Polymer/liquid crystal (LC) composites offer a unique opportunity to study polymerizations in ordered media, specifically the potential effect mesophase order can have on polymer properties including molecular weight. To develop successful polymer/LC composites for display applications, it is important to understand the effect of mesophase order on polymer molecular weight in order to optimize the electrooptic (EO) properties of the polymerALC composite. Polymer molecular weight may be influenced in a LC by changes in polymerization rate as LC order is modulated and by chain transfer. This work focuses on the photopolymerization of an aliphatic monoacrylate monomer, decyl acrylate (DA), both in the ordered LC phases of 8CB as well as in isotropic solutions with LC and co-solvent.When DA is polymerized using the LC as the solvent, enhanced polymerization rates and polymer molecular weights are observed in the highly ordered smectic phase compared to the less ordered nentatic and isotropic phases. When conducted strictly in an isotropic environment using a co-solvent with increasing 8CB percentages, a dramatic decrease in the polymerization rate and a significant reduction of the polymer molecular weight is observed, implying degradative chain transfer to the LC. NMR results show that this chain transfer is a result of hydrogen abstraction from the liquid crystals, which leads to the reduction in the polymerization rate with increasing 8CB concentration. The most likely site of hydrogen abstraction is from the benzyl hydrogens of the alkyl chain of 8CB. This chain transfer also plays a role for polymerizations performed in the ordered phases of the LC. Chain transfer appears to be less significant when polymerizations are conducted in the smectic phase due to the anti-parallel association of the LC molecules. When polymerizations occur in the less ordered phases, chain transfer dominates leading to a large reduction in polymer molecular weight and polymerization rate. (C) 2003 Elsevier Science Ltd. All rights reserved.

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.

To develop viable polymer stabilized liquid crystal systems. it is crucial to understand the factors that affect polymer nanostructure evolution. This work examines the influence of the photopolymerization of aliphatic and fluorinated monoacrylate monomer within a room temperature smectic liquid crystal (LC). Additionally, the effect of LC order on polymerization kinetics, monomer and polymer organization. and the effect of the polymer on LC properties have been examined. Through this work, insight has been gained regarding the impact that the introduction of a fluorinated monoacrylate monomer has on polymerization kinetics, LC organization, and monomer, polymer segregation and organization within a polymer, LC system. Fluorinated moieties lower the surface energy of the monomer to enhance segregation between the smectic layers of the LC as compared with an analogous aliphatic monomer. Additionally, the enhanced segregation significantly increases the polymerization rate in the smectic phase and drives the continued segregation of the fluorinated polymer during and after polymerization. Fluorination also leads to the formation of an ordered polymer nanostructure if polymerized in ordered LC phases, This ordering is particularly evident when the fluorinated monomer is polymerized in the smectic phase in which the monomer is organized between the smectic layers of the LC. In addition, the ordered polymer structure found with the fluorinated monomer in the smectic phase leads to continued birefringence above the clearing point of the LC due to surface interactions between the LC and the ordered fluorinated polymer. The continued birefringence offers an exceptional opportunity to examine how factors such as polymer molecular mass and UV light intensity affect the overall polymer morphology of these polymer, LC systems. As the initiator concentration and UV light intensity are decreased, longer polymer chains form lattice-type morphologies: whereas, shorter polymer chains form smoother morphologies that more closely mirror the texture of the LC smectic phase.

Polymer stabilized liquid crystalline systems (PSLCs) have been of considerable research interest due to their great potential in liquid crystal display applications. Of particular importance in the properties of PSLC systems is the evolution of the polymer nanostructure. The unique characteristics of fluorinated monomers not only may provide unique polymer nanostructure but also may enhance desirable properties of PSLC materials. This study focuses on the polymerization and polymer nanostructure of low surface energy fluorinated materials in a liquid crystalline solvent. Enhanced polymerization rates are observed as the order of the liquid crystalline solvent is increased with particularly pronounced acceleration for a fluorinated monoacrylate. This behavior is primarily due to segregation both before and after polymerization. Fluorinated monomers segregate between the smectic layers of the liquid crystal comparable to segregation behavior of analogous aliphatic monomers. The monomer structure has a significant impact on the polymer segregation behavior in these polymer/liquid crystalline composites as well. Network polymer structures, obtained from both aliphatic and fluorinated diacrylate monomers, phase separate from the liquid crystal while linear fluorinated polymer structures remain segregated between the smectic layers of the liquid crystal. Not only does this linear polymer remain between the smectic layers and retain its segregation behavior throughout the polymerization, but the polymer is also ordered to a much greater degree than the monomer. This ordered structure significantly alters the polymer/LC interaction and leads to birefringence at temperatures well above the liquid crystalline isotropic clearing point.