MACROMOLECULAR CLUSTERS IN POOR-SOLVENT POLYMER-SOLUTIONS

We apply our previously proposed ''Gaussian cluster'' model [J. Chem. Phys. 104, 1626 (1996)] to the study of macromolecular association and aggregation, under poor-solvent conditions (T<Theta). The properties of the macromolecular clusters are studied as a function of the number of chains v=2,3,4,..., of the attractive two-body excluded-volume par ameter z and of the repulsive three-body parameter K-1. In the tempera ture window between its upper stability temperature and the coil-globu le transition temperature for the single chains, a cluster of 2 divide d by 10 chains can have a smaller radius of gyration than an isolated polymer chain, provided K-1 is sufficiently small. We suggest that thi s parameter may be estimated by considering the relative separations b etween the Theta, the critical and the collapse temperatures. We compu te the equilibrium distribution of the chains in a dilute solution amo ng all possible cluster sizes (including v=1, the isolated chains); po pulation of the clusters v greater than or equal to 2 increases with K -1, but is generally rather small. Below the coil-globule transition t emperature, anywhere on the dilute side of the two-phase region of the phase diagram, there is no free-energy barrier to polymer aggregation and precipitation: a single collapsed chain is a ''critical nucleus'' and the spinodal practically coincides with the binodal. (C) 1997 Ame rican Institute of Physics.