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By using the time-resolved small-angle light scattering (TRSALS) technique, we present the real-time measurement of the physical gelation process for a crystalline polymer. To investigate the growth kinetics, a complete picture of the gel structural formation should be differentiated into the nucleation and growth of the microgels, the diffusive aggregation of the microgels, the percolation in cluster−cluster aggregation process, and the late-stage coarsening by the Ostwald ripening process. We propose some phenomenological functions to describe the hierarchical structure of the nucleation gels. The modeling of the late-stage gel structure could be built upon the following three relevant categories:  the structure of the primary particle, the nonfractal local structure of a random packing of the nearest neighbors, and the fractal correlations between the particles constituting the aggregates. The model is able to reproduce the overall behavior of H v and V v scattering intensity distributions over the experimental q range and holds the truth of the gel structural development in the late-stage coarsening process.

By using the time-resolved small angle light scattering (TRSALS) technique, we present the first real-time measurement of the physical gelation process for a crystalline polymer. The finding is that the light scattering patterns show a unique feature in the H v and the Vv scattering for PVDF gel electrolytes. More significant is the fact that the initial Hv scattering pattern with a four-leaf-clover shape gradually turns into a final pattern with 2-fold structure, composed of an X-type and a four-crescentmoon shape at low and high q ranges, respectively. The experimental results are noteworthy in that they show the characteristics of the special birefingent transition, i.e., the anisotropic-to-isotropic transition in the gelation process. Our observations illustrate a novel aspect in the structural formation of physical gels. During the course of the gelation we observed the existence of three distinct time regimes: (i) a nucleation and growth stage, where the droplet formation can be interpreted in terms of the simultaneous formation of crystallites or fibril texture with the growth of the birefringent droplets and the crystallites or the fibrils act as junction points in the droplets, leading to approximate dispersion of the hard spheres in the solutions; (ii) the dynamic cluster-to-percolation transition stage, where the large-scale concentration fluctuation is triggered by the hard spheres diffusion and aggregation to form the macroscopic percolation structures and where this main characteristics of the process is simultaneously accompanied by the anisotropic-to-isotropic transition; (iii) a ripening process in the late stage of gelation that causes the arrangement and growth of microstructures such as crystallites or fibrils to be highly concentrated. It is clear that these experimental results are entirely different from previous understandings of spinodal gels. We may proceed from these results to conclude that the formation of the birefringent droplets and the colloid aggregation dominate the physical gelation process in the present work.

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 [η]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.[η]C ∼ 1.5), the gelation behavior depends on the competition between transitions of the sol-gel type and the dynamic percolationto-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. [η]C ∼ 4), the initial stage phase separation controls mainly the structural formation of PVC gels. (4) As the concentration is further increased more than [η]C ∼ 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.

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