Comparison of the gelation dynamics for polystyrenes prepared by conventional and living radical polymerizations: A time-resolved dynamic light scattering study

The structure and dynamics for the two types of polystyrene gels prepared by conventional and living radical polymerizations have been investigated by time-resolved dynamic light scattering (TRDLS). The reaction was initiated with the monomer solutions with and without a reversible addition-fragmentation chain transfer (RAFT) agent and was monitored through the gelation process. In the absence of a cross-linker, the reaction process was characterized by the appearance of a maximum when the excess scattering intensity divided by concentration was plotted as a function of reaction time. The intensity maximum was ascribed to an increase in the molecular weight and subsequent suppression of the concentration fluctuations, which was qualitatively reproduced by the Flory-Huggins free energy formula. As the concentration became higher, the time-intensity correlation functions (ICFs) exhibited two modes, suggesting the formation of larger aggregates in addition to the cooperative mode. The intensities were successfully decomposed into the two components by taking account of the relative amplitude of the fast and slow modes. The difference of the time-course between the two types of polymerizations was clearly detected by combining TRDLS and gel permeation chromatography, GPC, data. For the cross-linking system, the kinetics was characterized by the universal gelation mechanism, namely: (1) divergence of the slow mode and (2) appearance of the power law in ICF around the gelation threshold regardless of the polymerization method. On the other hand, the real time intensity decomposition analysis revealed striking differences in the gelation mechanism between the two types of polymerization. For example, the slow mode appeared at an early stage of reaction for the conventional system and sustained its amplitude through the gelation process. For the living (RAFT) system, the appearance of the slow mode was limited in a narrower range of the reaction time with a much smaller amplitude.