Reconstruction of the Interface of Oxidatively Functionalized Polyethylene and Derivatives on Heating

Oxidation of low-density polyethylene film with aqueous chromic acid results in a material having hydrophilic carboxylic acid and ketone groups in a thin oxidatively functionalized interface. This interface is indefinitely stable at room temperature. On heating under vacuum, it rapidly becomes hydrophobic and similar in its wettability to unfunctionalized polyethylene film. The progression of the contact angle with water from the initial value to the final value follows kinetics that suggest that the polar functional groups disappear from the interface by diffusion. The magnitude of the apparent diffusion constant derived from these studies can be described approximately by an Arrhenius equation over a significant portion of the temperature range explored, with an Arrhenius activation energy of diffusion of approximately equals 50 kcal/mol. Comparison of the properties of interfaces composed of carboxylic acid groups with those containing other species demonstrates that the structure of the interfacial groups also significantly influences the rate of reconstruction.