Phase transitions in polymethylene single chains from Monte Carlo simulated annealing

Folding of single isolated polymethylene chains was examined with Monte Carlo simulated annealing. All carbon and hydrogen atoms were considered explicitly by using van der Waals plus torsional potential functions. The calculations allowed continuous variation in each torsional degree of freedom. In addition, we examined the consequences of simplifying the calculations by restricting the carbons to be on a diamond lattice. Ensembles of conformations for larger chains (C_NH_2N+2, with N = 16, 32, 64) all show a transition from a random coil at high temperature to a globular form at low temperature. For longer chains (N ≥ 64), this globularization transition is well described by the mean-field theory of Sanchez.