Strength Development at Incompatible Polymer Interfaces
Mechanical strength of incompatible polymer interfaces welded above T$\sb{\rm g}$ has been investigated as a function of time, temperature, and composition. Three pairs of polymers were used: polystyrene-polymethylmethacrylate, polystyrene-co-acrylonitrile-polymethylmethacrylate, and polystyrene-co-acrylonitrile-polycarbonate. For each pair, the weld strength, as measured by G$\sb{\rm IC}$, attained a constant value which increased with welding temperature. While only five to ten percent of bulk G$\sb{\rm IC}$ values, these plateau values 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 interaction parameter was a minimum. These results are in agreement with a model of the interface based on current molecular theories of incompatible interfaces coupled with a chain pull out microstructural deformation mechanism, which predicts that G$\sb{\rm IC}$ increases inversely with the interaction parameter, X. Microscopic investigation of the fracture surfaces revealed dissimilar fracture surfaces for a given pair, with evidence of stick-slip crack growth. X-ray photoelectron spectroscopy revealed residues of one polymer on the other's surface, indicating cohesive fracture occurred to some extent; in each case, the residue was always from the polymer with the lower entanglement density.
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign