Sharp and Fast: Sensors and Switches Based on Polymer Brushes with Adsorption-Active Minority Chains

Abstract

We propose a design for polymer-based sensors and switches with sharp switching transition and fast response time. The switching mechanism involves a radical change in the conformations of adsorption-active minority chains in a brush. Such transitions can be induced by a temperature change of only about ten degrees, and the characteristic time of the conformational change is less than a second. We present an analytical theory for these switches and support it by self-consistent field calculations and Brownian dynamics simulations.Multicomponent polymer brushes offer promising perspectives for the design of smart responsive materials with a wide range of applications in nano-and biotechnology [1,2]. For example, mixed polymer brushes comprising approximately equal amounts of hydrophobic and hydrophilic polymers have been used to fabricate surfaces with switchable wettability [3,4]. If the polymers phase separate along the direction perpendicular to the brush, the surface properties can switch between the properties of the two polymer species. The transition occurs on a temperature interval of ∆T ≈ 30K. However, the response times are relatively slow on the time scale of minutes to hours, due to the existence of kinetically frozen metastable states with lateral nanoscale segregation.In this letter we propose a new class of brush-based switches, which rely on a radical conformational change of adsorption-active minority chains in a brush. The basic mechanism of the transition is illustrated in Fig. 1a). Consider a brush of polymers with chain length N b containing a small amount of minority chains with length N > N b , which undergo an adsorption transition on the substrate. In the absence of the brush, the adsorption transition (in the limit N → ∞) is continuous. Due to the interaction with the brush, it becomes first order at N, N b → ∞, and at finite chain length the chain end distribution of the minority chain becomes bimodal. A small change in temperature or solvent composition may thus lead to a sharp transition from an adsorbed state, where the switch chain is completely hidden inside the brush, to an exposed state, where the free end of the switch chain is localized at the outer surface of the brush. If each minority chain has an active group attached to its free end, the brush switches between two states: one where all active groups are fully hidden inside the brush, near the solid substrate, and one where they are exposed at the outer brush surface. The active end-groups can serve as sensors triggering an immune-like response or a detectable change in optical properties.The proposed switches possess two main advantages.First, the transition is sharp and can be induced by a temperature change of only about ten degrees, as attested and utilized by polymer chromatography in mixed eluents [5]. Second, the characteristic time scale for conformational changes is small; below, we estimate it to be well below a second. Hence the rate of change in brush properties is limited by the rate of change in the external c...

Stimuli-Responsive Brushes with Active Minority Components: Monte Carlo Study and Analytical Theory

“…The switching time is one major issue for the quality of sensors or other responsive materials. In our previous work [17], we performed overdamped Brownian dynamics simulations of a single switch chain in a static brush potential to estimate the switching time. By tracking the position of the free end bead of the minority chain, we found that this bead could jump between two well separated positions (which correspond to the exposed and adsorbed state, respectively) during a time about 10 6 τ mono , where τ mono is the characteristic relaxation time of a single monomer, which is typically on the order of 10 −9 seconds for flexible chains.…”

“…IV presents a more detailed derivation of the theory for the adsorption-active minority chain sketched in Ref. [17]. In Sec.…”

“…In a recent letter, we have proposed a new class of brush-based switches [17], which rely on a radical conformational change of individual adsorption-active minority chains in an otherwise inert brush.…”

“…[17] rely on several assumptions. First, the presence of the minority chains was taken to have no effect on the surrounding polymer brush.…”

Theoretical approaches to starlike polymer brushes in Θ-solvent

“…Then, scientists are now turning increasingly interested in their vital roles in biological systems [5][6][7]. Apart from that, branched polymers offer a large number of free ends that may be functionalized, serving in nano-technical applications such as environment-responsive surface layers [8][9][10][11][12][13][14], or as antifouling barriers [15][16][17] on sensitive surfaces. When grafted onto colloids, they lead to an improved stability against coagulation, featuring a thinner polymeric layer than colloids decorated with linear chains [18].…”