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

Abstract

The paper reports the mechanical behaviour of two polymers, poly(vinylidenē uoride) (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 in¯uence 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 in¯uence of the carbon dioxide plasticization is pronounced and a signi®cant initial decrease in the hardness is reported. Subsequently, the hardness increases with increasing gas pressure, which is attributed to the action of an e ective 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.

Microscopic origin of the high-strain mechanical response of poled and non-poled poly(vinylidene fluoride) in the β-phase

“…The values of E 0 obtained in our case are 30 GPa for the non-poled sample and 840 GPa for the poled one. The value obtained for the non-poled samples is lower than the one observed in [25] as expected due to the difference in the experimental conditions. The strong increase of the Young's modulus at 0 K with poling reflects on the one side, the increase of strength due to the electromechanical coupling, but on the other hand the value is extremely big due to lower thermo-mechanical thermal stability of the poled sample.…”

“…The value of the parameter a (units K À1 ) is 0.013 for the non-poled longitudinal sample, 0.021 for the poled parallel sample and 0.026 for the non-poled transversal sample. The value obtained for the non-poled longitudinal samples is the same as the one obtained by in compression experiments in similar samples [25]. The values of E 0 obtained in our case are 30 GPa for the non-poled sample and 840 GPa for the poled one.…”

“…3). The value of the initial hardness obtained for the non-poled sample stretched along the longitudinal direction -H 0 = 95 MPa -is half from the one observed in previous compression experiments [25] but shows the same temperature dependence as indicated by the value of the b-parameter, 0.012 K À1 . The dipolar interactions between crystallites and the crystallites and the amorphous media though the interface increases hardness but decreases its thermal stability: H 0 = 179 MPa, b = 0.015 K À1 for the poled longitudinal sample.…”

“…where E 0 is the modulus for a T = 0 K and a is a fitting parameter indicating the thermal variation of the modulus [25]. All fittings were obtained with R > 0.996.…”

“…An exponential-type decay of the plastic component of the hardness with increasing temperature (Eq. (5)), has been empirically established for the micro-hardness behavior of various semicrystalline and glass polymer [25,28]:…”