DYNAMICS OF A NEMATIC LIQUID-CRYSTAL CONSTRAINED BY A POLYMER NETWORK- A PROTON NMR-STUDY

We report on the proton spin-lattice relaxation measurements in the is otropic and nematic phases of the liquid crystal 4'-pentyl-4-cyanobiph enyl (5CB), constrained by a low-concentration polymer network. Above the nematic-isotropic transition, a relatively small impact of the net work in the MHz regime and a huge increase in the relaxation rate in t he kHz regime are explained consistently by processes in the weakly or ientationally ordered surface layer. A dynamic process with the correl ation time similar to 4X10(-6) s is detected. The orientational order parameter in the surface layer is found to be almost temperature indep endent, and is estimated as S-0 similar to 0.1. The large internal sur face-to-volume ratio of the liquid crystal in the dispersion, required for the explanation of experimental data, supports the simplified mod el which envisions the network elements as tiny fibers with nanometer radius. Comparison between. NMR and optical data reveals mutual cross linking of the fibers. Measurements of the molecular self-diffusion co nstant indicate that fibers are distributed inhomogeneously on the sub micrometer scale. (C) 1995 American Institute of Physics.