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Photopolymerizable diacrylate monomers dissolved in fluid-layer smectic A and smectic C liquid crystal (LC) hosts exhibited significant spatial segregation and orientation that depend strongly on monomer structure. Small, flexible monomers such as 1,6-hexanediol diacrylate (HDDA) oriented parallel to the smectic layers and intercalated, whereas rod-shaped mesogen-like monomers such as 1,4-di-(4-(6-acryloyloxyhexyloxy)benzoyloxy)-2-methylbenzene (C6M) oriented normal to the smectic layers and collected within them. Such spatial segregation caused by the smectic layering dramatically enhanced photopolymerization rates; for HDDA, termination rates were reduced, whereas for C6M, both the termination and propagation rates were increased. These polymerization precursor structures suggest novel materials-design paradigms for gel LCs and nanophase-separated polymer systems.
In pursuit of novel LC phase behavior and properties, a number of polymer-LC composites have been developed. Some composites make use of LC polymers ( 1 ), whereas others are formed by phase separation of the polymer and LC to produce LC droplets [polymer-dispersed LCs (PDLCs) (2, 3)]. Another group of these composites that show great promise is formed by the polymerization of monomer solutes in an LC solvent (4). These polymer-LC gel systems can yield electro-optically bistable chiral nematic devices [polymer-stabilized LCs (PSLCs) (5)] and ferroelectric LC gels [(PSFLCs) (6, 7)], which combine fast electro-optic response (8) with polymer-induced mechanical stabilization (9). Research to date on the formation and structure of polymer-LC gels has focused on the macroscopic phase behavior and optical properties of the resulting composites (10). Little is known, however, about the roles that the monomer segregation and subsequent polymerization behavior play on the ultimate performance of the polymer-LC gel.
We report results on the effect of diacrylate monomer structure on the spatial organization of monomer-LC mixtures prior to polymerization and thus the effect of monomer segregation and structure on polymerization kinetics. This work was initially motivated by observations of a dramatically enhanced rate of photopolymerization of LC acrylate monomers ( 1 ) and decreased termination rate (12) in LC phases, which suggests that the inherent order in LCs can significantly alter chemical reaction behavior and kinetics (I3). The fluidlike environment in LCs not only permits molecular motion, diffusion, and chemical reaction, but also is both spatially anisotropic (orientational ordered) and spatially inhomogeneous (for example, layered). We find distinctive structure-dependent positional and orientational ordering of the...