Relationship between Segmental Relaxation of Polystyrene Films and Stick-Slip Behavior during Dynamic Wetting of Liquid Droplets on Their Surfaces.

Abstract

A novel method was previously reported for detecting the glass transition of thin polystyrene (PS) films by correlating the relationships between the temperature-dependent viscoelasticity of the PS films and stick-slip behavior on their surfaces during dynamic wetting of liquid droplets. In the present study, the frequency dependence of the stick-slip behavior is investigated. The results show that the stick-slip behavior of liquid dynamic wetting on PS films is dependent on the contact line velocity, which is related to the deformation frequency of the PS surface during the moving liquid front. The stick-slip behavior was revealed to be determined by a dimensionless parameter (ξ), which is the ratio of the PS segmental relaxation time (τα) and the characteristic time (τc) for PS surface deformation near the droplet contact line. When ξ is close to 1 (τα ≈ τc), the Δθ (jumping angle), a scale of the stick-slip behavior, reaches a maximum. This correlation between Δθ and ξ demonstrates that the stick-slip behavior is related to the energy dissipation caused by the PS α-relaxation process, and the peak temperature (or frequency) in Δθ corresponds to the α-relaxation temperature (time) of the polymer. These results strongly demonstrate that the utilization of the stick-slip behavior is a creditable method, similar to dynamic viscoelastic measurement, for probing the glass transition and segmental relaxation of thin polymer films.