(83a) Random Liquid Crystalline Copolymers Consisting of Prolate and Oblate Liquid Crystal Monomers

Soft polymeric materials with complex shape transformations in response to environmental cues have shown great potential in applications ranging from drug delivery to soft robotics. A variety of polymeric materials, including hydrogels, shape memory polymers, and dielectric elastomers have been reported to exhibit stimuli-responsive shape deformations. Thermally responsive hydrogels, such as poly(N-isopropylacrylamide), consist of a three-dimensional network of lightly crosslinked polymer chains that can reversibly change their shape between a swollen hydrated state and a shrunken dehydrated state. The intrinsic mechanical properties of stimuli-responsive hydrogels, however, are typically the same in all directions, leading to isotropic deformations. Another group of shape changing materials, called shape memory polymers, can maintain temporary shapes with kinetically trapped polymer states with low conformational entropy and can return to their original shape when triggered by an external stimulus. Although recent technological advances have observed the development of double- and triple-shape memory polymers, the fact that these materials can only return from a deformed state to the original, memorized state and are not able to deform out of the memorized state means that shape memory polymers does not allow for predetermined and reversible shape deformations. Dielectric elastomers, another well-known class of shape changing polymers, show deformations caused by Coulombic forces, with a contraction parallel and an expansion perpendicular to the electric field direction. However, their reliance on a directional and high voltage makes non-uniform deformations on the microscale challenging.

Liquid crystal elastomers (LCEs) capitalize on the interplay between the self-association of liquid crystal (LC) functional groups and the entropic conformation of polymer backbones, resulting in phase-dependent macroscopic shape deformations. Past studies have shown that LCEs are capable of exhibiting a rich palette of reversible and well-programmed anisotropic shape changes in response to a wide range of external stimuli, including heat, solvents, light, and magnetic and electric fields, and have attracted great interest for their potential in the design and synthesis of responsive biomedical devices, soft actuators and robotics, and active surface structures. LC moieties can be incorporated into polymer chains as either part of the polymer backbone (main-chain LCE) or the pendant functional group (side-chain LCE). Previous studies have shown that in LC phases, the conformation of LC polymer chains can deviate from statistically spherical random coils, depending on the configuration of the pendant LC functional groups, as seen in Scheme 1. For prolate LC polymers, polymer backbones align along the orientation of the mesogenic groups (called the director), resulting in a longer radius of gyration parallel to the LC director (R_∥) than that perpendicular to the LC director (R_⊥). This anisotropy gives rise to a uniaxial contraction of the respective LCE parallel to the LC director above the LC–isotropic phase transition temperatures (T_LC–I). In contrast, for oblate LC polymers in LC phases, the radius of gyration parallel to the director is shorter than the radius perpendicular, that is R_∥ < R_⊥, which results in a uniaxial elongation of the LCE parallel to the director above the T_LC–I. Though the polymer chain conformation and the consequent shape deformations of LCEs made purely of one configuration of LC monomer, either prolate or oblate, have been extensively studied, to the best of our knowledge, the effect of the copolymerization of LC monomers with different configurations on the polymer conformation and the consequent LCE shape deformation remains hidden.

In this work, we report the synthesis of random LC copolymers consisting of a combination of prolate and oblate reactive LC monomers. We demonstrate that the orientational order of LC functional groups is destabilized in random LC copolymers consisting of both prolate and oblate monomers, whereas a random insertion of the same configuration of LC monomers, e.g., oblate monomers, preserves the packing of the LC pendant groups on the polymer backbone. Furthermore, we illustrate the control over both the direction and magnitude of thermally triggered shape deformations of random LC copolymers by tuning LC monomer configurations and chemical compositions. Overall, our results not only provide insights into how LC monomer configurations and the chemical composition of random LC copolymers affect the thermal and mechanical properties of the formed LCE through the coupling among LC pendant groups, but also demonstrate a new design principle to program deformation behaviors of LCE structures by patterning local LC monomer configurations.

Scheme 1. Polymer chain conformation and consequent mechanical deformation of LC polymers made of LC monomers with different configurations: (A) prolate configuration with R_∥ > R_⊥ and (B) oblate configuration with R_∥ < R_⊥ in LC phases. R_∥ and R_⊥ represent the radius of gyration of polymer backbones parallel and perpendicular to the LC director, respectively. Red and blue rods represent the pendant LC functional groups of prolate and oblate LC polymers, respectively.