Grafted Polymer Layers Research Articles

Morphology of Polymer Brushes Infiltrated by Attractive Nanoinclusions of Various Sizes

Addition of nanoparticles can control the morphologies of grafted polymer layers that are important in a variety of natural and artificial systems. We study the morphologies of grafted polymer layers interacting attractively with nanoparticle inclusions, as a function of particle size and the interaction strength, using self-consistent field theory and Langevin dynamics simulations. We find that the addition of nanoparticles causes distinctive changes in the layer morphology. For sufficiently strong interaction/binding, increasing the concentration of nanoparticles causes a compression of the polymer layer into a compact, low height state, followed by a subsequent rebound and swelling at sufficiently high concentrations. For nanoparticles of small size, the compression of the layer is sharp and occurs over a narrow range of nanoparticle concentrations. The transition region widens as the nanoparticle size increases. The transition is initiated via a dense layer of tightly bound monomers and nanoparticles near the grafting surface, with a low density region above it. For nanoparticles much larger than the characteristic graft spacing in the brush, the behavior is reversed: the nanoparticles penetrate only the dilute region near the top of the polymer layer without causing the layer to collapse.

Nanobiocomposite Adhesion: Role of Graft Length and Temperature in a Hybrid Biomimetic Approach

Cellulose microspheres bearing poly(ε-caprolactone) grafts of different molecular weights were investigated to evaluate the effect of graft length on the interfacial properties. Surface force and friction measurements were performed using an atomic force microscope in colloidal probe mode. The maximum interaction distance and adhesion is dependent on the temperature and the time in contact via a diffusion controlled mechanism. The effects are highest for the longer grafts, and molecular weight thresholds were found to lie between 21 and 34 kDa at 25 °C and between 9 and 21 kDa at 40 °C. The interpenetration of the graft into a matrix leads to "hidden length" contributions to adhesion, analogous to those in natural biocomposites. The nanotribology results display Amontonian behavior, and the friction force at zero applied load is higher at the graft-graft interface than for a bare cellulose sphere interacting with the graft. These results clearly demonstrate the benefits of the grafted polymer layer on the adhesion, toughness, and resistance to shear in the design of cellulosic nanobiocomposites.

Collective ordering of colloids in grafted polymer layers

We present Monte Carlo simulations of colloidal particles pulled into grafted polymer layers by an external force. The insertion free energy for penetration of a single colloid into a polymer layer is qualitatively different for surfaces with an ordered and a disordered distribution of grafting points and the tendency of colloidal particles to traverse the grafting layer is strongly size dependent. In dense colloidal suspensions, under the influence of sufficiently strong external forces, colloids penetrate and form internally ordered, columnar structures spanning the polymer layer. The competition between the tendency for macro-phase separation of colloids and polymers and the elastic-like penalty for deforming the grafted layer results in the micro-phase separation, i.e. finite colloidal clusters characterized by a well-defined length scale. Depending on the conditions, these clusters are isolated or laterally percolating. The morphology of the observed patterns can be controlled by the external fields, which opens up new routes for the design of thin structured films.

Modeling of equilibrium and kinetics of human polyclonal immunoglobulin G adsorption on a tentacle cation exchanger

AbstractAdsorption of human immunoglobulin G (IgG) on a commercial cation exchanger with a grafted polymer layer was investigated at pH 4.5 and in the NaCl concentration range of 0–150 mM. Adsorption equilibrium was determined in static batch experiments and verified in batch uptake experiments. Parameters of the Langmuir isotherm were estimated for each salt concentration separately. The batch uptake experiments provided also the estimates of effective pore diffusion coefficients of IgG for individual protein and salt concentrations. The values of the effective pore diffusion coefficient depended strongly on both factors. They increased by about 5–15 times with the NaCl concentration and decreased about three times with the protein concentration. The quality of the estimated parameters was confirmed by frontal experiments described by the general rate model of chromatography.

Fabrication of dual-responsive cellulose-based membrane via simplified surface-initiated ATRP

An independently temperature- and pH-responsive membrane was developed by simultaneously grafting poly(N-isopropylacryamide) (PNIPAAm) and poly[(2-(diethylamino)ethyl methacrylate] (PDEAEMA) from different sides of a crosslinked cellulose membrane. The synthesis was simplified by using surface-initiated activators regenerated by electron transfer for atom-transfer radical polymerization in a diffusion device. The grafted membrane was heterostructured. The grafted polymer layer thickness was linearly related to reaction time. The wettabilities of the double-membrane sides responded individually and reversibly to temperature and pH. The surface grafted with PNIPAAm shifted from hydrophilicity to hydrophobicity above the lower critical solution temperature. The PDEAEMA side was hydrophilic in acidic aqueous solution and hydrophobic under basic conditions. This dual-response cellulose membrane has potential applications in water treatment, separations, and other membrane applications.

Equilibrium and kinetics of protein binding on ion-exchange cellulose membranes with grafted polymer layer

AbstractThe performance of weak and strong anion- and cation-exchange membrane adsorbents with a grafted gel layer (Sartobind Q, D, S, and C) was investigated using six proteins: bovine serum albumin, human serum albumin, α-lactalbumin, β-lactoglobulin, lysozyme, and myoglobin. Static binding experiments were used to assess the effect of pH and buffer concentration and to determine the adsorption isotherms for selected membrane/protein combinations. The equilibrium data were duly described either by the Langmuir or Freundlich isotherms. Dynamic binding experiments were carried out for the same membrane/protein combinations in a broad range of linear flow velocity. Both the dynamic binding capacity at 10 % breakthrough and the final binding capacity at complete breakthrough were independent of the flow velocity despite strong dispersion of the adsorption zone. A good match between the equilibrium data from static and dynamic experiments was obtained for the anion exchangers. The correlation between the dynamic binding capacity and protein molecule size was observed for the strong cation exchanger. This was due to the different accessibility of the gel layer for the protein molecules.

The Effects of the Reaction Conditions on the Grafting Degree of SiO<sub>2</sub>-g-PMAA Particle

Modified MPS-SiO2 particle was obtained by the bonding of 3-methacryloxypropyl trimethoxysilane (KH-570) on the surface of silica gel particle. The methacrylic acid (MMA) monomers were grafted on the surface of MPS-SiO2 particle to prepare the grafting particles SiO2-g-PMAA. The effects of reaction conditions on the graft degree were explored. The results indicate that the MMA monomers can be easily grafted on the surface of silica gel particle by using the method of graft polymerization. During the graft polymerization, the grafted polymer layer is a hindrance to the subsequent graft polymerization. Then the grafting degree of the polymer under a certain condition has a limiting value. The reaction conditions, such as monomer concentration, the amount of initiator, reaction temperature, have remarkably influence on the graft polymerization of SiO2-g-PMAA.

Toward remote-controlled valve functions via magnetically responsive capillary pore membranes

Polyethyleneterephthalate track-etched membranes with a pore diameter of 650nm were functionalized via surface-initiated atom transfer radical polymerization with grafted poly(2-hydroxyethylmethacrylate). Grafted chain length and density were varied. Superparamagnetic nanoparticles (Fe3O4; core diameter 15nm) were selectively covalently coupled to the end groups of the grafted chains. The membranes were characterized by grafting degree, X-ray photoelectron spectroscopy, electron microscopy, zeta potential and pore size in dry state via gas flow/pore dewetting permporometry. The results confirmed that all functionalization steps were well controlled. Water permeability measurements allowed estimation of the hydrodynamic pore diameter of the membranes, and, hence, the hydrodynamic polymer layer thickness on the pore walls. The water permeability of the nanoparticle hybrid membranes was then measured in a static or an alternating external magnetic field. Significant and reversible decreases of permeability were observed, with the largest effects for membranes with high polymer grafting density and long polymer chains (hydrodynamic layer thickness up to 100nm). The maximum change in effective pore diameter was only 6%. However, the estimated change of swollen polymer layer thickness (originally between 60 and 100nm) was up to 13nm. The functionality of the membranes can be tuned by variations of straightforward parameters such as pore size or grafted chain lengths. The study is also relevant as a model system for altering the effective thickness of grafted polymer layers on a surface by an external magnetic field for other applications, for instance in microfluidic systems.

Towards flame retardant cotton fabrics by atmospheric pressure plasma-induced graft polymerization: Synthesis and application of novel phosphoramidate monomers

The synthesis of two novel phosphoramidate flame retardant monomers for application to cotton textiles using atmospheric pressure dielectric barrier discharge (DBD) plasma was achieved and the molecular structures were characterized using +ESI-MS and FT-IR. The effects of plasma exposure time, monomer concentration, crosslinker chemistry, crosslinker concentration and photoinitiator concentration on grafting yield were investigated and the process was optimized for highest grafting yield. SEM images of the treated fabrics showed a grafted polymer layer covalently bonded to the polymer structure of cotton that survived a lengthy extraction process to remove ungrafted homopolymer and/or unreacted monomer. Thermogravimetric analysis showed that fabrics treated with the monomers demonstrated an increased thermal stability relative to untreated cotton. Vertical burn testing demonstrated the effectiveness of the grafted phosphorus-containing polymer at promoting char formation even though the fabrics were not self-extinguishing. Measurement of the amount of phosphorus present on the treated samples confirmed that the add-on using this application strategy was too low for self-extinguishing properties, but the rest of the evidence suggested that these compounds have the potential for use in generating flame retardant cotton substrates provided that a more suitable application strategy is developed to further improve grafting yield.

Core–shell magnetic chitosan particles functionalized by grafting: Synthesis and characterization

A method for preparing magnetic composite particles with a core containing iron oxide encapsulated in crosslinked chitosan (CS) and a functionalized synthetic copolymer shell is proposed and optimized. The magnetic material is produced in situ. Surface vinyl groups are introduced by attaching glycidyl methacrylate (GMA) to the hydroxyl or amino chitosan residues via ether or N-alkyl bonds in acidic medium. These groups are used in a subsequent synthesis step as starting points for free radical copolymerization of GMA, ethylene glycol dimethacrylate (EGDMA) and poly(ethylene glycol methylmethacrylate) (PEG-MMA). The epoxy rings are opened in the final step with ethylenediamine (EDA). The results confirm that a grafted polymer layer has been anchored on the surface by covalent bonds. Sorption batch experiments using copper (II) ion solutions were conducted in order to compare the performance of the synthesized materials as heavy metal adsorbents. The functionalized particles show superior adsorption capacity compared to the ungrafted material.

Preparation and Characterization of PMAA/SiO<sub>2</sub> Composite Particle Via Emulsion Polymerization

Silica gel particle was chemically modified by using 3-methacryloxypropyl trimethoxysilane (KH-570) as coupling agent. Then poly methyl acrylic acid (PMAA) was grafted on the surface of particles MPS-SiO2 with the method of Emulsion Polymerization, and grafting particles PMMA/SiO2 were prepared. The grafting particle PMAA/SiO2 was characterized by using FT-IR, TG and SEM. The experimental results show that the graft polymerization of methyl acrylic acid (MAA) on onto the surface of silica gel particles can successfully realized using the method of “graft from”. During the graft polymerization, the grafted polymer layer is a hindrance to the subsequent graft polymerization. When the grafted polymer layer reach to a certain density via overlapping and entwisting, a kinetic barrier will be built up. Hence, the grafting degree of the polymer under a certain condition has a limiting value.

Ion exchange membrane adsorption of bovine serum albumin—Impact of operating and buffer conditions on breakthrough curves

Abstract The relevant phenomena underlying the adsorption of proteins on ion exchange membranes with grafted polymer layers are not well understood yet. In this work the influence of operating and buffer conditions on the adsorption of bovine serum albumin (BSA) on anion and cation exchange membrane adsorbers is systematically investigated in order to find indications on the relevant mechanisms. The experimental results obtained at different buffer conditions are analyzed using the steric mass action model. Large differences were observed between the adsorption behavior of BSA on anion and cation exchange membranes. On the anion exchange membranes the dynamic binding capacity at 10% breakthrough was independent of the applied flow rate and decreased with increasing salt concentration. This in contrast to the cation exchange membranes, for which the dynamic binding capacity was strongly dependent on the flow rate and for which an optimum in binding capacity was observed at an intermediate salt concentration. Especially, the results obtained on cation exchange suggest a strong role of non-ideal effects such as steric hindrance and electrostatic repulsion in membrane adsorption. An improved understanding of these effects is required for further optimization of membrane adsorber materials.

Substrate-Independent Method for Growing and Modulating the Density of Polymer Brushes from Surfaces by ATRP

We describe a method for grafting PEG-based polymer chains of variable surface density using a substrate independent approach, allowing grafting from virtually any material substrate. The approach relies upon initial coupling of a macroinitiator to plasma polymer treated surfaces. The macroinitiator is a novel random terpolymer containing ATRP initiator residues, strongly negatively charged groups, and carboxylic acid moieties that facilitate covalent surface anchoring. Surface-initiated ATRP (SI-ATRP) using polyethylene glycol methyl ether methacrylate (PEGMA) at different concentrations led to grafted surfaces of controlled thickness in either the "brush" or "mushroom" morphology, which was controlled by the abundance of initiator residues in the macroinitiator. Grafted polymer layer structure was investigated via direct interaction force measurements using colloid probe atomic force microscopy (AFM). Equilibrium, hydrated graft layer thicknesses inferred from the highly repulsive AFM force data suggest that the polymer brush graft layer contained polymer chains which were fully stretched. Since the degree of stretching resulted in layer thicknesses approaching the polymer contour length, the polymer brushes studied must be very close to maximum graft density. Grafted layers where the polymer molecules were in the mushroom regime resulted in much thinner layers but the chains had greater chain entropic freedom as indicated by strongly attractive bridging interactions between tethered chains and the silica colloid probe. Use of this experimental methodology would be suitable for preparing grafted polymer layers of a preferred density free from substrate-specific linking chemistries.

Preparation and characterization of poly(glycidyl methacrylate) grafted from magnesium hydroxide particles via SI-ATRP

In order to improve the compatibility of magnesium hydroxide particles [Mg(OH)2] and polymer matrix, poly(glycidyl methacrylate) (PGMA) grafted from magnesium hydroxide particles were synthesized via surface-initiated atom transfer radical polymerization (SI-ATRP). In this work, two approaches for the immobilization of ATRP initiator on the magnesium hydroxide particles surface were compared and selected. The density of initiator was significantly increased by the method of introducing more hydroxyl groups via ATRP of 2-hydroxyethyl methacrylate (HEMA) on the surface. The percentage of bromine atom for the initiator-functionalized magnesium hydroxide particles [Mg(OH)2-g-PHEMA–Br] reached to 1.75%, compared to 0.48% for Mg(OH)2–Br determined by XPS analysis. The surface-initiated ATRP of glycidyl methacrylate (GMA) can be conducted in a controlled manner, as revealed by the linear kinetic plot, linear increase of number average molecular weight (Mn) with monomer conversions, and the relatively narrow molecular weight distributions (Mw/Mn∼1.4) of PGMA chains. The percentage of grafting PG (%) and the thickness of the grafted polymer layer increased with the increasing of polymerization time and reached to 116.6% and 197.6nm after 300min respectively. As for the polymerization with different initial monomer concentration, the number average molecular weights (Mn) and weight average molecular weights (Mw) of PGMA increased with the increasing of initial monomer concentration. TGA indicated that the initial decomposition temperature of Mg(OH)2-g-PHEMA–PGMA composite particles (253°C) was much lower than that of unmodified magnesium hydroxide particles (337°C).

Zwitterionic sulfobetaine-grafted poly(vinylidene fluoride) membrane surface with stably anti-protein-fouling performance via a two-step surface polymerization

A zwitterionic polymer, poly(3-(methacryloylamino) propyl-dimethyl-(3-sulfopropyl) ammonium hydroxide) (poly(MPDSAH)) was successfully grafted in high density from the surface of poly(vinylidene fluoride) (PVDF) hollow fiber membrane via a two-step polymerization. Poly(2-hydroxyethyl methacrylate) (poly(HEMA)) chains were firstly grafted from outside surface of PVDF membrane through atom transfer radical polymerization (ATRP) to provide the initiation sites for subsequent cerium (Ce (IV))-induced graft copolymerization of polyMPDSAH in the presence of N,N′-ethylene bisacrylamide (EBAA) as a cross-linking agent. Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS) confirmed that the EBAA could stimulate zwitterionic polymers grafting onto the membrane surface. The dense poly(MPDSAH) layers on the PVDF membrane surface were revealed by the scanning electron microscope (SEM). The mechanical property of PVDF membrane was improved by the zwitterionic surface layers. The gravimetry results indicated the grafting amount increased to 520μg/cm2 for a copolymerization time of more than 3h. Static and dynamic water contact angle measurements showed that the surface hydrophilicity of the PVDF membranes was significantly enhanced. As the grafting amount reached 513μgcm−2, the value of contact angle dropped to 22.1° and the amount of protein adsorption decreased to zero. The cyclic experiments for BSA solution filtration demonstrated that the extent of protein fouling was significantly reduced and most of the fouling was reversible. The grafted polymer layer on the PVDF membrane showed a good stability during the membrane cleaning process. The experimental results concluded a good prospect in obtaining the sulfobetaine-modified PVDF membranes with high mechanical strength, good anti-protein-fouling performance, and long-term stability via the two-step polymerization.

Formation of pH-responsive polymer composite membranes by plasma-induced graft polymerization method

The structure and the transport properties of polymer composite membranes consisting of a porous substrate and a polymer layer obtained by plasma-induced graft polymerization method were studied. It has shown that the presence of the polymer layer on the surface of porous substrate leads to changing its transport properties - the water permeability of the formed composite membranes substantially depends on the solution pH. These changes are caused by convertible conformational transitions of macromolecules of the grafted polymer layer from an expanded state into a compact one which is in turn caused by the degree of ionization of the functional groups on the surface of this layer.

Nanomechanical properties of polymer brushes by colloidal AFM probes

Nanomechanical properties of end grafted polymer layers were studied by AFM based, colloidal probe compression measurements. Zwitterionic poly(sulfobetaine methacrylate) (PSBMA) brush was grafted from planar Si surface and poly(methyl methacrylate) (PMAA) brush was grown on colloidal probe by surface initiated atom transfer radical polymerization. PMAA brush was further modified with adhesion promoting arginyl-glycyl-aspartic acid (RGD) peptide sequences. Force–distance curves were obtained for systems where the polymer brushes were probed on unmodified surfaces or face to each other. For each systems the grafting density of the polymer brush was determined applying a ‘box’ like polymer brush model based on the theory by de Gennes. ‘Average’ grafting density was calculated in cases when two polymer brushes face each other: RGD functionalized PMAA or PMAA against PSBMA. For our systems the values for the grafting density was between 0.04 and 0.11 nm −2. Furthermore the measured approach force–distance curves were fitted according to the Hertz model and the apparent Young’s modulus was determined for all measurements being in a range of around 250 kPa at physiological conditions.

Protein adsorption and transport in polymer-functionalized ion-exchangers.

A wide variety of stationary phases is available for use in preparative chromatography of proteins, covering different base matrices, pore structures and modes of chromatography. There has recently been significant growth in the number of such materials in which the base matrix is derivatized to add a covalently attached or grafted polymer layer or, in some cases, a hydrogel that fills the pore space. This review summarizes the main structural and functional features of ion exchangers of this kind, which represent the largest class of such materials. Although the adsorption and transport properties may generally be used operationally and modeled phenomenologically using the same methods as are used for proteins in conventional media, there are noteworthy mechanistic differences in protein behavior in these adsorbents. A fundamental difference in protein retention is that it may be portrayed as partitioning into a three-dimensional polymer phase rather than adsorption at an extended two-dimensional surface, as applies in more conventional media. Beyond this partitioning behavior, however, the polymer-functionalized media often display rapid intraparticle transport that, while qualitatively comparable to that in conventional media, is sufficiently rapid quantitatively under certain conditions that it can lead to clear benefits in key measures of performance such as the dynamic binding capacity. Although possible mechanistic bases for the retention and transport properties are discussed, appreciable areas of uncertainty make detailed mechanistic modeling very challenging, and more detailed experimental characterization is likely to be more productive.

Polymer Brushes by the “Grafting to” Method

This paper focuses on the attachment of densely grafted polymer layers (polymer brushes) to various inorganic and polymeric substrates by the "grafting to" method. A brief overview of synthesis of polymer brushes by the method is first provided, with emphasis on chemical approaches to polymer attachment. The second part of the paper covers the synthesis of polymer layers via a recently developed macromolecular anchoring layer approach. Several examples of application of the grafting technique are presented for generation of hydrophobic, hydrophilic, gradient, and switchable surfaces.

Modified Anopore™ hybrid membranes for the microfiltration of cellulose acid hydrolysis mixtures

Three classes of nonfouling substances, poly[poly(ethylene glycol) methyl ether methacrylate] (POEM), poly(sulfobetaine methacrylate) (PSBMA), and perfluoroalkyl tagged poly(ethylene glycol) surfactants (fPEG) were covalently tethered onto the surface of Anopore™ membranes (nominal pore size 0.2 μm) to form a series of organic–inorganic hybrid membranes. These membranes were then used for the microfiltration of cellulose acid hydrolysis mixtures. Flux recoveries of over 80% after three filtration cycles were observed for the hybrid membranes. Thus the fluxes of the three synthesized hybrid membranes could be largely recovered by washing with methanol and water after the microfiltration of cellulose hydrolysis mixtures. In contrast the bare membranes showed much lower flux recoveries. It was also shown that the grafted polymer layer improves the stability of the membranes in acidic environment. The synthesized membranes were also characterized by scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS), which were used to understand the morphology and chemical compositions of the membrane surfaces, and the results of these methods correlate with the microfiltration data presented. Mathematical modeling of the microfiltration data indicates that the pore constriction model best describes the fouling behavior of the bare Anopore™ membrane, while the cake filtration model fits the data most satisfactorily for the three groups of hybrid membranes. The current work presents an alternative approach to membrane design for biomass related separations.