Variations in molecular compactness and chain entanglement during the compression of grafted polymers

Abstract

Characterizing the effect of geometrical confinement on mean polymer shape is an important step towards understanding and controlling molecular behaviour at interfaces. In this work, we study the configurational transitions and molecular shape changes that take place when a grafted polymer (or mushroom) is compressed by a hard plane. The polymer is modelled as a single. permanently-grafted chain with a Lennard-Jones interaction between monomer beads. For this model, we have monitored molecular size, asphericity, and chain entanglements as a function of compression, from the regime of self-avoiding walks to the regime of collapsed polymers. With these tools. we show that strong confinement can produce chain compactization and disentanglement even in the presence of a mild attractive interaction. Our results provide limit values to the degree of compression and monomer attraction that is necessary to deform strongly collapsed polymer mushrooms. Schematic representation of chain entanglements as a function of molecular size and monomer-monomer interaction. The surface has a bifurcation point (denoted by B), marking the shape transition from a soft to a hard polymer. At weak interactions, we find a two-sheet surface associated with the compressing behaviour of soft mushrooms, In this case, the compactization of free mushrooms corresponds to paths on the lower surface, whereas the transition to flat polymers takes place on the upper surface. (The direction of compression is indicated by an arrow in the drawing.) For strong interactions, we find only a one-sheet surface associated with the behaviour of hard mushrooms, On this surface, polymer shape is not affected by weak confinement, but strong confinement leads directly into flat polymers. In order to compare with Figs. 7-10, the letters a and b denote the limits of low and high compression, L = 100 A and L = 10 A, respectively.