Quantitative Analysis of Thickness and pH Actuation of Weak Polyelectrolyte Brushes
Journal article, 2018
Polymer brushes are widely used as surface coatings for various inert, functional, or responsive interfaces. If the polymer can alter its protonation state (a polyelectrolyte (PE)), the brush can switch between a collapsed and swollen state with pH, which enables applications such as nanoscale actuators. However, changes in brush height as the polymer alters its charge state are not straightforward to measure accurately. Here, we show how surface plasmon resonance can be used to determine the thickness of PE brushes both in their charged and neutral states. We use different methods to measure the heights of brushes consisting of poly(acrylic acid) and the polybasic poly(2-(diethylamino)ethyl methacrylate), both prepared by atom transfer radical polymerization. We find polymers in solution that can act as refractive index probes, which do not interact with the grafted polyelectrolytes, thus providing an "exclusion height" of the brush. Importantly, the angular reflection spectrum can be used to directly identify if a probe is indeed noninteracting. Furthermore, using different noninteracting probes results in small but significant changes (∼10%) in the exclusion height as long as the probe is reasonably large (approximately >2 kg/mol). These differences cannot be attributed to probe charge. Data from multiple brushes show that the relative height increase (at physiological ionic strength), i.e., the "collapse ratio" upon charging due to pH alterations, increases with the absolute brush height. In addition, we show that the plasmonic response to the pH switching of the polyelectrolyte brush is opposite to the response of hydrophilic polymer brushes collapsing at the lower critical solution temperature. This phenomenon is explained by an increase in refractometric constant upon charging. Our study shows that surface plasmon resonance is an excellent tool for characterizing polyelectrolyte brushes and provides useful insights into pH actuation not easily obtained by other methods.