Branching and cross-linking in radiation-modified poly(tetrafluoroethylene): A solid-state NMR investigation

Fluoropolymers exhibit a number of favorable properties such as high-temperature stability, chemical resistivity, and a low surface energy. However, the possibility to incorporate fluoropolymers into other materials is limited because of the low surface energy. Modification procedures such as radiation modification can be applied to overcome these difficulties. High-resolution fluorine-19 solid-state NMR using high-speed magic-angle sample spinning has been applied to investigate the structural changes resulting from irradiation of poly(tetrafluoroethylene) (PTFE) with high-energy electrons under vacuum. PTFE irradiated at room temperature mainly shows chain scission. PTFE irradiated at high temperatures, above the melting point, under vacuum, however, contains branches as well as cross-links. Evidence for cross-links in this material is derived from structural information. A quantitative analysis of the data is used to establish the density of side soups and branches. The excess of branches to end groups is interpreted as cross-links.