Miscibility behavior for blends of semiflexible chains
- Univ. of Chicago, IL (United States)
Polyolefin blends exhibit widely differing miscibilities with either upper or lower critical temperatures. A solubility parameter theory explains the phase behavior of some of these blends, but not the lower critical temperatures. Fredrickson and coworkers use incompressible Flory-Huggins theory with Gaussian fluctuations to argue that immiscibility arises primarily from differences in stiffness between chains. Schweizer and coworkers test this stiffness assertion with the molecular based compressible PRISM theory. They model polyolefin blends as linear chains of tangent spheres with identical attractive energies {var_epsilon} for both species. A bending energy E{sub b}(i) (i=1,2) determines the chain stiffness for each blend component. The PRISM calculations imply that enthalpic contributions dominate the polyolefin blend miscibilities and that stiffness alone cannot fully explain experiments. Freed and Dudowicz employ the lattice cluster theory (LCT) to determine the importance of monomer molecular structure on the miscibility of polyolefin blends. Each CH{sub n} group (n = 0 to 3) in a polyolefin chain is modeled as a united atom occupying one lattice site, and, therefore, monomers extend over several lattice sites. They consider only one van der Waals attraction {var_epsilon}, between all nearest neighbor united atom groups to focus on the influence of monomer structure on polyolefin miscibilities, so stiffness only enters the theory through the excluded volume constraints. The LCT computations show that differences in both monomer structure and enthalpic contributions affect blend miscibility. We seek to provide a direct comparison of the LCT and the PRISM theory predictions and to determine the experimentally relevant constant pressure polyolefin phase diagrams that are not readily accessible to integral equation methods.
- OSTI ID:
- 602844
- Report Number(s):
- CONF-961108--
- Country of Publication:
- United States
- Language:
- English
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