Concentration and chain-length dependence of thermodynamic interactions in polyethylene isotope blends

✍ Scribed by Buckley Crist

• Publisher: John Wiley and Sons
• Year: 1997
• Tongue: English
• Weight: 195 KB
• Volume: 35
• Category: Article
• ISSN: 0887-6266
• DOI: 10.1002/(sici)1099-0488(199712)35:17<2889::aid-polb12>3.0.co;2-3

✦ Synopsis

Small-angle neutron scattering (SANS) measurements of interactions in polymer blends, x NS , generally depend on blend concentration f, even though x NS is evaluated with a model that assumes that the thermodynamic interaction parameter x FH Å x NS is independent of f. Londono et al. have reported x NS to increase by Ç 41 when f drops below 0.05 in polyethylene isotope blends. The relation between scattering and thermodynamics is addressed with incompressible Flory-Huggins theory wherein the thermodynamic interaction parameter x may vary with concentration f and degree of polymerization N; here x NS (f) x x(f). For polyethylene isotope and similar polyolefin blends, the strong upward curvature of x NS implies a modest (ca. 30%) increase of x. Macroscopic phase behavior is unaffected because the shape of the binodal remains essentially unchanged. The f-dependence of x NS in turn depends on N, leading to the following empirical expression for the thermodynamic interaction parameter: x(f, N) Å b 0 (2g/Nf 1 f 2 )(f 1 ln f 1 / f 2 ln f 2 ). For polyethylene isotope blends at 155ЊC, b Å 2.85 1 10 04 and g Å 0.15. Simple Flory-Huggins behavior with x FH Å b is recovered when N approaches infinity. The source of the fand N-dependent second term is not known.