Self-Avoiding Flexible Polymers Under Spherical Confinement

Snapshot of a polymer chain of N = 2048 monomers confined within a spherical cavity at monomer concentration Φ ≈ 0.03.
Snapshot of a polymer chain of N = 2048 monomers confined within a spherical cavity at monomer concentration Φ ≈ 0.03.

The May 2006 issue of Nano Letters features the work of Erik Luijten and post-doc Angelo Cacciuto on polymers under confinement.

If flexible polymer chains are confined to a narrow space, their entropy decreases, and their free energy increases accordingly. Knowledge of the excess free energy of confined polymers is an important ingredient in understanding a broad range of biological and industrial problems: it affects the behavior of colloids in suspension, it determines the forces required in flow-injection moulding, and it controls the escape time of polymers from cavities.

Decades ago, theoretical predictions indicated how the free energy would change with the degree of confinement and suggested that this dependence is essentially unaffected by the specific geometry of the confinement, i.e., whether a polymer is squeezed between two plates, into a nanotube, or into a spherical cavity. Other theoretical predictions, however, suggested that there might be a difference. Using computer simulations, Cacciuto and Luijten now addressed this long-standing controversy and showed that polymers in spherical cavities respond much more strongly to confinement than polymers in other geometries. Their calculations show that the old theory is correct for polymers between plates and in tubes, whereas the competing theory provides the correct results for polymers in a closed environment.

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