The conformation of thermoresponsive polymer brushes probed by optical reflectivity

TL;DR

This paper introduces an optical reflectivity technique to analyze the conformation and phase behavior of thermoresponsive polymer brushes in water, providing insights into their structure, hydration, and hysteresis for smart coating applications.

Contribution

The study demonstrates a novel optical method for in-depth, quantitative analysis of thermoresponsive polymer brushes, revealing phase separation and hysteresis phenomena.

Findings

Spectral reflectance quantifies brush conformation and hydration.

Molecular parameters influence hydration and phase behavior.

Vertical phase separation occurs near the lower critical solution temperature.

Abstract

We describe a microscope-based optical setup that allows us to perform space-and time-resolved measurements of the spectral reflectance of transparent substrates coated with ultrathin films. This technique is applied to investigate the behavior in water of thermosensitive polymer brushes made of poly(N-isopropylacrylamide) grafted on glass. We show that spectral reflectance measurements yield quantitative information about the conformation and axial structure of the brushes as a function of temperature. We study how molecular parameters (grafting density, chain length) affect the hydra-tion state of a brush, and provide one of the few experimental evidence for the occurrence of vertical phase separation in the vicinity of the lower critical solution temperature of the polymer. The origin of the hysteretic behavior of poly(N-isopropylacrylamide) brushes upon cycling the temperature is also clarified. We thus demonstrate that our optical technique allows for in-depth characterization of stimuli-responsive polymer layers, which is crucial for the rational design of smart polymer coatings in actuation, gating or sensing applications.