Photopolymerization in Pluronic Lyotropic Liquid Crystals: Induced Mesophase Thermal Stability

Nanostructured polymers are of significant interest due to their potential use in applications ranging from ultrafiltration membranes to catalysis supports. To date, much research has focused on using small molecular weight surfactants that form lyotropic liquid crystals (LLCs) as polymerization templates or polymerizing reactive LLCs to form such nanostructure in polymers. LLC phases formed from small molecular weight surfactants, however, are in and of themselves neither thermally nor mechanically stable, often rendering it difficult to preserve the LLC structure throughout polymerization. A relatively new class of surfactant molecules, Pluronic block copolymers, may afford increased flexibility in nanostructure development when used as polymerization templates. This work has focused on the photopolymerization of water and oil-soluble monomers in Pluronic lyotropic liquid crystals. In particular, the effect that LLC order and monomer segregation behavior has on monomer polymerization rate has been examined. The order of the LLC mesophase has a significant impact on polymerization rate with faster polymerization occurring in the normal phases for the oil-soluble monomer hexanediol diacrylate (HDDA) and in the inverse phases for the water-soluble monomer poly(ethylene glycol) diacrylate (PEG-400-DA). In addition, the effect of monomer addition and polymerization on LLC order has been characterized, with retention of LLC order before and after photopolymerization. Mesophase thermal stability, or the persistence of an LC phase at elevated temperatures, has also been examined for the polymer/LLC hydrogels to determine how polymer nanostructure differences affect fundamental LLC properties. Interestingly, when PEG-400-DA is polymerized in the continuous domains of the hexagonal mesophase, the clearing point is increased more than 40 degreesC. However, when HDDA is polymerized in the discontinuous domains of hexagonal phase, the mesophase thermal stability increases only 25 degreesC. Conversely, when HDDA is polymerized in the continuous region of the inverse hexagonal phase, the clearing point increases more than 60 degreesC. The difference in polymer nanostructure that evolves during the polymerizations directly impacts the mesophase thermal stability. The LLC polymerization also affects the physical properties of polymer/Pluronic hydrogels with significant differences in compressive modulus observed based on the LLC phase in which the nanostructured polymer is generated