PHASE-DIAGRAM OF BRANCHED POLYMER COLLAPSE

The phase diagram of the collapse of a two-dimensional infinite branch ed polymer interacting with the solvent and with itself through contac t interactions is studied from the q --> 1 limit of an extension of th e q-state Potts model. Exact solution on the Bethe lattice and Migdal- Kadanoff renormalization group calculations shows that there is a line of theta transitions from the extended to a single compact phase. The theta line, governed by three different fixed points, consists of two lines of extended-compact transitions which are in different universa lity classes and meet in a multicritical point. On the other hand, dir ected branched polymers are shown to be completely determined by the s trongly embedded case and there is a single theta transition which is in the directed percolation universality class.