Phase Effects on the Polymerization of a Styrloxy Cholesteric Liquid Crystalline Monomer

Abstract

A styryloxy liquid crystalline monomer was synthesized and photopolymerized via free radical and cationic initiators with continuous and pulsed light sources. A phase description of monomer/polymer mixtures was used to help assist in the interpretation of the polymerization data. The monomer (a cholesteryl ester) exhibits isotropic, cholesteric nematic, and crystalline phases. For conversions of up to about 20% polymer, the polymerization media appear to exhibit the same cholesteric phase as the monomer, allowing polymerization rates to be measured in a purely cholesteric nematic phase. Free-radical polymerization exotherms show rate accelerations at conversions corresponding to compositions where changes are observed in the medium opacity. No accelerations are observed for cationic polymerization: Differences are attributed to the types of termination processes in free-radical and cationic polymerization. While polymerization rate changes differ for the free-radical and cationic polymerizations, real-time laser light scattering indicates that polymerization induced phase changes occur at the same degree of conversion in both cases. Rates for free-radical polymerization were also measured as a function of temperature. Arrhenius plots indicate that the polymerization activation energy at low conversions is the same for polymerization in the cholesteric nematic and isotropic phases.