Slow dynamics in gels

Abstract

Shear modulus data are presented for a polyurethane system at different stages of the gelation process. Data were collected over a wide frequency domain covering both the α relaxation and the internal mode relaxation. At the gel point, both the loss modulus G″ and the storage modulus G′ have a power law frequency dependence over four decades with an exponent 0.70 close to the value predicted by percolation theory assuming Rouse dynamics. After the gel point G′ exhibits a gel modulus which increases with increasing reaction extent, while G″ continues to have a power law behaviour at low frequencies with an exponent that decreases initially to 0.5 and then increases to 0.7 for the fully grown gels. The presence of slow dynamical processes in gels is visible in most reported dynamic mechanical measurements on different types of gels and is not expected by the standard model of polymeric gels. A number of possible explanations are mentioned, but we conclude that the slow dynamics in gels are at present an unexplained experimental observation.