Wavelength-selective photo-polymerization of shape memory liquid-crystal elastomer

Abstract

The present application relates to a process of preparing a shape-memory crosslinked liquid crystal elastomers film, the process comprising the steps of pre-crosslinking a liquid crystal oligomer mixture comprising a liquid crystal oligomer, a crosslinker, and initiators under a first stimulus; deforming the pre-crosslinked oligomer mixture; and crosslinking the deformed oligomer mixture under a second stimulus. The present application also relates to a liquid crystal oligomer mixture to make a shape-memory crosslinked liquid crystal elastomers film.

Description

PATENTAtorney Docket No: TUOE.P2007WQ / 00652565WAVELENGTH-SELECTIVE PHOTO-POLYMERIZATION OF SHAPE MEMORY LIQUID-CRYSTAL ELASTOMERFIELD

[0001] The present application relates to wavelength- selective photo-polymerization of shape-memory crosslinked liquid-crystal elastomer film.BACKGROUND

[0002] Liquid crystal elastomers (LCEs) are a class of smart soft active polymers that have a crosslinked elastomeric network, in which self-organizing mesogenic groups are incorporated. When exposed to an external stimulus, the combination of LCEs’ properties of entropy elasticity and liquid-crystalline (LC) order results in outstanding mechanical and optical reversible actuation. As such, LCEs have been used in various fields such as mechanical actuators and sensors, dynamic surfaces, soft-robotics, haptics, tissue engineering, and micro-electromechanical systems (MEMS), among others.

[0003] Various techniques can be used to fabricate LCEs with shape memory, such as the multi-step processes for permanent LC monodomain programming of the LCE film.

[0004] Yakacki et al. have developed a two-stage thiol-acrylate Michael addition and photopolymerization (TAMAP) reaction. TAMAP uses a thiol-acrylate “click” reaction to partially crosslink the LC polydomain into a gel-like state, which is then aligned into a monodomain. Followed by a photo-polymerization reaction based on the excess acrylate groups remaining unreacted in the first step. This second crosslinking step stabilizes the monodomain.

[0005] The limitation in this method is that thiol-acrylate “click” reaction uses a base catalyst, which initiates the reaction as soon as it is introduced to the solution. This is a major disadvantage of this procedure, heavily limiting the time for maneuvering of the solution before initial crosslinking. Thus, making this method unsuitable for large-scale production and restricting its applicability for multiple coating techniques. Moreover, if the reaction rate exceeds the solvent evaporation rate, another issue might emerge, as the trapping of solvent could lead to irregularities in the LCE film.

[0006] WO2023219504 similarly discloses the production of a shape memory polymer for haptic feedback purposes using a 2-step crosslinking methodology based on a Michael addition step followed by a free-radical polymerization step.

[0007] El-Ghayoury et al. disclose the use of wavelength selective photopolymerization of acrylates and oxetanes to produce double crosslinked networks. However, no liquidPATENTAtorney Docket No: TUOE.P2007WQ / 00652565 crystalline moieties were introduced in these systems to provide shape programming properties. Journal of Polymer Science, Part A: Polymer Chemistry, 41(4), 469-475.

[0008] Hoekstra et al. disclose a wavelength selective photopolymerization method to create interdigitated liquid crystalline networks, where a difunctional acrylate and a difunctional oxetane reactive mesogen were polymerized to create two interlocking networks. However, these networks were not covalently bonded to each other and were therefore not suitable for two-way shape memory actuation. Angew. Chem. Int. Ed. 2021, 60, 10935-10941.

[0009] Thus, improvement is needed to create crosslinked liquid crystal elastomer films with shape-memory behavior that can overcome the current challenges in creating such films.SUMMARY

[0010] The present application relates to a process of preparing a shape-memory crosslinked liquid crystal elastomers film, the process comprising the steps of pre-crosslinking a liquid crystal oligomer mixture comprising a liquid crystal oligomer, a crosslinker, and initiators under a first stimulus; deforming the pre-crosslinked oligomer mixture; and crosslinking the deformed oligomer mixture under a second stimulus. The present application also relates to a liquid crystal oligomer mixture comprising a liquid crystal oligomer, a crosslinker, and initiators having the chemical structure, wherein the different functional groups of the crosslinker react separately when different stimuli are applied to the liquid crystal oligomer mixture.BRIEF DESCRIPTION OF THE FIGURES

[0011] FIG. l is a schematic representation of the steps involved in fabricating a shapememory crosslinked liquid crystal elastomer film.

[0012] FIGS. 2 A and B (A) chemicals used for this method’s reaction mixture; (B) absorption spectra of the two photo-initiators.

[0013] FIG. 3 is a FTIR measurements of the liquid crystal oligomer mixture before crosslinking, after the pre-crosslinking step, and after the crosslinking step.

[0014] FIG. 4 shows the actuation strain over temperature of two shape-memory crosslinked liquid crystal film based on different reactive mesogen that were programmed by stretching.

[0015] FIGS. 5 A, B, and C: (A) Radial alignment of a dot sample observed via crosspolymerised optical microscopy (POM). (B) Main view interferometry scan of the surface of a dots sample. (C) Depth as a function of length plot showing actuation of a dots sample.PATENTAttorney Docket No: TUOE.P2007WQ / 00652565

[0016] FIGS. 6 A, B, C, D: Planar alignment of a stretched sample observed by POM: in (A) the sample makes a 45° angle relative to the cross polarizers orientation, and in (B) the sample is aligned (0°) with them. Actuation in the form of shrinking of the LCE film when heated, while also lifting a weight (i.e, clip). (C) Picture taken at room temperature (RT). (D) Picture taken at ~60 °C, where a clear decrease in the length of the polymer film is visible.DETAILED DESCRIPTION OF THE EMBODIMENTS

[0017] As mentioned above, various challenges exist in creating a dual-crosslinking liquid crystal elastomer film with shape-memory behaviors. The challenges lies in various aspects, such as selecting the proper precursors, choosing the suitable reaction chemistry, limiting or eliminating the use of solvents, reducing the reaction steps, and scaling up the production.

[0018] The LCE fabrication method described herein is distinguished for its simplicity, controllability, precision, lack of time-dependency, scalability, and remarkable capability of influencing LC behavior. These features make it a promising and superior choice for synthesizing and programming LCEs compared to predecessor techniques.

[0019] One aspect of the described method herein is a two-step wavelength-selective photopolymerization reaction (TWSP), which introduces a process for programming reversible LCEs with shape memory. Contrary to the existing method that uses reactions such as Micheal addition, the crosslinking steps in the described method are photo reactions. Photo reactions allow for far greater spatial-temporal control over the crosslinking steps. Thus, making it a promising and superior choice for synthesizing and programming LCEs compared to predecessor techniques. The high controllability of both crosslinking steps allows for the creation of more intricate and even localised three-dimensional (3D) patterns and shapes, making this invention promising for both dynamic surface feedback and soft robotics.

[0020] Another aspect of the present application is the use of liquid crystals in the network. Liquid crystal elastomers are a class of smart polymers consisting of an elastomeric network in which self-organizing mesogenic groups are incorporated. When exposed to an external stimulus, the combination of its properties of entropy elasticity and liquid-crystalline order results in outstanding mechanical and optical reversible actuation.

[0021] These external stimuli include, but are not limited to, temperature, light, electric field, magnetic field, and mechanical strain / stress.

[0022] When the external stimulus is temperature, the LCEs can be considered as thermal- responsive. For these LCEs, there is an important temperature called nematic-isotropicPATENTAttorney Docket No: TUOE.P2007WO / 00652565 transition temperature. This is the temperature at which a liquid crystal transitions from the nematic phase, where the molecules are oriented in a parallel but not fixed position, to the isotropic phase, where the molecules are randomly oriented and behave like a conventional liquid.

[0023] The mechanism behind the thermal-responsive LCEs with reversible shape memory properties is as follows. First, the LC polydomain is pre-crosslinked through a free-radical photo-polymerization under a first wavelength light irradiation in the isotropic phase. Next, the pre-LCE film, in the nematic phase, is uniformly aligned (e.g., stress-based deformation), i.e., the mesogens are re-oriented along a selected direction (director). Then, the film, still in the nematic phase, is illuminated with ultraviolet (UV) light (< 400 nm), which photoinitiates a cationic ring-opening polymerization of oxetanes or epoxides to crosslink the film. In the end, a permanently programmed monodomain LCE film capable of large and reversible “hands-free” actuation under thermal stimulus is obtained.Embodiments

[0024] In some embodiments, the process of preparing a shape-memory crosslinked liquid crystal elastomers film comprising the steps of pre-crosslinking a liquid crystal oligomer mixture comprising a liquid crystal oligomer, a crosslinker, and initiators under a first stimulus; deforming the pre-crosslinked oligomer mixture; and crosslinking the deformed oligomer mixture under a second stimulus.

[0025] In some embodiments, the liquid crystal oligomer is a diacrylate mesogen covalently linked by chain extenders, such as dithiol chain extenders. In some embodiments, the number of mesogens is greater than the number of chain extenders.

[0026] In some embodiments, the ratio between the total number of mesogens and the total number of chain extenders is greater than 1.01 : 1, e.g. from 1.01 : 1 to 1.05:1, 1.05: 1 to 1.10: 1, 1.10: 1 to 1.15: 1, and 1.15: 1 to 1.20: 1.

[0027] In some embodiments, the mesogen is a diacrylate mesogen, e.g., diacrylate 4-(3- acryloyloxyhexyloxy)-benzoic acid 2-methyl-l,4-phenylene ester, diacrylate 4-(6- (acryloyloxy)hexyloxy)phenyl 4-(6-(acryloyloxy)hexyloxy)benzoate, diacrylate l,4-di(4-(3- acryloyloxypropyloxy)benzoyloxy)-2-methylbenzene, diacrylate 4-((4-(6- (acryloyloxy)hexyloxy)phenoxy)carbonyl)phenyl 4-(6-(acryloyloxy)hexyloxy)benzoate, diacrylate 1,4-Phenylene bis(4-(6-(acryloyloxy)hexyloxy)benzoate, and diacrylate 4-((4-(3- (Acryloyloxy)propoxy)phenoxy)carbonyl)phenyl 4-(3-(acryloyloxy)propoxy)benzoate.

[0028] In some embodiments, the chain extender is a dithiol chain extender, e.g., 3,6-dioxa- 1,8-octanedithiol, ethylene glycol bis(3-mercaptopropionate), l,4-bis(3-mercaptobutyryloxy)-PATENTAttorney Docket No: TUOE.P2007WQ / 00652565 butane, 1,2-ethanedithiol, 1,3 -propanedithiol, 1,4-butanedithiol, 1,5 -pentanedi thiol, 1,6- hexanedithiol,l,7-heptanedithiol 1,8-octanedithiol, 1,9-nonanedithiol, and 1,10-decanedithiol.

[0029] In some embodiments, the crosslinker comprises a molecule that has at least two functional groups, wherein each functional group comprises an acrylate, a methacrylate, a vinyl, an oxetane, or an epoxide. In some embodiments, the crosslinker comprises a spacer, e.g., a polyether or an alkyl chain. In some embodiments, the crosslinker comprises, e.g., 2- propenoicacid,2-[(3-ethyl-3-oxetanyl)methoxy]ethylester, glycidyl acrylate, and glycidyl methacrylate.

[0030] In some embodiments, the crosslinker comprises a molecule that has at least two functional groups. Each functional group may independently comprise an acrylate, a methacrylate, a vinyl, an oxetane, or an epoxide. This configuration allows for selective reactivity under different stimuli, enabling controlled crosslinking behavior in the liquid crystal oligomer mixture. The presence of multiple, distinct functional groups facilitates stepwise or sequential crosslinking, which is advantageous for programming shape-memory properties into the resulting elastomer film.

[0031] In some embodiments, the initiators are photo initiators, e.g., Irgacure 819, Irgacure 2022, triarylsulfonium hexafluorophosphate salts, and bis(4-tert-butylphenyl)iodonium perfluoro- 1 -butanesulfonate .

[0032] In some embodiments, the deforming step can be a stress-based deformation, firee- volume based deformation, or deformation based on a decrease in order parameter. A stressbased deformation may include mechanical stretching, compression, or pressing with a patterned mold to induce alignment of the liquid crystal mesogens along a preferred axis. Free-volume based deformation may involve manipulating the polymer matrix to create voids or cavities that allow mesogens to reorient, such as by swelling, indentation, or localized pressure. Deformation based on a decrease in order parameter may be achieved by thermal or photonic stimulation that disrupts the liquid crystalline alignment, followed by mechanical manipulation to re-establish a new orientation. These deformation techniques enable the programming of anisotropic structures and facilitate the formation of monodomain alignment, and for achieving reversible shape-memory behavior in the crosslinked liquid crystal elastomer film.

[0033] In some embodiments, the first stimulus is a light wave with a wavelength being from 400 nm to 450 nm.

[0034] In further embodiments, the wavelength of the first stimulus may be selected from one or more of the following sub-ranges: from 400 nm to 410 nm, from 410 nm to 420 nm, fromPATENTAttorney Docket No: TUOE.P2007WQ / 00652565420 nm to 430 nm, from 430 nm to 440 nm, and from 440 nm to 450 nm. In yet other embodiments, the wavelength may be selected from: from 405 nm to 425 nm, from 410 nm to 430 nm, or from 415 nm to 435 nm.

[0035] In some embodiments, the second stimulus is a light wave with a wavelength being from 200 nm to 400 nm.

[0036] In further embodiments, the wavelength of the second stimulus may be selected from one or more of the following sub-ranges: from 200 nm to 250 nm, from 250 nm to 300 nm, from 300 nm to 350 nm, and from 350 nm to 400 nm. In yet other embodiments, the wavelength may be selected from: from 280 nm to 320 nm or from 320 nm to 360 nm. These sub-ranges enable selective activation of cationic ring-opening polymerization based on the absorption profile of the reactive groups and the corresponding photo-initiators.

[0037] In some embodiments, the crosslinked liquid crystal film has a transition temperature ranging from 5 °C to 90 °C, depending on the chemical composition and mesogen alignment. This transition temperature corresponds to the nematic-isotropic phase change, which governs the actuation behavior of the shape-memory material.

[0038] In further embodiments, the transition temperature may be selected from one or more of the following sub-ranges: from 5 °C to 30 °C, from 10 °C to 40 °C, from 20 °C to 50 °C, from 30 °C to 60 °C, from 40 °C to 70 °C, from 50 °C to 80 °C, from 60 °C to 90 °C, from 20 °C to 70 °C, from 10 °C to 60 °C, from 5 °C to 45 °C, or from 35 °C to 85 °C. These subranges allow for tailoring the thermal responsiveness of the film to specific application environments, such as wearable devices, soft robotics, or haptic interfaces

[0039] n some embodiments, the pre-polymerization step is performed at a temperature below the nematic-isotropic transition temperature (TNI) of the pre-crosslinked oligomer mixture. As used herein, TNI refers to the temperature at which the liquid crystal transitions from the nematic phase — where mesogens are directionally aligned — to the isotropic phase — where mesogens lose orientation and behave like a conventional liquid. The prepolymerization temperature may range from 20 °C to 80 °C, and in further embodiments may be selected from one or more of the following sub-ranges: from 20 °C to 40 °C, from 30 °C to 50 °C, from 40 °C to 60 °C, from 50 °C to 70 °C, from 60 °C to 80 °C, from 25 °C to 55 °C, from 35 °C to 65 °C, from 45 °C to 75 °C, or from 20 °C to 60 °C. These temperature windows ensure that the mesogens remain in the nematic phase during polymerization, preserving their alignment and shape programming.

[0040] In some embodiments, the deforming step is performed at a temperature from -5 °C to 20 °C. In further embodiments, the temperature may be selected from one or more of thePATENTAttorney Docket No: TUOE.P2007WQ / 00652565 following sub-ranges: from -5 °C to 0 °C, from 0 °C to 5 °C, from 5 °C to 10 °C, from 10 °C to 15 °C, and from 15 °C to 20 °C. The temperature at deformation should allow the mesogens remain in the nematic state and can be effectively aligned.

[0041] In some embodiments, the polymerization step is performed at a temperature from -5 °C to 20 °C. In further embodiments, the temperature may be selected from one or more of the following sub-ranges: -5 °C to 0 °C, 0 °C to 5 °C, 5 °C to 10 °C, 10 °C to 15 °C, and 15 °C to 20 °C. These sub-ranges temperature may control crosslinking of the aligned mesogens while minimizing thermal disruption of the induced order.

[0042] In some embodiments, the shape-memory crosslinked liquid crystal elastomers film is obtained using a method that comprises two steps. Wherein in a first step is a pre-crosslinking step, wherein a loosely crosslinked network is obtained by free radical polymerization between liquid crystalline diacrylates, a dithiol and a molecule that comprises an acrylate group as well as an oxetane or an epoxide group. And wherein in a second step the material is deformed to orient the liquid crystalline molecules, which is then fixed by a second photo crosslinking reaction via cationic ring opening polymerization.

[0043] In some embodiments, the shape-memory crosslinked liquid crystal elastomers film made by reacting a liquid crystalline diacrylate with a dithiol in the presence of a base catalyst, this short polymer chain will from now on be referred to as a liquid crystal oligomer. Wherein the ratios are chosen such that the liquid crystal oligomers terminate in acrylate groups on either side. A loosely crosslinked network is then obtained by free radical polymerizations between the previously formed oligomers and a molecule that comprises an acrylate group as well as an oxetane group in a first crosslinking stage. And in a second stage the material is subjected to a deformation that orients the liquid crystalline chains which are then fixed by a second crosslinking reaction via cationic ring opening polymerization.

[0044] In some embodiments, the pre-crosslinked liquid crystal mixture is deformed by pressing a mold onto the pre-crosslinked mixture.

[0045] In some embodiments, the pre-crosslinked liquid crystal mixture is deformed by stretching it, elongating the material over one axis and thereby inducing a planar alignment. Wherein after fixing the order in a second crosslinking step a contraction-based actuation is obtained upon exposure to a stimulus that decreases the order parameter.

[0046] In some embodiments, the pre-crosslinked liquid crystal mixture is deformed by twisting. Wherein after fixing the order in a second crosslinking step, a twisting deformation is obtained upon exposure to a stimulus that decreases the order parameter.PATENTAttorney Docket No: TUOE.P2007WQ / 00652565

[0047] In some embodiments, the liquid crystal oligomer mixture comprises mesogenic monomers with difunctional groups as described above. The mesogens may be present in the oligomer mixture in an amount ranging from 50 wt% to 90 wt%, for example 54.6 wt%, 76.2 wt%, or 79.5 wt%, depending on the desired transition temperature and actuation behavior of the final elastomer film.

[0048] In further embodiments, the mesogen content may be selected from one or more of the following sub-ranges: from 50 wt% to 55 wt%, from 55 wt% to 60 wt%, from 60 wt% to 65 wt%, from 65 wt% to 70 wt%, from 70 wt% to 75 wt%, from 75 wt% to 80 wt%, from 80 wt% to 85 wt%, from 85 wt% to 90 wt%, from 54 wt% to 76 wt%, from 76 wt% to 80 wt%, from 79 wt% to 90 wt%, or from 50 wt% to 79.5 wt%.

[0049] In some embodiments, the mesogenic monomers with difunctional groups is selected from diacrylate compounds, such as diacrylate 4-(3-acryloyloxyhexyloxy)-benzoic acid 2- methyl-l,4-phenylene ester and diacrylate 4-(6-(acryloyloxy)hexyloxy)phenyl 4-(6- (acry 1 oyl oxy )hexy 1 oxy )b enzoate .

[0050] In some embodiments, the liquid crystal oligomer mixture comprises a chain extender as described above. The chain extender may be present in the reaction mixture in an amount ranging from 10 wt% to 30 wt%, for example 16.8 wt% or 19.9 wt%, to control the oligomer length and terminal functionality, which influences the crosslinking density and mechanical properties of the elastomer.

[0051] In further embodiments, the chain extender content may be selected from one or more of the following sub-ranges: from 10 wt% to 15 wt%, from 15 wt% to 20 wt%, from 20 wt% to 25 wt%, from 25 wt% to 30 wt%, from 16 wt% to 20 wt%, from 18 wt% to 22 wt%, from 10 wt% to 19.9 wt%, from 16.8 wt% to 30 wt%, from 10 wt% to 16.8 wt%, from 19 wt% to 25 wt%, from 15 wt% to 30 wt%, or from 10 wt% to 25 wt%.

[0052] In some embodiments, the chain extender is selected from dithiol compounds, such as 3,6-dioxa-l,8-octanedithiol.

[0053] In some embodiments, the crosslinker comprises a molecule having at least two functional groups, such as acrylate and oxetane groups. Suitable crosslinkers include 2- Propenoicacid,2-[(3-ethyl-3-oxetanyl)methoxy]ethylester, 3,3-(Oxybis(methylene))bis(3- ethyloxetane), and 3-methyl-3-oxetanemethanol.

[0054] In some embodiments, the crosslinker may be present in the liquid crystal oligomer mixture in an amount ranging from 15 wt% to 25 wt%, for example 17.9 wt%, to enable wavelength-selective dual-stage polymerization.PATENTAttorney Docket No: TUOE.P2007WO / 00652565

[0055] In further embodiments, the crosslinker content may be selected from one or more of the following sub-ranges: from 15 wt% to 17 wt%, from 17 wt% to 19 wt%, from 19 wt% to 21 wt%, from 21 wt% to 23 wt%, from 23 wt% to 25 wt%, from 15 wt% to 20 wt%, from 17.5 wt% to 22.5 wt%, from 15 wt% to 17.9 wt%, from 17.9 wt% to 25 wt%, from 18 wt% to 24 wt%, from 16 wt% to 23 wt%, or from 15 wt% to 22 wt%.

[0056] In some embodiments, the initiators comprise photo-initiators selected from radical and cationic types. Suitable examples include Phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (Irgacure 819) and triaryl sulfonium hexafluorophosphate salts.

[0057] In some embodiments, the photo-initiators may be present in the liquid crystal oligomer mixture in an amount ranging from 1 wt% to 10 wt%, for example 3.6 wt% for Irgacure 819 and 7.1 wt% for the sulfonium salt solution, to enable selective activation under blue and UV light, respectively.

[0058] In further embodiments, the photo-initiator content may be selected from one or more of the following sub-ranges: from 1 wt% to 3 wt%, from 3 wt% to 5 wt%, from 5 wt% to 7 wt%, from 7 wt% to 9 wt%, from 9 wt% to 10 wt%, from 3.6 wt% to 7.1 wt%, from 1 wt% to 3.6 wt%, from 7.1 wt% to 10 wt%, from 2 wt% to 6 wt%, from 4 wt% to 8 wt%, from 1 wt% to 5 wt%, or from 5 wt% to 10 wt%.Examples

[0059] Synthesis of liquid crystal oligomer: an oligomer that comprises a reactive mesogen and a chain extender is first synthesized via thiol-ene addition catalyzed by a base catalyst. The LC mitogens chosen and tried for this synthesis were RM82 and C6BAPE with the chemical structures as follows. The chain extender is 2,2'-(Ethylenedioxy)diethanethiol (DODT), and the base catalyst is l,8-Diazabicyclo[5.4.0]undec-7-ene (DBU). The reaction is shown as below.C6BAPE

[0060] This process has been proven to work for both 1 :0.9 and 1 :0.85 ratios of mesogemchain extender. Figure 2 shows a schematic representation of the previously explained process.PATENTAtorney Docket No: TUOE.P2007WQ / 00652565

[0061] FIG. 1 shows the mixture of unreacted monomers, by initiating the free radical polymerization (2) with blue light, the acrylate and thiol groups in the system convert to form a loosely crosslinked network where the polymer chains and liquid crystalline moieties are randomly oriented (3). After deforming the material (4), for example by stretching, the loosely crosslinked network becomes ordered, aligning the liquid crystal moieties. The formed order is then fixed by converting the unreacted oxetane groups via cationic ring opening polymerization by exposing the material to UV-light (6). Resulting in a more densely crosslinked, aligned shape memory polymer.

[0062] Synthesis shape-memory crosslinked liquid crystal elastomers film: The liquid crystal oligomer is subject to a two-step wavelength-selective photo-polymerization reaction will consist of the synthesized oligomer combined with an acrylate-oxetane crosslinker and two photo-initiators, as shown in FIG 2 (A). Note that this procedure uses 25 wt.% of acrylate- oxetane crosslinker relative only to the oligomer’s mass. FIG. 2 (B) presents the absorption spectra of each of the photo-initiators to better illustrate this new method. In the precrosslinking step, as explained prior, a free-radical photo-polymerization of acrylates will occur. For this, 4 wt.% of the Irgacure 819 photo-initiator is used. Note that Irgacure 819 absorbs wavelengths in the blue range (450-500 nm) of the solar spectra. On the other hand, for the crosslinking step, 8 wt.% of the triarylsulfonium hexafluorophosphate salts (THPS) photo-initiator, which absorbs UV wavelengths (< 400 nm), is utilized for the initiation of the cationic ring opening photo-polymerization of oxetanes. The distinct non-overlapping absorption ranges of the two photo-initiators make this procedure possible. Moreover, the usage of a crosslinker, which possesses both acrylate and oxetane groups, which can react independently at different stages of the process, is also crucial for the method’s viability.

[0063] The compounds in for the synthesis are as follows:• Monomer 1 mesogenic monomer diacrylate 4-(3-acryloyloxyhexyloxy)-benzoic acid 2-m ethyl- 1,4-phenylene ester• Monomer 2 mesogenic monomer diacrylate 4-(6-(acryloyloxy) hexyloxy) phenyl 4-(6-(acryloyloxy)hexyloxy) benzoate• Monomer 3 chain extender dithiol chain extender 3, 6-di oxa- 1,8- octanedi thiol• Reagent 3 crosslinker 2-Propenoicacid,2-[(3-ethyl-3-oxetanyl) methoxy] ethylesterPATENTAtorney Docket No: TUOE.P2007WO / 00652565Reagent 5 3,3-(Oxybis(methylene)) bis(3-ethyloxetane)Reagent 6 3 -methyl-3-oxetanem ethanol• Reagent 7 photoinitiator Phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (Irgacure 819)• Reagent 8 photoinitiator triarylsulfonium hexafluorophosphate salts (in a50wt% solution in propylene carbonate)• Reagent 9 base catalyst l,8-Diazabicyclo[5.4.0]undec-7-ene.

[0064] For the liquid crystal elastomer synthesis, the mixture of 54.6wt% of reagent 1, 16.8wt% of reagent 3, 17.9wt% of reagent 4, 3.6wt% of reagent 7, and 7.1wt% of reagent 8 is coated on a substrate. The pre-crosslinking step was then performed by free-radical photopolymerization of acrylates under 20 mW / cm2of blue light irradiation (400-450 nm) at 60 °C for 5 min, in a nitrogen atmosphere. After which mixture was cooled down to 0 °C and it was deformed by pressing a mold with pillars into the surface to create dents in the loosely crosslinked polymer and in doing so create order in the network. Then the crosslinking step was then performed via cationic ring-opening photo-polymerization of the unreacted oxetane groups under 30 mW / cm2of UV light (< 400 nm) for 10 min.

[0065] Alternatively, a liquid crystal oligomer is first synthesized. In this approach 79.5 wt% of reagent 1 was combined with 19.9 wt% of reagent 2 and both reagents were dissolved in dichloromethane. Reagent 9 was then added in catalytic amounts. The mixture was left for at least 5 hours to fully react, after which solvent was evaporated to obtain the liquid crystal oligomer.

[0066] To fabricate the crosslinked liquid crystal film, 71.4wt% of liquid crystal oligomer was combined with 17.9 wt% of reagent 4, 3.6 wt% of photo initiator 7, and 7. lwt% of photo initiator 8 and dissolved in dichloromethane. The solution was then coated onto a substrate and solvent was evaporated. The material was then exposed to 20 mW / cm2of blue light irradiation (400-450 nm) at 60 °C for 5 min in a nitrogen atmosphere to perform the precrosslinking step. The resulting loosely crosslinked film was then cooled down to 0 °C and it was deformed by stretching the material to 135.3% strain, inducing a planar alignment. The crosslinking step cationic ring opening polymerization was then performed under 30 mW / cm2of UV light (< 400 nm) for 10 min and thereby permanently programming the alignment.PATENTAttorney Docket No: TUOE.P2007WQ / 00652565

[0067] In an alternative approach, to obtain a material with a lower transition temperature, a liquid crystal oligomer was synthesized using 76.2wt% of reagent 2 and 23.2wt% of reagent 2. Both reagents were dissolved in dichloromethane and reagent 9 was then added in catalytic amounts. The mixture was left for at least 5 hours to fully react, after which solvent was evaporated to obtain the oligomer. After which the same aforementioned steps were followed to create a shape memory polymer.

[0068] In FIG. 3, Fourier transform infrared (FTIR) spectra of the material before any crosslinking (1), after the first crosslinking step (2) and after the second crosslinking step (3) are demonstrated. At the wavenumbers corresponding to acrylate groups (4), the peaks disappear after the first crosslinking step. While the peak corresponding to the oxetane group (5) remains visible after the first crosslinking step but decreases significantly after the second crosslinking step. Thus, proving that the free radical polymerization of acrylate groups and the radical ring opening polymerization of oxetane groups were carried out individually.

[0069] In FIG. 4, the actuation strain in relation to temperature of samples with a contractionbased actuation is plotted. The samples were made by stretching the material after the first crosslinking step and then fixing the imposed alignment by performing the second crosslinking step. These results were obtained by performing a constant force measurement in a dynamic mechanical thermal analyzer while heating at a constant rate. The material made with reagent 1 actuates over a broad temperature range, most of its contraction occurs between 30 and 90°C (1). While the material made with reagent 2 actuates over a lower and narrower temperature range with most of its actuation between 10 and 40°C (2).

[0070] In an alternative approach, to gain materials with varying transition temperatures, mixtures of reagents 1 and 2 with different ratios were used to fabricate oligomers.

[0071] In an alternative approach, to influence the transition temperature, reagents 5 and 6 were added to the procedure to change the liquid crystal content and the crosslink density of the final network. With this approach, shape memory polymers with a variety of actuation temperatures can be obtained.

[0072] As of this moment, with this new method, we can already fabricate a stable thermal- responsive LCE film capable of large reversible “hands-free” actuation. Two types of alignment were tried, either by stretching (stretched sample) or stamping with a mould containing uniform, regularly spaced dots (dots sample). As shown in FIG. 5 (A), the dots sample’s alignment was found to be radial, as expected, via cross-polymerised optical microscopy (POM). FIG. 5 (B) and (C) illustrate the dots sample’s reversible actuation fromPATENTAttorney Docket No: TUOE.P2007WO / 00652565 the cavity at room temperature (RT) to flat when heated, at approximately 80 °C, was measured via interferometry (Sensofar).

[0073] The rotation of a stretched sample from 45° (FIG. 6 (A)) to 0° (FIG. 6 (B)) orientation relative to the cross polarisers in POM allowed for the confirmation of the sample's planar alignment. The actuation of this sample was observed by its shrinkage when heated. The original length of the sample at RT in seen in FIG. 6 (C), and when heated at ~60 °C its shrinkage is observed in FIG. 6 (D).

[0074] Moreover, this stretched sample was also measured via dynamic mechanical analysis (DMA). With a constant stress / force applied, it was possible to measure the change in the strain of the material as the temperature increased. Thus, in other words, the actuation of the material under temperature variation was measured. Note that the slope of this curve corresponds to the sharpness of the sharpness of the transition.

[0075] Changes may be made in the above compositions and processes without departing from the scope hereof. It should thus be noted that the matter contained in the above description or shown in the accompanying drawings should be interpreted as illustrative and not in a limiting sense. The following claims are intended to cover all generic and specific features described herein, as well as all statements of the scope of the present method and system, which, as a matter of language, might be said to fall therebetween.

Claims

PATENTAtorney Docket No: TUOE.P2007WQ / 00652565CLAIMSWhat is claimed is:

1. A process of preparing a shape-memory crosslinked liquid crystal elastomers film, the process comprising the steps of: pre-crosslinking a liquid crystal oligomer mixture comprising a liquid crystal oligomer, a crosslinker, and initiators under a first stimulus; deforming the pre-crosslinked oligomer mixture; and crosslinking the deformed oligomer mixture under a second stimulus.

2. The process according to claim 1, wherein the liquid crystal oligomer comprises diacrylate mesogens covalently linked by dithiol chain extenders.

3. The process according to any one of claims 1 or 2, wherein the crosslinker comprises a molecule having at least two functional groups, each functional group independently selected from the group consisting of acrylate, methacrylate, vinyl, oxetane, and epoxide.

4. The process according to any one of claims 1 to 3, wherein the initiators comprise photo initiators.

5. The process according to any one of claims 1 to 4, wherein the deforming step comprises a stress-based deformation.

6. The process according to any one of claims 1 to 5, wherein the first stimulus is a light wave with wavelength being from 400 nm to 450 nm.

7. The process according to any one of claims 1 to 6, wherein the second stimulus is a light wave with wavelength being from 200 nm to 400 nm.

8. The process according to any one of claims 1 to 7, wherein the crosslinked liquid crystal film has a transition temperature from 5 °C to 90 °C.

9. The process according to any one of claims 1 to 8, wherein the deforming step is performed at a temperature from -5 °C to 20 °C.

10. The process according to any one of claims 1 to 9, wherein the liquid crystal oligomer mixture comprises mesogenic monomers in an amount ranging from 50 wt% to 90 wt%.

11. The process according to any one of claims 1 to 10, wherein the liquid crystal oligomer mixture comprises a chain extender in an amount ranging from 10 wt% to 30 wt%.PATENTAtorney Docket No: TUOE.P2007WQ / 0065256512. The process according to any one of claims 1 to 11, wherein the liquid crystal oligomer mixture comprises a crosslinker in an amount ranging from 15 wt% to 25 wt%.

13. The process according to any one of claims 1 to 12, wherein the liquid crystal oligomer mixture comprises photo-initiators in an amount ranging from 1 wt% to 10 wt%.

14. A liquid crystal oligomer mixture for a shape-memory crosslinked liquid crystal film, the liquid crystal oligomer mixture comprising: a liquid crystal oligomer comprising diacrylate mesogens covalently linked by dithiol chain extenders; a crosslinker comprising an acrylate, methacrylate, a molecule with a vinyl group and an oxetane group, a molecule with a vinyl group and an epoxide group, or combinations thereof; and photo initiators; wherein the different functional groups of the crosslinker react separately when different stimuli are applied to the liquid crystal oligomer mixture.

15. The liquid crystal oligomer mixture of claim 14, wherein the diacrylate mesogen is selected from the group consisting of diacrylate 4-(3-acryloyloxyhexyloxy)-benzoic acid 2- m ethyl- 1,4-phenylene ester, diacrylate 4-(6-(acryloyloxy)hexyloxy)phenyl 4-(6-(acry 1 oyl oxy )hexy 1 oxy )b enzoate, di aery 1 ate 1 , 4 -di (4-(3 -aery 1 oy 1 oxy propyl oxy )b enzoy 1 oxy )- 2-methylbenzene, diacrylate 4-((4-(6-(acryloyloxy)hexyloxy)phenoxy)carbonyl)phenyl 4-(6- (acryl oyl oxy)hexyl oxy )b enzoate, diacrylate 1,4-Phenylene bis(4-(6-(acryl oyl oxy)hexyl oxy )b enzoate, and diacrylate 4-((4-(3- (Acryloyloxy)propoxy)phenoxy)carbonyl)phenyl 4-(3-(acryloyloxy)propoxy)benzoate.

16. The process according to any one of claims 14 to 15, wherein the dithiol chain extender is selected from the group consisting of 3,6-dioxa-l,8-octanedithiol, ethylene glycol bis(3-mercaptopropionate), l,4-bis(3-mercaptobutyryloxy)- butane, 1,2-ethanedithiol, 1,3 -propanedithiol, 1,4-butanedithiol, 1,5 -pentanedi thiol, 1,6- hexanedithiol,l,7-heptanedithiol 1,8-octanedithiol, 1,9-nonanedithiol, and 1, 10-decanedithiol.

17. The process according to any one of claims 14 to 16, wherein the crosslinker is selected from the group consisting of 2-propenoicacid,2-[(3-ethyl-3- oxetanyl)methoxy]ethylester, glycidyl acrylate, and glycidyl methacrylate.

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