MOLECULAR MODELING OF MICELLE FORMATION AND SOLUBILIZATION IN BLOCK-COPOLYMER MICELLES .2. LATTICE THEORY FOR MONOMERS WITH INTERNAL DEGREES OF FREEDOM

A self-consistent mean-field lattice theory of the micellization and s olubilization properties of poly(ethylene oxide)-poly(propylene oxide) block copolymers is described. The polymer segments are allowed to as sume both polar and nonpolar conformations (corresponding to the gauch e and trans rotations of the C-C and C-O bonds), which permits the dep endence of the segment-segment interactions on temperature and composi tion to be accounted for in a physically realistic manner. The phase d iagrams of poly(ethylene oxide) and poly(propylene oxide) in water, bo th of which exhibit lower critical solution temperatures, can be repro duced semiquantitatively. The predictions of the theory compare favora bly with published light scattering results on the aggregation behavio r of block copolymers and with our experimental results for the solubi lization of naphthalene in these micelles as a function of polymer com position and molecular weight. The dependence of the micelle-water par tition coefficient on polymer composition is not simply related to the proportion of the hydrophobic constituent but depends on the detailed micelle structure. The strong effect of the molecular weight and PPO content of the polymer on the amount of naphthalene solubilized observ ed experimentally was interpreted in terms of the model results.