Sub-Glass Transition Motions in Linear and Cross-Linked Bisphenol-Type Epoxy Resins by Deuterium Line Shape NMR

Abstract

Deuterium solid-state echo delay line shapes are used to establish the local motions occurring in linear and cross-linked epoxy systems based on Bisphenol A. Four isotopically labeled samples were prepared: linear and cross-linked resins with deuterated methylene units, and linear and cross-linked resins with deuterated phenylene units. Two local motions are observed in all four samples: trans−gauche isomerization of the methylene units and π flips of the phenylene units. The solid-state echo delay line shapes were simulated to obtain the rates of motion, the amplitudes of motion, and apparent distribution of rates, and the apparent temperature dependence of the rates. The two jump motions differ by about an order of magnitude in rate, with the π flip process being the faster of the two. Both have similar distributions of relaxation times given by a stretched exponential correlation function with an exponent of 0.6−0.7. The apparent activation energy for the π flip process is 50 kJ/mol and for the trans...