NONUNIVERSALITY IN THE COLLAPSE OF 2-DIMENSIONAL BRANCHED POLYMERS

In this paper we study the complete phase diagram of a model of intera cting branched polymers. The model we consider is a lattice animal one , where the collapse transition can be driven both by a contact fugaci ty between two occupied nearest neighbours and by a fugacity related t o each occupied edge. Using a Potts model formulation of the problem w e conjecture the existence of two different universality classes for t he theta transitions (with thermal exponents, nu and phi, equal to (1/ 2, 2/3) and (8/15, 8/15)), separated by a higher-order percolation poi nt. We also present convincing numerical evidence for these exponent v alues using a transfer-matrix approach. We discuss the possibility of a collapse-collapse transition and we predict the behaviour of our mod el when an adsorbing surface is included.