Activated and nonactivated desorption from polymer surfaces

This dissertation addresses the interaction between dipolar molecular adsorbates and crystalline polymer surfaces. The goals were to investigate and compare the interactions of water with the ferroelectric copolymer poly(vinylidene fluoride (PVDF) – trifluoroethylene (TrFE)) and poly(methylvinylidene-cyanide) (PMVC) a strongly dipole ordered polymer. As a reference, we also studied other dipolar molecular adsorbates such as bromoform and chloromethane, to further elucidate the interactions with P(VDF-TrFE). The study of adsorbate surface interactions on polymers is an emerging discipline, and still in infancy despite the long history of surface science. Our main focus is the water molecule as an adsorbed and absorbed dipolar molecule. The size of the molecule facilitates absorption on and desorption from the bulk of a crystalline polymer. We propose that on the microscopic scale, dipole interactions matter and affect the surface physical chemistry at the polymer surfaces, as does lattice strain caused by water absorption. Surface dipoles can affect the binding site of water species adsorbed at the surface and sterically hinder or enhance desorption of adsorbed or absorbed water. We find that perturbations of local surface dipoles can affect desorption of absorbed water and polymer lattice strain plays an important role. Prior studies have shown that the electronic structure of the ferroelectric P(VDF-TrFE) films is locally altered with incident UV radiation suggesting metastable excited states that may involve dipole reorientation. Light polarization dependent photo-assisted thermal desorption helps demonstrate that water desorption from surface and bulk can be influenced by the formation of electronic metastable states. Changes in local dipole orientation and the formation of long lived metastable states affect the strength of the coupling between the dipoles of water molecules and the dipoles of the copolymer P(VDF-TrFE). These effects were not observed for water absorption and adsorption on PMVC adding the necessary supporting evidence that dipole rotation can alter the desorption kinetics for absorbed water. The water desorption from PMVC is an intrinsically activated process by the strain in the polymer, while the water desorption from P(VDF-TrFE) is not. This suggests that dipole rotation in the polymer substrate may play a key role