(45i) Adhesion Hysteresis of Polystyrene Thin Films
Adhesion between polymer films depends on the thermodynamic surface energies of the two films, the rates of approach and separation, and the previous history of the films. Here, we present adhesion hysteresis of glassy polystyrene films measured with a surface forces apparatus (SFA). Adhesion hysteresisâan increase in the effective interfacial energy over the thermodynamic equilibrium interfacial energyâhas previously been attributed to configurational effects, e.g., interdiffusion: diffusion of polymer chain ends across an interface. It has been noted that, for liquid-like polymer films (T greater than Tg), the geometry of the films in the adhesive junction changes over the course of an adhesion cycle (film compression followed by separation and adhesive failure/jump from contact). However, the relationship between morphological changes in glassy, solid-like polymer films and adhesion hysteresis has not been reported, and previous work suggests that at temperatures lower than Tg by more than 20°C even short polymers (MW = 1,300 Da) behave like solids. Nevertheless, we report changes in surface morphology of films of low molecular weight polystyrene at temperatures well below Tg (MW = 2,330 Da, Tg - T = 47°C) during adhesion cycles, with concomitant changes in adhesion forces. Furthermore, films of higher molecular weight polystyrene (MW = 280,000 Da) do not experience morphological changes. We propose that negative stress at the contact edge of the adhesive junction increases the rate of polymer interdiffusion, resulting in stick-slip detachment and plastic deformation of the lower molecular weight polymer films, and that the increased entanglement of the higher molecular weight polymers prevents this deformation. This relationship between surface morphology and adhesion hysteresis has implications for understanding adhesion and friction between polymer films.