Vapor uptake in polymer materials:Simulations and theory

Polymer brushes are a class of coatings consisting of end-anchored polymer chains. These brushes can be synthesized by chemically bonding entire polymer chains to a surface, or by growing the chains from initiating groups on the surface. Since they are strongly bound to the surface, brush coatings contain a high concentration of polymer even when immersed in a favorable solvent. This leads to a high osmotic pressure in the brush, which produces a range of technologically interesting properties. Proposed applications for brushes include fouling-resistant layers, surfaces with low friction and adhesion, and coatings that would increase the selectivity and absorption capacity of sensors and separation processes. Additionally, polymer molecules are generally responsive to changes in their environment, and retain this property in brush form. Polymer brushes can therefore also be employed as "smart" materials, whose properties can be switched on demand or made dependent on environmental conditions. While early polymer brush research focused on brushes in liquid environments, the use of polymer brushes in air or solvent vapors has also become a topic of interest in the last decades. Experimental studies have shown that many of the interesting properties of polymer brushes in liquid also extend to brushes in solvent vapor. However, fundamental research into vapor swelling of brushes is relatively limited. In this thesis, the validity of several simple but previously untested assumptions is examined using coarse-grained molecular dynamics simulations as the primary tool

Fundamentals and Applications of Polymer Brushes in Air

For several decades, high-density, end-tethered polymers, forming so-called polymer brushes, have inspired scientists to understand their properties and to translate them to applications. While earlier research focused on polymer brushes in liquids, it was recently recognized that these brushes can find application in air as well. In this review, we report on recent progress in unraveling fundamental concepts of brushes in air, such as their vapor-swelling and solvent partitioning. Moreover, we provide an overview of the plethora of applications in air (e.g., in sensing, separations or smart adhesives) where brushes can be key components. To conclude, we provide an outlook by identifying open questions and issues that, when solved, will pave the way for the large scale application of brushes in air

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

Polymer brushes can absorb vapors from the surrounding atmosphere, which is relevant for many applications such as in sensing and separation technologies. In this article, we report on the absorption of binary mixtures of solvent vapors (A and B) with a thermodynamic mean-field model and with grand-canonical molecular dynamics simulations. Both methods show that the vapor with the strongest vapor-polymer interaction is favored and absorbs preferentially. In addition, the absorption of one vapor (A) influences the absorption of another (B). If the A-B interaction is stronger than the interaction between vapor B and the polymers, the presence of vapor A in the brush can aid the absorption of B: the vapors absorb collaboratively as friends. In contrast, if the A-polymer interaction is stronger than the B-polymer interaction and the brush has reached its maximum sorption capacity, the presence of A can reduce the absorption of B: the vapors absorb competitively as foes

Vapor sorption in binary polymer brushes: The effect of the polymer-polymer interface

Polymer brushes attract vapors that are good solvents for polymers. This is useful in sensing and other technologies that rely on concentrating vapors for optimal performance. It was recently shown that vapor sorption can be enhanced further by incorporating two incompatible types of polymers A and B in the brushes: additional vapor adsorbs at the high-energy polymer-polymer interface in these binary brushes. In this article, we present a model that describes this enhanced sorption in binary brushes of immiscible A-B polymers. To do so, we set up a free-energy model to predict the interfacial area between the different polymer phases in binary brushes. This description is combined with Gibbs adsorption isotherms to determine the adsorption at these interfaces. We validate our model with coarse-grained molecular dynamics simulations. Moreover, based on our results, we propose design parameters (A-B chain fraction, grafting density, vapor, and A-B interaction strength) for optimal vapor absorption in coatings composed of binary brushes

Concentrating Vapor Traces with Binary Brushes of Immiscible Polymers

Vapors in the air around us can provide useful information about our environment, but we need sensitive vapor sensors to access this information, especially because those vapors are often present at very low concentrations. We report molecular dynamics simulations of a concept that can significantly increase the sensitivity of vapor sensors at low concentrations. By coating the sensor surfaces with end-anchored immiscible polymers, surface-bound polymer blends are formed that can concentrate vapors, reaching sorption enhancements of more than one order of magnitude, especially at low vapor concentrations

Polymer Brush Friction in Cylindrical Geometries

Polymer brushes are outstanding lubricants that can strongly reduce wear and friction between surfaces in sliding motion. In recent decades, many researchers have put great effort in obtaining a clear understanding of the origin of the lubricating performance of these brushes. In particular, molecular dynamics simulations have been a key technique in this scientific journey. They have given us a microscopic interpretation of the tribo-mechanical response of brushes and have led to the prediction of their shear-thinning behavior, which has been shown to agree with experimental observations. However, most studies so far have focused on parallel plate geometries, while the brush-covered surfaces might be highly curved in many applications. Here, we present molecular dynamics simulations that are set up to study the friction for brushes grafted on the exterior of cylinders that are moving inside larger cylinders that bear brushes on their interior. Our simulations show that the density distributions for brushes on the interior or exterior of these cylinders are qualitatively different from the density profiles of brushes on flat surfaces. In agreement with theoretical predictions, we find that brushes on the exterior of cylinders display a more gradual decay, while brushes on the interior of cylinders becomes denser compared to flat substrates. When motion is imposed, the density profiles for cylinder-grafted brushes adapt qualitatively differently to the shear motion than observed for the parallel plate geometry: the zone where brushes overlap moves away from its equilibrium position. Surprisingly, and despite all these differences, we observe that the effective viscosity is independent of the radius of the brush-grafted cylinders. The reason for this is that the viscosity is determined by the overlap between the brushes, which turns out to be insensitive to the exact density profiles. Our results provide a microscopic interpretation of the friction mechanism for polymer brushes in cylindrical geometries and will aid the design of effective lubricants for these systems

Enhanced vapor sorption in block and random copolymer brushes

Polymer brushes in gaseous environments absorb and adsorb vapors of favorable solvents, which makes them potentially relevant for sensing applications and separation technologies. Though significant amounts of vapor are sorbed in homopolymer brushes at high vapor pressures, at low vapor pressures sorption remains limited. In this work, we vary the structure of two-component polymer brushes and investigate the enhancement in vapor sorption at different relative vapor pressures compared to homopolymer brushes. We perform molecular dynamics simulations on two-component block and random copolymer brushes and investigate the influence of monomer miscibility and formation of high-energy interfaces between immiscible monomers on vapor sorption. Additionally, we present absorption isotherms of pure homopolymer, mixed binary brush and 2-block, 4-block, and random copolymer brushes. Based on these isotherms, we finally show that random copolymer brushes absorb more vapor than any other architecture investigated thus far. Random brushes display enhanced sorption at both high and low vapor pressures, with the largest enhancement in sorption at low vapor pressures

Sorption Characteristics of Polymer Brushes in Equilibrium with Solvent Vapors

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

Vapor Swelling of Polymer Brushes Compared to Nongrafted Films

Polymer brushes, coatings of polymers covalently end-grafted to a surface, have been proposed as a more stable alternative to traditional physisorbed coatings. However, when such coatings are applied in settings such as vapor sensing and gas separation technologies, their responsiveness to solvent vapors becomes an important consideration. It can be anticipated that the end-anchoring in polymer brushes reduces the translational entropy of the polymers and instead introduces an entropic penalty against stretching when vapor is absorbed. Therefore, swelling can be expected to be diminished in brushes compared to nongrafted films. Here, we study the effect of the anchoring-constraint on vapor sorption in polymer coatings using coarse-grained molecular dynamics simulations as well as humidity-controlled ellipsometry on chemically identical polymer brushes and nongrafted films. We find a qualitative agreement between simulations and experiments, with both indicating that brushes certainly swell less than physisorbed films, although this effect is minor for common grafting densities. Our results imply that polymer brushes indeed hold great potential for the intended applications