NATURE OF THE COLLAPSE TRANSITION FOR POLYMERS

We solve models for self-avoiding walks on Husimi lattices of coordina tion number q = 2(sigma + 1), sigma = 1,2,... made up of squares. An a ttractive interaction is introduced between first-neighbor sites incor porated into the walk or between bonds on opposite edges of elementary squares. For sigma > 1 one polymerized phase is present in the phase diagram and the Theta point is a tricritical point. For sigma = 1, how ever, two distinct polymerized phases appear, and the Theta point is a tricritical point (attractive sites) or a critical end point (attract ive bonds).