Binary Polymer Brushes Research Articles

Surface Orthogonal Patterning and Bidirectional Self-Assembly of Nanoparticles Tethered by V-Shaped Diblock Copolymers.

We investigated the surface orthogonal patterning and bidirectional self-assembly of binary hairy nanoparticles (NPs) constructed by uniformly tethering a single NP with multiple V-shaped AB diblock copolymers using Brownian dynamics simulations in a poor solvent. At low concentration, the chain collapse and microphase separation of binary polymer brushes can lead to the patterning of the NP surface into A- and B-type orthogonal patches with various numbers of domains (valency), n = 1-6, that adopt spherical, linear, triangular, tetrahedral, square pyramidal, and pentagonal pyramidal configurations. There is a linear relationship between the valency and the average ratio of NP diameter to the polymers' unperturbed root-mean-square end-to-end distance for the corresponding valency. The linear slope depends on the grafting density and is independent of the interaction parameters between polymers. At high concentration, the orthogonal patch NPs serve as building blocks and exhibit directional attractions by overlapping the same type of domains, resulting in self-assembly into a series of fascinating architectures depending on the valency and polymer length. Notably, the 2-valent orthogonal patch NPs have the bidirectional bonding ability to form the two-dimensional (2D) square NP arrays by two distinct pathways. Simultaneously patching A and B blocks enables the one-step formation of 2D square arrays via bidirectional growth, whereas step-by-step patching causes the directional formation of 1D chains followed by 2D square arrays. Moreover, the gap between NPs in the 2D square arrays is related to the polymer length but independent of the NP diameter. These 2D square NP arrays are of significant value in practical applications such as integrated circuit manufacturing and nanotechnology.

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.

Surface-Initiated Zerovalent Metal-Mediated Controlled Radical Polymerization (SI-Mt0CRP) for Brush Engineering.

ConspectusThe surface-tethered polymer brush has become a powerful approach to tailoring the chemical and physical properties of surfaces and interfaces and revealed broad application prospects in widespread fields such as self-cleaning, surface lubrication, and antibiofouling. Access to these diverse functional polymer brushes is highly dependent on versatile and powerful surface-initiated controlled radical polymerization (SI-CRP) strategies. However, conventional SI-CRP typically requires oxygen exclusion, large amounts of catalysts and monomer solution, and a long reaction time, making it time-consuming and sophisticated. When using a two-plate system consisting of an initiator-bearing substrate and a metal plate, we and our collaborators introduced surface-initiated zerovalent metal-mediated controlled radical polymerization (SI-Mt0CRP). In the SI-Mt0CRP setup, a metal(0) plate (Cu, Fe, Zn, or Sn) is placed proximately to an initiator-functionalized substrate and forms a confined polymerization system which considerably simplifies the synthesis of a wide range of polymer brushes with high grafting densities over large areas (up to the meter scale).In comparison to classical SI-ATRP (catalyzed by metal salts), SI-Mt0CRP demonstrates oxygen tolerance, high controllability, good retention of chain-end functionality, and facile recyclability of the metal catalysts (i.e., metal foil/plate). Taking advantage of the confined geometry of the SI-Mt0CRP setup, polymer brushes with various conformations and architectures are easily accessible while consuming only microliter volumes of monomer solution and without complicated operations under ambient conditions. Owing to these attractive characteristics, SI-Mt0CRP has become a versatile technique for functionalizing materials for targeted applications, ranging from the areas of surface science to materials science and nanotechnology.In this Account, we summarize the recent advances of SI-Mt0CRP catalyzed by zerovalent metals (e.g., Cu, Fe, Zn, and Sn) and highlight the intrinsic advantages of the featured experimental setup, compared with the "classical" SI-CRP in which metal salt, powder, or wire is applied. We further discuss the synthetic features and proposed mechanism of SI-Mt0CRP while emphasizing the various external technologies' (including "on water" reaction, galvanic replacement, lithography, and capillary microfluidic) integrated polymerization systems. We also describe structural polymer brushes, including block copolymers, patterned and gradient structures, and arrayed and binary polymer brushes. Finally, we introduce the diverse polymer brushes that have been prepared using these techniques, with a focus on targeted and emerging applications. We anticipate that the discussion presented in this Account will promote a better understanding of the SI-Mt0CRP technique and advance the future development of practical surface brushing.

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.

Surface Coassembly of Binary Mixed Polymer Brushes and Linear Block Copolymer Chains.

Binary mixed polymer brushes (BMPBs) are two different homopolymer chains that are covalently anchored to the solid surfaces at high grafting densities. One feature of the BMPBs is the unique ability to make surface phase separation under external stimuli. In this research, we demonstrate that different surface nanostructures can be fabricated by surface coassembly of BMPBs and free block copolymer (BCP) chains. Polystyrene/poly(2-(dimethylamino)ethyl methacrylate) (PS/PDMAEMA) BMPBs on silica particles (PS-PDMAEMA-SiO2) are synthesized by a two-step "grafting to" approach. PDMAEMA-b-PS block copolymer (BCP) chains and PS-PDMAEMA-SiO2 make surface self-assembly and a variety of surface nanostructures are formed in methanol. The grafting densities of PS and PDMAEMA brushes, solvent, and the BCP structures all exert significant influences on the surface morphology. With an increase in PDMAEMA grafting density, the surface structures change from perforated layers, to rods, and to spherical surface micelles (s-micelles). The PS grafting density also exerts an effect on the formation of the surface nanostructures. At low PS grafting density, sparsely distributed s-micelles are produced, and at high density, densely distributed s-micelles are observed. Based on transmission electron microscopy and scanning electron microscopy results, a surface phase diagram is constructed, which provides a guide to the surface morphology control.

Determination of pepsin by capillary electrophoresis using mixed polymer coated capillary with switchable properties toward protein adsorption/desorption.

In this work, a simple online preconcentration method for quantitative detection of pepsin was realized by using the binary mixed polymer brushes coated capillary with switchable properties toward protein adsorption. Firstly, the binary mixed polymer brushes were prepared by grafting poly(2-methyl-2-oxazoline) and poly(4-vinylpyridine) onto the inner wall of the capillary through a polydopamine anchor. Then the coatings were characterized by X-ray photoelectron spectrometer and electroosmotic flow measurement. The results indicated that the composition of coating could be controlled by varying the feed ratio of poly(2-methyl-2-oxazoline) to poly(4-vinylpyridine) and the inner surface charge could be tuned toward the change of pH and ionic strength. The results showed that when the poly(2-methyl-2-oxazoline)/poly(4-vinylpyridine) mass ratio was 80/20, the highest online preconcentration effect was obtained and the sensitivity enhancement factor was 6.3. Moreover, satisfactory sensitivity (limit of detection: 7.5ng/mL) and good repeatability were obtained with the online preconcentration method. The polymer-coated capillary was still stable for online preconcentration and detection of pepsin after 50 consecutive runs. Last, the proposed method was used successfully to online preconcentrate pepsin in the saliva matrix.

A binary mixed polymer brush coating with adjusted hydrophobic property to control protein adsorption

Binary mixed polymer brushes based on PGMA-b-PNIPAM-b-PSt and PMOXA-r-GMA were used as thermoresponsive coating to control protein adsorption.

Capillary Microfluidic-Assisted Surface Structuring.

A facile and universal oxygen-tolerant, capillary microfluidic-derived, controlled radical polymerization for surface structuring (gradient and patterned polymer brushes) is reported. A syringe pump and a filter paper sheet are used as capillary microfluidic to supply the reaction solution (monomer, solvent, and ligand) to a sandwich-shaped setup by placing a flat copper plate onto an ATRP initiator-modified substrate and resulting in gradient polymer brush formation with controlled thickness, steepness, and grafting area, polymers are showing the high chain-end fidelity. Two different polymer brushes (binary polymer brushes) can be simultaneously grown from both ends of the initiator modified substrate by using this method, which can be used to study the interfacial properties of different polymer brushes.

Switchable Antimicrobial and Antifouling Coatings from Tannic Acid-Scaffolded Binary Polymer Brushes

Surface biofouling has been a serious environmental and ecological problem for centuries. In this study, pH-sensitive poly(2-diisopropylaminoethyl methacrylate)-b-poly(2-methacryloyloxyethyl phosph...

Binary polymer brush patterns from facile initiator stickiness for cell culturing.

We report a new initiator stickiness method to fabricate micropatterned binary polymer brush surfaces, which are ideal platforms for studying cell adhesion behavior. The atom transfer radical polymerization (ATRP) initiator, ω-mercaptoundecyl bromoisobutyrate (MUDBr), is found to adsorb on several hosting polymer brushes, including poly[oligo(ethylene glycol)methyl ether methacrylate] (POEGMA), poly(2-hydroxyethyl methacrylate) (PHEMA), and poly(glycidyl methacrylate) (PGMA) brushes. Based on the initiator stickiness, micropatterned initiator molecules are printed onto a layer of homogenous hosting polymer brushes via microcontact printing (μCP), and then, vertically, a patterned second layer of polymer brushes is grown from the initiator areas. With this simple, fast, and additive method, we demonstrate the fabrication of various binary polymer brushes, and show their applications for patterning cell microarrays and controlling cell orientation. This new approach to generating binary polymer brushes shows great potential for the manipulation of interfacial phenomena, facilitating a range of applications from semiconductors and lubrication to fundamental cell biology studies.

Facile Fabrication of Hierarchically Thermoresponsive Binary Polymer Pattern for Controlled Cell Adhesion.

A versatile platform allowing capture and detection of normal and dysfunctional cells on the same patterned surface is important for accessing the cellular mechanism, developing diagnostic assays, and implementing therapy. Here, an original and effective method for fabricating binary polymer brushes pattern is developed for controlled cell adhesion. The binary polymer brushes pattern, composed of poly(N-isopropylacrylamide) (PNIPAAm) and poly[poly(ethylene glycol) methyl ether methacrylate] (POEGMA) chains, is simply obtained via a combination of surface-initiated photopolymerization and surface-activated free radical polymerization. This method is unique in that it does not utilize any protecting groups or procedures of backfilling with immobilized initiator. It is demonstrated that the precise and well-defined binary polymer patterns with high resolution are fabricated using this facile method. PNIPAAm chains capture and release cells by thermoresponsiveness, while POEGMA chains possess high capability to capture dysfunctional cells specifically, inducing a switch of normal red blood cells (RBCs) arrays to hemolytic RBCs arrays on the pattern with temperature. This novel platform composed of binary polymer brush pattern is smart and versatile, which opens up pathways to potential applications as microsensors, biochips, and bioassays.

Fabrication of microstructured binary polymer brush "corrals" with integral pH sensing for studies of proton transport in model membrane systems.

Binary brush structures consisting of poly(cysteine methacrylate) (PCysMA) "corrals" enclosed within poly(oligoethylene glycol methyl ether methacrylate) (POEGMA) "walls" are fabricated simply and efficiently using a two-step photochemical process. First, the C-Cl bonds of 4-(chloromethyl)phenylsilane monolayers are selectively converted into carboxylic acid groups by patterned exposure to UV light through a mask and POEGMA is grown from unmodified chlorinated regions by surface-initiated atom-transfer radical polymerisation (ATRP). Incorporation of a ratiometric fluorescent pH indicator, Nile Blue 2-(methacryloyloxy)ethyl carbamate (NBC), into the polymer brushes facilitates assessment of local changes in pH using a confocal laser scanning microscope with spectral resolution capability. Moreover, the dye label acts as a radical spin trap, enabling removal of halogen end-groups from the brushes via in situ dye addition during the polymerisation process. Second, an initiator is attached to the carboxylic acid-functionalised regions formed by UV photolysis in the patterning step, enabling growth of PCysMA brushes by ATRP. Transfer of the system to THF, a poor solvent for PCysMA, causes collapse of the PCysMA brushes. At the interface between the collapsed brush and solvent, selective derivatisation of amine groups is achieved by reaction with excess glutaraldehyde, facilitating attachment of aminobutyl(nitrile triacetic acid) (NTA). The PCysMA brush collapse is reversed on transfer to water, leaving it fully expanded but only functionalized at the brush-water interface. Following complexation of NTA with Ni2+, attachment of histidine-tagged proteorhodopsin and lipid deposition, light-activated transport of protons into the brush structure is demonstrated by measuring the ratiometric response of NBC in the POEGMA walls.

Antifouling and Antimicrobial Coatings from Zwitterionic and Cationic Binary Polymer Brushes Assembled via “Click” Reactions

Controlled architecture of bifunctional polymers on surfaces is highly challenged because of the stringent reaction conditions or tedious operations required for surface modification. Herein, a simple and effective method was developed to assemble zwitterionic and cationic binary polymer brushes onto polydopamine-anchored stainless steel (SS) surfaces. Zwitterionic poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) was first graft polymerized from the functionalized SS surface via thiol–ene “click” reaction. Alkynyl-modified cationic poly(2-(methacryloyloxy) ethyl trimethylammonium chloride) (alkynyl-PMETA) was subsequently introduced via azide–alkyne “click” reaction. After the grafting of PMPC/PMETA binary polymer brushes, the resulting functionalized SS surfaces can cooperatively reduce the adhesion of Gram-positive bacteria Staphylococcus aureus (S. aureus) and Gram-negative bacteria Pseudomonas sp., as well as the attachment of microalgae Amphora coffeaeformis. In addition, the binary polymer brus...

A novel approach for UV-patterning with binary polymer brushes

A mixed self-assembled monolayer (SAM) of an initiator (3-(2-bromo-2-isobutyryloxy)propyl triethoxysilane) for atom transfer radical polymerization (ATRP) and an agent (6-(triethoxysilyl)hexyl 2-(((methylthio)carbonothioyl)thio)-2-phenylacetate) for reversible addition-fragmentation chain transfer (RAFT) polymerization was constructed on the surface of a silicon wafer or glass plate by a silane coupling reaction. When a UV light at 254nm was irradiated at the mixed SAM through a photomask, the surface density of the bromine atom at the end of BPE in the irradiated region was drastically reduced by UV-driven scission of the BrC bond, as observed by X-ray photoelectron spectroscopy. Consequently, the surface-initiated (SI)-ATRP of 2-ethylhexyl methacrylate (EHMA) was used to easily construct the poly(EHMA) (PEHMA) brush domain. Subsequently, SI-RAFT polymerization of a zwitterionic vinyl monomer, carboxymethyl betaine (CMB), was performed. Using the sequential polymerization, the PCMB and PEHMA brush domains on the solid substrate could be very easily patterned. Patterning proteins and cells with the binary polymer brush is expected because the PCMB brush indicated strong suppression of protein adsorption and cell adhesion, and the PEHMA brush had non-polar properties. This technique is very simple and useful for regulating the shape and size of bio-fouling and anti-biofouling domains on solid surfaces.

Micrometre and nanometre scale patterning of binary polymer brushes, supported lipid bilayers and proteins.

Binary polymer brush patterns were fabricated via photodeprotection of an aminosilane with a photo-cleavable nitrophenyl protecting group. UV exposure of the silane film through a mask yields micrometre-scale amine-terminated regions that can be derivatised to incorporate a bromine initiator to facilitate polymer brush growth via atom transfer radical polymerisation (ATRP). Atomic force microscopy (AFM) and imaging secondary ion mass spectrometry (SIMS) confirm that relatively thick brushes can be grown with high spatial confinement. Nanometre-scale patterns were formed by using a Lloyd's mirror interferometer to expose the nitrophenyl-protected aminosilane film. In exposed regions, protein-resistant poly(oligo(ethylene glycol)methyl ether methacrylate) (POEGMEMA) brushes were grown by ATRP and used to define channels as narrow as 141 nm into which proteins could be adsorbed. The contrast in the pattern can be inverted by (i) a simple blocking reaction after UV exposure, (ii) a second deprotection step to expose previously intact protecting groups, and (iii) subsequent brush growth via surface ATRP. Alternatively, two-component brush patterns can be formed. Exposure of a nitrophenyl-protected aminosilane layer either through a mask or to an interferogram, enables growth of an initial POEGMEMA brush. Subsequent UV exposure of the previously intact regions allows attachment of ATRP initiator sites and growth of a second poly(cysteine methacrylate) (PCysMA) brush within photolithographically-defined micrometre or nanometre scale regions. POEGMEMA brushes resist deposition of liposomes, but fluorescence recovery after photobleaching (FRAP) studies confirm that liposomes readily rupture on PCysMA "corrals" defined within POEGMEMA "walls". This leads to the formation of highly mobile supported lipid bilayers that exhibit similar diffusion coefficients to lipid bilayers formed on surfaces such as glass.

Mixed brush made of 4-arm stars and linear chains: MD simulations.

We investigate the structural properties of binary polymer brushes, composed of functional 4-armed star polymers and chemically identical linear polymers of different molecular weights. The molecular dynamics simulations confirm recent self-consistent field studies, in which a considerable potential of these systems for the design of switchable surfaces has been claimed. The length of the linear chains serves as a control parameter, which, while passing over a critical value, induces a sharp transition of the molecular conformation. We investigate these transitions at different grafting densities and summarize our findings in a phase diagram. The temperature dependence of the brush structure is investigated in a non-selective solvent, and non-trivial variations of the surface composition are observed. The quantity of these latter effects would be insufficient to build switchable systems, and we argue that a minor quantity of solvent selectivity would suffice to enable the desired feature of an environment-responsive coating.

Biomimicking Nano-Micro Binary Polymer Brushes for Smart Cell Orientation and Adhesion Control.

A new biomimetic surface named nano-micro binary polymer brushes is fabricated by large-area bench-top dip-pen nanodisplacement lithography technique. It is composed of gelatin-modified poly(glycidyl methacrylate) nanolines which are spaced by microstripes of poly(N-isopropylacrylamide). Cells are not only adhered and oriented well on the re-used surface, but also detachable from the surface with well-preserved extracellular matrix and aligned morphology.

Stimuli-Responsive Cellulose Nanocrystals for Surfactant-Free Oil Harvesting.

Cellulose nanocrystals with grafted binary polymer brushes (CNC-BPB), poly(oligoethylene glycol) methacrylate (POEGMA) and poly(methacrylic acid) (PMAA), were prepared by cerium-mediated polymerization in aqueous solution. The physical properties of CNC-BPB can be controlled by external triggers, such as temperature and pH, which can be utilized to stabilize and destabilize oil-water emulsions. By virtue of the modifications, these bifunctionalized CNCs diffused to the oil-water interface and stabilized the oil droplets at high pHs. When the pH was lowered to 2, strong hydrogen bonding between POEGMA and PMAA chains grafted on the CNC induced the coalescence of the emulsion droplets, resulting in the phase separation of oil and water. For emulsions stabilized by CNC-POEGMA and free PMAA mixtures, instantaneous coalescence was not observed at low pHs. Successive stabilization-destabilization over 5 cycles was demonstrated by modulating the pH with the addition of acid or base without any loss in efficiency. This work demonstrates that functional sustainable nanomaterials can be used for small scale oil-water separations, particularly for oil droplet transportation and harvesting of lipophilic compounds.

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.

Binary Polymer Brushes of Strongly Immiscible Polymers.

The phenomenon of microphase separation is an example of self-assembly in soft matter and has been observed in block copolymers (BCPs) and similar materials (i.e., supramolecular assemblies (SMAs) and homo/block copolymer blends (HBCs)). In this study, we use microphase separation to construct responsive polymer brushes that collapse to generate periodic surfaces. This is achieved by a chemical reaction between the minor block (10%, poly(4-vinylpyridine)) of the block copolymer and a substrate. The major block of polystyrene (PS) forms mosaic-like arrays of grafted patches that are 10-20 nm in size. Depending on the nature of the assembly (SMA, HBC, or neat BCP) and annealing method (exposure to vapors of different solvents or heating above the glass transition temperature), a range of "mosaic" brushes with different parameters can be obtained. Successive grafting of a secondary polymer (polyacrylamide, PAAm) results in the fabrication of binary polymer brushes (BPBs). Upon being exposed to specific selective solvents, BPBs may adopt different conformations. The surface tension and adhesion of the binary brush are governed by the polymer occupying the top stratum. The "mosaic" brush approach allows for a combination of strongly immiscible polymers in one brush. This facilitates substantial contrast in the surface properties upon switching, previously only possible for substrates composed of predetermined nanostructures. We also demonstrate a possible application of such PS/PAAm brushes in a tunable bioadhesion-bioadhesive (PS on top) or nonbioadhesive (PAAm on top) surface as revealed by Escherichia coli bacterial seeding.