PHASE-TRANSITIONS OF SINGLE SEMISTIFF POLYMER-CHAINS

We study numerically a lattice model of semiflexible homopolymers with nearest neighbor (nn) attraction and energetic preference for straigh t joints between bonded monomers. For this we use a new Monte Carlo al gorithm, the ''pruned-enriched Rosenbluth Method'' (PERM). It is very efficient both for relatively open configurations at high temperatures and for compact and frozen-in low-T states. This allows us to study i n detail the phase diagram as a function of nn attraction epsilon and stiffness x. It shows a theta-collapse line with a transition from ope n coils (small epsilon) to molten compact globules (large epsilon) and a freezing transition toward a state with orientational global order (large stiffness x). Qualitatively this is similar to a recently studi ed mean-field theory [S. Doniach, T. Garel, and H. Orland (1996), J. C hem. Phys. 105(4), 1601], but there are important differences in detai ls. In contrast to the mean-field theory and to naive expectations, th e theta-temperature ino eases with stiffness x. The freezing temperatu re increases even faster, and reaches the theta-line at a finite value of x. For even stiffer chains, the freezing transition takes place di rectly, without the formation Of an intermediate globular state. Altho ugh being in conflict with mean-field theory, the latter had been conj ectured already by Doniach ct ai. on the basis of heuristic arguments and of low-statistics Monte Carlo simulations. Finally, we discuss the relevance of the present model as a very crude model for protein fold ing.