STRENGTH OF POLYSTYRENE POLY(METHYL METHACRYLATE) INTERFACES

The fracture strength of polystyrene (PS) and poly(methyl methacrylate) (PMMA) interfaces was measured as a function of welding temperatures with a wedge cleavage fracture geometry. The fracture strength of 42.5 +/- 4.5 J/m2 is independent of the welding temperature in the range from 120 to 150-degrees-C. This behavior agrees with a simple scaling argument in combination with Helfand's thermodynamic theory of incompatible mixing. Calculations of the equilibrium interpenetration depth (d(infinity)) of the incompatible interface using short-time welding data of symmetric interfaces yield a value with the same order of magnitude as determined by neutron reflection experiments of Higgins et al. and Russel et al. We calculate 60 +/- 20 angstrom for a short-time welding case in comparison to Higgins' 25-angstrom result and Russel's 50-angstrom result. Helfand's theory predicts 27 angstrom for a PS-PMMA interface. Finally, on the basis of optical and X-ray photoelectron spectroscopy surface analysis of fractured PS-PMMA surfaces, less than 30% coverage of PS was on the PMMA side, indicating some cohesive fracture of PS, but no PMMA was transferred to the PS side. This occurred regardless of whether crazed or clear fracture areas were examined. An optical dye study supported this result, with as much as 20% of the PMMA crazed fracture surfaces testing positive for PS when high concentrations of the dye were used. Unlike the XPS study, PS was not observed on clear PMMA fracture surfaces. This may be due to optical intensity detection limits. We conclude that there is probably a combination of adhesive and cohesive failure, as has been reported. 6