The gelation of PVC: Characterization and control

Poly(vinyl chloride), PVC, can be gelled from 10% (w/w) solutions in a number of solvents so as to give model networks where the junctions are directly accessible by crystallographic techniques, and where the extent and properties of the network can be correlated with the nature of these junctions. The resulting network structure has been studied as a function of polymer crystallinity and polymer-solvent interaction, and a correlation between gel-forming ability and solubility parameters was found. As in previous and accompanying work [1, 2], the gel system was found to consist of two crystal populations, the gel-forming micellar (B) and the better-developed, presumably lamellar (A) crystals incorporated in the gel. The thermal behavior of wet and dried gels was followed by DSC and by high-temperature x-ray diffraction, in the course of which the distinctness of the two crystal classes became apparent also as regards thermal stability. The gelation behavior of PVC is discussed in the wider perspective of polymer crystallization and in relation to plasticization and the stereochemistry of PVC. The explicit connections between industrial plasticization and the present gelation behavior are being pointed out, hopefully with benefit for the understanding and control of the former. The crystallization effects themselves raise further questions as regards the necessity of long syndiotactic runs in the chain (as opposed to the usually envisaged random distribution of stereoisomers) or alternatively the incorporation of substantial amounts of isomeric defects into the syndiotactic lattice. Arguments for and against these obviously fundamental alternatives are invoked.