Dynamic behavior of polymer surface and the time dependence of contact angle

Time-dependent contact angles were measured by depositing sessile drops of water on the polymer surfaces and monitoring the drop shape as a function of time. It was found that contact angles decreased sharply with contact time and the equilibrium contact angle was finally attained after a certain time. Values of starting (θ_s) and equilibrium contact angles (θ_e) obtained by the sessile drop method depend on polymer properties. The Wilhelmy plate technique was used to measure advancing and receding contact angles. The variations of starting (θ_s) and equilibrium contact angles (θ_e), advancing (θ_a) and receding contact angles (θ_r) have been studied on the oxidized surface of polymers containing polybutadiene block to explore the cause of time-dependence in contact angle measurement and the meaning of θ _s and θ _e. The results showed the linear relationships between starting (θ_s) and advancing contact angles(θ_a), the equilibrium(θ_e) and receding contact angles(θ_r). The similar relationship was also established between the contact angle hysteresis(θ_a-θ_r) and differences (θ_s-θ_e) in starting contact angles and equilibrium contact angles. Therefore, time-dependence in contact angle measurement was mainly attributed to the surface reconstruction when water drops were deposited on polymer surfaces. The starting contact angle was contributed by the hydrophobic component on polymer surface and the equilibrium contact angle mainly by the hydrophilic component of polymer. These results not only demonstrated the interdependency between two contact angle measurements, the sessile drop method and the Wilhelmy plate technique, but also provided the experimental evidence to explain the cause of time-dependent contact angle. This may also provide a new method to study dynamic behavior of polymer surface.