Near-Theta Polymers in a Cylindrical Space

Y Jung and C Hyeon and BY Ha, MACROMOLECULES, 53, 2412-2419 (2020).

DOI: 10.1021/acs.macromol.9b02370

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 Theta- 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-Theta polymers. Our results suggest that the confinement enhances self-avoidance, turning a near-Theta solvent into a good solvent. This finding has a number of nontrivial consequences. First, it induces the linear ordering of a near-Theta chain, as if the chain is in a good solvent. Second, under strong confinement, the chain size, R-parallel to , scales with the cylinder diameter, D, approximately as R-parallel to approximate to Na(D/a - 1)(-4/3), where N is the number of monomers and a the monomer size. This is distinct from R-parallel to approximate to 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-Theta solvent. Finally, we show how these findings are related to long-range bond-bond correlations observed for single polymers or polymer melts.

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