COUPLED ENTHALPIC-PACKING EFFECTS ON THE MISCIBILITY OF CONFORMATIONALLY ASYMMETRIC POLYMER BLENDS

A systematic numerical study of a minimalist model of conformationally and interaction asymmetric model binary blends has been carried out b ased on a computationally convenient formulation of PRISM theory and t he free energy route to the thermodynamics. Effective chi-parameters b ased on the correlated enthalpic contribution to the free energy of mi xing are computed within a constant volume and conformationally ideal description. The model calculations are carried out for molecular para meters which span the plausible range for hydrocarbon materials such a s the polyolefins. Distinct regimes of miscibility behavior are found depending on whether the conformational and interaction asymmetries re inforce or tend to compensate. The former case is the most common expe rimental situation, and large positive chi-parameters are predicted du e to the nonadditive influence of unfavorable bare energetic interacti ons and local packing differences between the stiffness asymmetric spe cies. The theory predicts many non-mean-fieid behaviors observed in re cent experiments such as failure of naive group contribution schemes, strong deuteration swap effects for random copolymer alloys, breakdown of the random copolymer theory for the effect of copolymer compositio n, and the possibility of unusual temperature dependences of, and appa rent entropic contribution to, the effective X-parameter. if the confo rmational and interaction asymmetries tend to anticorrelate or compens ate, then the theory predicts the possibility of negative chi-paramete rs associated with blend composition-dependent local packing rearrange ments. This novel feature suggests several design strategies for achie ving miscible high molecular weight polyolefin blends by rationally ma nipulating conformational and dispersion force properties of the polym er molecules. The blend theory is also compared with a microscopic ver sion of a pure component-based solubility parameter theory. Overall, a remarkable agreement is found in the asymmetry reinforcement regime, but significant mixing ''irregularities'' can occur especially in the asymmetry compensation regime.