Memory effects in polymer brushes showing co-nonsolvency effects

Abstract

Densely packed polymer chains grafted to a substrate, especially polymer brushes, have been studied intensively. Of special interest are systems that react to changes in external conditions or”remember” previous conditions. With this focus, we explore the properties of PNiPAAm brushes and relate published work to own results. The co-nonsolvency effect leads to a collapse of a PNiPAAm brush for a certain mixing ratio of ethanol in water. This also influences the wetting behavior of PNiPAAm brushes. We show that through prewetting of a brush with different liquids (water and ethanol), the contact angle of subsequent water drops changes significantly. To explain this change, the swelling of the brush was investigated with spectroscopic ellipsometry and the orientation of the molecules at the surface with sum-frequency generation (SFG). Only little change in swelling was found. The SFG measurements reveal in the ethanol prewetted case a well ordered hydrophobic methyl layer at the interface, which is consistent with the contact angle measurement.

Introduction

A hundred years ago Staudinger published his article “Über Polymerisation”, which could be regarded as the start for the ever-growing field of polymer research [1] and its integration in various modern research fields. One special property of polymers is that they can be responsive to different stimuli, e.g., pH [[2], [3], [4]], ionic strength [5], light [6,7], magnetic or electric fields [[8], [9], [10]], temperature [11,12] and more. These bulk effects are also applicable when using the polymers as a coating on substrates.

Coating surfaces with polymer layers is either done by physical interaction between the polymer layer and the substrate, i.e., relying on adhesion, or by chemical bonding of the macromolecules to the substrate. Chemical bonding is especially useful for nanoscopically thick polymer films which needs precise control of the properties. Here, unique properties can be achieved as shown in [13] and references therein. When grafting polymers densely enough to a surface, their steric interaction stretches them away form the surface. The resulting coating is called a polymer brush [[13], [14], [15]], because the polymer chains are stretched away from the surface similar to bristles in a brush. Two of the major methods to create polymer brushes are grafting to and grafting from [16]. In the grafting-to method, finished end-functionized polymer chains are grafted to a surface. In contrast, in the grafting-from method the polymerization is initialized at the surface and the polymer chains grow on the substrate monomer by monomer. The grafting-from method leads to more polydisperse polymers on the surface, but allows for a higher grafting density and thicker brushes compared to the grafting-to method. Besides polymer brushes, also polymer networks with optional functionalization can be attached to the surface [[17], [18], [19]]. Currently, many promising applications of polymer brushes (and some analogous systems) emerge [20,21], including anti-icing, sensing [22,23], anti-fouling [5], self-cleaning surfaces [24,25], protein adsorption [26], cell application [[27], [28], [29], [30]], or drug release [31,32].

The prospect of this work is threefold:

• to review work in the field of responsive and in particular adaptive surfaces, with regards to wetting

• to give a selected overview of the work on memory and co-nonsolvency effects in polymer brushes

• to show some pathways to get a better understanding of how brushes showing co-nonsolvency respond to the external conditions

One focus is on the effect that prewetting has on the properties of the brush. To make changes in the properties of the brush visible, the water contact angle after prewetting is measured. Here, the co-nonsolvency effect might influence the behavior. The collapse of the thin brush layer on the scale of tens of nanometers will then change the behavior of the macroscopic contact angle of the drop that is in the millimeter scale. In addition to contact angle measurements, sum-frequency generation and spectroscopic ellipsometry measurements are presented. This method combination allows us to analyze the impact of swelling of the brush and the molecular orientation at the brushes surface. Comparing changes of the brush due to prewetting, i.e., swelling the brush for some time, enables the discussion about mechanisms leading to contact angle variation. This discussion is then embedded into the current research of this field.