HOW FAR IS FAR FROM CRITICAL-POINT IN POLYMER BLENDS - LATTICE CLUSTER THEORY COMPUTATIONS FOR STRUCTURED MONOMER, COMPRESSIBLE SYSTEMS

Although the lattice cluster theory (LCT) incorporates many features w hich are essential in describing real polymer blends, such as compress ibility, monomer structures, local correlations, chain connectivity, a nd polymer-polymer interactions, it still remains a mean field theory and is therefore not applicable in the vicinity of the critical point where critical fluctuations become large. The LCT, however, permits fo rmulating the Ginzburg criterion, which roughly specifies the temperat ure range in which mean field applies. The present treatment abandons the conventional assumptions of incompressibility and of composition a nd the molecular weight independent effective interaction parameter ch i(eff) upon which all prior analyses of the Ginzburg criterion are bas ed. Blend compressibility, monomer structure, and local correlations a re found to exert profound influences on the blend phase diagram and o ther critical properties and, thus, exhibit a significant impact on th e estimate of the size of the nonclassical region. The LCT is also use d to test various methods which employ available experimental data in computations of the Ginzburg number Gi. The reduced temperature tau = \T-T(c)\/ defining the range of the validity of mean field theory (tau > tau(MF)) and the onset of the Ising-type scaling regime (tau > tau( crit) are quite different, and renormalization group estimates of tau( MF) and tau(crit) are presented as a function of Gi to more precisely specify these scaling regimes.