Polymer Durability and Radiation Effects - American Chemical Society

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Chapter 14

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Effects of Simulated Space Environments on Piezoelectric Vinylidene Fluoride-Based Polymers 1

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Tim R. Dargaville - , Roger L . Clough , and Mathew Celina * 1

Sandia National Laboratories, Albuquerque, NM 87185 Under contract to SNL Current address: School of Physical and Chemical Sciences, Queensland University of Technology, Brisbane 4001, Australia 2

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Piezoelectric polymers based on polyvinylidene fluoride (PVDF) are of interest as adaptive materials for large aperture space-based telescopes. In this study, two piezoelectric polymers, PVDF and P(VDF-TrFE), were exposed to conditions simulating the thermal, radiative and atomic oxygen conditions of low Earth orbit. The degradation pathways were governed by a combination of chemical and physical degradation processes with the molecular changes primarily induced via radiative damage, and physical damage from temperature and atomic oxygen exposure, as evident from depoling, loss of orientation and surface erosion. The piezoelectric responsiveness of each polymer was strongly dependent on exposure temperature. Radiation and atomic oxygen exposure caused physical and chemical degradation, which would ultimately cause terminal damage of thin films, but did not adversely affect the piezoelectric properties.

© 2008 American Chemical Society

Celina and Assink; Polymer Durability and Radiation Effects ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

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154 High performance polymers have been recognized as a key component in the emerging technology of exceptionally large-area spacecraft, such as solar sails, arrays and large telescope mirrors. The intrinsic low density, functionality, processibility and flexibility of certain polymers make them attractive considering the storage and weight limitations of launch vehicles. Unfortunately, the hostile environment of space is extremely damaging to organic polymers, often severely restricting their performance and lifetimes (7-5). This is especially concerning given the great cost in launching the spacecraft into orbit and possible limitations on the duration or success of missions. Thin film piezoelectric polymers have the potential to be used in revolutionary large diameter film-based primary mirror adaptive optics for low Earth orbit (LEO) based telescopes. Using piezoelectric polymers has the advantage of incorporating the actuation mechanism directly into the thin film such that any mirror shape changes can be adjusted in real time. This allows for correction/compensation of misalignment errors, temperature fluctuations and even focus shifting (6). To achieve maximum weight savings, a mirror made from a piezoelectric polymer may not have any protective shielding, and could therefore be exposed to atomic oxygen (AO), vacuum ultraviolet (VUV) radiation, and the temperature extremes of LEO. We are interested in examining how piezoelectric polymers based on the vinylidene fluoride backbone will perform in LEO conditions with the overall materials selection and performance requirements having been previously considered (7,8). In this paper we have addressed the issue of piezoelectric performance of PVDF and copolymers of vinylidene fluoride and trifluoroethylene (P(VDFTrFE)) over temperature ranges simulating the LEO environment, and examined the effects of radiation (gamma and vacuum ultraviolet) and atomic oxygen.

Experimental The polymers used were poled piezoelectric films 30 ± 2 μπι PVDF from MSI and 28 ± 2 μηι P(VDF-TrFE) from Ktech Corp. The details of the temperature annealing and AO/VUV experiments have been reported elsewhere (9-11).

Results and Discussion Effects of Temperature It is generally accepted that commercial PVDF devices should not be used above 80°C due to deterioration of the piezoelectric performance. In LEO the

Celina and Assink; Polymer Durability and Radiation Effects ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

155 temperature of the material may exceed 80°C if it has full sun exposure and a low emittance to absorbance ratio. With this in mind, we exposed films of piezoelectric PVDF and P(VDF-TrFE) to elevated temperatures and then measured the residual piezoelectric response via the d piezoelectric coefficient. In Figure 1 the d coefficient is plotted against the annealing temperature for the PVDF homopolymer and for 20 and 30 % TrFE copolymers. Included in these plots are the respective DSC traces. The loss in the d of the homopolymer above 80°C does not correspond with any significant transition in the DSC; instead we propose that the loss in piezoelectric response is due to thermal contraction (as evidenced by a decrease in the film area (9)) of the highly stretched films causing concomitant randomization of the dipoles. The two TrFE copolymers exhibit gradual decreases in their d coefficients at low annealing temperature followed by a rapid reduction at 100 and 125°C for the 30 and 20 % TrFE copolymers, respectively. The DSC traces indicate that the loss in d coefficient corresponds with the Curie transition (T ; the point at which the crystalline phase changesfromthe polar form to the non-polar form). Clearly the copolymer with 20 % TrFE content is superior in temperature performance to the 30 % TrFE copolymer. It is noteworthy to mention that the d loss profiles in Figure 1 are independent of annealing time beyond several hours. For example, the PVDF homopolymer has the same d coefficient after annealing at 120°C for one day as it does after 1 year. This is very encouraging since even with a limited piezoelectric response, the materials will still deform when an electric field is applied, although the magnitude of the deformation per unit field will be less than before heat exposure. In LEO the temperatures a material experiences in the Earth's shadow may be as low as minus 100°C (4). To examine the performance of the PVDF homopolymer and 20 % copolymer at sub-ambient temperatures we measured the remanent polarization (P ; the polarization with no electric field applied) which was extracted from D-E hysteresis loops. These results are plotted in Figure 2. When cooled, the homopolymer experienced a drop in P at higher temperature compared with the TrFE copolymer. Instead of a loss of polarization, as observed during thermal annealing, the diminished P at low temperature can be attributed to an increased energy barrier for dipole switching resulting in lower observed polarization due to changes of molecular mobility and T as a function of temperature. To confirm that the results from d and remanent polarization measurements translate into bimorph performance across a wide temperature range, small rectangular bimorphs were fabricated to mimic small sections of a thin film telescope mirror. An electric field was applied to the bimorphs and the maximum deflection measured. This deflection was converted to d i (the piezoelectric response in the plane parallel to the deflection) (12) and plotted (Figure 3) between -95 and +80°C. Superimposed on the plots are the moduli of 33

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Celina and Assink; Polymer Durability and Radiation Effects ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

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Celina and Assink; Polymer Durability and Radiation Effects ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

Celina and Assink; Polymer Durability and Radiation Effects ACS Symposium Series; American Chemical Society: Washington, DC, 2007. 33

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Figure 1. Loss in the d piezoelectric coefficient as a function of annealing temperature correlated with DSC endotherms for the PVDF homopolymer and two TrFE copolymers.

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158 the two non-annealed materials measured using differential mechanical analysis (DMA). The magnitude of the bimorph deflection over the temperature range studied clearly depends on the modulus of the material. Below the glass transition temperature the bimorphs stiffen and the deflection decreases, while at high temperature any thermal depoling is compensated by the softening of the materials and results in greater deflection. A similar correlation between the d i coefficient (determined from the polarization when a stress was applied) and the modulus for PVDF was reported by Wang (13). The TrFE copolymer has a much less pronounced glass transition at approximately -30°C and as a result, has improved low temperature deflection (and higher P (Figure 2)) compared with the homopolymer. The bimorphs were also annealed at 110 and 140°C and the deflection measured. The resulting loss in deflection potential agrees well with the d annealing experiments, i.e. the copolymer can withstand higher temperatures without depoling. 3

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Figure 2. Remanent polarization of PVDF and P(VDF o-TrFE o) as a function of temperature. 8

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Effects of Vacuum UV Irradiation and Atomic Oxygen In addition to the large thermal extremes in LEO, materials may also be exposed to atomic oxygen (AO) and the full solar spectrum, which extends into the highly energetic vacuum UV range (115 - 200 nm). The high flux of atomic oxygen (AO), (approximately 10 atoms/cm -s with an orbital speed of 8 km/s) formed by photodissociation of the small concentration of residual molecular oxygen in LEO, will cause surface pitting and erosion, while the VUV exposure may induce deleterious radiation events (5). 15

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Celina and Assink; Polymer Durability and Radiation Effects ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

Celina and Assink; Polymer Durability and Radiation Effects ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

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Figure 3. Effect of temperature on d coefficients and storage moduli (E*) of a PVDF homopolymer bimorph and a P(VDF -TrFE o) bimorph. a) unannealed; b) annealed 24 hrs at 110 °C; c) annealed 24 hrs at 140