PVC can form thermoreversible gels in a large number of solvents among which diesters. Gelation, i.e., formation of an infinite three-dimensional network, occurs above a so-called critical gelation concentration, Cgel. Below this concentration, finite aggregates (or clusters) are formed whose size depends upon the preparation concentration, and diverges at Cgel. Recent studies on PVC aggregates have provided a simple description of these systems in terms of a longitudinal fractal dimension of the fibers constituting the network and allowed one to account for the exponent of the elastic modulus-concentration relation observed for the gels. Clearly, the molecular morphology of the gel is closely related to that of the aggregates.A recent neutron scattering study on the molecular structure of gels of PVC and of chemically-modified PVCs has given strong support to the occurrence of a PVC-diester complex in the less stereoregular sequences.This complex is thought to arise from electrostatic interactions between the polarized C=O bonds of the diesters and the polarized H-C-Cl bonds of PVC.The existence of such a type of complex allows one to account for the high elastic modulii of these gels. Consequently, by replacement of a few chlorine atoms by hydrogen, the electrostatic PVC-diester interaction must eventually vanish which entails a subsequent decrease of the fraction of the complex together with a drop of the elastic modulus, which is experimentally observed.The neutron scattering study further reveals that the basic fibrillar structure is little affected while the number of contacts between fibers is significantly diminished. The main effect arising from chemical modification is a cross-sectional growth of the fibers.

The PVC fibrils are thought to be composed of three types of domains:

(1) tiny crystallites made up with the highly syndiotactic sequences,

(2) organized domains build up from the PVC-DEO complex,

(3) disordered domains.

Due to constraints that inevitably exist at the interfaces between the crystal-complex domains or the crystal-disordered domains, and that essentially arise from solvent occlusion, the cross-sectional growth of the fibrils is limited. The chemical modification has been shown to impede the formation of a PVC-DEO complex, which most probably results in the formation of “polyethylene-like” interactions. Expulsion of solvent molecules from this domain is therefore expected, which should diminish the stress imposed upon the aforementioned interfaces. As a result, fibrils are liable to grow larger in cross section. The cross-sectional growth appears therefore to be consistent with the disappearance of the PVC-DEO complex. As fibrils grow larger in the gel at constant polymer concentration the number of contacts between them inevitably decreases. This explains why the elastic modulus decreases with increasing the degree of chemical modification as its magnitude is directly related to the number of junctions per unit volume. Investigating the structure of aggregates prepared below the critical gelation concentration therefore provides information that has a direct bearing upon the gel properties, including the rheological properties.