Influence of short chain branching on the miscibility of binary polymer blends: Application to polyolefin mixtures

The lattice cluster theory (LCT) is used to study the influence of monomer structure, i.e., short chain branching, on the miscibility of binary polymer blends. The systems are chosen as corresponding to united atom models of the monomer structures in the binary polyolefin blends studied by Graessley and co-worker. We focus on the short chain branching by using a compressible blend model with a single interaction energy for all united atom groups. The polyolefin blend miscibilities are found to correlate well with a monomer structure dependent branching parameter r, the excess volume, and the SANS interaction parameter chi(eff). Our calculations also demonstrate that except for very immiscible polyolefin blends both the ''entropic'' and ''enthalpic'' portions of chi(eff) are relevant and comparable to each other, with an increasing dominance of the entropic part over the enthalpic contribution when the blend miscibility improves. Computations are also provided for the pressure dependence of the blend miscibilities. Solubility parameters and stiffness asymmetries do not correlate with our computed blend miscibilities. Although quantitative comparisons with experiment will at least require the use of monomer dependent interaction energies, the present LCT computations permit us to understand certain experimental trends.