Self-organization of cholesterol-side-chain liquid crystalline polymers by tailoring the main chain structure and flexible spacer length

Abstract

Three kinds of side-chain liquid crystalline polymers (SCLCPs), in which cholesteryl mesogens (Chol) were linked to polyacrylate (PCholAC-m), polymethacrylate (PCholMC-m), and poly(2-vinylbenzene-1,4-dioate) (PCholVA-m) backbones through methylene spacers of different lengths (m = 0, 2, 4, 6, 8, and 10), were successfully synthesized using free-radical polymerization. The phase behavior and structure of the polymers were investigated in detail using DSC, POM, and SAXS. The results indicated that the Chol-SCLCPs displayed interesting self-organization by varying the main-chain structure and spacer length. The polymers initially form a smectic A phase, except for PCholAC-0 without a liquid crystalline phase. Next, polymers PCholAC-m (m = 2, 4, 6) formed a two-layer smectic A phase in which the alkyl tail was overlapped (SmA₂). Two-phase coexisting structures were observed in PCholAC-m (m = 8, 10), including a bilayer smectic A phase in which the alkyl tail was inserted into the mesogens (SmA_d) and a single-layer smectic A phase in which the mesogens were overlapped (SmA₁). Similar results were observed for PCholMC-m. Furthermore, PCholMC-0 possessed a stable bilayer SmA₂ owing to the methyl steric effect of the main chain. PCholVA-m (m = 0, 2, 4) samples showed a well-defined smectic A phase in which the backbone was squeezed by the parallel side chains on both sides. PCholVA-6 and PCholVA-10 exhibited a SmA_d phase. The two-phase coexisting structures were also found in PCholVA-8. Finally, the glass transition temperature of the polymers decreased with increasing flexible spacer length because the flexible spacer acted as an internal plasticizer in the system. However, changes in the polymer clearing points demonstrated the diversity resulting from the different smectic A phase structures.