ON THE N-SCALING OF THE GINZBURG NUMBER AND THE CRITICAL AMPLITUDES IN VARIOUS COMPATIBLE POLYMER BLENDS

The susceptibility related to composition fluctuations in polymer blen ds can be described by a crossover function derived by Belyakov et al. [2, 3] near and far from the critical temperature T(c). This has quit e recently been demonstrated by us using neutron and light scattering data [1]. In this paper we discuss consequences of this analysis with respect to the accomplishment of the mean-field approximation far from T, and the N scaling (N is the degree of polymerization) of the criti cal amplitudes of the bare correlation length and susceptibility respe ctively and the Ginzburg number, Gi. A quite unexpected N dependence o f Gi was found : polymer blends with N < 30 give the same Gi as low mo lecular liquids and at about N = 30 an increase of Gi of more than one order of magnitude which is synonymous to an increase of the 3d-Ising regime is observed. For N greater than or similar to 30 a Gi is-propo rtional-to I/N2 scaling is found in a range up to N congruent-to 400. The observed N scaling of Gi can be explained by the large entropic co ntribution of the Flory-Huggins parameter GAMMA(sigma) related to the compressibility or free volume of the system in comparison with the co mbinatorial entropic term, which was obtained from the analysis of the scattering results with the crossover function.