Entropically Influenced Reconstruction at the PBD-ox/Water Interface: The Role of Physical Cross-Linking and Rubber Elasticity

Oxidation of the surface of syndiotactic 1,2-polybutadiene (PBD) films with aqueous KMnO 4 /K 2 CO 3 at room temperature produced PBD-ox, containing a range of oxidized functionality. When heated against water, the surface of PBD-ox became more hydrophobic, a result contrary to what would be expected if enthalpic forces dominated the reconstruction. Initially, the hydrophilicity of the surface varied reversibly as a function of temperature, reflecting reversible changes in the relative concentrations of hydrophilic and hydrophobic groups at the interface. Eventually, however, the surface remained hydrophilic against water, independent of temperature. The temperature dependence of this phenomenon suggests the importance of entropy in determining the state of minimum interfacial free energy in this system. This entropic effect is attributable to rubber elasticity arising from crystallinity in the polymer, and its loss is associated with a change in the amount and type of crystallinity.