Probe Diffusion during Sol–Gel Transition of a Radical Polymerization System Using Isorefractive Dynamic Light Scattering

Probe diffusion in a gelation process provides unique information such as local viscosity and sol/gel fraction that general scattering and rheological measurements do not provide. In this study, we revisited a gelation process of radical copolymerization of monomers and cross-linkers by conducting a series of probe diffusion experiments with dynamic light scattering (DLS). By using an isorefractive solvent to the gel system, we exclusively monitored the dynamics of gold nanoparticles during its real-time gelation process at multiscattering angles. The obtained time-correlation functions (g²(τ) – 1) were analyzed by fitting with empirical stretched exponential functions. The ratio of mobile particles to the total particles, the relaxation time of mobile particles, and the heterogeneity of their dynamics were obtained as the fitting parameters. With those fitting parameters, the gel point, heterogeneity of local environment, and the local viscosity were evaluated. In addition, a unique up-and-down transition was found in the relaxation time, suggesting the local viscosity that the particles feel changes drastically around the gel point. This transition point in the relaxation time matches the gel point for homogeneous gels but showed a systematic deviation in heterogeneous gels by changing q^–1 and the size of probe particles.