Effects of phase separation on structural characteristics of poly(vinyl chloride) physical gels

The effects of phase separation on structural characteristics of poly(vinyl chloride)/chlorobenzene (PVC/ClBz) physical gels were studied through the time-resolved light scattering, pulsed NMR, and the gelation kinetic analyses. The present study clarifies the characteristics of PVC solutions at various concentration regions and their influence on the gel structure formed from the spinodal decomposition of the solutions. According to the physical meaning of the [eta ]C value capable of expressing the characteristics of PVC solutions, one can divide the chain aggregation behaviors into four regions with increasing PVC concentration. (1) At the concentration less than the macroscopic percolation transition limit, the polymer-rich phase transforms into isolated droplets, and the gelation cannot occur. (2) When the concentration is close to the critical gelation concentration C-gel* (ca. [eta ]C similar to 1.5), the gelation behavior depends on the competition between transitions of the sol-gel type and the dynamic percolation-to-cluster type; moreover, the structure and properties of gels are mainly dominated by the evolution of the later-stage phase separation. (3) At the concentration exists between the C-gel* and the chain overlapped concentration C* (ca. [eta ]C similar to 4), the initial stage phase separation controls mainly the structural formation of PVC gels. (4) As the concentration is further increased more than [eta ]C similar to 4, i.e., the overlapping between chains coils is present in this region, the influence of phase separation on PVC/ClBz gelation would be weakened. It should be noted that the C* value is higher than the C-gel* value in this work, implying that the chain overlapping is not a prerequisite for the gelation of PVC solutions to undergo liquid-liquid phase separation. Thus, the aggregation behavior of PVC solutions in region 3 was focused in order to emphasize the effect of initial phase separation on gelation. As a result, the gelation mechanism and the structural characteristics of PVC gels can be interpreted well by our proposed model.