The phase behaviors of adsorbed polymethylene chains

The phase behaviors of a single adsorbed polymethylene chain are investigated by using molecular dynamics simulations. In the free space, it is confirmed in our calculation that the isolated polymer chain exhibits a disordered coil state at high temperatures, and collapses into a condensed state at low temperatures, i.e., the coil-to-globule transition, and finite chain length effects are considered since the critical region may depend on the chain length. When the chain is adsorbed on an attractive surface, however, the equilibrium properties may not only depend on chain length but also depend on the adsorption energy. For short chain of N = 40 monomers, a coil-to-globule transition is found for weak adsorption energy of w = 2.5 kcal/mol, but the critical temperature is lower than the free chain, and for strong adsorptions of w = 3.5 and 4.5 kcal/mol, the structures at low temperatures are adsorbed hairpin like, so we may call the transition process coil-to-hairpin transition. For long chains of N = 80 monomers and N = 120 monomers, the critical regions are the same for the free chains both at T = 265 K, and for the adsorption energies of w = 2.5, 3.5, and 4.5 kcal/mol, the curves of the heat capacities are smooth when T > 200 K, and while T < 200 K, the values of the heat capacities decrease as the temperatures decreasing, so the transition may be from loose globular structures to compact globular structures, and for more stronger adsorption energy of w = 6.5 and 8.5 kcal/mol, the critical regions are obvious and they are coil-to-crystal like transitions.