Temperature Dependent Stress Relaxation in a Model Diels-Alder Network

The effect of temperature on the complex shear modulus (G*(omega)) of a model reversible covalent network formed by the Diels-Alder reaction was studied. The gel temperature of 119 degrees C and the functional group conversion at this temperature were determined by the Winter-Chambon criterion. The complex modulus of the cross-linked network was measured from 110 degrees C to 121 degrees C, near the gel temperature, to determine the frequency ranges over which stress relaxation could occur. The crossover time was found to have a strong dependence on temperature (Ea similar to 260 kJ mol(-1)); greater than would be expected from a typical thermally-activated retro-Diels-Alder process. Low frequency scaling of G*(omega) over the experimental frequency and temperature range was interpreted to be a result of the existence of a distribution of transient clusters in these thermoreversible covalent gels.