Polymer Folding in Cylindrical Confinement

A polymer chain with sufficiently short-range interactions can undergo a direct first-order-like folding transition from an expanded coil to an ordered crystallite. This all-or-none transition is analogous to the folding transition exhibited by many small proteins [1] and is of interest for smart materials applications. Here we investigate the effects of cylindrical confinement on this transition. We study a flexible square-well-sphere chain end-tethered at the base of a hard-wall cylinder of diameter D. We carry out Wang-Landau simulations to construct the density of states, and thus the thermodynamics, for this system. For a wide cylinder an entropic stabilization of the folded state is observed [2]. However, as the cylinder diameter approaches the size of the folded state we find a destabilization effect. For cylinder diameters smaller than the native ground-state, the chain folds into a different, higher energy, ground state ensemble and the T vs D phase diagram displays non-monotonic behavior as the system is forced into different ground states for different ranges of D. The folding transition remains first-order-like until the cylinder is so narrow that only 1D folding is possible. [1] J. Chem. Phys. 145, 174903 (2016); [2] Macromolecules 50, 6967 (2017).