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Hardnesses of poly(vinylidene) fluoride and polyamide in high-pressure gas ambience

Abstract The paper reports the mechanical behaviour of two polymers, poly(vinylidene fluoride) (PVDF) and polyamide 11 (PA11), under high-gas-pressure (carbon dioxide and argon atmospheres) media at 42°C by using a normal indentation test. The experimental technique, an indentation method, for the measurement of the indentation hardness is described and the corresponding data are reported as a function of the temperature and pressure. Argon interacts poorly with both polymers and the data are similar; the hardness increases with increase in the pressure. It is shown that with PVDF, which has a relatively low glass transition temperature (-100°C), the influence of the plasticization due to carbon dioxide upon the hardness is negligible; the hardness data for argon and carbon dioxide are comparable. Subsidiary studies indicate that in the carbon dioxide and indeed argon environments no increase in crystallinity is detected after exposure to the gas environment. However, with PA11, which has a glass transition temperature (20-50°C), closer to the ambient, the influence of the carbon dioxide plasticization is pronounced and a significant initial decrease in the hardness is reported. Subsequently, the hardness increases with increasing gas pressure, which is attributed to the action of an effective hydrostatic constraint. The data for the argon environment for PA11 show no evidence of plasticization and only an increase in the hardness with increasing hydrostatic pressure. Again, no evidence of enhanced gas-induced crystallinity is observed.