Brush Grafting Research Articles

The role of solvent quality, inhomogeneous polymer brush composition, grafting density and number of free chains on the viscosity, friction between surfaces, and their scaling laws

Abstract Brushes made up of mixtures of polymers are modeled using mesoscale simulations under shear and the viscosity and the friction coefficient are obtained as functions of the chains’ grafting density, brush composition, free chain concentration, and solvent quality. We find that brushes made up of mixtures of polymers can reduce to a greater extent the friction coefficient in comparison to a mono-chain brush and that both the viscosity and the friction coefficient obey scaling laws that depend only on solvent quality. These results shed light on microscopic mechanisms that affect viscosity and friction in polymeric brushes, which are findings of interest for research on nanotribology.

Enhancement of the growth of polymer brushes via ATRP initiated from ions-releasing indium tin oxide substrates

Surface-initiated atom transfer radical polymerization (SI-ATRP) was used for grafting of model brushes (PNIPAM and PMMA) from various planar surfaces: indium tin oxide (ITO), glass and silicon oxide. All polymerizations for a given monomer were conducted at the same conditions in the presence of a sacrificial initiator. Both PNIPAM and PMMA brushes were found to be thicker when grafted from ITO with respect to the other surfaces demonstrating a significant influence of the ITO substrate on SI-ATRP kinetics. It was shown that ITO significantly increases the polymerization rate of the grafted chains due to not negligible desorption of metal ions which may undergo redox reactions also with the copper catalyst. Moreover, the molar masses of the grafted PNIPAM and PMMA chains were found generally larger (3–6 times for PNIPAM) than the ones of free polymers generated in solution, regardless of the substrate type, indicating significant differences in the rates of SI-ATRP and ATRP in the solution.

Atomistic Study of Zwitterionic Peptoid Antifouling Brushes.

Using molecular dynamics (MD) simulations, we study the molecular behavior and hydration properties of a set of zwitterionic "peptoid" brushes, grafted on a rutile surface, that has been previously reported to exhibit excellent resistance against protein adsorption and cell attachment. Peptoids are novel poly( N-substituted glycine) peptide mimics with theside chains attached to amide nitrogens. They constitute a unique model polymer system because hundreds of side chains have been demonstrated, and the exact chain length and sequence order of the residues/monomers may be specified in experiments. In this report, we vary the brush grafting density as well as the side chain/polymer molecular volume. We include in our study polysarcosine as an uncharged comparison with a small polymer chain cross-section. Sarcosine is the simplest peptoid residue with only a nominally hydrophobic methyl group as side chain, but is also reported to exhibit high antifouling performance. Overall, we show in detail how molecular volume and hydration effects are intertwined in a zwitterionic polymer brush. For example, the zwitterionic design significantly promotes extended chain conformations and could actually lower the overall electrostatic potential. Some properties promoted by the balanced charges, such as chain flexibility and hydration, increase more prominently at "low" to "intermediate" chain densities. These and other observations should provide insight on the molecular behavior of peptoids and inform the design of zwitterionic antifouling polymer brushes.

Improved separation performance and durability of polyamide reverse osmosis membrane in tertiary treatment of textile effluent through grafting monomethoxy-poly(ethylene glycol) brushes

To improve the separation performance and durability of the aromatic polyamide thin-film composite reverse osmosis membrane in tertiary treatment of textile effluent, grafting of hydrophilic polymer brushes was performed in this work through sequential surface treatment using acidic aqueous glutaraldehyde and monomethoxy-poly(ethylene glycol) (MPEG) solutions. ATR-FTIR and XPS analyses confirmed that MPEG brushes were grafted on membrane surface at the sites of NH group of amide linkages and end amino groups in the polyamide active layer. The modification was found to enhance membrane fouling resistance, water permeability and rejections to both COD and conductivity in tertiary treatment of textile effluent. The steady-state flux of the modified membrane to secondary textile effluent was higher by 24.0% compared with the virgin membrane and comparable with that of the commercial membrane BW30FR. The grafting of MPEG brushes was also found to make the membrane less susceptible to chlorine. After ten times of intensified chlorine exposure, i.e. total chlorine exposure of 42,000 ppmh NaClO of 35.0 °C and pH 9.0, the membrane grafted with MPEG brushes maintained COD and conductivity rejections of 97.9% and 98.5%, respectively, to textile effluent, showing better durability against chlorine than the virgin membrane and compared membrane BW30FR.

Engineering Lubricious, Biopassive Polymer Brushes by Surface-Initiated, Controlled Radical Polymerization

Surface-initiated controlled radical polymerization enables the fabrication of biopassive polymer brushes with interfacial, physicochemical properties that can be independently varied across a single substrate. Poly[(oligoethylene glycol) methacrylate] (POEGMA) brushes were synthesized by surface-initiated atom transfer radical polymerization (SI-ATRP), locally varying the exposure of initiator-functionalized surfaces to the polymerization solution to yield POEGMA brush thickness gradients. A combination of variable-angle spectroscopic ellipsometry (VASE) and atomic force microscopy (AFM) demonstrated that brush swelling, grafting density, nanomechanical properties, and biopassivity towards protein adsorption all remained constant within a thickness range between 20 and 90 nm. However, the nanotribological properties of POEGMA brushes, investigated by lateral force microscopy (LFM), were found to vary progressively along the gradient, thinner brushes showing significantly lower friction than thicker and m...

Reversible modulation of the redox activity in conducting polymer nanofilms induced by hydrophobic collapse of a surface-grafted polyelectrolyte

We present the covalent modification of a Pani-like conducting polymer (polyaminobenzylamine, PABA) by grafting of a polyelectrolyte brush (poly [2-(methacryloyloxy)-ethyl-trimethylammonium chloride], PMETAC). As PABA has extra pendant amino moieties, the grafting procedure does not affect the backbone nitrogen atoms that are implicated in the electronic structure of the conducting polymers. Moreover, perchlorate anions interact very strongly with the quaternary ammonium pendant groups of PMETAC through ion pairing. Therefore, the grafting does not only keep the electroactivity of PABA in aqueous solutions but it adds the ion-actuation properties of the PMETAC brush to the modified electrode as demonstrated by contact angle measurements and electrochemical methods. In this way, the conjugation of the electron transfer properties of the conducting polymer with the anion responsiveness of the integrated brush renders perchlorate actuation of the electrochemical response. These results constitute a rational integration of nanometer-sized polymer building blocks that yields synergism of functionalities and illustrate the potentialities of nanoarchitectonics for pushing the limits of soft material science into the nanoworld.

Polyelectrolyte Brush-Grafted Polydopamine-Based Catalysts with Enhanced Catalytic Activity and Stability.

Three types of surface treatments, namely, polyethyleneimine (PEI) coating, short PEI (S-PEI) grafting, and long PEI (L-PEI) grafting, were performed on polydopamine (Pdop)-based catalysts to enhance their catalytic activity and stability. Brush-grafted catalysts were prepared by the stepwise synthesis of Au and short (or long) PEI brushes on Pdop particles (PdopP/Au/S- or L-PEI grafting). PEI-coated Pdop-based catalysts (PdopP/Au/PEI coating) were also prepared as non-brush-grafted catalysts. Among the surface-treated PdopP/Au catalysts, the brush-grafted catalysts (S-PEI and L-PEI grafting) exhibited excellent and stable catalytic performance because the brush grafting enabled the protection of the catalysts against harsh conditions, effective transfer of reactants to the catalysts, and confinement of reactants around the catalysts. The brush-grafted catalysts could also more effectively decompose larger dyes than the non-brush-grafted catalysts. The process-to-effectiveness of PEI coating is the best because the release of Pdop from PdopP/Au was moderately inhibited by the presence of only one layer of PEI coating on the PdopP/Au. Thus, this approach could be an alternative method to enhance the stability of PdopP/Au catalysts.

Preparation of active antibacterial biomaterials based on sparfloxacin, enrofloxacin, and lomefloxacin deposited on polyethylene

ABSTRACTBacterial contamination is one of the biggest drawbacks of implanted biomaterials, which may cause nosocomial infection. Avoiding bacterial adhesion onto the biomaterial surface by preparing active antibacterial biomaterials is an accurate solution. In this study, three of the fluoroquinolones: sparfloxacin, enrofloxacin, and lomefloxacin were selected due to their broad bactericidal effect and immobilized onto low‐density polyethylene surface at two different pH values (pH 3 and pH 6), after tailoring the surface by plasma treatment followed by grafting of polymer brush of N‐allylmethylamine. Surface wettability test and morphological investigations were carried out by water contact angle measurement and scanning electron microscopy analysis, respectively. Chemical characterizations were carried out by Fourier transform infrared spectra and X‐ray photoelectron spectroscopy. Antibacterial activity was checked against Staphylococcus aureus and Escherichia coli strains and results revealed that prepared surfaces were more effective against E. coli than S. aureus especially those prepared at pH 6. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46174.

Direct polymer brush grafting to polymer fibers and films by surface-initiated polymerization

Direct surface modification of polymer fibers and films by surface-initiated polymerizations has been investigated. The introduction of initiating sites on the polymer materials and successive polymerization produce surface-tethered polymer chains on the polymer surface. The surface-selective modification controls the surface properties such as wetting, lubrication and anti-fouling without sacrificing the bulk performances. Researches on grafting polymer chains to five types of polymers, poly(methyl methacrylate)-based copolymer, Br-containing polyolefins, poly(butylene terephthalate), poly(vinylidene fluoride-co-trifluoroethylene) and poly(ether-ether ketone) are reviewed.

Linear minority chain in a star brush: The coil-to-flower transition

Conformational transition in a single “minority” linear polymer chain inserted into a polymer brush made of arm-grafted chemically identical stars (star brush) is studied theoretically. It is shown that the chain can undergo a transition between conformations of the grafted coil and the “flower” consisting of a strongly stretched “stem” passing through the brush and the coil-like “head” on the top of the brush. The coil-to-flower transition can be provoked by changing internal parameters of the system (the chain length or the brush grafting density) or external conditions (the solvent strength). At sparse grafting, the coil-to-flower transition occurs continuously as a second-order-like phase transition, whereas at moderate and high grafting density, when the brush has a “two-layer” structure, the transition has the features of the first order phase transition. This opens a route for designing “molecular switches” based on a linear chain inserted in a star brush.

Dopant macroinitiator for electropolymerisation and functionalisation of conducting polymer thin films

Grafting of polymer brushes from conducting polymer (CP) thin films by controlled radical polymerisation provides a versatile route for the synthesis of functional, electroactive surfaces, with applications in diverse fields. However, drawbacks of this approach include difficulty in upscaling synthesis due to the need for specialised CP precursor monomers functionalised with initiation sites. We herein describe an alternative approach to the synthesis of CP-based polymer brushes whereby ATRP initiation sites are attached to a macrodopant incorporated into CP films during electropolymerisation. The facile electropolymerisation of commonly studied CPs (PEDOT, PPy, and PTTh) with an initiator-functionalised macrodopant - poly(styrene sulfonate-co-(2-bromopropionyloxy ethyl methacrylate)) (PeesBro) – is demonstrated. The composite polymer films thus synthesised were used as substrates for grafting of hydrophilic polymer brushes. Although PSS is commonly used as a macrodopant in CP films, its initiator-functionalised derivatives have not previously been utilised in this manner. Despite the elegance of this approach, the authors’ knowledge, there have been no previous examples reported of utilising macromolecular dopants as initiators for subsequent grafting of polymer brushes.

Efficacy of antifouling modification of ultrafiltration membranes by grafting zwitterionic polymer brushes

Abstract We studied the efficacy of grafting zwitterionic polymer brushes for the antifouling modification of ultrafiltration (UF) membranes. Poly(vinylidene fluoride) (PVDF) UF membranes were modified by surface initiated atom transfer radical polymerization (SI-ATRP) to graft poly(sulfobetaine methacrylate) (PSBMA) brushes to the membrane surface. Protein adsorption tests and chemical force microscopy show that a large brush layer thickness (greater than 100 nm) is necessary in order to impart effective fouling resistance. In dynamic fouling experiments with bovine serum albumin (BSA) as a model foulant, however, the modified membranes exhibited only a slight increase of flux recovery after fouling compared to pristine PVDF membranes. Despite the thick PSBMA brush layer, this low water flux recovery indicates that internal fouling takes place within the membrane pores. To prevent internal fouling, we grafted PSBMA brushes to both the surface and internal structure of the membranes. Nevertheless, dynamic fouling experiments again showed only a minute improvement in water flux recovery. While a thick brush layer is required for effective fouling resistance, we note that the small pore size of UF membranes imposes a fundamental limit on the brush layer thickness inside the pores. Finally, we discuss the challenges of polymer brush grafting for antifouling UF membrane modification and suggest possible alternative methods to create fouling resistant UF membranes.

Effects of the Surface Morphology and Conformations of Lignocellulosic Biomass Biopolymers on Their Nanoscale Interactions with Hydrophobic Self-Assembled Monolayers.

The effects of the morphology and conformations of the surface biopolymers present on lignocellulosic biomass as well as their steric hindrance on enzymatic adsorption to biomass surfaces remain elusive. In a step to better understand these effects, nanoscale steric forces between a model surface that represents the hydrophobic residues of a cellulase enzyme and a set of reference lignocellulosic substrates were measured using atomic force microscopy (AFM) in liquid media. The reference substrates investigated were prepared by kraft, sulfite, and organosolv pulping pretreatment methods and varied in their surface lignin, xylan, and acetone extractives' contents. Measured steric forces were quantified through fitting to a model developed to describe polyelectrolytes brushes in terms of a brush thickness and a brush grafting density. Our data indicated that cellulose microfibrils extend from the microfibril matrix leading to a long-range steric repulsion and low attractive forces to the hydrophobic model of the enzyme, suggesting that steric hindering can be a possible mechanism for nonproductive binding of enzymes to cellulose. When the amount of xylan increased in the absence of lignin, steric repulsions between the hydrophobic model of the enzyme, and biomass biopolymers decreased as a result of collapsed cellulose microfibrils and adhesion forces increased. This suggests that leaving a small amount of xylan after biomass pretreatment can help improve enzymatic binding to cellulose. Irrespective of the type of lignin present on biomass, grafting densities increased and brush thicknesses decreased compared to those of lignin-free substrates. When compared to lignin-free substrates, lignin-containing substrates had higher attractive forces and lower steric repulsive forces. In addition, AFM images of the reference substrates in the wet and dry states showed that lignin precipitates on the biomass surface where kraft lignin had the highest particle size leading to a limited accessibility of the enzyme to the cellulose in biomass. When the effects of lignin precipitate size, the adhesion force, and steric forces on nonproductive enzymatic binding were all considered, our results indicate that organosolv pretreatment should be the treatment of choice to minimize enzymatic nonproductive binding to lignin.

Stereoregular Brush Polymers and Graft Copolymers by Chiral Zirconocene-Mediated Coordination Polymerization of P3HT Macromers.

Two poly(3-hexylthiophene) (P3HT) macromers containing a donor polymer with a polymerizable methacrylate (MA) end group, P3HT-CH2-MA and P3HT-(CH2)2-MA, have been synthesized, and P3HT-(CH2)2-MA has been successfully homopolymerized and copolymerized with methyl methacrylate (MMA) into stereoregular brush polymers and graft copolymers, respectively, using chiral ansa-zirconocene catalysts. Macromer P3HT-CH2-MA is too sterically hindered to polymerize by the current Zr catalysts, but macromer P3HT-(CH2)2-MA is readily polymerizable via either homopolymerization or copolymerization with MMA in a stereospecific fashion with both C2-ligated zirconocenium catalyst 1 and Cs-ligated zirconocenium catalyst 2. Thus, highly isotactic (with mm% ≥ 92%) and syndiotactic (with rr% ≥ 93%) brush polymers, it-PMA-g-P3HT and st-PMA-g-P3HT, as well as well-defined stereoregular graft copolymers with different grafted P3HT densities, it-P(M)MA-g-P3HT and st-P(M)MA-g-P3HT, have been synthesized using this controlled coordination-addition polymerization system under ambient conditions. These stereoregular brush polymers and graft copolymers exhibit both thermal (glass and melting) transitions with Tg and Tm values corresponding to transitions within the stereoregular P(M)MA and crystalline P3HT domains. Acceptor molecules such as C60 can be effectively encapsulated inside the helical cavity of st-P(M)MA-g-P3HT to form a unique supramolecular helical crystalline complex, thus offering a novel strategy to control the donor/acceptor solar cell domain morphology.

Modulation of cell adhesion to conductive polymers

Our work on conductive polymer (CP) systems grafted with stimuli-responsive polymer brushes is motivated by the prospect of precisely controlling cellular behaviour by tailored smart interfaces. Here, the effects on cell adhesion by applying a potential to poly(3,4-ethylenedioxythiophene) (PEDOT) during both protein coating and cell culture is investigated. The results highlight the importance of pre-adsorbing fibronectin in this case, especially for the reduced polymer which binds protein strongly. The effects of changing the surface chemistry of the PEDOT electrode by grafting of brushes by atom transfer radical polymerisation (ATRP) is also investigated. Specifically, the composition of the salt-sensitive poly(oligo(ethylene glycol methyl ether methacrylate))-based brushes was tailored to control the level of cell adhesion to the interface. The composition, and also the length of the grafted brushes was seen to be important to the cell adhesion. It is also demonstrated how PEDOT films grafted with a protein and cell rejecting brush can be converted to a cell adhesive state by attaching an integrin ligand to the brush to mediate cell adhesion.

Zwitterionic polymer brush grafting on anodic aluminum oxide membranes by surface-initiated atom transfer radical polymerization

A feasible processing of zwitterionic polymer-grafted anodic aluminum oxide (AAO) membranes by surface-initiated atom transfer radical polymerization (SI-ATRP) and the geometric effect were investigated.

Nanotribological properties of nanostructured poly(cysteine methacrylate) brushes.

The nanomechanical properties of zwitterionic poly(cysteine methacrylate) (PCysMA) brushes grown from planar surfaces by atom transfer radical polymerisation have been characterised by friction force microscopy (FFM). FFM provides quantitative insights into polymer structure-property relationships and in particular illuminates the dependence of brush swelling on chain packing in nanostructured materials. In ethanol, which is a poor solvent for PCysMA, a linear friction-load relationship is observed, indicating that energy dissipation occurs primarily through ploughing. In contrast, in a good solvent for PCysMA such as water, a non-linear friction-load relationship is observed that can be fitted by Derjaguin-Muller-Toporov (DMT) mechanics, suggesting that the relatively small modulus of the swollen polymer leads to a large contact area and consequently a significant shear contribution to energy dissipation. The brush grafting density was varied by using UV photolysis of C-Br bonds at 244 nm to dehalogenate the surface in a controlled fashion. The surface shear strength increases initially as the brush grafting density is reduced, but then decreases for UV doses greater than 0.5 J cm-2, reaching a limiting value when the brush thickness is ca. 50% that of a brush monolayer. Below this critical grafting density, a collapsed brush layer is obtained. For nm-scale gradient brush structures formed via interferometric lithography, the mean width increases as the period is increased, and the lateral mobility of brushes in these regions is reduced, leading to an increase in brush height as the grafted chains become progressively more extended. For a width of 260 nm, the mean brush height in water and ethanol is close to the thickness of a dense unpatterned brush monolayer synthesised under identical conditions. Both the surface shear stress measured for PCysMA brushes under water and the coefficient of friction measured in ethanol are closely correlated to the feature height, and hence to the chain conformation.

AFM Study of Polymer Brush Grafted to Deformable Surfaces: Quantitative Properties of the Brush and Substrate Mechanics

Polymers brushes (polymer chains end-tethered to a substrate) have been extensively studied with atomic force microscopy (AFM). Force–indentation curves are collected while squeezing the sample surface with the AFM probe. The analysis of these curves allows obtaining the equilibrium brush thickness and grafting density by using an appropriate model for mechanical deformation of the brush. However, this approach becomes inaccurate when the substrate is deformable, which is frequently the case. In this situation, the collected force–distance curves include information about both the brush and substrate deformations. Here we describe a method which takes into account both the brush and substrate deformations. Quantitative accuracy of the presented method is demonstrated by applying the method to measuring a poly(ethylene oxide) brush grafted to the cross-linked poly(2-vinylpyridine) substrate swollen in aqueous media of different acidity. By analyzing the AFM force curves, we simultaneously obtain the grafti...

Modeling RAFT Gelation and Grafting of Polymer Brushes for the Production of Molecularly Imprinted Functional Particles

SummaryRAFT precipitation polymerization was here used for the molecular imprinting of 5‐fluorouracil (a drug used in cancer treatment) followed by grafting of polymer functional brushes in the particles surface. Materials combining high molecular recognition capabilities and stimulation triggered by changes in the surrounding environment (e.g. pH/temperature) have thus been sought. Mathematical modeling of the RAFT copolymerization of vinyl/divinyl monomers was used to describe the crosslinking process involved in the molecular imprinting step. The size of the grafted polymer functional brushes was estimated using models for the calculation of molecular weight distribution in RAFT homopolymerization. One of the goals of this study consists in the development of polymer reaction engineering tools for the design of synthesis conditions leading to tailored molecularly imprinted particles with responsive features. A first sketch of the two steps involved in the production of the MIP responsive particles is here presented. However, the need for new kinetic studies accounting for specificities of the molecular imprinting and surface extension processes (e.g. presence of template during molecular imprinting and solvent effects) is here identified. Measurements of the dynamics of gelation and brushes growth are also needed to perform model validation. Improvement on the modeling of RAFT gelation with heterogeneous conditions avoiding some simplifications and the amelioration of the numerical efficiency in the calculation of the MWD of RAFT polymers are other issues to be addressed in future works.

Polyethyleneimine brushes effectively inhibit encrustation on polyurethane ureteral stents both in dynamic bioreactor and in vivo

Polyurethane (PU) ureteral stents have been widely used as biomedical devices to aid the flow of the urine. Due to the biofilm formation and encrustation complications it has been hindered their long term clinical usage. To overcome these complications, in this study, cationic polyethyleneimine (PEI) brushes grafted on PU stents and their performances were tested both in a dynamic biofilm reactor system (in vitro) and in a rat model (in vivo). Thus, we hypothesized that PEI brushes inhibit bacterial adhesion owing to the dynamic motion of brushes in liquid environment. In addition, cationic structure of PEI disrupts the membrane and so kills the bacteria on time of contact. Cationic PEI brushes decreased the biofilm formation up to 2 orders of magnitude and approximately 50% of encrustation amount in respect to unmodified PU, in vitro. In addition, according to Atomic Absorption Spectroscopy (AAS) results, approximately 90% of encrustation was inhibited on in vivo animal models. Decrease in encrustation was clearly observed on the stents obtained from rat model, by Scanning Electron Microscopy (SEM). Also, histological evaluations showed that; PEI brush grafting decreased host tissue inflammation in close relation to decrease in biofilm formation and encrustation. As a results; dual effect of anti-adhesive and contact-killing antibacterial strategy showed high efficiency on PEI brushes grafted PU stents both in vitro and in vivo.