First-order phase transition in unfolding a collapsed polymer: A histogram Monte Carlo simulation

The phase transition associated with stretching a collapsed polymer chain (consisting of N monomers) is investigated in detail using the histogram Monte Carlo simulation method and the bond-fluctuation model. The distribution functions for the number of contacts and end-to-end distances are obtained. The free energy profile associated with the transition is explicitly computed. Our results on the energy cumulants and free energy profiles provide direct evidence for the first-order nature of the phase transition. The phase transition occurs in the whole poor solvent regime below the $\Theta$ point, $\chi >{\chi }_c.$ The free energy barrier at the transition is found to scale with $N\left(\chi -{\chi }_c\right).\mathrm{}$ Phase diagram for the first-order transition phase boundary is also obtained. We find that the mean-field theory results give only a qualitatively correct picture for the phase transition.