Sorption Characteristics of Polymer Brushes in Equilibrium with Solvent Vapors

Abstract

While polymer brushes in contact with liquids have been researched intensively, the characteristics of brushes in equilibrium with vapors have been largely unexplored, despite their relevance for many applications, including sensors and smart adhesives. Here, we use molecular dynamics simulations to show that solvent and polymer density distributions for brushes exposed to vapors are qualitatively different from those of brushes exposed to liquids. Polymer density profiles for vapor-solvated brushes decay more sharply than for liquid-solvated brushes. Moreover, adsorption layers of enhanced solvent density are formed at the brush–vapor interface. Interestingly and despite all of these effects, we find that solvent sorption in the brush is described rather well with a simple mean-field Flory–Huggins model that incorporates an entropic penalty for stretching of the brush polymers, provided that parameters such as the polymer–solvent interaction parameter, grafting density, and relative vapor pressure are varied individually.

Friends, Foes, and Favorites: Relative Interactions Determine How Polymer Brushes Absorb Vapors of Binary Solvents

“…For a single-component vapor, the absorption of a vapor depends on the strength of the interaction between this vapor and the brush . For a component in a binary vapor, this trend also holds.…”

“…Brush swelling by vapors depends on multiple system properties. In a previous study 29 from our group, simulations showed that the interparticle interactions, partial vapor pressure, and grafting density all influence the absorption of single-component vapors in contact with brushes. It was found that absorption is described relatively well by using a simple mean field model based on the Flory−Huggins theory of solvation.…”

“…Moreover, we assume that the brush composition is homogeneous throughout the brush. However, inhomogeneities perpendicular to the grafting surface are not expected for collapsed brushes since density profiles of collapsed brushes often show a sharp brush/gas interface 28 as well as a nearly height-independent density 24 , 29 (see also Figure S1 ). Large inhomogeneities parallel to the surface are prevented by the anchoring of the polymer chains; the minimum and maximum distances between chains due to this end attachment provide natural limits to the brush composition.…”

Concentrating Vapor Traces with Binary Brushes of Immiscible Polymers

“…The absorption isotherm of the pure brush (orange squares) is in line with isotherms found in solvophilic brushes. 23 We note that the isotherm of the bare surface (wall) has a concave rather than convex shape and that this difference contributes to the enhanced absorption, especially at low concentrations. Vapor adsorbing on a bare surface forms fewer attractive interactions with neighboring particles compared with vapor absorbing in brushes: For surfaces, these neighboring particles are located on the surface underneath the adsorbing particle, whereas for brushes, they are present around the absorbing particle.…”

All-Atom Molecular Dynamics Simulations of the Temperature Response of Densely Grafted Polyelectrolyte Brushes

“…In fact, several studies have shown that this is a reasonable assumption for densely grafted PE brushes . Under this condition, the brush height can be expressed as , where H is the equilibrium brush height, σ g is the PE brush grafting density, N is the number of Kuhn monomers per chain, and ϕ is the PE volume fraction. Notice that ϕ is assumed to be a constant throughout the brush height, in line with the Alexander-de-Gennes model.…”