High resolution 13C nuclear magnetic resonance in cured epoxy polymers. Rotating frame relaxation

Abstract

The combination of cross-polarization, dipolar decoupling and magic angle spinning produces high resolution ¹³C n.m.r. spectra in organic solids. Carbon rotating frame relaxation is complicated by the presence of the strongly interacting proton spin system: spin–spin and spin–lattice processes will both promote relaxation. Dipolar order is created when the proton system is spin locked and is rapidly destroyed by magic angle spinning. Some implications of these spin–spin mechanisms on determination of ¹³C T_1ρ are examined and illustrated by an experimental study on a model cured epoxy polymer, diglycidyl ether of bisphenol A (DGEBA) cured with piperidine. For this specimen at room temperature we find that for r.f. fields above ≈ 40 kHz, the decay of the carbon spin lock magnetization after the first 100 µs or so is not dominated by spin–spin relaxation and can reflect molecular motion. Over the temperature range 242–324 K, the T_1ρ for the methyl carbon is described by an activation energy of 11 kJ mol^–1.