A Novel Aspect on Structural Formation of Physical Gels

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 V v scattering for PVDF gel electrolytes. More significant is the fact that the initial H v 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-crescent-moon 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.