Near‑Θ Polymers in a Cylindrical Space

The advent of single-molecule manipulations has renewed our interest in understanding chain molecules in confined spaces. The conformation and dynamics of these molecules depend on the degree of confinement and self-avoidance. A distinguishing feature of weakly self-avoiding polymers (e.g., DNA) in a cylindrical space is the emergence of the so-called extended de Gennes regime. On the other hand, an earlier study indicates that slit confinement enhances the self-avoidance of a Θ-polymer, for which the two-body (monomer–monomer) interaction vanishes. Using molecular dynamics simulations, we study how cylindrical confinement modulates the self-avoidance of near-Θ polymers. Our results suggest that the confinement enhances self-avoidance, turning a near-Θ solvent into a good solvent. This finding has a number of nontrivial consequences. First, it induces the linear ordering of a near-Θ chain, as if the chain is in a good solvent. Second, under strong confinement, the chain size, R ∥, scales with the cylinder diameter, D, approximately as R ∥ ≈ Na(D/a – 1)−4/3, where N is the number of monomers and a the monomer size. This is distinct from R ∥ ≈ Na(D/a)−1 as suggested by the conventional picture, in which the second virial coefficient, B 2, remains unchanged upon confinement. In contrast, enhanced self-avoidance is not easily felt by the confinement free energy unless B 2 is large enough, outside the regime of a near-Θ solvent. Finally, we show how these findings are related to long-range bond–bond correlations observed for single polymers or polymer melts.