Effect of nuclear radiation on the mechanical relaxation behavior of polytetrafluoroethylene

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Abstract
The dynamic mechanical behavior of 60Co irradiated polytetrafluoroethylene (PTFE) has been investigated at audio frequencies over the approximate temperature range of 80 to 600†K for irradiation doses up to 200 Mrad. The data presented herein, together with data obtained during earlier studies of the effects of radiation on the specific volume and thermal conductivity of PTFE, demonstrate that rather low radiation doses cause pronounced structural changes leading to a more densely packed and more highly crystallized material. Irradiation produces major alternations of the mechanical spectra over the entire temperature range and subsequent postirradiation heat treatment produces additional significant changes, presumably by inducing further ordering of the structure. No significant changes were found in the mechanical relaxation behavior for PTFE samples subjected to equivalent absorbed doses of two different types of nuclear radiation. Analyses of the mechanical relaxation data using several different measures of relaxation strength indicate that the γ-relaxation is associated with motion in disordered regions of the polymer and may possibly have multiple character; that an additional relaxation process may occur in the temperature region lying between the γ- and β-relaxations; that the β-relaxation is associated with motion of chain molecules in the ordered regions of the polymer; and that the mechanism for the α-relaxation, which may also be multiple in nature, cannot be conveniently described in terms of a single molecular process associated solely with the crystalline phase or with the amorphous phase. Differential scanning calorimeter (DSC) studies over a temperature range encompassing both the room-temperature (⊂ 292 and ⊂ 303°K) crystalline transitions and the crystal-melt transition near 600° K largely substantiate the conclusions (derived from the mechanical property studies) of the occurence of radiation-induced recrystallization in bulk PTFE.