Critical opalescence points to thermodynamic instability: relevance to small-angle X-ray scattering of resorcinol-formaldehyde gel formation at low pH

During the formation at low pH of resorcinol-formaldehyde gels with a structure in the micrometre range, small-angle X-ray scattering exhibits a non-monotonic intensity variation as a function of reaction time. The data are analyzed in terms of scattering by statistical fluctuations of polymer concentration, the amplitude of which is maximal close to the critical point for phase separation between polymer and solvent. The data do not carry any morphological information, but they unambiguously show that the driving force of the gel formation is a thermodynamic instability of the polymerizing solution.