Influence of the spacer on the phase behaviors of side-chain liquid crystalline copolymers based on triphenylene discotic mesogen unit

Abstract

Based on two analogous triphenylene liquid crystalline monomers (MTS and MT6S), a novel series of binary copolymers poly{[3,6,7,10,11-pentakis(hexyloxy)-2-oxytriphenylene]methacrylate-co-6-[3,6,7,10,11-pentakis(hexyloxy)-2-oxytriphenylene]hexyl methacrylate} (poly(MTS_x-co-MT6S_y)) were prepared by free radical polymerization using AIBN as initiator. The random nature of the copolymers was expected on the basis of the assumed similar reactivities due to the analogous triphenylene monomers. The phase behaviors of copolymers were studied by DSC, POM and 1D WAXD. The results showed that the content of PMT6S has a significant effect on the liquid crystalline (LC) phase behaviors and phase structures of copolymers. The comparison between PMTS and PMT6S indicates that the content of the spacer was crucial to determine the LC structures. The PMTS formed a stable columnar nematic phase (Φ_N) and the PMT6S exhibited a stable hexagonal columnar phase (Φ_H). After copolymerization, the glass transition temperature and the phase transition temperature of the copolymers from the LC phase to isotropic phase both decreased with the molar fraction of PMT6S in the feed. The samples whose molar content of PMT6S was below 75% formed stable Φ_N, similar to the property of PMTS. The samples whose molar content of PMT6S was above 80% presented the symmetry Φ_H, similar to the property of PMT6S. Through copolymerization, one can better understand the interrelation of microstructures and how Tp mesogenic orders constitute the key basis for various applications.