Changes in rotational motion of nitroxide spin probes in epoxy adhesives during the cure reaction

The decrease in rotational mobility of nitroxide spin probes 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and 4-(4-nitrobenzoyloxy)-2,2,6,6-tetramethylpiperidine-1-oxyl (Nitro-TEMPO), dispersed in Epon 828/MBCH (4,4-methylenebiscyclohexylamine) epoxy, was studied during the cure reaction. The larger probe, Nitro-TEMPO, exhibits the T 50G transition at a temperature 8°C above the T g of the fully cured epoxy. During isothermal cure, the T 50G transition of Nitro-TEMPO occurs reproducibly at the pregel transition in the viscoelastic time-temperature-transformation diagram. The smaller probe, TEMPO, crosses T 50G at 20°C below T g in the fully cured epoxy, and therefore shows no T 50G process during the cure reaction. Detailed analyses of the rotational correlation time τ R and the rotational anisotropy N were carried out during the cure reaction for TEMPO. The results indicate that an abrupt change in the rotational relaxation behaviour of TEMPO occurs near the pregel transition, interpreted as a hindering of motion around the minor rotational axis. At the vitrification transition, we determine unique values τ R = 4.3 × 10 −9s and N = 10.5 for TEMPO in Epon 828/MBCH. Investigations of composite samples filled with carbon black and glass beads indicate no significant differences in the cure time dependence of the rotational mobility of the spin probe in filled versus neat material.