performance though their geometries and mesogen alignment distributions were different.

The fabrications of three LCEs with 3D programmed shapes, which were nominated as “3D-S-LCE”, “3D-E-LCE” and “3D-SE-LCE” respectively, are expounded in “S-III” of the ESI†. For the fabrication of “3D-S-LCE”, the partly crosslinked polydomain LCE synthesized in first step crosslinking was in disc shape with a 3D pattern of Heilongjiang university logo on one surface, which was stamped by a badge during the first step crosslinking. The patterned surface was mechanically pressed to be flat. Then the flattened surface was fixed by the second step crosslinking, as shown in Scheme S4. For the fabrication of “3D-E-LCE”, the partly crosslinked polydomain LCE synthesized in first step crosslinking was in disc shape with flat surfaces. A 3D plum blossom was mechanically embossed on its one surface by a plum blossom plastic mould (No.1 plastic mould) and then fixed by the second step crosslinking, as shown in Scheme S5. For the fabrication of “3D-SE-LCE”, the partly crosslinked polydomain LCE synthesized in first step crosslinking had a stamped 3D pattern of Heilongjiang university logo on one surface. The patterned surface was mechanically embossed to be another 3D plum blossom shape by another plum blossom plastic mould (No.2 plastic mould) and then fixed by the second step crosslinking, as shown in Scheme S6. The thermal-actuation behaviors of “3D-S-LCE”, “3D-E-LCE” and “3D-SE-LCE” were examined on a program-controlled hot stage, as shown in Figure 5, Moive S4, S5 and S6 of the ESI†. For the “3D-S-LCE”, the initial stamped 3D pattern of Heilongjiang university logo formed in first step crosslinking re-appeared on its surface when it was heated to 80 ℃. Upon cooling to room temperature, the surface resumed flatness state. For the “3D-E-LCE”, upon heating to 80 ℃, the 3D plum blossom disappeared and the surface became flat. Upon cooling to room temperature, the surface resumed its 3D plum blossom pattern. For the “3D-SE-LCE”, the 3D plum blossom pattern on its surface changed to be the 3D pattern of Heilongjiang university logo formed in first step crosslinking. Upon cooling to room temperature, the surface changed back to 3D plum blossom pattern. The fabricated LCEs with 3D programmed shapes had complicated topological structures and spatial distributions of mesogen alignments which were difficult to be clearly measured. Therefore, it is very hard to foresight their shape changing modes by mean of determining the LC orientation profiles. However, by using the capability of two-way shape memory, reversible shape morphings between arbitrary complicated 3D shapes predetermined by mechanical shaping procedures can be conveniently programmed and achieved.

4. Conclusions

To sum up, we presented the result of direct programming complex reversible shape morphing in polysiloxane main-chain LCEs by using mechanical programming process, which was inspired by the principle of using “mechanical programming process” to achieve programmable shape changing behavior in SMPs. In the approach, polydomain LCEs were first synthesized with distinct geometries, which were not limited to flat shapes, via a sol-gel processed hydrosilation reaction. The programmed arbitrary shaped LCEs were prepared by mechanical programming, such as stretching, pressing, stamping or embossing, the polydomain LCEs and subsequent the second crosslinking step. After second crosslinking to permanently fix the programmed LCE shape and the alignment distribution of mesogens formed by mechanical programming process, the resulting LCEs exhibited reversible shape morphing between the initial and programmed shapes, which were induced by the LC phase transition, upon heating and cooling process. This facile and versatile strategy of realizing programmable shape morphing performance utilized the property of two-way memory between the first network structure and the second network structure generated in the two-step crosslinking process, and thus jumped out of the suffering of meticulous modulating the spatial distribution of mesogen alignments. This strategy is suitable for polysiloxane main-chain LCEs due to their well processability, elasticity, and well reactivity during preparation, which made them to be easily fabricated into different initial shapes, and programmed into various complex shapes. Our work should widen the prospect for the design and application of LCE materials in smart actuators, biomedical devices, soft robotics and multi-functional structures, etc, where arbitrary and easily programmed shape morphings are needed.