Nuclear magnetic relaxation and low temperature molecular mobility in organic glasses

Abstract

Proton spin–lattice relaxation times T₁ at 15 and 30 MHz, and relaxation times in the rotating frame T_1ρ, have been measured over the temperature range 20–230 K for cyclohexane, cyclohexene, 1,3‐cyclohexadiene, benzene, and substituted benzenes in glasses made of simple organic molecules. Results for different glass‐forming systems with guest molecules of various concentrations and for molecules in the cavities of a zeolite, have been compared. Asymmetric logT₁⁻¹ vs 1/T curves with a relaxation rate that decays more slowly at the low temperature side have been observed for all samples. This indicates high molecular mobility in the mixed glasses. An explanation is offered in terms of intramolecular dipolar interaction and exponential correlation functions which are distributed by the Cole–Davidson distribution. Various parameters have been derived, and models for this behavior have been discussed.