Kinetics of spinodal decomposition in chemically crosslinked gels

We present time-resolved small angle light scattering measurements of spinodal decomposition in a covalently crosslinked N-isopropylacrylamide gel. We find that for shallow quenches and early times the linear Cahn-Hilliard-Cook (CHC) theory can be used to describe the time evolution of the scattered intensity as well as the turbidity. Surprisingly, we find that the initial length scale on which fluctuations grow is much larger than the concentration correlation length and the diffusion constant characteristic of the phase separating gel is much slower than the collective diffusion constant. It appears that the initial process of phase separation in these gels is driven by fluctuations on micron sized regions, perhaps representing large-scale variations in the degree of swelling. We have also obtained the range of validity of the linear CHC theory, and find that the break-down occurs earlier at higher wave-numbers and deeper quenches.