Neutral Polymer Brushes Research Articles

Molecular dynamics simulations of the nickel removal from crude oil by neutral and charged spherical polymer brushes

Nickel in crude oil adversely affects the refining process for rapidly deactivating the hydrogenation and cracking catalysts. Polymer brushes bearing high-density ligands have shown promising nickel removal ability. In this work, submicron polybutadiene (PB) grafted by polyacrylamide (PAM@PB) was prepared and characterized by the transmission electron microscope, dynamic light scattering, and elemental analysis, which was also compared with the PB latex grafted by polyacrylic acid (PAA@PB) synthesized in our previous work. The nickel removal rates by both brushes were evaluated through the electric desalination process. Theresults show that the nickel removal capacity of PAA@PB is higher than that of PAM@PB. By the molecular dynamics simulations, the stretching and competitive chelation of polymer brush models PAA@PB-M and PAM@PB-M were investigated. Charged polyacrylic acid brushes usually have a higher thickness layer and a lower ligand density under the electrostatic repulsion and hydrogen bonding. The electrostatic interaction plays a more important role than the van der Waals interactions on nickel removal. PAA@PB-M is more attractive against Ni2+ ions than PAM@PB-M. The inevitable formation of stable sandwiched π-π stacking structures between Ni2+ ions and multiple porphyrin rings causes great difficulty in nickel removal during the electric desalination process. Adding protons to the oil phase can reduce the formation of the sandwiched structures and further improve nickel removal rates. In this paper, molecular dynamics simulation is innovatively applied to the mechanism of demetallization of polymer brushes, revealing the differences in structure and demetallization between charged and neutral polymer brushes, which provides guidance for the design of core–shell polymer brushes with high demetallization efficiency.

Rapid characterization of neutral polymer brush with a conventional zetameter and a variable pinch of salt.

The fundamental and practical importance of particle stabilization has motivated various characterization methods for studying polymer brushes on particle surfaces. In this work, we show how one can perform sensitive measurements of neutral polymer coating on colloidal particles using a commercial zetameter and salt solutions. By systematically varying the Debye length, we study the mobility of the polymer-coated particles in an applied electric field and show that the electrophoretic mobility of polymer-coated particles normalized by the mobility of non-coated particles is entirely controlled by the polymer brush and independent of the native surface charge, here controlled with pH, or the surface-ion interaction. Our result is rationalized with a simple hydrodynamic model, allowing for the estimation of characteristics of the polymer coating: the brush length L, and the Brinkman length ξ, determined by its resistance to flows. We demonstrate that the Debye layer provides a convenient and faithful probe to the characterization of polymer coatings on particles. Because the method simply relies on a conventional zetameter, it is widely accessible and offers a practical tool to rapidly probe neutral polymer brushes, an asset in the development and utilization of polymer-coated colloidal particles.

Mechanics of polymer brush based soft active materials– theory and experiments

A brush-like structure emerges from stretching of long polymer chains, densely grafted on to the surface of an impermeable substrate. The structure arises due to the competition between conformational entropic elasticity of polymer chains and excluded volume interactions leading to intra and interchain monomer repulsions. Recently, soft materials based on stimuli responsive polymer brushes have been developed to produce controllable and reversible large bending deformation of the host substrates. To understand such systems and improve their functional properties, we study the stress distribution in a brush, and develop surface stress-curvature relation for an elastic beam of a soft material grafted with a neutral polymer brush. In the strongly stretched brush regime, we combine mean field theory from polymer physics with a continuum mechanics model and show that the residual stress variation in a brush is a quartic function of distance from the grafting surface, with a maximum occurring at the grafting surface. By idealizing a brush as a continuum elastic surface layer with residual stress, we derive a closed form expression for surface stress and surface elasticity of the layer as a function of brush parameters, such as graft density and molecular weight. A generalized continuum beam model accounts for the Young–Laplace and Steigmann–Ogden curvature elasticity correction terms, and yields a surface stress-curvature relation, that contains existing relations in the literature as special cases. Further, we report experiments on a thermoresponsive random copolymer brush, Poly(N- isopropylacrylamide)-co-Poly(N,N-Dimethylacrylamide) (PNIPAm-co-PDMA) brush, grafted on one side of a plasticized poly(vinyl chloride) (pPVC) thin film. Estimated surface stress from measured curvature is on the order of −10N/m, and its magnitude decreases gradually, and reversibly, on increasing ambient temperature from 15 °C to 55 °C.

Phosphorene-directed self-assembly of asymmetric PS-b-PMMA block copolymer for perpendicularly-oriented sub-10 nm PS nanopore arrays

Few-layer black phosphorus, also known as phosphorene, is a new two-dimensional material which is of enormous interest for applications, mainly in electronics and optoelectronics. Herein, we for the first time employ phosphorene for directing the self-assembly of asymmetric polystyrene-block-polymethylmethacrylate (PS-b-PMMA) block copolymer (BCP) thin film to form the perpendicular orientation of sub-10 nm PS nanopore arrays in a hexagonal fashion normal to the interface. We experimentally demonstrate that none of the PS and PMMA blocks exhibit preferential affinity to the phosphorene-modified surface. Furthermore, the perpendicularly-oriented PS nanostructures almost stay unchanged with the variation of number of layers of few-layer phosphorene nanoflakes between 15–30 layers. Differing from the neutral polymer brushes which are widely used for chemical modification of the silicon substrate, phosphorene provides a novel physical way to control the interfacial interactions between the asymmetric PS-b-PMMA BCP thin film and the silicon substrate. Based on our results, it is possible to build a new scheme for producing sub-10 nm PS nanopore arrays oriented perpendicularly to the few-layer phosphorene nanoflakes. Furthermore, the nanostructural microdomains could serve as a promising nanolithography template for surface patterning of phosphorene nanoflakes.

Controlled Growth of Metal-Organic Frameworks on Polymer Brushes.

Polymer brushes are for the first time used to induce the synthesis of metal-organic frameworks (MOFs). The semi-fixed polymer chains provide a confined environment, which allows a mild growth of MOFs in between polymer chains to give surface-attached spherical MOF nanoparticles, in contrast to the larger MOF cubes/plates formed simultaneously in solution. Polymer brushes bearing carboxylate acid functionalities are indispensable for the formation of surface bound MOFs, while no MOF nanoparticles are observed on neutral polymer brushes. Characterization of the resultant MOF/polymer brushes hybrid film indicates the formation of crystalline MOF structure. The dimension of surface-attached MOFs can be fine-tuned from 20 nm to 1.4 μm simply by varying the structural parameter of polymer brushes and the nucleation duration. The method is not only applicable to the synthesis of MOF-5 and MIL-125, but shows great potential for the preparation of other surface-attached MOFs.

Interacting Polyelectrolyte Brushes Grafted in Two Bilayers: Molecular Dynamics Simulations

Using molecular dynamics simulations, we study interacting polyelectrolyte brushes that are grafted to two parallel surfaces (quasi-Planar Membrane). The interactions between brushes are important, for instance, in stabilization of dispersions against flocculation. We simulate the relative shear motion of both neutral and polyelectrolyte end-grafted polymer brushes. The flexible neutral polymer brush is treated as a bead-spring model, and the polyelectrolyte brush is treated the same way except that each bead is charged and there are counter ions present to neutralize the charge. We investigate the friction coefficient, monomer density, and brush penetration for the two kinds of brushes with both the same grafting density and the same normal force under good solvent conditions.

The Weak Interaction of Surfactants with Polymer Brushes and Its Impact on Lubricating Behavior

We study the weak interaction between polymers and oppositely charged surfactants and its effect on the lubricating behavior and wettability of polymer brush-covered surfaces. For cationic (PMETAC) and anionic (PSPMA) brushes, a gradual transition from ultralow friction to ultrahigh friction was observed upon adding oppositely charged surfactant as a result of the electrostatic and hydrophobic interactions between surfactant and polymer. The surfactant exchange led to a strong dehydration of the brush and a concomitant increase in friction. Upon adding surfactant above the CMC, we find a reduction in friction for the anionic brushes, while the cationic brushes maintain a high friction. This difference between the two lubrication systems suggests a different interaction mechanism between the polymers and the surfactants. For zwitterionic (PSBMA) and neutral (POEGMA) polymer brushes, where electrostatic and hydrophobic interactions could be negligible, there is nearly no surfactant uptake and also no effect...

Membrane rigidity induced by grafted polymer brush.

The contribution of neutral polymer brush to the curvature elasticity of the grafting surface is investigated theoretically. Using self-consistent field theory, we accurately evaluate the dependence of bending modulus on parameters including chain length, Flory-Huggins parameter and grafting density and reveal the importance of solvent. The results show that the brush-induced bending modulus follows a complex dependence on grafting density and Flory-Huggins parameter, while it obeys a simple power law with chain length as N(3). The method is further applied to calculate the polymer brush's contribution to the elastic properties of PEG-grafted lipid monolayers.

Macroscopic lateral heterogeneity observed in a laterally mobile immiscible mixed polyelectrolyte-neutral polymer brush.

We studied mixed poly(ethylene oxide) (PEO) and poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) brushes. The question we attempted to answer was: when the chain grafting points are laterally mobile, how will this lateral mobility influence the structure and phase behavior of the mixed brush? Three different model mixed PEO/PDMAEMA brush systems were prepared: (1) a laterally mobile mixed brush by spreading onto the air-water interface a mixture of poly(ethylene oxide)-poly(n-butyl acrylate) (PEO-PnBA) and poly(2-(dimethylamino)ethyl methacrylate)-poly(n-butyl acrylate) (PDMAEMA-PnBA) diblock copolymers (the specific diblock copolymers used will be denoted as PEO113-PnBA100 and PDMAEMA118-PnBA100, where the subscripts refer to the number-average degrees of polymerization of the individual blocks), (2) a mobility-restricted (inseparable) version of the above mixed brush prepared using a PEO-PnBA-PDMAEMA triblock copolymer (denoted as PEO113-PnBA89-PDMAEMA120) having respective brush molecular weights matched with those of the diblock copolymers, and (3) a different laterally mobile mixed PEO and PDMAEMA brush prepared from a PEO113-PnBA100 and PDMAEMA200-PnBA103 diblock copolymer combination, which represents a further more height-mismatched mixed brush situation than described in (1). These three mixed brush systems were investigated by surface pressure-area isotherm and X-ray (XR) reflectivity measurements. These experimental data were analyzed within the theoretical framework of a continuum self-consistent field (SCF) polymer brush model. The combined experimental and theoretical results suggest that the mobile mixed brush derived using the PEO113-PnBA100 and PDMAEMA118-PnBA100 combination (i.e., mixed brush System #1) undergoes a lateral macroscopic phase separation at high chain grafting densities, whereas the more height-mismatched system (System #3) is only microscopically phase separated under comparable brush density conditions even though the lateral mobility of the grafted chains is unrestricted. The macroscopic phase separation observed in the laterally mobile mixed brush system is in contrast with the microphase separation behavior commonly observed in two-dimensional laterally mobile charged small molecule mixtures. Further study is needed to determine the detailed morphologies of the macro- and microphase-separated mixed PEO/PDMAEMA brushes.

Switching Transport through Nanopores with pH-Responsive Polymer Brushes for Controlled Ion Permeability

Several nanoporous platforms were functionalized with pH-responsive poly(methacrylic acid) (PMAA) brushes using surface-initiated atom transfer radical polymerization (SI-ATRP). The growth of the PMAA brush and its pH-responsive behavior from the nanoporous platforms were confirmed by scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, and atomic force microscopy (AFM). The swelling behavior of the pH-responsive PMAA brushes grafted only from the nanopore walls was investigated by AFM in aqueous liquid environment with pH values of 4 and 8. AFM images displayed open nanopores at pH 4 and closed ones at pH 8, which rationalizes their use as gating platforms. Ion conductivity across the nanopores was investigated with current-voltage measurements at various pH values. Enhanced higher resistance across the nanopores was observed in a neutral polymer brush state (lower pH values) and lower resistance when the brush was charged (higher pH values). By adding a fluorescent dye in an environment of pH 4 or pH 8 at one side of the PMAA-brush functionalized nanopore array chips, diffusion across the nanopores was followed. These experiments displayed faster diffusion rates of the fluorescent molecules at pH 4 (PMAA neutral state, open pores) and slower diffusion at pH 8 (PMAA charged state, closed pores) showing the potential of this technology toward nanoscale valve applications.

The kinetic friction coefficient of neutral and charged polymer brushes

The compression-dependence of the friction coefficients of neutral and charged polymer brushes at the same surface coverage is studied using dissipative particle dynamics (DPD) at two different shear rates. The chemical potential of the solvent particles is kept constant along the compression curve in order to mimic the experimental conditions. We conclude that the kinetic friction coefficient between charged brushes is higher than that of neutral brushes at low compressions and smaller at high compressions. These differences are small. We also show that it is possible to simulate ultra-low friction coefficients comparable with experiments using the smallest shear rate accessible in a mesoscale simulation. The unexpected behavior of the shear deformation-induced structural heterogeneities in charged polymer brushes calls for further experiments to elucidate this local reorganization of the ions in adsorbed charged polymers.

Comparison of the kinetic friction of planar neutral and polyelectrolyte polymer brushes using molecular dynamics simulations

We have simulated the relative shear motion of both neutral and polyelectrolyte end-grafted polymer brushes using molecular dynamics. The flexible neutral polymer brush is treated as a bead-spring model, and the polyelectrolyte brush is treated the same way except that each bead is charged and there are counterions present to neutralize the charge. We investigated the friction coefficient, monomer density, and brush penetration for both polyelectrolyte and neutral brushes with both equal grafting density and equal normal force under good solvent conditions. We found that polyelectrolyte brushes had a smaller friction coefficient and monomer penetration than neutral polymer brushes with the identical grafting density and chain length, and the polyelectrolyte brushes supported a much higher normal load than the neutral brushes for the same degree of compression. Charged and neutral brushes with their grafting densities chosen so that they support the same load exhibited approximately the same degree of interpenetration, but the polyelectrolyte brush exhibited a significantly lower friction coefficient. We present evidence that the reason for this is that the extra normal force contribution provided by the counterion osmotic pressure that exists for polyelectrolyte brushes permits them to support the same load as an identical neutral polymer brush of higher grafting density. Because of the resulting lower monomer density for the charged brushes, fewer monomer collisions take place per unit time, resulting in a lower friction coefficient.

Loops, tails and trains: A simple model for structural transformations of grafted adsorbing neutral polymer brushes

The structure of grafted adsorbing polymers on surfaces is described as a statistical ensemble of loops generated by an one-dimensional random walk perpendicular to the surface. The configuration of each chain is considered as a succession of closed loops ended by an open loop (a tail). The probability of formation of each individual loop is the product between the probability of first return to the surface and a Boltzmann factor containing the free energy of the Flory-Huggins kind, which is approximated by the minimum free energy of all possible configurations of that loop. At high grafting densities, the attractive interactions between monomers and surface control the fraction of polymer belonging to either closed loops or tails, hence the formation of a stretched grafted brush. At low grafting densities, the increase of that interaction above a critical value generates an abrupt collapse of the brush on the surface. Whereas for long polymers (with more than about 100 Kuhn segments), the structure of the brush can be determined, in general, only via Monte-Carlo sampling, it is argued that the two structural transitions indicated above can be well predicted by simple approximations.

The induction of thrombus generation on nanostructured neutral polymer brush surfaces

Surface induced thrombus generation is a major clinical concern associated with vascular medical devices and implants. Here, we show that high graft density hydrophilic non-charged poly (N,N-dimethylacrylamide) (PDMA) brushes prevent the initiation of blood coagulation on synthetic surfaces. Using a multi-faceted analysis approach, we have identified that PDMA brushes greater than 0.27 chains/nm 2 graft density showed this highly desired property. Non-specific protein adsorption is greatly reduced on high density brushes compared to bare surface as evident from isothermal titration calorimetry, gel electrophoresis, and proteomic analyses. We have identified approximately 129 proteins of various types on bare and PDMA brush coated surfaces at a range of surface concentrations. Thromboelastography, platelet activation, and aggregation analyses show that only high graft density brushes are neutral to blood coagulation. Since the polymer brush synthesis can be adapted to most currently used biomedical materials, these results have significant impact in the design of highly hemocompatible surfaces.

Molecular dynamics simulations of end-grafted centipede-like polymers with stiff charged side chains

We use molecular dynamics simulations to investigate centipede-like polymers with stiff charged side chains, end-grafted to a planar wall. The effect of the grafting density and the Bjerrum length on the conformational behaviour of the brush is examined in detail. In addition, we make a comparison of centipede-like polyelectrolyte (CPE) brushes with neutral centipede-like polymer (NCP) and linear polyelectrolyte (LPE) brushes. At weak electrostatic interaction, the main chains of the CPE chains adopt a strongly stretched conformation, and the monomer density profiles of side chains exhibit a clear oscillatory behaviour. With increasing Bjerrum length, the CPE brush undergoes a collapse transition. Compared to the CPE brushes, the counterion condensation effect is stronger for the LPE brushes, regardless of whether the electrostatic interaction is weak or strong and of whether the grafting density is low or high. Additionally, it is shown that the architecture of the grafted chains makes a weak contribution to the counterion condensation at strong electrostatic interaction. We also find that the electrostatic repulsion between charged side chains can enhance the stiffness of the main chains and thus limit the range of movement of the free-end monomers.

Charge-driven and reversible assembly of ultra-dense polymer brushes: formation and antifouling properties of a zipper brush

We investigated a new type of polymer brushes: the zipper brush. By adsorbing a diblock-copolymer with one charged block and one neutral block to an oppositely charged polyelectrolyte brush, a neutral polymer brush is formed on top of an almost neutral complex layer of polyelectrolytes. This neutral brush can be adsorbed in minutes and desorbed in seconds to restore the original polyelectrolyte brush. The zipper brush can be used, for example, as an antifouling layer to prevent protein adsorption. These characteristics are shown by fixed-angle optical reflectometry for the system of poly(N-methyl-2-vinyl pyridinium)-block-poly(ethylene oxide) (P2MVP-PEO) adsorbed to a poly(acrylic acid) (PAA) brush. After the diblock-copolymer has adsorbed (at pH 6), the charges of the PAA brush are almost completely compensated by the charges of the P2MVP block. The grafting density of the formed neutral brush can be controlled by the chain length and grafting density of the PAA brush, and by the chain length of P2MVP block. As the P2MVP blocks used in this study are much smaller than the PAA chains in the brush, the grafting density of the PEO brushes are found to be a multiplication of that of the PAA brush, and much higher grafting densities (up to 1.59 chains per nm2) can be obtained than previously reported for polymer brushes prepared by adsorption. At low pH, when the PAA is uncharged, only a few mg m−2 adsorb probably because of hydrogen bonding between uncharged PAA and PEO. As the pH increases the PAA becomes more charged and the adsorbed amount of diblock-copolymer increases strongly until pH 6 and then levels off between pH 6 and pH 10. At pH 6 and 10 high salt concentrations (above 250 mM) are needed to significantly reduce the adsorbed amount. The antifouling properties of the zipper brush were tested by exposing the brush to a number of different protein solutions. For the proteins lysozyme, fibrinogen, bovine serum albumin, and b-lactoglobuline the zipper brush completely prevented any adsorption, for cytochrome C a small amount of adsorption was observed.

Interaction of Particles with a Polydisperse Brush: A Self-Consistent-Field Analysis

Two complementary theoretical approaches are used to study the effect of polydispersity oil (anti)fouling properties of a neutral polymer brush. Polydispersity is described using the Schulz-Zimm distribution. The Scheutjens-Fleer self-consistent-field (SF-SCF) formalism is used to consider the interaction between a single particle and a polydisperse brush with grafting density sigma, focusing on the influence of the polydispersity index. The larger the polydispersity, the easier it is for a small particle (with radius R similar to 1/(2 root sigma)) to penetrate the brush. Hence, the monodisperse brush is better suited to protect a surface against the adsorption of small particles compared to a corresponding polydisperse brush. The brush grafting density, however, remains the most important parameter for timing the brush antifouling properties against small particles. For large particles (modeled as a flat wall.) an opposite effect of polydispersity is found: it is harder to compress a polydisperse brush than it corresponding monodisperse brush, and thus a polydisperse brush is better suited to protect the Surface against adsorption of large particles. A less-detailed approach, based on the stacking of Alexander-de Gennes boxes, is used to study the adsorption of many particles into a polydisperse brush. Consistent with the single-particle data generated by the SF-SCF theory, for weak attraction between the particles and the brush the absolute adsorbed amount remains low but increases strongly as a function of polydispersity (from M-w/M-n = 1-2 by a factor of 2-4). Obviously, at higher attraction between the particles and the brush the adsorption increases, but a less strong dependence on the polydispersity index is observed.

Friction and Normal Interaction Forces between Irreversibly Attached Weakly Charged Polymer Brushes

Polyelectrolyte brushes were built on mica by anchoring polystyrene-poly(acrylic acid) (PS-b-PAA) diblock copolymers at a controlled surface density in a polystyrene monolayer covalently attached to OH-activated mica surfaces. Compared to physisorbed polymer brushes, these irreversibly attached charged brushes allow the polymer grafting density to remain constant upon changes in environmental conditions (e.g., pH, salt concentration, compression, and shear). The normal interaction and friction forces as a function of surface separation distance and at different concentrations of added salt (NaCl) were investigated using a surface forces apparatus. The interaction force profiles were completely reversible both on loading and receding and were purely repulsive. For a constant polymer grafting density, the influence of the polyelectrolyte charges and the Debye screening effect on the overall interaction forces was investigated. The experimental interaction force profiles agree very well with scaling models developed for neutral and charged polymer brushes. The variation of the friction force between two PAA brushes in motion with respect to each other as a function of surface separation distance appeared to be similar to that observed with neutral brushes. This similarity suggests that the increase in friction is associated with an increase in mutual interpenetration upon compression as observed with neutral polymers. The effect of the PAA charges and added ions was more significant on the repulsive normal forces than on the friction forces. The reversible characteristics of the normal force profiles and friction measurements confirmed the strong attachment of the PAA brushes to the mica substrate. High friction coefficients (ca 0.3) were measured at relatively high pressures (40 atm) with no surface damage or polymer removal.

Theory of Friction between Neutral Polymer Brushes

A type of static friction between two polymer brush coated surfaces resulting from fluctuations from mean-field theory is found, but with creeplike motion for forces below the force of static frict...

Synthesis and Swelling Behavior of pH-Responsive Polybase Brushes

We synthesize polybase brushes and investigate their swelling behavior. Poly(2-(dimethylamino)ethyl methacrylate)) (PDMAEMA) brushes are prepared by the "grafting from" method using surface-initiated Atom Transfer Radical Polymerization to obtain dense brushes with relatively monodisperse chains (PDI = 1.35). In situ quaternization reaction can be performed to obtain poly(2-(trimethylamino)ethyl methacrylate)) (PTMAEMA) brushes. We determine the swollen thickness of the brushes using ellipsometry and neutron reflectivity techniques. Brushes are submitted to different solvent conditions to be investigated as neutral brushes and weak and strong polyelectrolyte brushes. The swelling of the brushes is systematically compared to scaling models. It should be pointed out that the scaling analysis of different types of brushes (neutral polymer and weak and strong polyelectrolyte brushes) is performed with identical samples. The scaling behavior of the PDMAEMA brush in methanol and the PTMAEMA brush in water is in good agreement with the predicted scaling laws for a neutral polymer brush in a good solvent and a polyelectrolyte brush in the osmotic regime. The salt-induced contraction of the quaternized brush is observed for high salt concentration, in agreement with the predicted transition between the regimes of the osmotic brush and the salted brush. From the crossover concentration, we calculate the effective charge ratio of the brush following the Manning counterion condensation. We also use PDMAEMA brushes as pH-responsive polybase brushes. The swelling behavior of the polybase brush is intermediate with respect to the behavior of the neutral polymer brush in a good solvent and the behavior of the quenched polyelectrolyte brush, as expected. The effective charge ratio of the PDMAEMA brush is determined as a function of pH using the scaling law of the polyelectrolyte brush in the osmotic regime.