Microphase Separation of Mixed Binary Polymer Brushes at Different Temperatures

Height-Switching Dynamics of Mixed Polymer Brushes with Polymers of Different Stiffnesses

Mixed brushes consisting of flexible and semiflexible polymers of the same chain length exhibit a height-switching phenomenon because of rigidity-dependent critical adsorption [Yang et al. Macromolecules 2020, 53, 7369]. Semiflexible polymers stand higher at weak surface attraction (high temperature), but they close to the attractive surface at strong attraction (low temperature). In this work, the height-switching dynamics of the mixed polymer brushes is studied by Metropolis Monte Carlo simulation. The height-switching time is calculated by a sudden change in the surface attraction. Two surface attraction change modes, i.e., the weak-to-strong mode where the attraction is changed from weak to strong and the strong-to-weak mode where it is changed from strong to weak, are investigated. Simulation results show that the height-switching time is related to the grafting density, the polymer stiffness, and surface attraction change mode. We find that the height-switching time is significantly decreased for the strong-to-weak mode. And our results also show that the height switching in the mixed polymer brushes is reversible.

Chains Stiffness Effect on the Vertical Segregation of Mixed Polymer Brushes in Selective Solvent

The microstructure of the binary polymer brushes in the selective solvent was studied using the numerical lattice self-consisting field approach. The case was considered when the selectivity to the solvent (the Flory-Huggins parameter χ) was varied only for one type of chains (responsive chains) while the others (non-responsive chains) remained hydrophilic (χ = 0). In such a brush, with an increase in the hydrophobicity of the responsive chains, a transition occurs between two two-layer microstructures. In the initial state the ends of the longer responsive chains are located near the external surface of the brush and those of non-responsive chains are inside the brush. When the hydrophobicity of the responsive chains becomes high enough then the reversed two-layer microstructure is formed, when the ends of non-responsive chains are located near the brush surface and the responsive chains collapse on the brush bottom. In contrast to previous works, the stiffness parameter (Kuhn segment length p) for one or for both types of chains was varied and its effect on the mechanism and characteristics of the transition was studied. If the stiffness of only responsive chains increases, then the transition occurs with the formation of an intermediate three-layer microstructure, where a layer of responsive chains is located between layers formed by non-responsive ones. If both types of chains have the same p, then the transition occurs gradually without the formation of an intermediate three-layer microstructure. For both cases, the effect of p on the critical value of χ*, corresponding to the transition point and on the steepness of the transition was investigated.

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.

Design of binary polymer brushes with tuneable functionality

Using coarse grained molecular dynamics simulations, we study how functionalized binary brushes may be used to create surfaces whose functionality can be tuned. Our model brushes consist of a mixture of nonresponsive polymers with functionalized responsive polymers. The functional groups switch from an exposed to a hidden state when the conformations of the responsive polymers change from extended to collapsed. We investigate quantitatively which sets of brush parameters result in optimal switching in functionality, by analyzing to which extent the brush conformation allows an external object to interact with the functional groups. It is demonstrated that brushes with species of comparable polymer lengths, or with longer responsive polymers than nonresponsive polymers, can show significant differences in their functionality. In the latter case, either the fraction of responsive polymers or the total grafting density has to be reduced. Among these possibilities, a reduction of the fraction of responsive polymers is shown to be most effective.

pH and Salt Response of Mixed Brushes Made of Oppositely Charged Polyelectrolytes Studied by in Situ AFM Force Measurements and Imaging

The response of mixed brushes made of poly(acrylic acid) and poly(2-vinyl pyridine) with a mixing ratio of about 60:40 was studied using atomic force microscopy (AFM) force measurements with colloidal probes and AFM imaging with a sharp tip in the pH range between 2.5 and 8 and at varying KCl concentrations up to 1 M. It was found that under all conditions a dense polyelectrolyte complex layer coexists with excess polyelectrolyte chains in varying swelling states depending on pH and salt concentration. The mixed brush thus combines typical features of polyelectrolyte brushes and complexes. So, the increase of the salt concentration not only led to a transition from osmotic to salted brush regime but also to salt-induced softening or partial decomposition of the complex layer. Attractive forces at high salt concentrations indicated the presence of P2VP chains in the swollen layer even at high pH values.

Homopolymer Nanolithography

Future progress in nanoscience and nanotechnology necessitates further development of versatile, labor-, and cost-efficient surface patterning strategies. A new approach to nanopatterning is reported, which utilizes surface segregation of a smooth layer of an end-grafted homopolymer in a poor solvent. The variation in polymer grafting density yields a range of surface nanostructures, including randomly organized pinned spherical micelles, worm-like structures, networks, and porous films. The capability to use the polymer patterns for site-specific deposition of small molecules, polymers, or nanoparticles is shown. This versatile strategy enables patterning of curved surfaces with direct access to the substrate and no need in changing polymer composition to realize different surface patterns.

Influence of Grafting Point Distribution on the Surface Structures of Y-Shaped Polymer Brushes in Solution

We report a simulated annealing study of surface structures of the Y-shaped copolymers grafted onto a planar substrate in nonselective solvents. The influences of the lateral size of the grafting surface and the distribution manner of the grafting point on the order degree of the ripple structures are investigated. Under uniformly distribution conditions, it is found that the well-defined ripple structures can be formed when the lateral size less than a threshold which depends on the solvent quality and grafting density. However, introducing a density fluctuation into the uniformly distribution grafting points in different ways, the defects with different degrees are observed in the ripple structures. The influence of the density fluctuations on the ripple phase are studied quantitatively. Furthermore, the possibility of the formation of surface structures with long-range order induced by directed self-assembly is investigated. The findings provide guidelines for fabricating patterned surfaces with highly ordered structures.

Binary and Bidisperse Polymer Brushes: Coexisting Surface States

In the present work, we consider polydispersity effects on a mixed polymer brush. Two types of polymer chains with different solvent selectivity being densely grafted together onto an impenetrable surface are forming a binary mixed polymer brush. Using a numerical quasi off-lattice self-consistent field method for heterogeneous chains we study the brush profile upon varying the strength of solvent selectivity (e.g., temperature) and the degree of polymerization of the two chain types (N1 and N2, respectively). For a monodisperse brush (N1 = N2) it is well-known, that the two types of polymers segregate into a two-layer structure, if the difference in solvent selectivity is increased. The state where the chains exposed to their good solvent forming the top layer of the brush can be frustrated for shorter chains and an inversion of the layering takes place. In the inverted state, the top layer is formed by long chains exposed to poor solvent covering the layer of shorter chains. By varying the solvent selectivity of the long chains we show that coexistence of the two states occurs,which indicates a discontinuous phase transition scenario for the switching process. We consider further the case of a very low fraction of short chains and find these chains to undergo a conformational transition of first order from a "coil" state, found deep inside the compact brush layer, to a "flower" state, stretching to the top of the brush upon varying the strength of the solvent selectivity. At the transition both states are found to be quasi-stable with an energy barrier of the order of the chain length in units of kBT. The discontinuous nature of the switching process by combining solvent selectivity and bidispersity can be of high interest for the creation of stimuli-responsive surfaces.

Lock/unlock mechanism of solvent-responsive binary polymer brushes: density functional theory approach

A density functional theory (DFT) approach based on a weighted density approximation has been employed to study the perpendicular microphase separation of symmetric binary polymer brushes with weak incompatibility in explicit solvents with different selectivities. Characterized by the relation between the grand potential and vertical structures (including nonlayered and layered structures), a dry binary brush can be categorized as W-type or U-type according to whether the characteristic relation contains a structure that undergoes spontaneous symmetry breaking. A W-type brush can memorize the selectivity of the induced solvent in one of its two layered structures after the removal of solvent, which can be seen as a kind of lock state with the nonselective solvent used as its key to unlock. A U-type brush is lockless but can adapt to the environment without the nonselective solvent's triggering. Also, the boundary described in chain-length-incompatibility space is investigated by the DFT approach, which also verifies that the spontaneous symmetry breaking of the W-type brush originates from the molecular contributions to asymmetry, such as the enthalpic contribution of incompatibility and the entropic contribution of chain connectivity.