NMR SELF-DIFFUSION STUDY OF ORGANIC GLASSES - COANP, MBANP, PNP, NPP

A pulsed field gradient proton spin-echo NMR self-diffusion study of o rganic glasses COANP, MBANP, PNP and NPP in their liquid and weakly su percooled states was performed. The NMR phase diagrams, based on the p roton NMR transverse relaxation time (T2) temperature hysteresis data of these materials, clearly give evidence of the onset of a glass phas e on cooling the isotropic liquids below their respective melting temp eratures. The self-diffusion data exhibit in the supercooled glassy st ate a non-Arrhenius behaviour and can be described in terms of the Vog el-Fulcher modification of the Arrhenius law, D = D(infinity) exp {-E( a)/[k(B) (T - T(VF))]}. The activation energies E(a) and Vogel-Fulcher temperatures T(VF) are 83.2 meV and 239 K for COANP, 66 meV and 249 K for MBANP, and 85 meV and 245 K for PNP, respectively. The flow visco sity data obtained for COANP in the same temperature region as well co nform to the Vogel-Fulcher behaviour, eta is-proportional-to exp{E(a)( eta))/[k(B)(T-T(VF))]}, with E(a)(eta) = 80.4 meV and T(VF) = 239 K. I n case of COANP D was found to increase with decreasing diffusion time in the supercooled (glassy) melt just below T(M) whereas no such beha viour was found above T(M).