REGULAR AND IRREGULAR MIXING IN BLENDS OF SATURATED-HYDROCARBON POLYMERS

Thermodynamic interactions in binary blends of model polyolefins were investigated by small-angle neutron scattering (SANS), cloud-point det erminations, and PVT measurements. Results for four microstructures-at actic polypropylene, head-to-head polypropylene, and two saturated sam ples of polyisoprenes with reduced 1,4 content-were incorporated into a solubility parameter (SP) scheme that had been developed earlier for various model copolymers of ethylene with propylene and l-butene. Com ponent SP values were assigned, after confirming their uniqueness by v arious tests of internal consistency, with SANS-determined interaction strengths for their blends with other components. As found earlier, t he SANS-based assignments agree rather well with independent estimates of SP from the pure component PVT properties. Several new instances o f irregular mixing-blends for which the interaction strength departs s ignificantly from the SP prediction-were identified, and some apparent patterns were noted. The suggested relationship between interaction s trength and mismatch of component segment lengths was also examined in some detail. What we find is an interesting global correlation betwee n segment length and SP assignment, whether obtained from SANS data or inferred from pure component PVT properties, but no connection at all between segment length mismatch in blends and either the temperature dependence of interaction strength or the presence of mixing irregular ity.