STRENGTH OF INCOMPATIBLE AMORPHOUS POLYMER INTERFACES

The mechanical strength of incompatible polymer interfaces welded above T(g) was investigated as a function of time, temperature, and composition. Three pairs of polymers were used: polystyrene-poly(methyl methacrylate), poly(styrene-co-acrylonitrile)-poly(methyl methacrylate), and poly(styrene-co-acrylonitrile)-polycarbonate. For each pair, the weld strength G1c, measured by wedge cleavage, attained a constant value which increased with welding temperature. While the plateau strengths are typically only about 5-10% of bulk G1c values, they are orders of magnitude greater than the work of adhesion calculated using intermolecular forces. For the copolymer-homopolymer pairs, the maximum plateau strength was reached when the Flory-Huggins interaction parameter, chi, was a minimum. These results are in agreement with a model based on Helfand's molecular theories of the structure of incompatible interfaces, coupled with a microscopic deformation mechanism, which predicts that G1c approximately 1/chi. X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) analysis of the fracture surfaces revealed dissimilar fracture surfaces for a given pair, with evidence of stick-slip crack growth. XPS revealed residues of one polymer on the other's surface, indicating cohesive fracture occurred to some extent. In each case, the cohesive fracture occurred in the polymer with the lower entanglement density and lower craze stress.