Cosolvency-Induced Switching of the Adhesion between Poly(methyl methacrylate) Brushes

We present a simple method to reversibly switch the adhesive force between two surfaces that are decorated with poly(methyl methacrylate) (PMMA) graft polymers. By employment of a PMMA/isopropanol/water or a PMMA/ethanol/water cosolvent system, we can tune the swelling of the brushes. In pure isopropanol or ethanol the polymer grafts are collapsed, and the adhesion is high when the contacting brushes are pulled apart. In an 80−20 vol % isopropanol−water or ethanol−water composition, the brushes are swollen. In these systems the adhesion is approximately 5 times smaller compared to the adhesion measured in the pure solvent systems. Moreover, we show that PMMA/isopropanol/water cosolvent systems perform better as switchable adhesives than PMMA/ethanol/water cosolvent systems. In the latter pulling events can arise when the swollen brushes are kept in contact for a longer time, such that the adhesion hysteresis can become large and the surface coating can be damaged due to bond-breaking events. C the magnitude of adhesion between two objects is of crucial importance for a wide variety of applications, which include: “pick-up and place” systems, gecko or mussel inspired tape, and biomedical glues. A versatile method of achieving such adhesion control is the functionalization of surfaces with polymeric materials, for example hydrogels, films, and especially brushes. These polymeric systems allow for reversible switching of the adhesive force between low and high values. Under good solvent conditions, a polymer has an extended coil conformation, while in poor solvents, polymers are in a collapsed globule state. Switching between the coil and globule state can be achieved by changing the effective solvent quality, for example by directly replacing the solvent, by cononsolvency, by UV light, by changing the temperature, by changing the oxidation state, or by adjusting the pH of the solvent. Upon end-anchoring polymers to a surface at a sufficiently high density, a so-called polymer brush is formed. When such brushes are in good solvents, the polymers stretch away from the surface, whereas in poor solvents, they form a collapsed, dense film on the surface. Swollen polymer brushes can act as efficient lubricants but can also find application in bioengineering or oil recovery. The adhesion between two contacting, brush-functionalized surfaces depends on the conformational state of the polymers, i.e., whether the brushes are swollen or collapsed. The adhesion between two collapsed polymer brushes is generally high and determined by intermolecular interactions between the contacting polymers. In contrast, the adhesion is often observed to be low under good solvent conditions because the attractive polymer−polymer interactions are now overshadowed by more favorable polymer−solvent interactions such that the polymers effectively repel each other. Keeping in mind potential applications of switchable adhesives, we anticipate that the necessity of a constant energy supply to maintain a constant high or low adhesion, as one would need for thermoresponsive polymers or polymers that change their conformation in an external electric field, would not be preferred. Also, removing components from the system (e.g., salt or one of the solvents in a solvent mixture) to trigger switching from the default state would be suboptimal, while this removal is not always practically feasible. Ideally, one would like to have the default adhesive state in pure solvent, while addition of a component triggers the adhesion switch. For example, for a “pick-up and place” system, the default state for pick-up asks for a high adhesive force and thus poor solvent conditions in pure solvent. A component addition should trigger brush swelling resulting in low adhesion and therefore deposition of the picked-up material. Triggering by cosolvency, in which a mixture of two poor solvents becomes a good solvent for a particular type of polymer, meets all demands described above for a “pick-up and place” system. Nevertheless, cosolvency has, to our knowledge, not been explored in the context of smart surface coatings. Cosolvency is a generic phenomenon that has been observed in a wide variety of polymers with different solvent mixtures and is commonly applied in the pharmaceutical industry and environmental engineering. The mechanism can Received: December 7, 2014 Accepted: December 22, 2014 Published: December 26, 2014 Letter pubs.acs.org/macroletters © 2014 American Chemical Society 75 DOI: 10.1021/mz500775w ACS Macro Lett. 2015, 4, 75−79 This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.