SILOXANE-BASED LIQUID CRYSTALLINE ELASTOMERS WITH DYNAMIC COVALENT BONDS

Abstract

The present invention relates to a siloxane-based liquid crystalline elastomer, preferably an exchangeable siloxane-based liquid crystalline elastomer, derived from monomers (A1), (B1) and (C1), wherein (C1) is an acyclic or cyclic vinyl siloxane, and (A1) and (B1) have the following formulae:

##STR00001##

wherein is a mesogen, and
R.sup.x and R.sup.y are independently selected from hydrogen or substituted or unsubstituted C.sub.1-12 alkyl;

##STR00002##

wherein is an organic group.

Claims

1. A siloxane-based liquid crystalline elastomer, preferably an exchangeable siloxane-based liquid crystalline elastomer, derived from monomers (A1), (B1) and (C1), wherein (C1) is an acyclic or cyclic vinyl siloxane, and (A1) and (B1) have the following formulae: ##STR00044## wherein is a mesogen, and R.sup.x and R.sup.Y are independently selected from hydrogen or substituted or unsubstituted C.sub.1-12 alkyl; ##STR00045## wherein is an organic group.

2. (canceled)

3. The siloxane-based liquid crystalline elastomer as claimed in claim 1 having a T.sub.c in the range 30 to 150° C., preferably 30 to 125° C.; and/or a T.sub.v in the range 150 to 300° C., preferably 150 to 280° C.; and/or a T.sub.g in the range −100 to 0° C., preferably −75 to −10° C.; and/or wherein the gap between T.sub.c and T.sub.v is in the range 100 to 350° C., preferably 100 to 300° C.

4-5. (canceled)

6. The siloxane-based liquid crystalline elastomer as claimed in claim 1, wherein monomer (C1) has a formula selected from (C1a) and (C1b): ##STR00046## wherein n is 0 or an integer from 1 to 20; and each R.sup.1, R.sup.2, R.sup.3, R.sup.4, R.sup.5, R.sup.6, R.sup.7, R.sup.8, R.sup.9, R.sup.10 and R.sup.11 are organic groups which may be the same or different, and preferably wherein monomer (C1) is selected from: ##STR00047##

7. (canceled)

8. The siloxane-based liquid crystalline elastomer as claimed in claim 1, wherein in monomer (B1) is an aliphatic or aromatic organic group, said organic group optionally containing at least one heteroatom, and preferably wherein monomer (B1) is HS—(CH.sub.2).sub.2—O—(CH.sub.2).sub.2—O—(CH.sub.2).sub.2—SH.

9. (canceled)

10. The siloxane-based liquid crystalline elastomer as claimed in claim 1, wherein in monomer (A1) is nematic or smectic, and preferably wherein monomer (A1) is selected from: ##STR00048## ##STR00049##

11. (canceled)

12. The siloxane-based liquid crystalline elastomer as claimed claim 1 comprising repeat units of formulae (A), (B), and (Ca) or (Cb): ##STR00050##

13. The siloxane-based liquid crystalline elastomer as claimed claim 1, having a failure strain of 100 to 500%, and/or wherein the actuation stroke after the fifth heating/cooling cycle is within +/−5% of the actuation stroke after the first heating/cooling cycle.

14. (canceled)

15. A composition comprising a siloxane-based liquid crystalline elastomer, preferably an exchangeable siloxane-based liquid crystalline elastomer, and a catalyst, preferably wherein the catalyst is a base, wherein said siloxane-based liquid crystalline elastomer is derived from monomers (A1), (B1) and (C1), wherein (A1) has a formula selected from ##STR00051## wherein is a mesogen, and R.sup.x and R.sup.y are independently selected from hydrogen or substituted or unsubstituted C.sub.1-12 alkyl; (B1) has a formula selected from ##STR00052## wherein is an organic group; and (C1) is an acyclic or cyclic vinyl siloxane or an acyclic or cyclic thiol siloxane.

16-20. (canceled)

21. The composition claimed in claim 15, wherein said siloxane-based liquid crystalline elastomer comprises repeat units of formulae (A), (B), and (Ca), (Cb), (Cc) or (Cd): wherein the repeat unit of formula (A) is ##STR00053## wherein the repeat unit of formula (B) is ##STR00054## wherein the repeat unit of formula (Ca) is ##STR00055## wherein the repeat unit of formula (Cb) is ##STR00056## wherein the repeat unit of formula (Cc) is ##STR00057## and wherein the repeat unit of formula (Cd) is ##STR00058##

22. (canceled)

23. The method of preparing a composition as claimed in claim 15, preferably conducted in one pot, comprising: (i) preparing a mixture comprising monomers of each of formula (A1), (B1), and (C1) as defined in claim 15, and a catalyst; (ii) polymerising the monomers of formula (A1) and (B1) to give an intermediate reaction mixture comprising a thiol-terminated oligomer, an acrylate-terminated oligomer, a vinyl-terminated oligomer, or a silane-terminated oligomer, monomers of formula (C1) and a catalyst; and (iii) photopolymerizing said intermediate reaction mixture to give said composition; or (i) preparing a mixture comprising monomers of each of formula (B1) and (C1) as defined in claim 20, and optionally a catalyst; (ii) photopolymerizing said mixture to give an intermediate reaction mixture comprising a thiol-terminated siloxane, a vinyl-terminated oligomer, or a silane-terminated siloxane, and optionally a catalyst; (iii) adding monomers of formula (A1) as defined in claim 20, and optionally a catalyst to said intermediate reaction mixture; and (iv) polymerising said intermediate reaction mixture to give said composition, wherein catalyst is added in at least step (i) or step (iii).

24-26. (canceled)

27. The method of making a moulded article comprising a composition as claimed in claim 15, comprising: (i) heating the composition to a temperature above the T.sub.v of the siloxane-based liquid crystalline elastomer; (ii) moulding the composition into a desired shape whilst applying a constant tensile stress to give a moulded composition having alignment (e.g. having a required pattern of alignment); and (iii) cooling the moulded composition to room temperature to give said moulded article.

28. The method as claimed in claim 27, wherein the step (ii) moulding is selected from shear extrusion (e.g. 3D printing), uniaxial alignment, surface alignment and injection moulding.

29. The method as claimed in claim 27, wherein said step (ii) moulding involves siloxane bond exchange within the siloxane-based liquid crystalline elastomer.

30. (canceled)

31. A moulded article comprising a composition as claimed in claim 15.

32. (canceled)

33. The composition as claimed in claim 15, wherein said siloxane-based liquid crystalline elastomer is derived from monomers (A1), (B1) and (C1), wherein (C1) is an acyclic or cyclic vinyl siloxane, and (A1) and (B1) have the following formulae: ##STR00059## wherein is a mesogen, and R.sup.x and R.sup.y are independently selected from hydrogen or substituted or unsubstituted C.sub.1-12 alkyl; ##STR00060## wherein is an organic group.

34. The composition as claimed in claim 15, wherein said siloxane-based liquid crystalline elastomer is derived from monomers (A1), (B1) and (C1), wherein (A1) has a formula selected from ##STR00061## wherein is a mesogen; (B1) has a formula selected from ##STR00062## wherein is an organic group; and (C1) is an acyclic or cyclic vinyl siloxane or an acyclic or cyclic thiol siloxane.

35. The composition as claimed in claim 15, wherein said siloxane-based liquid crystalline elastomer comprises repeat units of formulae (A), (B), and (Ca), (Cb), (Cc) or (Cd): wherein the repeat unit of formula (A) is ##STR00063## wherein the repeat unit of formula (B) is ##STR00064## wherein the repeat unit of formula (Ca) is ##STR00065## wherein the repeat unit of formula (Cb) is ##STR00066## wherein the repeat unit of formula (Cc) is ##STR00067## and wherein the repeat unit of formula (Cd) is ##STR00068##

36. The method as claimed in claim 23, comprising: (i) preparing a mixture comprising monomers of each of formula (A1), (B1), and (C1), wherein (C1) is an acyclic or cyclic vinyl siloxane, and a catalyst: ##STR00069## wherein R.sup.x and R.sup.Y are independently selected from hydrogen or substituted or unsubstituted C.sub.1-12 alkyl; (ii) polymerising the monomers of formula (A1) and (B1) to give an intermediate reaction mixture comprising a thiol-terminated oligomer, monomers of formula (C1) and a catalyst; and (iii) photopolymerizing said intermediate reaction mixture to give said composition; or (i) preparing a mixture comprising monomers of each of formula (B1) and (C1), wherein (C1) is an acyclic or cyclic vinyl siloxane, and optionally a catalyst: ##STR00070## wherein is an organic group; (ii) photopolymerizing said mixture to give an intermediate reaction mixture comprising a thiol-terminated siloxane, and optionally a catalyst; (iii) adding monomers of formula (A1) and optionally a catalyst to said intermediate reaction mixture: ##STR00071## wherein , R.sup.x and R.sup.Y are independently selected from hydrogen or substituted or unsubstituted C.sub.1-12 alkyl; and (iv) polymerising said intermediate reaction mixture to give said composition, wherein catalyst is added in at least step (i) or step (iii).

Description

BRIEF DESCRIPTION OF THE FIGURES

[0204] FIG. 1 shows the reaction scheme for the thiol-acryalte/thiol-ene click chemistry used in the examples of this application.

[0205] FIG. 2a shows the general mechanism of siloxane exchange enabled by acid or base catalyst.

[0206] FIG. 2b shows two possible routes of siloxane exchange enabled by acid or base catalyst for the xLCEs of the present invention: the siloxanolate catalyst breaks the ring and terminates the linear 4-functional siloxane crosslink (“ring opening”), or two ring-crosslinks join into a single 8-functional ring, which may later exchange into two different 4-crosslinks due to its flexibility (“ring merging”).

[0207] FIG. 3 shows differential scanning calorimetry (DSC) of xLCE networks of the present invention on heating. xLCE networks with different crosslinking density were tested and FIG. 3 shows the glass-(T.sub.g) and the nematic-isotropic (T.sub.c) transition temperature variation with composition.

[0208] FIG. 4a shows scaled stress-relaxation σ(t)/σ.sub.max for the 40%-crosslinked xLCE at T=190° C., and several concentrations of catalyst.

[0209] FIG. 4b shows stress relaxation curves for the 40%-crosslinked xLCE at 1 wt % of TMA-Si catalyst, and several temperatures. Dashed lines are the fits with exponential function, which produce the relaxation time τ=1/β.

[0210] FIG. 5a shows the Arrhenius plots for the relaxation time τ(T) for different xLCE networks (i.e. the 20%, 40% and 100% crosslinked networks). The slope of the linear fitting gives the bond strength ΔG≈28 kcal/mol, and the additive constant gives the ‘rate of attempts’ ω.sub.0.

[0211] FIG. 5b shows a comparison of the scaled stress relaxation at 200° C. for the 20%, 40% and 100% crosslinked networks.

[0212] FIG. 6a shows how strain changes with temperature in a sample of the 40%-crosslinked xLCE under constant stress.

[0213] FIG. 6b shows the results of programming an aligned monodomain in the 40%-crosslinked xLCE.

[0214] FIG. 6c shows how strain changes with temperature in samples of the 40% crosslinked xLCE under constant tensile stress, where the xLCE has been prepared with different types of catalyst.

[0215] FIG. 7a shows the initial polydomain 40%-crosslinked xLCE (top) and the uniaxially aligned monodomain 40%-crosslinked xLCE, programmed by its plastic flow to 100% elongation (bottom).

[0216] FIG. 7b shows two microscopy images between crossed polars of the uniaxially aligned monodomain 40%-crosslinked xLCE.

[0217] FIG. 7c shows an X-ray image of the uniaxially aligned monodomain 40%-crosslinked xLCE.

[0218] FIG. 8a shows one cycle of heating-cooling (over the range −50° C. to 90° C.) of the uniaxially aligned monodomain 40%-crosslinked xLCE, demonstrating the classical reversible thermal actuation of LCE.

[0219] FIG. 8b shows the cyclic contraction-extension of the uniaxially aligned monodomain 40%-crosslinked xLCE during 11 of the heating cycles shown in FIG. 8a.

[0220] FIG. 8c shows the actuation strain plotted against temperature for the uniaxially aligned monodomain 40%-crosslinked xLCE, showing the reproducibility of actuation and also the extent of thermal hysteresis at the applied heating rate of 3°/min.

[0221] FIG. 9 shows the appearance of a thermally molded continuous strip, which combines three different xLCE materials: the 20%, 40%, and 100% crosslinked material, at various temperatures.

EXAMPLES

Materials

[0222] Diacrylate liquid crystal (LC) monomer, RM82, was purchased from Wilshire Technologies, Inc.

[0223] 2,2′-(Ethylenedioxy)diethanethiol (EDDT), 2,4,6,8-tetramethyl-2,4,6,8-tetravinyl cyclotetrasiloxane (TMTVCTS), triethylamine (TEA), Irgacure I-651, toluene, and tetrahydrofuran were purchased from Sigma-Aldrich.

[0224] Tetramethylammonium siloxanolate (TMA-Si) was purchased from Gelest.

Measurement Methods

[0225] Differential Scanning Calorimetry (DSC) [0226] DSC4000 PerkinElmer was used to obtain the transition temperatures. Samples with ≈10 mg were loaded into standard aluminum DSC pans. The samples were heated to 120° C. at 10° C. min-1, held isothermally for 5 min to undo the thermal history, and cooled to −50° C. at 10° C. min-1. Then samples were heated again to 120° C. to obtain the data. T.sub.g could be found at the step change in the slope of the heat flow signal and T.sub.c could be obtained at local minimum of the endothermic peak. The sample was run three times. [0227] Stress Relaxation Measurements [0228] DMAQ800 (TA instruments) was used to characterize the relaxation behavior of siloxane crosslinked LCE. Samples with dimensions of ≈15 mm×5 mm×0.9 mm were tested. All of the samples were tested under constant uniaxial strain 3% imposed at t=0, the strain was held constant isothermally for 180 min at 170, 180, 190, 200, or 210° C. Prior to imposing the strain, samples were kept at the desired temperature for 5 min. Samples were annealed at 80° C. for 12 h before the relaxation test. [0229] Iso-Force Measurements [0230] DMAQ800 (TA instruments) was used to characterize the plastic flow of siloxane crosslinked LCE induced by siloxane bond exchange as a function of temperature. Samples with dimensions of ≈15 mm×5 mm×0.9 mm were tested. All of the samples were tested under constant uniaxial stress of 14, 35, 65, 96, 0r 146 kPa imposed at t=0, the stress was held constant while the temperature was ramped at 2° C./min until 260° C. Prior to imposing the stress, samples were kept at the desired temperature for 5 min. Samples were annealed at 80° C. for 12 h before the relaxation test. [0231] Programing Monodomain Measurements [0232] DMAQ800 (TA instruments) was used to align polydomain samples into monodomain via creep test. Samples with dimensions of ≈15 mm×5 mm×0.9 mm were tested. All samples were tested under constant uniaxial stress of 50, 100, 150, or 200 kPa imposed at t=0, the stress was held constant isothermally at 250° C. until the strain reached 100%. Prior imposing the stress, samples were kept at the desired temperature for 2 min. After reaching 100% strain the samples were kept starched while cooling to room temperature. Samples were annealed at 80° C. for 12 h before the relaxation test. [0233] Wide Angle x-Ray Scattering (WAXS) [0234] The phase of the monodomain LCE at room temperature was characterized using a Philips diffractometer using a Philips Copper target (PW-2233/20) with the wavelength of 0.154 nm. The beam size was ˜0.7×0.7 mm.sup.2 with flux of 4×10{circumflex over ( )}.sup.9 X-ray/s. The distance between the sample and the imaging area was 100 mm. The sample (0.5 mm×6.5 mm and 20 mm) was exposed to the x-ray source for 20 seconds. [0235] Actuation Measurements [0236] Discovery DMA850 (TA instruments) was used to measure the actuation performance for the monodomain film. Rectangular samples measuring approximately 15 mm×5 mm×0.5 mm were tested in tensile mode. To measure actuation strain, a constant stress (12 kPa) was applied to the LCE film; each sample was heated and cooled at least 11 times from 100 to −50° C., at 3° C. min-1. [0237] Welding Conditions [0238] Moore hydraulic press (Birmingham, England) was used to hot press the LCE samples. Samples were first held at 250° C. for 5 min before applying a load of 0.5 ton. The samples were allowed to cool to room temperature under the applied load.

LCE Network Preparation Method

[0239] LCE networks were prepared using a one pot two-step thiol-acrylate/thiol-ene reaction sequence. First, LC oligomers were prepared via a self-limiting thiol-acrylate Michael addition between a mesogenic diacrylate (RM82) and an isotropic dithiol (EDDT). The Michael addition was catalyzed via TMA-Si or TEA. By controlling the molar ratio of thiol to acrylate, thiol-terminated oligomers were obtained. The di-thiol oligomer was then radically crosslinked with vinyl siloxane crosslinker, TMTVCTS. Reaction progress was monitored by Fourier-transform IR spectroscopy (using a Nicolet 750 Magna FTIR spectrometer with KBr beam splitter and an MCR/A detector) and swelling and gel fraction experiments. The experimental method is outlined below.

[0240] In a 25 ml vial the intended amount of catalyst TMA-Si (0.1, 0.3, 1, or 3 wt %), was initially dissolved in a mixture of solvent (20 wt % THF and 20 wt % toluene), and to this solution RM82 was added and heated to 80° C. until fully dissolved. After the mixture was cooled down to room temperature, I-651 (1.5 wt %), EDDT, and TMTVCTS were added and mixed vigorously using a vortex mixer. The solution of monomers was degassed using a vacuum chamber and then quickly transferred into a mold (two glass sides with 1 mm spacer coated with ran-x, an anti-sticking agent). The monomer mixture was kept at 50° C. to fully oligomerize via Michael addition reaction for 12 h. Then the thiol-terminated oligomer was photopolymerized with TMTVCTS via 365 nm UV light for 15 min at 50° C. The ratio of thiol, acrylate, and vinyl molar functional groups was kept constant in all samples. The molar ratio used was 1.0 diacrylate:1.4 dithiol:0.4 vinyl, unless otherwise noted. After the polymerization was compete, the samples were removed from the mold and placed in a vacuum oven at 80° C. for 12 h to remove the solvents.

[0241] LCE networks having different crosslinking densities were also be prepared using the above method, but by varying the molar ratio of the reactants. As outlined in Table 1 below, the material compositions of the LCE networks prepared were characterized by the mol fraction of reacting bonds, thiol-acrylate and thiol-vinyl, always taking the content of mesogenic di-acrylate RM82 monomer as 100% (or 1 molar ratio). As such, the lowest crosslinking density network prepared, labelled as “20% crosslinked”, has 20% (or 0.2 molar ratio) of vinyl bonds on 4-functional ring-siloxane crosslinks, and accordingly, the stoichiometric amount of 120% (or 1.2 molar ratio) of thiols on the di-functional chain extender EDDT (see Table 1). At the opposite end, the highest crosslinked network prepared, labelled as “100% crosslinked”, has 100% vinyl bonds (1:1 with acrylate bonds of the mesogens), and accordingly 200% (or 2 molar ratio) of thiols. For instance, according to this nomenclature, the “100% crosslinked” network has exactly two RM82 mesogens per crosslink, that is, on average network strands contain just one RM82 rod between two thiols. In the same way, the “20% network” has its strands, on average, with 5 RM82 rods separated by thiol spacers.

TABLE-US-00001 TABLE 1 Network Mass of Mass of Mass of description RM82 EDDT TMTVCTS 20% crosslinked 1 0.3341 0.0511 (20% TMTVCTS) 40% crosslinked 1 0.3898 0.1023 (40% TMTVCTS) 60% crosslinked 1 0.4454 0.1534 (60% TMTVCTS) 80% crosslinked 1 0.5011 0.2045 (80% TMTVCTS) 100% crosslinked 1 0.5568 0.2557 (100% TMTVCTS)

Example 1

[0242] The DSC results of the series of materials outlined above is shown in FIG. 3 (i.e. the 20%, 40%, 60%, 80% and 100% crosslinked materials). The glass transition (T.sub.g) is around −30° C. with very little change observed even when the crosslinking density is significantly increased. This is thought to be attributed to flexibility of the siloxane crosslinker and reduction of the rigid mesogenic units. On the other hand, it can be seen that the reduction of these mesogenic units reduces the nematic-isotropic transition (T.sub.c). It is noted that even the “100% crosslinked” LCE has a broad range of the liquid-crystalline phase below T.sub.c˜32° C.

Example 2

[0243] FIG. 4 shows the results of a typical stress-relaxation in the xLCE, which takes place after an instant fixed-strain is imposed on the sample (maintaining the constant temperature). The results are presented via a scaled relaxation function σ(t)/σ.sub.max, in order to focus purely on the time dependence.

[0244] The normalized stress as a function of time for 40% TMTVCTS samples containing various TEA and TMA-Si concentrations is shown in FIG. 4a. In this example, which is conducted at T=190° C., networks with 3 wt % of TMA-Si catalyst, and with a total of 1 wt % of a catalyst mixture of TMA-Si and TEA in ratios 1:0, 0.3:0.7, 0.1:0.9 and 0:1, respectively, are compared. The slowest relaxation is seen in the 1% TEA sample (labelled as 0% TMA-Si in the plot), however, an increasing fraction of TMA-Si makes the bond exchange faster. Both of these amines can trigger the relaxation of the siloxane elastomer, however, TEA is a more volatile catalyst at elevated temperature. Therefore, it has a slower stress relaxation compared to TMA-Si.

[0245] The fitting of such scaled stress relaxation curves with the basic exponential relaxation for 1% TMA-Si is shown in FIG. 4b. This exercise provides the characteristic relaxation time τ for each material and temperature. As expected, increasing the temperature accelerated the relaxation, and at 210° C. the elastomer was found to be fully relaxed after 7000 s due to its internal plastic flow.

Example 3

[0246] To study the influence of the siloxane concentration on the stress relaxation, siloxane crosslinked networks containing various siloxane concentrations (e.g. 20, 40, and 100 functional mol %) were tested, with each network having the same amount of catalyst (1 wt % of TMA-Si). The relaxation time data for various samples were then collated at different temperatures to generate the Arrhenius plot shown in FIG. 5 (i.e. τ(T) is plotted on the logarithmic scale, and the data is then fitted with the activation law ln[τ]=const+ΔG/k.sub.BT).

[0247] Referring to FIG. 5a, the data shows a single value of activation energy ΔG≈28 kcal/mol (or 116 kJ/mol), which corresponds to about 45 k.sub.BT at room temperature, and is in good agreement with the results of Xie et al. (Adv. Mater. 2019, 31 (11), 1807326) who used 0.1 wt % of sodium octanoate as catalyst in a much higher siloxane concentration elastomer (Sylgard 184 PDMS). In comparison, in the work of Leibler et al. (Science (80-.). 2011, 334 (6058), 965-968), the transesterification with the zinc acetate catalyst had an activation energy ΔG≈20 kcal/mol (or 34 k.sub.BT). It is expected that the single value of activation energy ΔG describes the macroscopic stress relaxation: this is a clear signature of the distinct reaction, in this case depicted in FIG. 2b.

[0248] Surprisingly, siloxane elastomers with very different concentration of crosslinker appear to have the same ‘rate of attempts’ wo in their relaxation behavior. This was confirmed by comparing the relaxation curves for these different networks at the same temperature (see FIG. 5b). Without wishing to be bound by theory, it is thought that this means the first exchange route depicted in FIG. 2b above (i.e. the ring opening mechanism) is the dominant process or perhaps even the only possible route for the bond exchange. This is because a relatively large amount of catalyst is used in this system (˜1 wt %) and the catalyst helps terminate the rings after their opening. Furthermore, it is thought that the contact of two siloxane-ring crosslinkers in the stretched network has a low probability, while the mobile TMA-Si catalyst can reach any location in the network. As the catalyst content was the same in the data shown in FIG. 5, so are the relaxation rates.

Example 4

[0249] FIG. 6 shows the results of the dynamic response of the 40% crosslinked xLCE prepared as above, due to the siloxane exchange reaction allowing plastic flow under stress, at a sufficiently high temperature. Referring to FIG. 6a, this test shows how the strain changes with temperature in the sample under constant tensile stress (NB: such a test is often incorrectly called “dilatometry” in the literature). In an LCE material, the effect of LCE thermal actuation produces a massive strain change on heating into the isotropic phase. This example is focused on the elastic-plastic transition of the exchangeable network, and so the starting temperature was set at 100° C. (i.e. well in the isotropic phase for the 40% crosslinked xLCE). A given stress (as labelled in FIG. 6a) was applied to the material, and the resulting extensional strain was then registered, which gives the value of the Young modulus of the material (E≈ 880 kPa). The temperature was then increased at a constant rate of 2°/min and the extensional strain monitored.

[0250] The results show that the classical rubber-elastic response is initially observed: as the (entropic) rubber modulus increases with temperature, at constant stress the strain decreases. However, as the temperature increases further, and the bond-exchange becomes more prominent, the plastic flow (creep) starts being noticeable. The region where the data deviates from the initial rubber-elastic decreasing slope is identified as the transition to plastic flow, the vitrification point T.sub.v: apparently it does not depend on the applied stress. Some creep under stress in a network with siloxane-exchange above 140-150° C. is to be expected, although the rapid flow only sets in at a much higher temperature (over 250° C.).

[0251] FIG. 6c shows how the strain changes with temperature in the 40% crosslinked xLCE sample under constant tensile stress where the xLCE has been prepared with different types of catalyst (either 1 wt % 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), 1 wt % sodium octanoate (Na.sup.+), or 1 wt % TMA-Si). The results show that the choice of base can influence the temperature at which rapid flow sets in, and therefore also the temperature gap between T.sub.c and T.sub.v.

Example 5

[0252] The regime of stress-induced plastic flow demonstrated in Example 4 can be used to program the xLCE materials into a monodomain aligned state. Referring to FIG. 6b, the sample of 40% crosslinked xLCE prepared as above is brought to a high temperature (T=250° C.) as suggested by the results of iso-stress test of Example 4. A constant tensile stress is then applied to a level labelled in FIG. 6b, and the sample is kept at this constant temperature and stress until its elongation reaches 100%. As can be clearly seen, this process happens faster at higher stress, but in all cases takes several minutes and allows easy control. The programmed sample is then removed from the stress and heating conditions.

[0253] 100% elongation of the sample is deemed sufficient to impart the fully uniaxial monodomain alignment to the xLCE, as confirmed by FIG. 7. FIG. 7a illustrates the uniaxially aligned monodomain sample and compares it with the initial polydomain xLCE. FIGS. 7b and 7c confirm this uniaxial alignment: FIG. 7b shows a pair of microscopy images between crossed polars, whilst FIG. 7c shows an X-ray image (nematic order parameter Q=0.62).

[0254] The programmed alignment is permanent as long the sample temperature is not allowed to raise above 140° C. (see FIG. 6a), when the residual creep would cause a gradual loss of alignment (which increases at even higher temperatures). However, it is possible to re-program the material to a different shape and state of alignment by a subsequent process.

Example 6

[0255] Having programmed the uniaxial monodomain alignment in the 40% crosslinked xLCE, its actuation response to reversible heating and cooling through the nematic-isotropic transition was examined. FIG. 8 illustrates different elements of this test, carried out in the DMA instrument under a low constant stress (of 12 kPa) to ensure the sample is straight and taut.

[0256] FIG. 8a focusses on one cycle of heating and cooling, over the range of −50° C. to 90° C. (T.sub.g≈−20° C. and T.sub.c≈60° C. for the 40% crosslinked xLCE). The sample starts rapid contraction when the temperatures approaches 30° C., and reaches the saturation strain of over 40% at around 70° C. (both values are clearly affected by the dynamics of temperature change). On cooling the cycle reverses. No creep of thermal degradation was expected to occur in the xLCE materials as the temperature never reached the levels where plastic creep might set in.

[0257] FIG. 8b illustrates the remarkable stability of this spontaneous contraction-expansion over 11 cycles of temperature. The same 11 cycles of heating and cooling are shown in FIG. 8c as actuation strain against temperature: all heating and all cooling strokes are on top of each other, however, a clear hysteresis of the nematic-isotropic transition can also be seen. To support this observation, FIG. 8c also shows the DSC scans (scaled, in a.u.) on heating and cooling, at the top of the plot, to illustrate where the glass and nematic transitions are in each direction.

[0258] The wide separation of the nematic transition and the vitrification temperature, at which the plastic creep starts to occur in the xLCE under stress is the reason for stability of the thermal actuation, and the programmed alignment pattern.

Example 7

[0259] The thermal molding of the xLCEs of the present invention were then demonstrated. Three different xLCE materials (with 20%, 40%, and 100% crosslinking density) were prepared as above into separate strips. The three strips were then molded together into one continuous sample by bringing the separate parts together at the required junctions and subjecting the assembly to high temperature (T=250° C.) and high pressure overnight. The remarkable thermal stability of the thiol-siloxane mesogenic system is noteworthy; few polymers will withstand several hours at 250° C. without any degradation. FIG. 9 illustrates the result of the molding, where it is impossible to distinguish the initial overlap regions in the molded sample (highlighted by circles). Referring to FIG. 9, at room temperature (22° C.) all three sections are in the polydomain nematic state, and so appeared white (i.e. they strongly scatter light). Then, on heating the strip, the sequential phase transitions into the isotropic phase were observed in different sections of the otherwise continuous polymer strip: first the 100% crosslinked section becomes isotropic (i.e. it appears transparent, therefore no longer scatters light), then the 40% section, and finally the 20% section so that the whole strip becomes isotropic by the time the temperature has been raised to 75° C.

[0260] This example demonstrates the capacity to mold together different xLCE materials containing exchangeable siloxane bonds and the appropriate catalyst. As such, the xLCEs of the present invention offer rich design options for complicated actuating shapes and constructions for practical applications.