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 T-h below the calorimetric glass transition temperature of the bulk T-g(bulk). Fracture energy G and fracture stress sigma 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 proportional to t(h)(1/2) and sigma proportional to t(h)(1/4) (t(h) is the healing time), and as G proportional to 1/T-h and a 1/T-h, a linear relationship between G(1/2) and sigma valid over a temperature range of T-h = T-g(bulk) - 23 degrees degrees C to T-h -T-g(bulk) -43 degrees C, has been found. The penetration depth of 0.5 nm corresponding to the value of G calculated using the measured value of sigma developed at T-h = T-g(bulk) - 53 degrees 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 polymerpolymer interfaces below T-g(bulk) 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. (c) 2005 Elsevier Ltd. All rights reserved.