Interplay between Liquid Crystalline Order and Microphase Segregation on the Self-Assembly of Side-Chain Liquid Crystalline Brush Block Copolymers

Herein we investigate the influence of competing self-organizing phenomena on the hierarchical self-assembly of liquid crystalline brush block copolymers (LCBBCs). A library of LCBBCs are synthesized using ring-opening metathesis polymerization (ROMP) of norbornene side-chain functionalized monomers comprising (1) cholesteryl mesogen with nine methylene spacer and (2) semicrystalline poly(ethylene glycol) (PEG). The self assembly of LCBBCs with variations in LC block content (7-80 wt %) are investigated in their melt state. All LCBBCs show two distinct thermal transitions corresponding to PEG semicrystalline phase and LC mesophases. Interestingly, the LCBBCs display a multilevel hierarchical structure evidenced by the results from X-ray scattering and transmission electron microscopy (TEM): (1) smectic A (SmA) mesophases (d = 3-7 nm) by the assembly of cholesteryl side chains and (2) microphase segregation into lamellar or cylinder (d = 40-75 nm) resulting from the incompatibility between LC moieties and PEG side chain. Surprisingly, the presence of microphase-segregated domains in LCBBCs prevents the formation of cholesteric mesophase in sharp contrast to side-chain liquid crystalline homopolymer (SCLCP) bearing the same mesogen and the flexible spacer. This could be attributed to very high surface to volume ratio at intermaterial dividing surface (IMDS) in LCBBCs, by which only LC layers (i.e., SmA mesophase) are favored to form at the IMDS. On the fundamental side, these LCBBCs are an interesting scaffold to explore the impact of interactions between LC order and microphase segregation of side-chain polymeric brushes on the self-assembly of LCBBCs. Moreover, these new LCBBC scaffolds will serve as a tool box for rational design of hierarchically organized functional materials for stimuli responsive applications.