Effect of polymer architecture on self-diffusion of LC polymers

Two LC side-group poly(methacrylates) were synthesized, and their melt dynamics were compared with each other and a third, main-chain side-group combined LC polymer. A new route was developed for the synthesis of the poly(methacrylate) polymers which readily converts relatively inexpensive perdeuteromethyl methacrylate to other methacrylate monomers. Self-diffusion data was obtained through the use of forward recoil spectrometry, while modulus and viscosity data were measured using rotational rheometers in oscillatory shear. Diffusion coefficients and complex viscosity were compared to previous experiments on liquid crystal polymers of similar architecture to determine the effect of side-group interdigitation and chain packing on center of mass movement. The decyl terminated LC side-group polymer possessed an interdigitated smectic phase and a sharp discontinuity in the self-diffusion behavior at the clearing transition. In contrast, the self-diffusion behavior of the methyl terminated LC side-group polymer, which possessed head-to-head side-group packing, was seemingly unaffected by the smectic-nematic and nematic-isotropic phase transitions. The self-diffusion coefficients of both polymers were relatively insensitive to the apparent glass transition. The presence of moderately fast sub-T-g chain motion was supported by rheological measurements that provided further evidence of considerable molecular motion below T-g. The complex phase behavior of the combined main-chain side-group polymer heavily influenced both the self-diffusion and rheological behavior. Differences between the self-diffusion and viscosity data of the main-chain side-group polymer could be interpreted in terms of the defect structure. (C) 1999 John Wiley & Sons, Inc.