Synthesis, nanostructures and dielectric properties of novel liquid crystalline block copolymers

Abstract

In this work, a series of diblock copolymers with a defined length of a soft poly(dimethylsiloxane) (PDMS) segment and different lengths of liquid crystalline (LC) poly[11-(4-cyano-4′-biphenoxy)undecyl methacrylate] (P11CBMA) segments were synthesized by using the atom transfer radical polymerization (ATRP) method. Their microphase separation behavior and dielectric properties were carefully investigated. The well-defined diblock copolymers PDMS_m-b-P11CBMA_n possess three different soft/rigid ratios (m = 62, n = 15, 22, and 40) and narrow molecular distributions (PDI ≤ 1.25). Due to the supramolecular cooperative motion effect, the copolymers can form lamellar morphology (W_LC = 57.8%), predominant lamellar morphology with PDMS cylindrical defects (W_LC = 66.8%), and PDMS cylinders embedded in the LC matrix (W_LC = 78.3%) after annealing at 160 °C (above T_i) under N₂ for 24 h, respectively. The dielectric properties of the block copolymers were strongly influenced by the microphase separated morphologies and orientational directions of mesogens. Compared to homopolymer P11CBMA, PDMS_m-b-P11CBMA_n showed higher dielectric constants due to the confinement effect of microdomain dimensions. The dielectric constants of diblock copolymers could be further improved by aligning mesogens parallel to the electric field through substrate treatments. These findings will inspire researchers to design and develop novel LC block copolymers for dielectric elastomer actuator applications.