Polymer brushes with reversibly tunable grafting density

TL;DR

This paper introduces a new type of responsive polymer brush with a controllable grafting density, achieved through external stimuli, enabling reversible and tunable surface properties demonstrated by numerical SCF calculations.

Contribution

The study presents a novel class of polymer brushes with reversible control over grafting density via external stimuli, combining microphase coexistence with numerical and scaling analysis.

Findings

Effective grafting density can be reversibly tuned by environmental changes.

Numerical SCF calculations confirm the microphase coexistence and tunability.

Large fluctuations in individual chain states do not affect the smooth variation of the overall grafting density.

Abstract

We propose a novel class of responsive polymer brushes, where the effective grafting density can be controlled by external stimuli. This is achieved by using end-grafted polymer chains that have an affinity to the substrate. For sufficiently strong surface interactions, a fraction of chains condenses into a near-surface layer, while the remaining ones form the outer brush. The dense layer and the more tenuous outer brush can be seen as coexisting microphases. The effective grafting density of the outer brush is controlled by the adsorption strength and can be changed reversibly and in a controlled way as a response to changes in environmental parameters. The effect is demonstrated by numerical SCF calculations and analyzed by scaling arguments. Since the thickness of the denser layer is about a few monomer sizes, its capacity to form a microphase is limited by the product of the brush chain length and the grafting density. We explore the range of chain lengths and grafting densities where the effect is most pronounced. In this range, the SCF studies suggest that individual chains inside the brush show large rapid fluctuations between two states that are separated by only a small free energy barrier. The behavior of the brush as a whole, however, does not reflect these large fluctuations, and the effective grafting density varies smoothly as a function of the control parameters.