Chemical Structures Formed in Electron Beam Irradiated Poly(tetrafluoroethylene‐co‐hexafluoropropylene) (FEP)

Abstract

Summary: The perfluorinated copolymer poly(tetrafluoroethylene‐co‐hexafluoropropylene) (FEP) was electron beam irradiated under vacuum at various temperatures ranging from room temperature to a temperature above the melting temperature of FEP. Changes of the chemical structure were analyzed by 19F solid‐state NMR and IR spectroscopy. Trifluoromethyl end groups were generated as a result of main chain scission at all irradiation temperatures studied. In addition, trifluoromethyl side groups in various environments and double bond structures were formed. Quantitative analysis showed that long‐chain branches were formed at irradiation temperatures above 200 °C. Furthermore, the hexafluoropropylene (HFP) units in FEP were found to be less sensitive to radiation than the perfluoropropyl vinyl ether (PPVE) units in poly[tetrafluoroethylene‐co‐(perfluoropropyl vinyl ether)] (PFA).19F solid‐state NMR spectra of FEP.magnified image19F solid‐state NMR spectra of FEP.

The Influence of the Irradiation Temperature on the Ratio of Chain Scission to Branching Reactions in Electron Beam Irradiated Polytetrafluoroethylene (PTFE)

“…19 F solid-state NMR spectra of few PTFE samples are shown in Figure 1. All the spectra are dominated by the strong -CF 2 -signal of the tetrafluoroethylene repeat units at À122 ppm.…”

“…All the spectra are dominated by the strong -CF 2 -signal of the tetrafluoroethylene repeat units at À122 ppm. The 19 detectable, the non-irradiated PTFE is concluded to have a unbranched structure and the number-average molecular weight exceeds 10 6 g Á mol À1 . [10] The additional weak signals in the spectra of the irradiated PTFE arise from different fluorine atoms in the chemical structures generated by radiation-induced reactions.…”

“…The new signals have been already observed and assigned during previous studies of irradiated perfluorinated polymers. [10,11,[14][15][16][17][18][19] The small signal at À82 ppm is assigned to -CF 3 end groups (-CF 2 -CF 3 ), which are generated as a result of main-chain scission. Short-chain branches (>CF-CF 3 ) as in poly(tetrafluoroethylene-cohexafluoropropylene) (FEP), give rise to signals of -CF 3 side groups (À68, À71 ppm), -CF 2 -groups adjacent to >CF-CF 3 structures (À112 ppm), and >CF-branching points (À184 ppm).…”

“…[10] Weak signals in the range of À55-À63 ppm are assumed to arise from -CF 3 groups in different environments such as a -CF 3 side group adjacent to a double bond -CF--C(CF 3 )-, a branched chain structure -C(-CF 3 ) 2 -, or a branched chain end -C(-CF 3 ) 3 . [19] Furthermore, the weak signal at À155 ppm is attributed to internal double bonds (-CF --CF -). [18] The PTFE samples were irradiated with the same dose of 500 kGy, but at different temperatures.…”

“…much improved. [7,8] It was shown that high-speed magicangle spinning (MAS) 19 F solid-state NMR provides detailed and quantitative information on changes in chemical structure occurring in PTFE as a result of irradiation. Evidence for branches and crosslinks in PTFE was derived directly from structural information using 19 F solid-state NMR.…”