The mechanical strength of incompatible polymer interfaces welded abov
e T(g) was investigated as a function of time, temperature, and compos
ition. Three pairs of polymers were used: polystyrene-poly(methyl meth
acrylate), poly(styrene-co-acrylonitrile)-poly(methyl methacrylate), a
nd poly(styrene-co-acrylonitrile)-polycarbonate. For each pair, the we
ld 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 intermol
ecular forces. For the copolymer-homopolymer pairs, the maximum platea
u strength was reached when the Flory-Huggins interaction parameter, c
hi, was a minimum. These results are in agreement with a model based o
n Helfand's molecular theories of the structure of incompatible interf
aces, 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 surfac
es revealed dissimilar fracture surfaces for a given pair, with eviden
ce of stick-slip crack growth. XPS revealed residues of one polymer on
the other's surface, indicating cohesive fracture occurred to some ex
tent. In each case, the cohesive fracture occurred in the polymer with
the lower entanglement density and lower craze stress.