The fluoropolymer poly(tetrafluoroethylene-co-hexafluoropropylene), FEP, was subjected to γ-irradiation to allow an investigation into the chemical, physical, and mechanical radiation-induced changes. XRD analysis indicated that the un-irradiated FEP is a semicrystalline copolymer with 34% crystallinity. 19F solid-state NMR data showed that un-irradiated FEP has a composition of 9:1 (TFE:HFP). The glass transition temperature range and the melting temperature range for un-irradiated FEP were found to be 357-360 K, and 532-538 K, respectively. In this project, a number of techniques were utilized to provide a comprehensive understanding of the radiation-induced changes in γ -irradiated FEP under various conditions. These techniques included ESR, solid-state 19F NMR, XRD, DMA, DSC, TGA, and Instron analysis.
ESR Spectroscopy studies indicated that the principal radicals formed in FEP following γ-radiolysis to low doses at 77 K and 300 K were the chain-end radical (-CF2-CF2*) and the mid-chain radical (-CF2-CF*-CF2-). In addition, the mid-chain radical (-CF2-C* (CF3)CF2-) was also believed to have a small contribution to the total number of radicals formed in FEP upon radiolysis. The total radical yields, G(R*), of FEP γ -irradiated at 77 K and 300 K were 0.2 ± 0.1 and 2.0 ± 0.1, respectively. Stepwise thermal annealing experiments indicated that the mid-chain alkyl radical was more stable than the chain-end radical.
The effect of re-irradiation of previously irradiated FEP samples was investigated by ESR spectroscopy. The radicals produced in FEP which had been previously irradiated to500 kGy at 300 K, 363 K, 423 K, and 523 K were identified as the chain-end radical (-CF2-CF2*), the mid-chain radical (-CF2-CF*-CF2-), and the mid-chain radical (-CF2-C*(CF3)CF2-). The total radical yields, G(R*), for FEP samples (pre-irradiated at 300 K, 363 K, 423 K, and 523 K) following further γ -radiolysis at 77 K and 300 K were determined to be (0.6, 0.7, 0.7, 0.7) and (1.7, 1.7, 1.7, 1.6), respectively. The ESR spectra for the re-irradiated FEP samples were different in shape to those obtained for FEP without pre-irradiation treatment. The differences in the spectra were attributed to the location of the radicals in the polymer matrix.
New structures formed in FEP upon radiolysis at different temperatures were identified using high-resolution 19F solid-state NMR spectroscopy. The new chain-end group (-CF2-CF3) was identified in the 19F solid-state NMR spectra of all irradiated FEP samples. In addition, the G-values for scission and crosslinking were calculated and it was found that branched structures are formed in FEP irradiated at temperatures above 500 K. The G-values for the chain-end, (-CF2-CF3), formed were found to increase with an increase in the irradiation dose. The G-value for the formation of chain-ends at temperatures below 500 K was determined to be 0.7. The G-value for branching in FEP irradiated temperatures above 500 K was calculated to be 1.3. This presence of branching in high temperature irradiated FEP was further confirmed by the presence of a -CF2- peak at -107 ppm.
XRD analysis indicated an initial increase in the crystallinity of FEP samples irradiated at temperatures below 500 K. Conversely, irradiation at temperatures above 500 K resulted in a decrease in crystallinity. The glass transition temperature measured by DMA for FEP irradiated at 363 K showed the maximum increase in Tg, which was attributed to greater chain mobility at temperatures close to the Tg of the un-irradiated FEP Samples irradiated above 500 K showed an initial increase in Tg followed by a decrease with increasing dose, which corresponded, to the decrease in crystallinity. Results from the heat of fusion as determined by DSC measurements showed an increase for FEP irradiated at temperatures below 500 K and a decrease for samples irradiated above 500 K. These results further confirm the crystallinity measurements obtained by XRD. TGA analysis was used to examine the thermal stability of un-irradiated FEP and FEP samples irradiated in different atmospheres. The irradiated FEP samples examined in a nitrogen atmosphere were found to be thermally more stable than those examined in air atmosphere. Mechanical analysis on irradiated FEP samples with an Instron Universal Tester illustrated a decrease in elongation at break and energy to break for all irradiated FEP samples compared to the un-irradiated FEP.