Brush Growth Research Articles

Surface-Initiated Photoinduced Iron-Catalyzed Atom Transfer Radical Polymerization with ppm Concentration of FeBr3 under Visible Light.

Reversible deactivation radical polymerizations with reduced amount of organometallic catalyst are currently a field of interest of many applications. One of the very promising techniques is photoinduced atom transfer radical polymerization (photo-ATRP) that is mainly studied for copper catalysts in the solution. Recently, advantageous iron-catalyzed photo-ATRP (photo-Fe-ATRP) compatible with high demanding biological applications was presented. In response to that, we developed surface-initiated photo-Fe-ATRP (SI-photo-Fe-ATRP) that was used for facile synthesis of poly(methyl methacrylate) brushes with the presence of only 200 ppm of FeBr3/tetrabutylammonium bromide catalyst (FeBr3/TBABr) under visible light irradiation (wavelength: 450 nm). The kinetics of both SI-photo-Fe-ATRP and photo-Fe-ATRP in solution were compared and followed by 1H NMR, atomic force microscopy (AFM) and gel permeation chromatography (GPC). Brush grafting densities were determined using two methodologies. The influence of the sacrificial initiator on the kinetics of brush growth was studied. It was found that SI-photo-Fe-ATRP could be effectively controlled even without any sacrificial initiators thanks to in situ production of ATRP initiator in solution as a result of reaction between the monomer and Br radicals generated in photoreduction of FeBr3/TBABr. The optimized and simplified reaction setup allowed synthesis of very thick (up to 110 nm) PMMA brushes at room temperature, under visible light with only 200 ppm of iron-based catalyst. The same reaction conditions, but with the presence of sacrificial initiator, enabled formation of much thinner layers (18 nm).

Open-to-Air RAFT Polymerization on a Surface under Ambient Conditions.

Oxygen (O2)-mediated controlled radical polymerization was performed on surfaces under ambient conditions, enabling on-surface polymer brush growth under open-to-air conditions at room temperature in the absence of metal components. Polymerization of zwitterionic monomers using this O2-mediated surface-initiated reversible addition fragmentation chain-transfer (O2-SI-RAFT) method yielded hydrophilic surfaces that exhibited anti-biofouling effects. O2-SI-RAFT polymerization can be performed on large surfaces under open-to-air conditions. Various monomers including (meth)acrylates and acrylamides were employed for O2-SI-RAFT polymerization; the method is thus versatile in terms of the polymers used for coating and functionalization. A wide range of hydrophilic and hydrophobic monomers can be employed. In addition, the end-group functionality of the polymer grown by O2-SI-RAFT polymerization allowed chain extension to form block copolymer brushes on a surface.

Shape Shifting of Cup Shaped Particles on Growing poly (2‐hydroxy ethyl methacrylate) Brushes by “Grafting From” Approach and Dissipative Particle Dynamics Simulation

AbstractControlled bending leading to shape shifted microparticles is indispensable for the targeted biomedical applications including advanced drug/cell delivery and has been one of the major research components in biomedical area for decades. Here, we report the shape‐shifting of cup‐shaped particles created by electrojetting from a blend of biodegradable polylactide (PLA) and biocompatible co‐polymer (poly[methylmethacrylate‐co‐2‐(2‐bromopropionyloxy)ethyl methacrylate] (poly(MMA‐co‐BEMA))) containing ATRP (Atom Transfer Radical Polymerization) initiating moiety, at a ratio of 75:25. Surface initiated ATRP of HEMA (2‐hydroxy ethyl methacrylate) was carried out for 1 hour to immobilize self‐crosslinkable poly(HEMA) brushes onto the cup shaped particles which underwent controlled bending post polymer brush growth leading to the formation of spherical ball with one small opening (hole). However, no change of shape was observed while growing non‐crosslinkable hydrophilic poly(DMAEMA) (poly(2‐(Dimethylamino)ethyl methacrylate)) brush from the surface of cup shaped particles. To understand the underlying phenomena of shape shifting, simulation studies were also performed. Dissipative particle dynamics (DPD) simulation of the ATRP process at the cup surface further confirmed that the use of HEMA monomer indeed led to the desired compact modified structure of the cup particle due to the crosslinking connectivity across the interface. We have also calculated the radial distribution function (RDF) and radius of gyration to study the structure of modified particles.

Boosting or moderating surface-initiated Cu(0)-mediated controlled radical polymerization with external additives

External additives regulate the copper disproportionation/comproportionation equilibrium to control polymer brush growth in surface-initiated Cu(0)-mediated controlled radical polymerization.

Controlled grafting of polymer brush layers from porous cellulosic membranes

Dense polymer brushes on porous supports present powerful routes toward fouling-resistant coatings and ultra-thin, mechanically-stable selective layers. However, characterization of polymer chains grafted from nonideal, planar substrates often relies on indirect methods to determine the length, density, and location of polymeric brushes. In this study, we explore new methods to control and characterize the properties of polymer brush layers. Starting from hand-cast cellulose films and commercial cellulose membranes, we use surface-initiated atom transfer radical polymerization to graft polymeric chains. We tailor brush density and simultaneously stabilize the support by varying the proportions of initiator and crosslinker esterified to the cellulose substrate. On films with grafted polyacrylic acid (PAA) chains, we use the silver-binding method to determine brush density. We analyze brush growth behavior by directly cleaving chains from the surface of films for analysis by size exclusion chromatography, uniquely observing the divergence of two distinct populations of differing length beyond a certain molecular weight. Such behavior would be important to consider if the alignment of block copolymer layers is absolutely necessary in an application. By controlled contact of polymerization solution with the top surface, we show that brush growth can be partially directed to the top surface for asymmetric cellulose membranes with relatively low molecular weight cutoffs. In a semi-quantitative method to locate brush growth, depleted uranium was bonded to grafted PAA chains, and targeted emission x-ray spectroscopy was conducted for comparison of uranium content in various regions. Finally, we used our developed methods and findings to inform the synthesis of homopolymer and diblock copolymer brush layers, which we then used as selective layers in pressure-driven filtration and diffusion cell experiments.

Growing Polymer Brushes from a Variety of Substrates under Ambient Conditions by Cu0-Mediated Surface-Initiated ATRP

Cu0-mediated surface-initiated atom transfer radical polymerization (Cu0 SI-ATRP) is a highly versatile, oxygen-tolerant, and extremely controlled polymer-grafting technique that enables the modification of flat inorganic surfaces, as well as porous organic and polymeric supports of different compositions. Exploiting the intimate contact between a copper plate, acting as a source of catalyst and reducing agent, and an initiator-bearing support, Cu0 SI-ATRP enables the rapid growth of biopassive, lubricious brushes from large flat surfaces, as well as from various organic supports, including cellulose fibers and elastomers, using microliter volumes of reaction mixtures, and without the need for deoxygenation of reaction mixtures or an inert atmosphere. Thanks to a detailed analysis of its mechanism and the parameters governing the polymerization process, polymer brush growth by Cu0 SI-ATRP can be precisely modulated and adapted to be applied to morphologically and chemically different substrates, setting up the basis for translating SI-ATRP methods from academic studies into technologically relevant surface-modification approaches.

Antibacterial response of polylactide surfaces modified with hydrophilic polymer brushes

Ultrathin polymeric films may completely change in their surface properties from bacteria loving to bacteria repelling which are essential for creating non-fouling surfaces. The covalent modifications of biodegradable polylactide (PLA) surface with alkyl bromide initiator followed by immobilization of three different types of polymers having potential antibacterial effects, i.e., poly(2-hydroxyethyl methacrylate) (PHEMA), Poly(2-[(methacryloyloxy)ethyl]trimethylammonium chloride) (PMETA) and poly(poly(ethylene glycol) methacrylate) (PPEGMA) were successfully conducted using surface-initiated atom transfer radical polymerization (ATRP) technique. High initiator density (1.72 initiator/nm2) on PLA surface determined by XPS led to the formation of smooth polymer brush as shown by AFM images which were further characterized by FTIR and contact angle measurement. Though linear brush growth was observed irrespective of monomers employed, ‘grafting yield’ was found to be highest for PHEMA brush (22.2 µg cm−2 at 6 h) under similar conditions indicating its high activity in the presence of efficient catalytic system. Finally, the antibacterial response of these brushes was evaluated against both gram-positive (Staphylococcus aureus) and gram-negative (Escherichia coli) bacteria and PMETA modified PLA surface was found to exhibit the highest inhibition of bacterial adhesion (> 97% for S. aureus). The novelty lies in the generation of polymer brushes with excellent antibacterial property onto biodegradable polylactide surface. The potential uses of these modified surfaces would include biomedical devices, food packaging, etc.

Entropic death of nonpatterned and nanopatterned polyelectrolyte brushes

ABSTRACTThe stability of nonpatterned and nanopatterned strong polyelectrolyte brushes (PEBs) is studied as a function of both brush character and the properties of a contacting liquid. High‐molecular‐weight PEBs of poly(4‐methyl vinylpyridinium iodide) (PMeVP) are synthesized using surface‐initiated radical‐chain polymerization. Nanopatterned brushes (NPBs) line with pattern sizes ranging from 50 to 200 nm are generated by patterning the initiator layer using deep‐ultraviolet photolithography followed by brush growth initiated from the patterned layer. Homogeneous PEBs with different degrees of charging and grafting densities are exposed to water and salt solutions with different temperatures for different periods. The degradation is monitored through dry‐state ellipsometry and atomic force microscopy measurements. Enhanced degrafting for more strongly swollen polymer brushes can be observed in agreement with an “entropic spring” model. Based on the results of the nonpatterned brushes, the NPBs are exposed to water at different temperatures and external salt content for varying periods of time. Counterintuitively, the NPBs show increased degrafting for smaller patterns, which is attributed to different polymer chain dynamics for nanobrushes and microbrushes. We investigate the influence of thermodynamic and kinetic parameters on the stability of (nanopatterned) PEBs and discuss the role of entanglements and formation of complexes in such films. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 1283–1295

Enzymatically Degassed Surface-Initiated Atom Transfer Radical Polymerization with Real-Time Monitoring.

Polymer brush coatings are frequently prepared by radical polymerization, a notoriously oxygen sensitive process. Glucose oxidase (GOx) can inexpensively enable radical polymerization in solution by enzymatically consuming oxygen as it oxidizes glucose. Here, we report the growth of polymeric brushes using GOx-assisted atom transfer radical polymerization (ATRP) from a surface while open to air. Specifically, we grew a set of biomedically relevant polymer brushes, including poly(oligo(ethylene glycol) methacrylate) (POEGMA), poly(2-dimethylaminoethyl methacrylate) (PDMAEMA), poly(sulfobetaine methacrylate) (PSBMA), and poly(2-(methylsulfinyl)ethyl acrylate (PMSEA). For each of these polymers, we monitored GOx-assisted and GOx-free ATRP reaction kinetics in real time using quartz crystal microbalance (QCM) and verified findings with localized surface plasmon resonance (LSPR). We modeled brush growth kinetics considering bimolecular termination. This model fit our data well ( r2 > 0.987 for all samples) and shows the addition of GOx increased effective kinetic chain lengths, propagation rates, and reproducibility. We tested the antifouling properties of the polymer brush coatings against human blood plasma and were surprised to find that coatings prepared with GOx repelled more plasma proteins in all cases than their GOx-free counterparts.

Surface-Initiated Cu(0)-Mediated CRP for the Rapid and Controlled Synthesis of Quasi-3D Structured Polymer Brushes.

Surface-initiated controlled radical polymerization mediated by Cu(0) plate (SI-Cu(0)plate-CRP) is an extremely effective and versatile technique for the synthesis of functional polymer brushes from vinyl monomers on planar substrates. The advantages of SI-Cu(0)plate-CRP in comparison to "classical" SI-CRP methods not only rely on the easy accessibility, handling, and recycling of the catalyst source, but also on the faster brush growth rates, and exceptionally high reinitiation efficiencies and grafting densitiesfor the obtained brushes. The confined geometry of the SI-Cu(0)plate-CRP reaction setup, with a Cu(0) plate placed in close proximity to the initiator bearing substrate, considerably simplifies the preparation of polymer brushes over large areas, and the fabrication of gradient, patterned and arrayed polymer brushes. In this viewpoint we summarize the recent developments and applications of SI-Cu(0)plate-CRP, emphasizing its mechanism, advantages, and standing challenges.

Substrate-Independent Approach to Dense Cleavable Polymer Brushes by Nitroxide-Mediated Polymerization.

High grafting density polymer brushes are grown on an inimer coating bearing nitroxide-mediated polymerization (NMP) inimers and glycidyl methacrylate (GMA). The inimer coating is cross-linked on the substrate to provide an initiator layer with needed stability during long exposure to organic solvents at moderate to high temperatures. Surface-initiated nitroxide-mediated polymerization (SI-NMP) is conducted to grow polystyrene (PS) brushes on the coating with a sacrificial layer designed to cleave the brushes. The cleaved brushes have larger molecular weights than the corresponding free polymers. The grafting density of the brushes is as high as 1.12 chains/nm2 throughout the brush growth, which is among the densest PS brushes reported so far. Atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) depth profiling are used to reveal the surface morphology and kinetics of the growth.

Modulation of Surface-Initiated ATRP by Confinement: Mechanism and Applications.

The mechanism of surface-initiated atom transfer polymerization (SI-ATRP) of methacrylates in confined volumes is systematically investigated by finely tuning the distance between a grafting surface and an inert plane by means of nanosized patterns and micrometer thick foils. The polymers were synthesized from monolayers of photocleavable initiators, which allow the analysis of detached brushes by size-exclusion chromatography (SEC). Compared to brushes synthesized under “open” polymerization mixtures, nearly a 4-fold increase in brush molar mass was recorded when SI-ATRP was performed within highly confined reaction volumes. Correlating the SI-ATRP of methyl methacrylate (MMA), with and without “sacrificial” initiator, to that of lauryl methacrylate (LMA) and analyzing the brush growth rates within differently confined volumes, we demonstrate faster grafting kinetics with increasing confinement due to the progressive hindering of CuII-based deactivators from the brush propagating front. This effect is especially noticeable when viscous polymerization mixtures are generated and enables the synthesis of several hundred nanometer thick brushes within relatively short polymerization times. The faster rates of confined SI-ATRP can be additionally used to fabricate, in one pot, precisely structured brush gradients, when volume confinement is continuously varied across a single substrate by spatially tuning the vertical distance between the grafting and the confining surfaces.

Novel Strategy for Photopatterning Emissive Polymer Brushes for Organic Light Emitting Diode Applications.

A light-mediated methodology to grow patterned, emissive polymer brushes with micron feature resolution is reported and applied to organic light emitting diode (OLED) displays. Light is used for both initiator functionalization of indium tin oxide and subsequent atom transfer radical polymerization of methacrylate-based fluorescent and phosphorescent iridium monomers. The iridium centers play key roles in photocatalyzing and mediating polymer growth while also emitting light in the final OLED structure. The scope of the presented procedure enables the synthesis of a library of polymers with emissive colors spanning the visible spectrum where the dopant incorporation, position of brush growth, and brush thickness are readily controlled. The chain-ends of the polymer brushes remain intact, affording subsequent chain extension and formation of well-defined diblock architectures. This high level of structure and function control allows for the facile preparation of random ternary copolymers and red–green–blue arrays to yield white emission.

Sub-Micron Patterning of Polymer Brushes by Controllable Deposition of Polyelectrolyte Monolayers

The controlled deposition of polyelectrolyte macroinitiators for polymer brush growth is investigated. High resolution patterns are created for periodic array gratings towards responsive polymer brush sensors. Further, controlling the segregation of the macroinitiator to the droplet edge by careful manipulation of the constituent solutes leads to sub-micron patterning of polymer brushes.

Submicron Patterning of Polymer Brushes: An Unexpected Discovery from Inkjet Printing of Polyelectrolyte Macroinitiators.

Using an electrostatic-based super inkjet printer we report the high-resolution deposition of polyelectrolyte macroinitiators and subsequent polymer brush growth using SI-ARGET-ATRP. We go on to demonstrate for the first time a submicron patterning phenomenon through the addition of either a like charged polyelectrolyte homopolymer or through careful control of ionic strength. As a result patterning of polymer brushes down to ca. 300 nm is reported. We present a possible mechanistic model and consider how this may be applied to other polyelectrolyte-based systems as a general method for submicron patterning.

Tailored conditions for controlled and fast growth of surface-grafted PNIPAM brushes

Abstract Surface-initiated Atom Transfer Radical Polymerization is an established technique for synthesis of polymer brushes via “grafting from” method. However, dynamic behavior of surface-tethered macromolecules during their growth requires more detailed studies to obtain uniform polymer brush films. It was addressed here by varying solvent composition in grafting of poly(N-isopropylacrylamide) brushes. Complementary in situ (quartz crystal microbalance) and ex situ (atomic force microscope, spectroscopic ellipsometry) techniques were employed to investigate the influence of methanol molar fraction (xMe) in methanol/water polymerization mixture on the brush growth. Grazing-angle polarized FT-IR spectroscopy was used to monitor macromolecular arrangement in the brushes. While achieving controlled, uniform and relatively fast growth of macromolecules is challenging a compromise may be reached by tuning solvent composition. The most balanced conditions with this respect were found in a narrow solvent composition window at ca. xMe ≈ 0.16 and below 0.31. The results may be applied for tailoring polymerization conditions of other polymer brush systems.

The Rise of Structural Osteoarthritis of the Hip: Commentary on an article by William Z. Morris, MD, et al.: "Capital Femoral Growth Plate Extension Predicts Cam Morphology in a Longitudinal Radiographic Study".

By the creative use of 80-year-old serial radiographs from the longitudinal Brush growth study in Cleveland, Ohio, Morris et al. have made an important contribution to our understanding of the evolution of proximal femoral cam morphology. One obvious potential limitation is the extrapolation of data from the 1940s to the present growth patterns of the proximal femoral physeal plate in adolescents, given improved nutrition, changing activity levels, and other unknown factors. However, the study participants were presumably children of parents with the financial capacity to have them monitored regularly by a pediatrician, and they participated in “health contests,” thereby permitting us to presume that they were comparable with today’s adolescents. Over fifty years ago, Murray1 described the “tilt deformity” of the femoral head and neck as a precursor of osteoarthritis of the hip. Numerous other studies have supported the theory that a substantial proportion of noninflammatory osteoarthritis is secondary to anatomical variations in hip development2. It is now 15 years since Ganz et al.3,4 linked …

Effect of Surface Charge on Surface-Initiated Atom Transfer Radical Polymerization from Cellulose Nanocrystals in Aqueous Media

Cellulose nanocrystals (CNCs) with different charge densities were utilized to examine the role of electrostatic interactions on surface-initiated atom transfer radical polymerization (SI-ATRP) in aqueous media. To this end, growth of hydrophilic uncharged poly(N,N-dimethylacrylamide) (PDMAM) brushes was monitored by electrophoresis, (1)H NMR spectroscopy, and dynamic light scattering (DLS). Molecular weight and polydispersity of PDMAM brushes was determined by GPC analysis of hydrolytically cleaved polymers. Initiator and polymer brush grafting densities, and thus, initiator efficiencies were derived from elemental analysis. Higher initiator efficiency of polymer brush growth was observed for CNCs with higher anionic surface sulfate half-ester group density, but at the expense of high polydispersity caused by inefficient deactivation. PDMAM grafts with number-average molecular weights up to 530 kDa and polydispersity indices <1.5 were obtained under highly diluted monomer concentrations. The role of surface chemistry on the growth of neutral polymer brushes from CNCs in water is emphasized and a model of the interfacial region at the onset of polymerization is proposed. The results presented here could have implications for other substrates that present surface charges and for the assumption that the kinetics of Cu-mediated SI-CRP are analogous to those conducted in solution.

Fabrication of Two-Component, Brush-on-Brush Topographical Microstructures by Combination of Atom-Transfer Radical Polymerization with Polymer End-Functionalization and Photopatterning.

Poly(oligoethylene glycol methyl ether methacrylate) (POEGMEMA) brushes, grown from silicon oxide surfaces by surface-initiated atom transfer radical polymerization (SI-ATRP), were end-capped by reaction with sodium azide leading to effective termination of polymerization. Reduction of the terminal azide to an amine, followed by derivatization with the reagent of choice, enabled end-functionalization of the polymers. Reaction with bromoisobutryl bromide yielded a terminal bromine atom that could be used as an initiator for ATRP with a second, contrasting monomer (methacrylic acid). Attachment of a nitrophenyl protecting group to the amine facilitated photopatterning: when the sample was exposed to UV light through a mask, the amine was deprotected in exposed regions, enabling selective bromination and the growth of a patterned brush by ATRP. Using this approach, micropatterned pH-responsive poly(methacrylic acid) (PMAA) brushes were grown on a protein resistant planar poly(oligoethylene glycol methyl ether methacrylate) (POEGMEMA) brush. Atomic force microscopy analysis by tapping mode and PeakForce quantitative nanomechanical mapping (QNM) mode allowed topographical verification of the spatially specific secondary brush growth and its stimulus responsiveness. Chemical confirmation of selective polymer growth was achieved by secondary ion mass spectrometry (SIMS).

Polyelectrolyte brushes: Water Content, Zeta Potential and Mechanical Properties

ABSTRACTPolymer brushes of poly[2-(methacryloyloxy)ethyl]trimethylammonium chloride (PMETAC) and poly(sulfo propyl methacrylate) (PSPM) were synthesized by Atomic Transfer Radical Polymerization from planar and colloidal surfaces. Polymer brush growth was followed by QCMD and the water content determined by combined QCMD and elipsometry. From the water content the percentage of water lost during the brush collapse with the ionic strength could be obtained.Highly charged PSPM brushes were indented by Atomic Force Microscopy at different ionic strengths. The force response was fitted to a phenomenological equation analogous to the equation of state of a compressible fluid. Internal energy and brush compressibility were obtained as a function of ionic strength.Spherical brushes of PMETAC and PSPM display an invariance of the zeta potential with ionic strength in the range from 20 mM to 200 mM NaCl, the zeta potential remains almost constant. This invariance can be explained applying a hairy surface approach.