Angle Resolved Thermal Desorption Studies for Poly(methylvinylidene cyanide) and Poly(vinylidenefluoride trifluoroethyle
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0947-A03-01
Angle Resolved Thermal Desorption Studies for Poly(methylvinylidene cyanide) and Poly(vinylidenefluoride trifluoroethylene) Carolina C. Ilie1, P. A. Jacobson1, Matt Poulsen1, Luis G. Rosa1, D. Sahadeva-Reddy2, James M. Takacs2, Stephen Ducharme1, and P. A. Dowben1 1 Physics and Astronomy, University of Nebraska at Lincoln, 116 Brace Laboratory of Physics, Lincoln, NE, 68588-0111 2 Chemistry, University of Nebraska at Lincoln, 552 Hamilton Hall, Lincoln, NE, 68588-0304 ABSTRACT We compare water adsorption and desorption on ferroelectric copolymer poly(vinylidene fluoride-trifluoroethylene), and the dipole ordered polymer poly(methylvinylidene cyanide). The angle-resolved thermal desorption spectra prove that the absorbed water species interacts more strongly with poly(vinylidene fluoride-trifluoroethylene) than with poly(methylvinylidene cyanide). The angle resolved thermal desorption spectra shows large deviations from the cosn θ distribution for light illuminated poly(vinylidene fluoride-trifluoroethylene). Water desorption from poly(methylvinylidene cyanide) deviates from the cosn θ distribution without illumination.
INTRODUCTION The desorption of a molecule from the surface, although simple enough to observe, is a poorly understood process. A deeper understanding of the physics of molecular thermal desorption is constrained by the limited number of angular resolved thermal desorption studies. We do know that the desorption energies for molecules leaving a surface can be dependent on the desorption angle [1,2]. Desorption should occur preferentially along the surface normal according to a cosn θ distribution, if ballistic phonons mediate molecular desorption. In fact, desorption rate of a surface species can deviate from the expected cosine law, and furthermore can even be enhanced in an off normal. Off normal desorption has been observed [2,3,4] and is likely due to steric effects. Activated desorption processes have been suggested since 1968 [5] to explain molecular desorption rates that are often strongly peaked along the surface normal. Although this model [5], like the model of different gas temperatures [6] fails to explain the experimental data [1], externally activated thermal desorption is known [3], although the microscopic mechanisms have not been directly identified. Here we demonstrate that there may be systems where both extrinsic and intrinsic activated molecular desorption processes should be considered. The interaction of water with the polymers poly(vinylidene fluoride-trifluoroethylene) (70:30), P(VDF-TrFE) [3,7,8], and poly(methylvinylidene cyanide), PMVC [9], have been previously investigated adsorption systems. The thermal desorption of water from P(VDF-TrFE) has been observed to be enhanced with UV illumination [3] suggesting that these systems provide a test of activation processes in thermal desorption. The absorbed water species interact more strongly with PVDF-TrFE than with PMVC [9], as indicated in Figure 1. The absorption of water is believed to distort the polymer chain
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