Phase Behavior of Mixed Polymer Brushes Grown from Ultrathin Coatings

Porous Morphology of High Grafting Density Mixed Polyelectrolyte Brushes Grown from a Y-Inimer Coating

Mixed A/B polyelectrolyte (PE) brushes of opposite charges are grown from a Y-shaped initiator-bearing coating to facilitate intimate mixing of the A and B polyelectrolytes in a 1:1 grafting ratio. The design of the Y-shaped inimer includes both ATRP and NMP initiators attached to a common Y-junction. A copolymer of a Y-shaped inimer with glycidyl methacrylate is cross-linked to the substrate resulting in a stable ultrathin coating decorated with Y-shaped initiators. Weak PE A/B mixed brushes based on poly(methacrylic acid)/poly(2-vinylpyridine) (PMAA/P2VP) with a high grafting density of ∼1 chain/nm2 are grown by surface-initiated ATRP and NMP, respectively. Detailed morphological characterization of the PMAA/P2VP brushes in response to pH changes reveals a nanoporous morphology under conditions that maximize complex coacervate formation between oppositely charged brushes. The charge ratio between the A and B brushes is varied via the composition of the brushes to further study the morphology evolution. The effect of intimate contact between the A and B brushes on the morphology is probed by comparing with a mixed A/B PE system with random fluctuations in grafting composition. A quantitative and qualitative study of the pore evolution with pH as well as charge composition is presented using a combination of atomic force microscopy, water contact angle measurement, and image analysis using Gwyddion software. These studies demonstrate that the porous morphology is enhanced and most uniform when the brushes are grown from the Y-inimer, indicating that a 1:1 grafting ratio and intimate contact between A and B brushes are essential.

Phase Behavior of Polydisperse Y-Shaped Polymer Brushes under Good Solvent Conditions

Y-shaped polymer brushes represent a special class of binary mixed polymer brushes, in which a combination of different homopolymers leads to unique phase behavior. While most theoretical and simulation studies use monodisperse models, experimental systems are always polydisperse. This discrepancy hampers linking theoretical and experimental results. In this theoretical study, we employed dissipative particle dynamics to study the influence of polydispersity on the phase behavior of Y-shaped brushes grafted to flat surfaces under good solvent conditions. Polydispersity was kept within experimentally achievable values and was modeled via Schulz-Zimm distribution. In total, 10 systems were considered, thus covering the phase behavior of monodisperse, partially polydisperse and fully polydisperse systems. Using such generic representation of real polymers, we observed a rippled structure and aggregates in monodisperse systems. In addition, polydisperse brushes formed a stable perforated layer not observed previously in monodisperse studies, and influenced the stability of the remaining phases. Although the perforated layer was experimentally observed under good solvent conditions and in the melt state, further confirmation of its presence in systems under good solvent conditions required mapping real polymers onto mesoscale models that reflected, for example, different polymer rigidity, and excluded volume effects or direct influence of the surface, just to mention a few parameters. Finally, in this work, we show that mesoscale modeling successfully describes polydisperse models, which opens the way for rapid exploring of complex systems such as polydisperse Y-shaped brushes in selective or bad solvents or under non-equilibrium conditions.

Shaping the Structure and Response of Surface-Grafted Polymer Brushes via the Molecular Weight Distribution

Breadth in the molecular weight distribution is an inherent feature of synthetic polymer systems. While in the past this was typically considered as an unavoidable consequence of polymer synthesis, multiple recent studies have shown that tailoring the molecular weight distribution can alter the properties of polymer brushes grafted to surfaces. In this Perspective, we describe recent advances in synthetic methods to control the molecular weight distribution of surface-grafted polymers and highlight studies that reveal how shaping this distribution can generate novel or enhanced functionality in these materials.

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.

Novel tetraphenylethylene (TPE)-functionalized nitroxide/alkoxyamine for nitroxide-mediated homogeneous and heterogeneous polymerizations

Tetraphenylethylene (TPE)-functionalized nitroxide/alkoxyamine realized the “in situ observation” of the NMP process for both homogeneous and heterogeneous polymerization systems.

Recent progress in creating complex and multiplexed surface-grafted macromolecular architectures

Surface-grafted macromolecules, including polymers, DNA, peptides, etc., are versatile modifications to tailor the interfacial functions in a wide range of fields. In this review, we aim to provide an overview of the most recent progress in engineering surface-grafted chains for the creation of complex and multiplexed surface architectures over micro- to macro-scopic areas. A brief introduction to surface grafting is given first. Then the fabrication of complex surface architectures is summarized with a focus on controlled chain conformations, grafting densities and three-dimensional structures. Furthermore, recent advances are highlighted for the generation of multiplexed arrays with designed chemical composition in both horizontal and vertical dimensions. The applications of such complicated macromolecular architectures are then briefly discussed. Finally, some perspective outlooks for future studies and challenges are suggested. We hope that this review will be helpful to those just entering this field and those in the field requiring quick access to useful reference information about the progress in the properties, processing, performance, and applications of functional surface-grafted architectures.