NMR self-diffusion study of organic glasses: COANP, MBANP, PNP, NPP

A pulsed field gradient proton spin-echo NMR self-diffusion study of organic glasses COANP, MBANP, PNP and NPP in their liquid and weakly supercooled states was performed. The NMR phase diagrams, based on the proton NMR transverse relaxation time (T2) temperature hysteresis data of these materials, clearly give evidence of the onset of a glass phase on cooling the isotropic liquids below their respective melting temperatures. The self-diffusion data exhibit in the supercooled glassy state a non-Arrhenius behaviour and can be described in terms of the Vogel-Fulcher modification of the Arrhenius law,D=D∞exp{−E a /[k B (T−T VF )]}. The activation energiesE a and Vogel-Fulcher temperaturesT 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 viscosity data obtained for COANP in the same temperature region as well conform to the Vogel-Fulcher behaviour, η∝exp{E a (η) /[k B (T−T VF )]}, withE a (η) =80.4 meV andT VF =239 K. In case of COANPD was found to increase with decreasing diffusion time in the supercooled (glassy) melt just belowT M whereas no such behaviour was found aboveT M .