The chemistry of xylylenes. XIII. The problem of crystallinity and crosslinking in poly(p‐xylylene)

The problem of crystallinity and crosslinking in poly(p‐xylylene) was re‐investigated with the use of polymer that was prepared by polymerization of p‐xylylene in solution. It was found that amorphous polymer is converted irreversibly to α‐crystalline polymer at 50–140°C. The α‐form in turn is converted irreversibly to the β‐form at 220–260°C. The rate of change and the ultimate proportion of change is dependent upon the temperature. The polymer appears to soften and reharden at 220–250°C. and subsequently to melt at 410–420°C. This is followed by rapid thermal degradation. It was shown that the properties associated with a three‐dimensional network are imparted to poly(p‐xylylene) by reactions that occur when the mixture of insoluble polymer and residual monomer (<0.01M) is warmed from −78°C. to room temperature. This result appears to be independent of reactions that commonly afford covalent crosslinks. On the other hand, there is a close correlation with the experimental conditions that afford low molecular weight cyclic poly(p‐xylylene). It is spectulated that some cycllization might occur in such a way that two or more chains of poly(p‐xylylene) from an interlaced structure which might be responsible for the properties of poly(p‐xylyene) heretofore attributed to covalent crosslinks.