INFLUENCE OF STIFFNESS, MONOMER STRUCTURE, AND ENERGETIC ASYMMETRIES ON POLYMER BLEND MISCIBILITIES - APPLICATIONS TO POLYOLEFINS

The generalization of the lattice cluster theory (LCT) to include expl icit trans-gauche energy differences is applied to study the combined influences of chain stiffness disparities, monomer molecular structure s, energetic asymmetries, and nonrandom mixing on the miscibilities of binary polymer blends. The combination of all these relevant physical features within a single theory enables testing various divers ent su ggestions concerning the dominant physical factors governing the misci bility of polyolefin blends. Thus, tests are presented of models ascri bing the observed miscibility patterns in polyolefin blends solely to entropic factors (stiffness disparities) or solely to enthalpic factor s (solubility parameter models). The LCT computations demonstrate the combined importance of both factors, as well as several others arising from monomer molecular structures and compressibility. An important a nd highly nontrivial ingredient in these tests is the novel computatio n of tile mean square radius of gyration for structured monomer chains . The LCT also provides partial teats of a model in which thermodynami cally equivalent semiflexible linear chains replace real polyolefin ch ains. In addition, we extend to semiflexible chains and to asymmetric polymerization indices a remarkable correlation between the binary ble nd critical temperature and a structural parameter that depends on the fractions of tri-and tetrafunctional united atom groups in the compon ent chains (for model blends ir which all van der Waals interactions a re equal). Several comparisons with experiment for polyolefin blends s erve to explain the molecular origins of observed deviations from solu bility parameter models for the phase behavior of blends containing po ly(isobutylene), as well as for the observed very weak variation of th e critical temperature with molecular weights observed in some experim ental blends.