molecules Review A Review on Liquid Crystal Polymers in Free-Standing Reversible Shape Memory Materials Zhibin Wen 1,* , Keke Yang 2,* and Jean-Marie Raquez 1,* 1 Laboratory of Polymeric and Composite Materials Center of Innovation and Research in Materials and Polymers, Materia Nova Research Center & University of Mons, 23 Place du Parc, B-7000 Mons, Belgium 2 Center for Degradable and Flame-Retardant Polymeric Materials (ERCEPM-MOE), National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610064, China * Correspondence: [email protected] (Z.W.); [email protected] (K.Y.); [email protected] (J.-M.R.); Tel.: +32-6537-3483 (Z.W.); +86-028-85410259 (K.Y.); +32-65-37-34-83 (J.-M.R.) Academic Editors: Laura Peponi and Valentina Sessini Received: 31 January 2020; Accepted: 3 March 2020; Published: 10 March 2020 Abstract: Liquid crystal polymers have attracted massive attention as stimuli-responsive shape memory materials due to their unique reversible large-scale and high-speed actuations. These materials can be utilized to fabricate artificial muscles, sensors, and actuators driven by thermal order–disorder phase transition or trans–cis photoisomerization. This review collects most commonly used liquid crystal monomers and techniques to macroscopically order and align liquid crystal materials (monodomain), highlighting the unique materials on the thermal and photo responsive reversible shape memory effects. Challenges and potential future applications are also discussed. Keywords: liquid crystal polymers; shape memory materials; reversible strain; thermal responsive; photo responsive 1. Introduction Liquid crystal polymers (LCPs) are of intense interest due to their unique anisotropic shape changing and mechanical properties [1]. During the past decade, researchers are examining and discovering their fascinating properties, such as rapid and large reversible actuation in order to make this class of materials good candidates for stimuli-responsive reversible shape memory materials [2–4]. Compared to traditional shape memory materials, one of the most remarkable properties of LCPs is a fully reversible, equilibrium phenomenon. It also has astonishingly large amplitude (200–300%), and can be stimulated by temperature change or irradiation by light [5]. The possibility of applications ranges from actuators and sensors [6,7] to artificial muscles [8] and active smart surface [9–11]. In this review, order–disorder phase transition of the thermal or trans–cis photoisomerization mechanisms of shape change of LCPs will be introduced in Section2. We summarize several pathways to prepare monodomain LCPs in Section3, highlighting the unique materials applied to free-standing reversible shape memory effects in Section4. 2. Mechanisms of Shape Change in LCPs The basic principle behind the shape change of LCPs depends on alignment directions and relevant phase transitions (nematic, cholesteric, smectic, or isotropic) [3]. The majority of studies on the actuation of LCPs based on thermally induced order–disorder phase transition are illustrated in Figure1a [2,12]. Materials exhibit anisotropic deformation along the director orientation as the order parameter (S) change above the phase transition temperature. For another photoinduced type, such as Molecules 2020, 25, 1241; doi:10.3390/molecules25051241 www.mdpi.com/journal/molecules Molecules 2020, 25, x 2 of 13 Molecules 2020, 25, 1241 2 of 13 parameter (S) change above the phase transition temperature. For another photoinduced type, such as azobenzene-containing LCPs, the stabilized rod-like trans azobenzene mesogens transfer to azobenzene-containing LCPs, the stabilized rod-like trans azobenzene mesogens transfer to unstable unstable bent cis isomers irradiation with UV light, resulting in the decrease of the order parameter. bent cis isomers irradiation with UV light, resulting in the decrease of the order parameter. Then, the Then, the volume shrinkage in the film surface cause bending behavior (Figure 1b) [13]. Based on the volume shrinkage in the film surface cause bending behavior (Figure1b) [ 13]. Based on the mechanisms, mechanisms, the widely studied monomers are adopted to fabricated LCPs illustrated in Figure 1c the widely studied monomers are adopted to fabricated LCPs illustrated in Figure1c [2,3,11,14,15]. [2,3,11,14,15]. FigureFigure 1. 1.( a(a)) MechanismsMechanisms ofof liquidliquid crystal polymers polymers (L (LCPs)CPs) shape shape change change of ofthe the polymer polymer chain chain susufferingffering from from anisotropic anisotropic deformation deformation to itsto coiledits coiled conformation conformation above above the nematic-to-isotropicthe nematic-to-isotropic phase transition.phase transition. Reproduced Reproduced with permission with permissi fromon [2]. from Copyright [2]. Copyright© 2010 WILEY-VCH. © 2010 WILEY-VCH. (b) Mechanisms (b) ofMechanisms the photoinduced of the photoinduced bending behavior bending in behavior the azobenzene-containing in the azobenzene-containing LCPs caused LCPs caused by a trans–cis by a photochemicaltrans–cis photochemical phase transition phase intransition the surface. in the Reproduced surface. Reproduced with permission with permission from [13]. from Copyright [13]. © 2006Copyright WILEY-VCH. © 2006 (c )WILEY-VCH. Chemical structures (c) Chemical of common structures liquid of crystal common monomers liquid [crystal2,3,11, 14monomers,15]. [2,3,11,14,15]. 3. Strategy for Preparation of Monodomain LCPs 3. StrategyTo observe for free-standingPreparation of reversible Monodomain actuation, LCPs macroscopic alignment of the mesogens is necessary to formTo a LCobserve monodomain. free-standing Up toreversible now, a variety actuation, of approaches macroscopic has alignment been used of to generatethe mesogens uniformly is macroscopicnecessary to LCPs. form a This LC sectionmonodomain. will discuss Up to thenow, commonly a variety of used approaches methods. has been used to generate uniformlyA two-step macroscopic crosslinking LCPs. techniqueThis section is will one discuss of the the most commonly commonly used implemented methods. and simplest methodsA totwo-step dictate alignment.crosslinking Multifunctional technique is one groups of the are most designed commonly in monomers implemented to fabricate and simplest a network controlledmethods byto reactdictate ratio alignment. [16–18]. Multifunctional In the initial step, groups a weakly are designed crosslinked in monomers elastic network to fabricate is formed a andnetwork aligned controlled by mechanical by react stretching ratio [16–18]. of the In polymerthe initial chains,step, a weakly against crosslinked the entropy. elastic The liquidnetwork crystal is phaseformed is further and aligned locked by in mechanical the conformation stretching of of the th backbonee polymer bychains, a following against the second entropy. crosslinking The liquid step. Küpfercrystal and phase Finkelmann is further locked introduced in the a conformation fast and a slow of the crosslinking backbone processby a following for polysiloxane-based second crosslinking LCPs. Anstep. optically Küpfer clear and film Finkelmann was fabricated introduced and fixeda fast with and ana slow external crosslinking load during process the secondfor polysiloxane- crosslinking stage.based The LCPs. highly An optically ordered monodomainclear film was fabricated of the sample and fixed has been with proven an external by the load X-ray during experiment the second [16 ]. crosslinking stage. The highly ordered monodomain of the sample has been proven by the X-ray However, it should be noted that a high internal stress in the network and a limited ordering of the experiment [16]. However, it should be noted that a high internal stress in the network and a limited mesogens inhibited this approach, because the first stage required a partly crosslinked network to ordering of the mesogens inhibited this approach, because the first stage required a partly crosslinked facilitate subsequent programming. network to facilitate subsequent programming. External fields alignments are the useful methods to prepare LCPs. Typically, the low viscous External fields alignments are the useful methods to prepare LCPs. Typically, the low viscous liquidliquid crystal crystal monomers monomers are are melted melted and and cooled cooled into into liquid liquid crystal crystal phase, phase, then then aligned aligned by externalby external fields (suchfields as (such surface as rubbingsurface rubbing [19,20], [19,20], photo alignmentphoto alignment [21,22 ],[21,22], electric electric [23], or[23], magnetic or magnetic alignment alignment [24,25 ]. Finally,[24,25]. the Finally, mesogens the mesogens are fixed byare further fixed by polymerization. further polymerization. The direction The ofdirection liquid crystalof liquid relative crystal to a substrate surface plane is critically determined by the nature of the surface. Rubbing on polyimide alignment layers has been used to fabricate a monodomain LCP film [26,27]. The monomeric mixtures Molecules 2020, 25, x 3 of 13 Molecules 2020, 25, 1241 3 of 13 relative to a substrate surface plane is critically determined by the nature of the surface. Rubbing on polyimide alignment layers has been used to fabricate a monodomain LCP film [26,27]. The are melted on a glass substrate coated with a rubbed polymer film (polyimide, poly (vinyl alcohol)), monomeric mixtures are melted on