Fracture energy-fracture stress relationship for weak polymer-polymer interfaces

The bulk thick films of high-molecular-weight atactic polystyrene (PS) were brought into contact at a small contact pressure ≤0.2 MPa at a constant healing temperature Th below the calorimetric glass transition temperature of the bulk Tgbulk. Fracture energy G and fracture stress σ of the auto-adhesive joints PS-PS were measured at ambient temperature in the T-peel test and the lap-shear joint geometry, respectively. In the framework of the diffusion controlled mechanism of the development of these two mechanical properties suggesting their evolution as G∝th1/2 and σ∝th1/4 (th is the healing time), and as G∝1/Th and σ∝1/Th, a linear relationship between G1/2 and σ, valid over a temperature range of Th=Tgbulk−23°CtoTh=Tgbulk−43°C, has been found. The penetration depth of 0.5 nm corresponding to the value of G calculated using the measured value of σ developed at Th=Tgbulk−53°C for 24 h was reasonably smaller than the thickness of the surface mobile layer of 1 nm predicted by Wool's rigidity percolation theory for thick bulk PS films. The feasibility of a full healing of polymer-polymer interfaces below Tgbulk has been discussed. The dependence of an apparent activation energy characterising the process of segmental motions at PS surfaces and interfaces on the approach and the physical property chosen for its calculation has been analysed.