Piezoelectric PVDF Materials Performance and Operation Limits in Space Environments
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Piezoelectric PVDF Materials Performance and Operation Limits in Space Environments Mathew C. Celina*, Tim R. Dargaville*, Pavel M. Chaplya, Roger L. Clough Organic Materials Dept. 1811, Sandia National Laboratories, Albuquerque, NM, 87185-1411, U.S.A. * under contract to Sandia National Laboratories ABSTRACT Piezoelectric polymers based on polyvinylidene fluoride (PVDF) are of interest for large aperture space-based telescopes. Dimensional adjustments of adaptive polymer films are achieved via charge deposition and require a detailed understanding of the piezoelectric material responses which are expected to suffer due to strong vacuum UV, γ-, X-ray, energetic particles and atomic oxygen under low earth orbit exposure conditions. The degradation of PVDF and its copolymers under various stress environments has been investigated. Initial radiation aging studies using γ- and e-beam irradiation have shown complex material changes with significant crosslinking, lowered melting and Curie points (where observable), effects on crystallinity, but little influence on overall piezoelectric properties. Surprisingly, complex aging processes have also been observed in elevated temperature environments with annealing phenomena and cyclic stresses resulting in thermal depoling of domains. Overall materials performance appears to be governed by a combination of chemical and physical degradation processes. Molecular changes are primarily induced via radiative damage, and physical damage from temperature and AO exposure is evident as depoling and surface erosion. Major differences between individual copolymers have been observed providing feedback on material selection strategies.
INTRODUCTION Large diameter PVDF thin film-based adaptive optics have been identified as alternative materials to overcome weight limitations in high-resolution spaced-based telescope systems. Recent studies on wireless shape control methods have demonstrated the feasibility of this technology [1]. Utilizing charge deposition for shape adjustments requires a detailed understanding of the piezoelectric material responses. Space applications also demand consistent, predictable, and reliable performance. While PVDF (as a generic material class covering various copolymers) has been identified as a suitable piezoelectric material, it is also well known that fluorinated polymers are sensitive to high energy radiation. Mechanical properties are expected to suffer with various types of radiation (vacuum UV, γ-, x-ray, charged particles) and extreme temperature fluctuations. Experiments carried out on the low earth orbit (LEO) long-duration exposure facility (LDEF) in the late 1980’s [2-4] and NASA’s experience with material performance for both satellite and space station applications, as well as feedback from the Hubble space telescope (HST) [5-7] have repeatedly shown polymer weaknesses in these environments. While radiation degradation studies of polymers are widely reported [8] there is little information available on piezoelectric performance and ex
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