Effect of photoinitiator segregation on polymerization kinetics in lyotropic liquid crystals

Through photopolymerization lyotropic liquid crystalline (LLC) phases may be templated onto organic polymers to yield highly complex nanostructures. In order to understand the unique polymerization behavior controlling structural development in LLC media, the polymerization kinetics in these systems have been studied using several commercially available photoinitiators. Although monomer segregation and diffusional restrictions largely govern the kinetics in these systems, the initiation may also be influenced by changing LLC order and composition. Nonpolar monomers, which partition to the oil soluble domains of the LLC phase typically display the fastest rate of polymerization in micellar aggregates. The rate decreases in phases with larger nonpolar domains due to decreasing localized double bond concentration. Polar monomers exhibit the opposite behavior. However, the segregation of photoinitiator may contribute to significantly different trends in polymerization behavior. Relatively mobile initiators, displaying favorable interaction with water, yield a trend in polymerization that is governed primarily by monomer and diffusional effects. When bulkier, hydrophobic initiators are used, the polymerization appears much less dependent on these effects. Rather than the decreasing rate usually observed at higher surfactant concentrations, polymerization of oil soluble monomers with the less mobile initiators shows the opposite trend of increasing rate at higher surfactant concentration. This behavior likely results from increasing initiator efficiency of the bulky, hydrophobic initiator in the surfactant rich environment.