Novel Fluoro-Terpolymers for Coatings Applications

Abstract

A series of thermally stable, elastomeric and hydrophobic fluoro-terpolymers of tetrafluoroethylene (TFE), vinyl acetate (VAc), and poly(dimethylsiloxane) methyl acrylate-terminated (PDMSMA), P(TFE-ter-VAc-ter-PDMSMA), were synthesized in supercritical carbon dioxide. Because the differences in reactivity ratios of TFE and PDMSMA precluded copolymerization, VAc was introduced into the feed resulting in terpolymers with yields as high as 64%, weight-average molar masses between 29 and 173 kg mol-1, and polydispersity between 1.8 and 3.7. DSC examination of the fluoro-terpolymers indicated a microphase separation consisting of P(TFE−VAc) and P(PDMSMA) domains for all terpolymer compositions. However, P(TFE-ter-VAc-ter-PDMSMA) terpolymers having TFE content greater than or equal to 55.2 mol % had additional semicrystalline domains, providing a Tg of −120 °C [P(PDMSMA) domains], a Tg of 28 °C [P(TFE−VAc) domains], and a Tm of 235 °C [PTFE domains]. Cross-linked terpolymers having 55.2 mol % of TFE lose only 5% of their mass after 14 days under continuous heating at 200 °C in air. As determined from stress/strain curves, the elastic modulus of the cross-linked terpolymer films increased with molar mass of the linear polymer, fluorocarbon composition, and curing time. According to dynamic advancing and receding water contact angle data, both un-cross-linked (112°/69°) and cross-linked (125°/81°) terpolymer films were more hydrophobic than TFE−VAc copolymer films (90°/65°). X-ray photoelectron spectroscopy revealed that both un-cross-linked and cross-linked films were surface-enriched with silicone (and not fluorine) at the air−polymer interface relative to bulk composition. These results suggest that PDMS pendant groups were oriented at the air surface, likely due to the combination of greater mobility and lower surface tension than −CF2− groups of TFE segments. These novel P(TFE-ter-VAc-ter-PDMSMA) terpolymers exhibit the properties associated with polysiloxanes and fluoropolymers, making them useful for coatings applications.