Colloidal dispersions in liquid crystals can serve as a soft-matter toolkit for the self-assembly of composite materials with pre-engineered properties and structures that are highly dependent on particle-induced topological defects. Here, we demonstrate that bulk and surface defects in nematic fluids can be patterned by tuning the topology of colloidal particles dispersed in them. In particular, by taking advantage of two-photon photopolymerization techniques to make knot-shaped microparticles, we show that the interplay of the topologies of the knotted particles, the nematic field and the induced defects leads to knotted, linked and other topologically non-trivial field configurations. These structures match theoretical predictions made on the basis of the minimization of the elastic free energy and satisfy topological constraints. Our approach may find uses in self-assembled topological superstructures of knotted particles linked by nematic fields, in topological scaffolds supporting the decoration of defect networks with nanoparticles, and in modelling other physical systems exhibiting topologically analogous phenomena.

Control of the mesophase in liquid crystalline elastomers (LCEs) is a critical aspect in harnessing their unique stimuli-responsive properties. Few studies have compared nematic and smectic main-chain LCEs in a direct way. Traditionally, it is believed that the mesogen core and synthetic route determines the phase behavior. In this study, we hypothesized that tuning the LC phases in main-chain LCE systems can be achieved by varying the spacer length while maintaining the same mesogen (RM257). By increasing the length of dithiol alkyl spacers containing two to eleven carbons along the spacer backbone (C2 to C11), we can modulate the mesophase from nematic to smectic, tailor the nematic to isotropic transition temperature between 90 and 140 °C, and increase the average work capacity from 128 to 262 kJ m. Phase nano-segregation resulting in the smectic C phase is achieved at room temperature for the C6, C9, and C11 spacers. In a shape switching system, this manifests in impressive actuation stroke of 700%. Upon heating from room temperature, these samples transition into the nematic and later, the isotropic phase. Furthermore, this segregation occurs along with polymer chain crystallinity, which increases the modulus of the networks by an order of magnitude; however, the crystallization rate is highly time dependent on the spacer length and can vary between 5 minutes for the C11 spacer and 24 hours for shorter spacers. This study presents several possibilities of a thiol-acrylate reaction in modulation of the thermomechanical and liquid-crystalline properties of LCEs and discusses their potential use for biomedical applications.

This work details a novel polyurethane liquid crystal elastomer (PULCE) with exchangeable carbamate functional groups that enable programming of a uniformly aligned monodomain sample through the application of external stress and simultaneous activation of dynamic bond exchange of the carbamate. PULCEs were synthesized using a thiol-Michael addition reaction, and the reversion of the carbamate group was observed by real-time FT-IR and mechanical analysis. Two independent phase transitions (isotropic–nematic and nematic–smectic) were employed to actuate two-way autonomous strains and multiple shape memory effects in a single system. In addition, thermally activated bond exchange engineered into the network transformed the permanent configuration of the material into various complex shapes. The programmed network topology and bond exchange conditions controlled the two-way autonomous shape changes. Coupling the shape memory effect of the polymer network with the plasticity induced by the thermally activated dynamic covalent chemistry in shape changing and shape memory materials will expand their applications and capabilities in emerging multifunctional devices.

The purpose of this study was to investigate the influence of cross‐linking on the thermomechanical behavior of liquid‐crystalline elastomers (LCEs). Main‐chain LCE networks were synthesized via a thiol‐acrylate Michael addition reaction. The robust nature of this reaction allowed for tailoring of the behavior of the LCEs by varying the concentration and functionality of the cross‐linker. The isotropic rubbery modulus, glass transition temperature, and strain‐to‐failure showed strong dependence on cross‐linker concentration and ranged from 0.9 MPa, 3 °C, and 105% to 3.2 MPa, 25 °C, and 853%, respectively. The isotropic transition temperature (Ti) was shown to be influenced by the functionality of the cross‐linker, ranging from 70 °C to 80 °C for tri‐ and tetra‐functional cross‐linkers. The magnitude of actuation can be tailored by controlling the amount of cross‐linker and applied stress. Actuation increased with increased applied stress and decreased with greater amounts of cross‐linking. The maximum strain actuation achieved was 296% under 100 kPa of bias stress, which resulted in work capacity of 296 kJ/m3 for the lowest cross‐linked networks. Overall, the experimental results provide a fundamental insight linking thermomechanical properties and actuation to a homogenous polydomain nematic LCE networks with order parameters of 0.80 when stretched. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 157–168

Two-stage thiol-acrylate Michael addition reactions have proven useful in programming main-chain liquid crystal elastomers (LCEs). However, the influence of excess acrylate concentration, which is critical to monodomain programming, has not previously been examined with respect to thermomechanical properties in these two-stage LCEs. Previous studies of thiol-acrylate LCEs have focused on polydomain LCEs and/or variation of thiol crosslinking monomers or linear thiol monomers. This study guides the design of monodomain LCE actuators using the two-stage methodology by varying the concentration of mesogenic acrylate monomers from 2 mol% to 45 mol% in stoichiometric excess of thiol. The findings demonstrate a technique to tailor the isotropic transition temperature by 44 °C using identical starting monomers. In contrast to expectations, low amounts of excess acrylate showed excellent fixity (90.4 ± 2.9%), while high amounts of excess acrylate did not hinder actuation strain (87.3 ± 2.3%). Tensile stress-strain properties were influenced by excess acrylate. Linear elastic behavior was observed parallel to the director with modulus increasing from 1.4 to 6.1 MPa. The soft elastic plateau was observed perpendicular to the director with initial modulus and threshold stresses increasing from 0.6 MPa to 2.6 MPa and 14 kPa to 208 kPa, respectively. Overall, this study examines the influence of excess acrylate on mechanical properties of LCE actuators.