Studies of poly(2,6-dimethyl-1,4-phenylene oxide) blends: 2. Poly(4-methylstyrene)

Blends of poly(2,6-dimethyl-1,4-phenylene oxide) (PMMPO) and poly(4-methylstyrene) (P4MS) were found to be compatible from a variety of experimental methods including calorimetric, density, and mechanical property measurements. Blend property behavior was similar to that widely reported for PMMPO/polystyrene (PS) blends. For each blend composition studied, a single glass transition temperature (T_g) was detected by differential scanning calorimetry. The compositional dependence of blend T_g was equally well represented by the empirical inverse rule of mixtures or by the Couchman thermodynamic expression. Density measurements of molded films suggested a mild excess volume of mixing that was slightly smaller than that observed for blends of PMMPO and PS. As in the case for PMMPO/PS, densification in the solid state may be associated with the observed mechanical property behavior of the PMMPO/P4MS blends. Initial modulus at each blend composition was larger than would be predicted by a simple weighted average of component polymer values. Tensile deformation changed from a ductile to a brittle mode of failure with increasing P4MS composition. The yield stress for ductile compositions and ultimate stress of brittle samples were both higher than found for the corresponding unblended polymers and higher than would be predicted from a simple additive relationship of weighted component properties. Blend impact strength determined by small strain rate tensile tests rapidly decreased to low levels with increasing P4MS composition. This drop in impact strength became more composition sensitive at higher loading rates during multiaxial deformation in an instrumented dart impact tester.