| Polymer surface dynamics have been traditionally described as an equilibrium between two states, hydrophilic and hydrophobic. The surface dynamics of polyisoprene and polybutadiene that had been treated with a water-vapor plasma, however, cannot be explained using this model. Instead, reconstruction between three states provides a model that adequately describes the surface behavior. The underlying physical mechanism of this reconstruction is believed to be aggregation upon cooling of polar groups from a dispersed state at the surface to create a hydrophilic surface, followed by extension of unmodified polymer chains to cover the polar aggregates. An Arrhenius analysis of the kinetics of reconstruction gave values of the activation energies for the forward steps as 3 ± 1 kcal/mol and 11 ± 2 kcal/mol, respectively, for polyisoprene.; In a separate study, tensile elongation of water-plasma-treated polyisoprene caused the surface to become more hydrophilic, consistent with exposure of underlying polar groups in the increased surface area. The hydrophobicity of the polymer surface returned when the strain was released. This reversibility damped out slowly over several cycles, but could be recovered by allowing the sample to relax in air. |
