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Changes in the melt viscosity of PTFE exposed to various radiation doses in air were measured using a capillary rheometer. The melt viscosity decreases dramatically after exposure to 2.5 MRad, but between 2.5 and 5 MRad there is an increase in viscosity which is attributed to the formation of branches and crosslinks. Above 5 MRad, the melt viscosity decreases with dose. Plots of log apparent melt viscosity vs. log apparent shear rate are given for PTFE exposed to various radiation doses. The data in each of these plots can be fit by a straight line (power law), and changes in the slopes of these lines are interpreted to indicate changes in the molecular weight distribution. It is postulated that the observed decrease in the slopes of these lines after irradiation is caused by a narrowing of the molecular weight distribution because of the decrease in the number of molecules at the high molecular weight end of the distribution. The melt viscosity of a PTFE sample irradiated to a given dose was less for a sample of low preirradiation crystallinity than for an otherwise identical sample of higher crystallinity. This crystallinity dependence of viscosity is related to differences in oxygen diffusion properties of crystalline and amorphous PTFE.
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Fisher et al. (1982) studied this question.
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