ORIGIN OF STRUCTURAL INHOMOGENEITIES IN POLYMER GELS

The structure of a cross-linked polymer network depends not only on it s chemical constituents but also on the conditions under which the gel is polymerized. Highly nonuniform spatial distributions of polymer ne twork concentration and cross-linking density are observed in most pol ymer gels. The inhomogeneities, as studied using light scattering, are shown to result from two origins: one from the dynamic critical fluct uations of the polymer solution at the onset of gelation, and the othe r from the domain formation due to the microphase separation. Both are directly related to the phase equilibrium properties of the gel durin g the gelation process. Those fluctuations of polymer density are froz en in the gel structure permanently. In addition to these permanent sp atial fluctuations, a polymer network undergoes thermal dynamical conc entration fluctuations which diverge at the critical point. These thre e types of fluctuations, two static and one dynamic, account for the n ature of gel inhomogeneities. The static spatial fluctuations reversib ly increase and diverge at the spinodal line, although they are perman ent inhomogeneities. A qualitative interpretation is given to account for this phenomenon.