SLOW DYNAMICS IN GELS

Shear modulus data are presented for a polyurethane system at differen t stages of the gelation process. Data were collected over a wide freq uency domain covering both the alpha relaxation and the internal mode relaxation. At the gel point, both the loss modulus G '' and the stora ge modulus G' have a power law frequency dependence over four decades with an exponent 0.70 close to the value predicted by percolation theo ry assuming Rouse dynamics. After the gel point G' exhibits a gel modu lus which increases with increasing reaction extent, while G '' contin ues 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 full y grown gels. The presence of slow dynamical processes in gels is visi ble in most reported dynamic mechanical measurements on different type s 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.