Density effects on collapse, compression and adhesion of thermoresponsive polymer brushes

TL;DR

This study investigates how grafting density influences the thermal collapse, swelling, and adhesion of PNIPAM polymer brushes using surface force measurements, revealing that density significantly affects swelling but not adhesion.

Contribution

It provides unprecedented resolution in grafting density effects on thermoresponsive polymer brushes and compares experimental results with theoretical predictions.

Findings

Swelling ratio decreases from ~7 to ~3 with increasing grafting density.

Dense brushes collapse gradually with temperature increase.

Adhesion between brushes is insensitive to molecular structure.

Abstract

We probe, using the Surface Forces Apparatus, the thermal response of poly(N-isopropylacrylamide) (PNIPAM) brushes of various grafting densities, grown from plasma-activated mica by means of surface-initiated polymerization. We thus show that dense thermoresponsive brushes collapse gradually as temperature is increased, and that grafting density greatly affects their ability to swell: the swelling ratio of the brushes, which characterizes the thickness variation between the swollen and the collapsed state, is found to decrease from $\sim 7$ to $\sim 3$ as the number of grafted chains per unit area increases. Such a result, obtained with an unprecedent resolution in grafting density, provides qualitative support to calculations by Mendez {\it et al.} [{\it Macromolecules} {\bf 2005} {\it 38}, 174]. We further show that, in contrast to swelling, adhesion between two PNIPAM brushes appears to be rather insensitive to their molecular structure.