LIMITS OI VALIDITY FOR MEAN-FIELD DESCRIPTION OF COMPRESSIBLE BINARY POLYMER BLENDS

We examine phase diagrams of model binary polymer blends and estimate the temperature range where mean field theory is applicable. The compu tations use both. compressible and incompressible Flory-Huggins theory and the corresponding random phase approximations. The size of the no nclassical:regime is estimated in terms of the Ginzburg number Gi for both the one and two phase regions. Model calculations indicate that c ompressibility (and thus pressure) can significantly affect widths of the nonclassical region and critical temperatures (T-crit) for phase s eparations. The qualitative scaling of T-crit and Gi (and hence the wi dths of the nonclassical regime) also differs between compressible and incompressible blends. Incompressible symmetric blends with polymeriz ation index N yield constant values for both the product NGi and the r atio T-crit/N, while compressible polymer mixtures produce NGi as nonc onstant and even possibly as varying nonmonotonically with chain lengt h. Pressure and entropic interaction variations are shown to change mi scibility limits and strongly to affect Ginzburg numbers for some blen ds. Model calculations provide insight into common experimental method s for determining the onset of the nonclassical domain.