Relaxation of crosslinked networks: theoretical models and apparent power law behaviour

The viscoelastic behaviour of crosslinked polymer networks is discussed. Models of the long time relaxation based on the retracing of dangling chains are described. It is pointed out that the theories all predict a power law time relation for the relaxation behaviour but differ in their predictions of the crosslink density dependence of the power law exponent. The experimental data of Chasset and Thirion are examined. It is shown that the power law relation works only over a limited time span, with deviations occurring at long times and more markedly for more highly crosslinked systems. Furthermore, the validity of time-crosslink density superposition for networks is confirmed, thereby precluding an exponent dependent on crosslink density in any power law representation of the data.