Materials Science and Technology of Polymers

Switchable Adhesion and Friction by Stimulus Responsive Polymer Brushes

Controlling friction and adhesion is relevant in nature and in our daily life. Such control can be achieved using stimulus responsive end-anchored polymers forming a brush. These brushes can adapt their physicochemical properties upon changing the surrounding environments, such as temperature, light, pH, electrical and magnetic fields. In this Thesis, we primarily use atomic force microscopy (AFM) to study the relation between the degree of solvation of the macromolecules in the brush and the friction and adhesion. In general, it is accepted that collapsed brushes promote high friction/adhesion and swollen brushes exhibit low friction/adhesion. However, our results show that brush swelling and adhesion/friction is not necessarily related. We show that the relation between swelling of a brush and its tribomechanical properties is rather complex, and that it can depend on the specific interactions in the contact: chemically identical/different brush-brush or brush-solid counter surface. Switchable adhesion and friction we observed by stimulus-responsive polymer brushes have potential applications in pick up-release system, robots and in biomedical systems.

Chapter 1 gives a short introduction to the topics related to the Thesis and the main research in the following chapters.

In the first paragraphs of Chapter 2, we give an introduction to the generic concept and definitions employed in this Thesis, including the polymer brush, AFM, adhesion, friction and stimulus-responsive polymers. In the second part of the chapter, we summarize literature studies on switchable friction and adhesion by stimulus-responsive polymer films, gels and brushes in contact with chemically identical or different counter surfaces. In the last paragraphs we discuss and give some examples on the relationship between friction and adhesion/adhesion hysteresis.

In Chapter 3, we describe the tribo-mechanical properties of thermally responsive poly(N-isopropyl acrylamide) (PNIPAM) brushes. By varying the temperature below, around and above the lower critical solution temperature (LCST), switchable friction and adhesion between the brush and a gold colloid are obtained. In particular, an enhanced dissipation and friction are observed near the LCST of PNIPAM, which we attribute to the stretching of partly collapsed polymer brush chains that adhere to the gold colloid. The results have important implications for the development of polymer-based switchable tribo-mechanical systems (such as smart tweezers or artificial muscles).

In Chapter 4, we explore the enhanced friction and dissipation of PNIPAM brushes due to the co-non-solvency effect: PNIPAM brushes are swollen in pure solvents, and precipitate in a certain solvent-cosolvent mixture. Both in water and in ethanol, the brushes are swollen and low friction is obtained. In 30% volume fraction of ethanol-water mixture, the brush collapses the most, but the friction is not the highest. Surprisingly, in 10-90 vol.% ethanol-water, a maximum in friction is observed, which is again related to the stretching of partially collapsed PNIPAM brushes. The highest friction is about two orders of magnitude larger than the lowest friction in ethanol.

Besides friction, the adhesion forces between PNIPAM brushes and various colloids, like silica, gold, polystyrene (PS), poly(methyl methacrylate) (PMMA), are also characterized by employing the co-non-solvency effect. Switchable adhesion can be obtained for all these tested colloids, but the magnitude of the adhesion-change varies. The application of co-non-solvency of PNIPAM brushes to pick up, move and release nanoparticles is studied in Chapter 5. In a water-ethanol mixture of 30% ethanol, the brush-particle adhesion is the highest such that particles can be picked up. In pure ethanol, the brushes swell and the adhesion is strongly reduced such that particle-release can be triggered. We employ the change in adherence to transfer silicon nanoparticles from one liquid into another. The advantage of the system is that the formation of aggregates is strongly reduced, which is an important problem in traditional methods of nanoparticle transfer. Here, the highest friction and adhesion are not found at the same composition of ethanol and water, which shows that they are not necessarily related.

In Chapter 6, cosolvency induced switchable adhesion between PMMA brushes is described: PMMA cannot be dissolved in water and alcohols, but can be swollen in a certain mixture of them. In poor solvents for PMMA, like water, ethanol and isopropanol, there is a high contact adhesion between PMMA brushes. However, in a mixture of water and one of the alcohols (v/v = 4/5) the brushes swell and a low adhesion between the brushes is observed. Compared with ethanol-water system, which exhibits numerous pulling events due to chains interdigitation, the isopropanol-water system is the better choice for capture and release system.

In Chapter 7, we describe a new and facile method to make zwitterionic poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) brushes using a poly(glycidyl methacrylate) (PGMA)-based macro-initiator. Our PGMA-PMPC brushes are shown to be very stable in pure water and salt solutions. Only 1% of the polymers detach after immersion for 4 weeks in saline solution or in artificial seawater. In contrast, PMPC brushes prepared using the traditional silane-based method show a strong ion-specific degrafting, which can be explained by the difference in polymer stretching in the different aqueous media. The stable PMPC brushes have potential application in biomedical engineering.

In Chapter 8, we show the specific ion effect on the hydration of zwitterionic PMPC brushes to control friction and adhesion. Via different methods to extract the swelling from our AFM results, we find an opposite effect of the addition of salts to the hydration behavior of PMPC brushes: Force-distance results show a decrease in swelling by adding salts. While AFM imaging results show the opposite trend, as the brushes in salt solution are less easily compressed under the applied load. The friction coefficient decreases by adding salts, and decreases more with the increase of the anion radius. In contrast, the adhesion between the PS colloid and the PMPC brush does not relate to brush swelling and instead may arise from electrostatic interactions. We conclude this chapter with an outlook to future experiments that can clarify our observations.