Aliphatic chain length effects on photopolymerization kinetics and structural evolution of polymerizable lyotropic liquid crystals

Abstract

Polymerizable lyotropic liquid crystals (LLCs) have the potential advantage of producing materials with anisotropic morphologies and nanometer size dimensions with potential in applications, such as drug delivery, catalysis, and tissue engineering. With the goal of more clearly understanding the factors involved in LLC phase retention after polymerization, this work describes the effect of aliphatic tail length in the amphiphilic molecule on order, polymerization kinetics, and structural evolution of reactive LLC systems. The polymerization kinetics are influenced by the LLC phase morphology with increases in the polymerization rate as the LLC phase increases in order. Enhanced polymerization rates are a result of an apparent increase in the termination and propagation rate parameters, induced by a local increase in monomer concentration due to segregation effects in the LLC phases. Polymerization in hexagonal phases formed from reactive amphiphiles with longer aliphatic tails exhibit lower polymerization rates when compared to systems with shorter aliphatic tails, suggesting that the order of the hexagonal phase decreases as the aliphatic tail increases. Two peaks are observed in the polymerization rate profiles when polymerizing in the hexagonal and cubic phases, with the second peak becoming more pronounced as the aliphatic tail length increases and the order of the LLC phase decreases. This effect appears to be due to changes in morphology during the polymerization process as the hexagonal phase morphology changes significantly during the polymer formation. Different behavior is observed for polymerizations in the lamellar phase for which the LLC phase is retained after polymerization.