Analysis of the influence of the polymerisation degree on the thermal properties of irradiation-modified poly(tetrafluoroethylene)

The influence of the polymerisation degree on the thermal properties of poly(tetrafluoroethylene) (PTFE) irradiated with 1.5 MeV electrons under various conditions was investigated. The polymerisation degree of radiation-degraded and radiation-crosslinked PTFE was determined by the analysis of chemical structure such as end groups and branching points using 19F NMR and IR methods. The melting heat and temperature observed in the as-polymerised and melt-crystallised states deviate from each other for high molecular weight PTFE. The differences vanish at polymerisation degree Xn<1600 produced by irradiation with dose of 100–200 kGy. The representation of melting temperature of the melt-crystallised state as function of Xn permits the estimation of the equilibrium melting temperature T°m as 605 K, which agrees with values given in the literature for the extended-chain crystals. We conclude that mainly extended-chain crystals were formed for PTFE of Xn≤1600 at the melt-crystallisation, whereas PTFE of Xn>1600 crystallises as folded-chain crystals. The lamellar thickness lc of the folded-chain crystals can be estimated as about 100 nm. The irradiation of PTFE in the melt results in crosslinking and branching with a strong influence on the thermal properties.