Abstract
Lyotropic liquid crystals (LLC's) have recently been employed as polymerization templates to yield highly ordered nanostructured materials with potential applications in regulated transport, ultrafiltration, and catalysis. With the goal of understanding the reaction mechanisms ultimately determining polymer morphology in these highly ordered systems, this study focuses on the influence of LLC order on photoinitiation efficiency and monomer segregation behavior. These phenomena have been elucidated through extensive examination of the polymerization kinetics utilizing photodifferential scanning calorimetry. The polymerization kinetics in these ordered liquids are highly dependent on phase morphology because liquid crystalline alignment has a profound effect on monomer segregation, and photoinitiation efficiency is a function of viscosity, polarity, and the diffusional constraints inherent in LLC phases. Through studies of the polymerization kinetics in different LLC phases using a free radical inhibitor, the relative initiation efficiencies of a number of photoinitiators were determined. By incorporating relative efficiency information in the analysis of propagation and termination rate parameters, the individual contributions of monomer segregation and photoinitiation behavior to the polymerization kinetics were differentiated. While segregation of polar monomers in the continuous phase leads to increased polymerization rate in more ordered LLC phases, nonpolar monomers exhibit the opposite trend due to segregation in the discontinuous region of the phases studied. Changes in photoinitiation efficiency either compound or negate the effect of monomer segregation on the polymerization rate depending on initiator structure and polarity. The initiation efficiency with relatively bulky, hydrophobic initiators increases substantially in more ordered systems while the efficiency of less hydrophobic initiators decreases slightly in more ordered phases.
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