Quantitative evaluation and interpretation of polymer-polymer interactions

The phase behavior of selected blends where at least one component is a random copolymer has been determined experimentally and analyzed in terms of a binary interaction model combined with appropriate thermodynamic theories such as the Flory-Huggins theory or the Sanchez-Lacombe equation-of-state theory. The temperature dependence of the binary interaction energy densities, Bij, determined in this study has also been investigated. With the introduction of an empirical scheme to account for the temperature dependence of the SanchezLacombe characteristic parameters, the temperature dependence of the experimental Flory-Huggins based interaction energy, Bij, can be well described by the lattice fluid theory using a constant bare interaction energy, * ∆Pij , in systems where specific interactions are absent. This implies that equation-of-state effects arising from volumetric contributions account for the most part of the temperature dependence of the observed interaction energies. The maximum amount of n-alkyl acrylate (xA) that can be incorporated into methyl methacrylate (MMA) copolymers while still preserving miscibility with SAN copolymers increases first and then decreases as the n-alkyl chain length of xA increases. Trends in the binary interaction energy densities involving the various n-alkyl acrylate units, BMMA/xA, BS/xA and BAN/xA, were compared with those predicted by the simple solubility parameter theory. The deviations from the geometric mean assumption, kij, were qualitatively explained in terms of monomer molecular characteristics. This study provides new experimental and theoretical results that do not support the existence of any “screening”, “asymmetric miscibility” or sequence distribution effects for the S/MMA pair that have been widely claimed in the recent literature. Therefore, the binary interaction model can be used with confidence for such systems. The interaction energy between S and n-butyl methacrylate (nBMA), BS/nBMA, determined in this study was found to be much larger than that reported in the literature. Deuteration effects were considered to be a possible reason for the above absolute difference. Attempts to correlate the binary interaction energies with structure parameters for repeat units are also reported here.