Controlling Network–Brush Interactions to Achieve Switchable Adhesion

Abstract

Polyelectrolytes are remarkable molecules because of their smart behavior. By altering the environmental pH value, polyelectrolytes can undergo conformational transitions, which, if controlled, enable a wide range of applications. Herein, we describe experiments whereby a polyacid gel and a polybase grafted to a silicon substrate can be used as a means of demonstrating reversible switching adhesion in aqueous solution. By changing the environmental pH value, we can control in situ whether the gel adheres to the grafted layer or whether it dissociates. Such control over adhesion may have applications in actuators, microfluidics, drug delivery, personal-care products, or even in the understanding of biological materials.If we consider nonpolyelectrolytic polymers, the adhesion between a grafted polymer layer (polymer brush) and a polymer network in the molten state is due to enthalpic interactions that force the brush into the gel as well as the entropy of the brush as it maximizes its conformations by forming a random or self-avoiding walk structure. As the brush is entangled between fixed cross-links, the only way for the brush and the network to come apart and disentangle is for the brush to diffuse along its own contour. [1][2][3][4] This is prohibited, all the more so given that all of the other grafted polymers would have to perform the same disentanglement at the same time. The only way to remove the adhesion is therefore to break bonds, leaving the system unusable.Polyelectrolytes in aqueous solution, however, often undergo a transition from a hydrophobic state, where they can collapse and fall out of solution, to an extended hydrophilic state. This property makes them useful as nanoactuators when they are attached to a surface. [5,6] The properties of hydrogels in good solvent conditions are perhaps even more dramatic, with swellings greater than an order of magnitude than in their dry state routinely observed. [7] We present herein a means of using the responsiveness to the pH value of polyelectrolytes to create reversible adhesion of two surfaces by studying the interaction of a poly[2-(dimethyl amino)ethyl methacrylate] (PDMAEMA, a polybase) brush-modified surface with a poly(methacrylic acid) (PMAA) gel. The brush and gel were equilibrated in water (initially at pH 7), then brought into contact, where it was found that there was good adhesion between the brush and the gel, a result previously obtained for two oppositely charged polyelectrolyte gels.[8] (The gel will not adhere to a silicon substrate without the brush at any pH value.) At equilibrium, the PMAA gel is partially charged and swollen to 112 % of its collapsed mass at pH 2 (approximately 300 % of its dry mass). There is also expected to be some charge on the PDMAEMA [9] brush, the free polymer of which has a pK a value of approximately 7. The two components of the system should be oppositely charged at the pH value at which equilibrium was achieved (pH 5.8). However, even at pH values of between 3 and 7, there is good adhesion between the br...

Creation of dense polymer brush layers by the controlled deposition of an amphiphilic responsive comb polymer

“…8), the densest brush stretches the most, as expected. The monatonic decay of the brushes with distance from the substrate is observed for all of the samples, except for the densest (0.10 chains/nm 2 ), a phenomenon observed in our previous work on polybase brushes synthesized using an ATRP grafting from method [1,36], and possibly explained by the effect of greater charge towards the free end of the chains [37]. The NR fits for the pD 4 condition all have χ 2 < 2.…”

“…The study of weak polyelectrolytes at surfaces is of particular interest due to their responsive nature. Applications of polyelectrolyte brushes are now starting to appear [1,2], and possible applications of temperature-responsive brushes [3,4], are also likely.…”

Determination of the molar mass of surface-grafted weak polyelectrolyte brushes using force spectroscopy

“…In recent years, polymer brushes have been shown to be a useful class of materials for many medical and biological applications [13,14]. For example, custom synthetic polymers have great potential in drug delivery and molecular recognition [15], and tethering polymer chains onto surfaces can effectively reduce friction [16,17], and control adhesion [18][19][20].…”

Salt Dependence of the Tribological Properties of a Surface-Grafted Weak Polycation in Aqueous Solution

“…There is a tension between the pH and salt dependence of polyelectrolytes, since pH is often considered to convey "smart" properties to the material, with an aim of triggering an environmental stimulus, e.g. for adhesion [17][18][19][20] and drug delivery [21,22]. The addition of salt screens ions and consequently reduces the effect of pH, which is particularly important in physiological environments.…”