Synthesis and liquid crystalline behavior of 2,5-disubstituted styrene-based random copolymers: Effect of difference in length of the rigid core on the mesomorphic behavior of mesogen-jacketed liquid crystalline polymers

A series of binary copolymers, poly{2,5-bis[(4-octadecyloxyphenyl)oxycarbonyl]-styrene-co-2,5-bis[(4′-octadecyloxybiphenyl)oxycarbonyl]-styrene} (poly(M-OC18-co-M-C18)), were synthesized via free radical polymerization. The random nature of the copolymers was expected on the basis of the assumed similar reactivities because of the analogous monomers. A combination of differential scanning calorimetry (DSC), polarized optical microscopy (POM) and wide-angle X-ray diffraction (WAXD) demonstrated that phase behavior of this series of copolymers were strongly composition dependent. The P-OC18 formed smectic A phase, re-entrant isotropic phase and column nematic phase, while the P-C18 only exhibited the stable smectic A phase. The comparison between P-OC18 and P-C18 indicates that the length of the rigid core is crucial to determine the liquid crystalline (LC) structures. After copolymerization, the glass transition temperature and the phase transition temperature of the copolymers from smectic phase to isotropic phase both increased with the molar fraction of 2,5-bis[(4′-octadecyloxybiphenyl)oxycarbonyl]-styrene (M-C18) in feed. In addition, when the feed was below 0.3, a column nematic phase was observed in high temperature. Namely, simply through copolymerization, we can greatly vary LC behavior of the mesogen-jacketed liquid crystalline polymers.