Publications
Evaluation of piezoelectric PVDF polymers for use in space environments
Thin polymer films have been identified as one of the major enabling technologies for future space-based systems. Potential devices include those based on piezoelectric bimorph polymers that deform when a charge is deposited, for example, from an electron gun. The thin-film and lightweight nature of the polymeric devices will allow them to be launched more readily and deployed to operating conditions once in orbit. Until now little work has been done aimed at investigating the performance of piezoelectric properties of PVDF and its copolymers and the prediction of their long-term stability in low Earth orbit (LEO) environmental conditions. In this paper, the piezoelectric properties of PVDF and the copolymers formed from polymerization of vinylidene fluoride and trifluoroethylene (TrFE) or hexafluoropropylene (HFP) have been studied over a broad temperature range simulating that expected in LEO. The temperatures experienced by unprotected polymers on low altitude spacecraft have previously been reported as ranging from approximately -100 C to +130 C as the polymer/spacecraft passes in and out of the Earth's shadow. To examine the effects of temperature on the piezoelectric properties of poled PVDF, P(VDF-TrFE) and P(VDF-HFP) the d{sub 33} piezoelectric coefficients and electric displacement-electric field (D-E) hysteresis loops were measured up to 160 C for the d{sub 33} measurements and from -80 to +110 C for the D-E loops. The room temperature d{sub 33} coefficient of PVDF homopolymer films, annealed for extended periods at 50, 80 and 125 C, dropped rapidly within a few days of heating, then remained unchanged for periods of up to 300 days. In contrast, the TrFE copolymer exhibited greater thermal stability than the homopolymer, with the d{sub 33} remaining almost unchanged from the pre-annealing value after heating at 50, 80 and 125 C. The HFP copolymer exhibited poor retention of d33 at temperatures above 80 C. For all three polymers short term annealing at 160 C reduced the d{sub 33} to zero. The decrease in d{sub 33} for the TrFE copolymer was correlated with the change in Curie temperature upon annealing of the copolymer, as observed by differential scanning calorimetry (DSC). Unlike radiation damage, which may occur from vacuum UV and atomic oxygen in LEO, the depoling of the polymers on exposure to elevated temperatures was attributed to a physical randomization of the morphology rather than a chemical degradation process. In situ D-E loop measurements over the temperature range -80 to +110 C showed that the remnant polarization of the TrFE copolymer was more stable than the PVDF homopolymer. These results suggest that the TrFE copolymer appears to have a better overall performance in thermal extremes compared with PVDF or an HFP copolymer.