Brush Length Research Articles

Assembly of Polymer-Grafted Magnetic Nanoparticles in Polymer Melts

Hydrophobic iron oxide nanoparticles grafted with hydrophobic polymer chains of varying molecular weights and graft densities are synthesized to underpin the role of brush entanglement and dipolar forces on creating nanostructures. Grafting density on magnetic nanoparticles is controlled in grafting-to method by changing the concentration of functionalized polymer in solution. The grafting density and brush length have varied systemically to observe the changes in nanostructures. Bridging between grafted chains and dipolar forces become effective only at low grafting density and result in long chains of particles. We demonstrate experimentally that structural transition of magnetic nanoparticles is controlled with the balance between grafted chain entanglements and dipolar forces.

Controlled immobilization of methyltrioxorhenium(VII) based on SI-ATRP of 4-vinyl pyridine from halloysite nanotubes for epoxidation of soybean oil

Poly(4-vinylpyridine) (P-4VP) brushes of different lengths were grafted onto halloysite nanotubes (HNTs) by surface-initiated atom transfer radical polymerization (SI-ATRP) and methyltrioxorhenium(VII) (CH3ReO3, MTO) was immobilized onto HNTs through P-4VP brushes to catalyze epoxidation of soybean oil. 2-Bromoisobutyryl bromine was chosen as the initiator of SI-ATRP and anchored by reacting with hydroxyl groups of HNTs. To reduce the effect of 4-vinylpyridine on the initiator, Me6tren as the ligand was introduced into the polymerization. The length of P-4VP brushes was controlled by the polymerization time. Through coordination between N in pyridine rings and MTO, a novel heterogeneous catalyst with different loadings of MTO for epoxidation of soybean oil was prepared. FTIR, TGA, TEM, GPC, UV–visible absorption spectroscopy and 1H NMR were used to characterize HNTs with P-4VP brushes and the heterogeneous catalysts. The results indicated that the length of P-4VP brushes increased with the polymerization time and MTO loading increased with P-4VP brush length. MTO molecules dispersed uniformly in P-4VP brushes and the heterogeneous MTO combined with H2O2 had good catalytic activity and selectivity to epoxidation of soybean oil. TOF of heterogeneous MTO increased with MTO loading and no ring-opening reaction occurred during epoxidation.

Blood Clearance and Biodistribution of Polymer Brush-Afforded Silica Particles Prepared by Surface-Initiated Living Radical Polymerization

The physiological properties of polymer brush-afforded silica particles prepared by surface-initiated living radical polymerization were investigated in terms of the circulation lifetime in the blood and distribution in tissues. Hydrophilic polymers consisting mainly of poly(poly(ethylene glycol) methyl ether methacrylate) were grafted onto silica particles by surface-initiated atom transfer radical polymerization that was mediated by a copper complex to produce hairy hybrid particles. A series of hybrid particles was synthesized by varying the diameter of the silica core and the chain length of the polymer brush to examine the relationship between their physicochemical and physiological properties. The hybrid particles were injected intravenously into mice to investigate systematically their blood clearance and body distribution. It was revealed that the structural features of the hybrid particles significantly affected their in vivo pharmacokinetics. Some hybrid particles exhibited an excellently prolonged circulation lifetime in the blood with a half life of ∼20 h. When such hybrid particles were injected intravenously into a tumor-bearing mouse, they preferentially accumulated in tumor tissue. The tumor-targeted delivery was optically visualized using hybrid particles grafted with fluorescence-labeled polymer brushes.

Dynamically cell separating thermo-functional biointerfaces with densely packed polymer brushes

Poly(N-isopropylacrylamide) (PIPAAm) brush grafted glass surfaces with various brush lengths were prepared as cell separating intelligent interfaces through a surface-initiated atom transfer radical polymerization (ATRP) with a CuCl–Me6TREN catalytic system and α-chloro-p-xylene as a free initiator in 2-propanol at 25 °C for 16 h. Characterization of the prepared surface was performed by X-ray photoelectron spectroscopy (XPS), attenuated total reflection Fourier transform infrared (ATR/FT-IR) spectroscopy, and gel permeation chromatography (GPC) measurement of PIPAAm in ATRP reaction solution for estimating the brush length. Phase transition behavior of PIPAAm in four cell culture media was also investigated by measuring the temperature-dependent turbidities. Prepared PIPAAm brush surfaces as cell separating intelligent interfaces were characterized by observing the adhesion and detachment behavior of four types of human cells: human umbilical vein endothelial cells (HUVECs), normal human dermal fibroblasts (NHDFs), human aortic smooth muscle cells (SMCs), and human skeletal muscle myoblast cells (HSMMs). The PIPAAm brush surface with a moderate brush length exhibited a proper cell adhesion and detachment behavior, while short-brush-surfaces scarcely detached cells and long-brush-surfaces scarcely adhered cells. The PIPAAm brush with a moderate brush length exhibited different cell detachment rates among individual cell types. Utilizing the different cell detachment properties, a mixture of green fluorescent protein (GFP) expressing HUVEC (GFP-HUVEC) and HSMM was separated. After being allowed to adhere on the surfaces at 37 °C for 24 h, the adhered cells on the surfaces were incubated at 20 °C. In the initial period of incubation at 20 °C, the GFP-HUVEC was released from the surface due to its prompt detachment property, and, in the subsequent period of incubation, HSMMs gradually detached themselves from the surface. These results indicated that a precisely designed PIPAAm brush functioned as an intelligent cell separating interface by utilizing the intrinsic cell detachment properties of individual cells.

Effective interactions between grafted nanoparticles in a polymer matrix

Molecular dynamics simulations were used to delineate the separation dependent forces between two polymer-grafted nanoparticles in a polymer melt, the associated potential of mean force (PMF), and the molecular origins of these forces. The nanoparticle radius (=5, in units of the size of the chain monomers) and grafted brush length (=10) were held constant, while the grafting density and the polymer matrix length were varied systematically in a series of simulations. We first show that simulations of a single nanoparticle do not reveal any signatures of the expected autophobic dewetting of the brush with increasing polymer matrix length. In fact, density distributions of the matrix and grafted chains around a single nanoparticle appear to only depend on the grafting density but not on the matrix chain length in the regime where autophobic dewetting is expected, i.e., when the matrix chain length is equal to or longer than the graft chain length. We thus conjecture that two nanoparticle simulations might be more illuminating in these situations. Indeed, the calculated forces between two nanoparticles in a melt show that increasing the matrix chain length from 10 to 70 causes the inter-nanoparticle potential of mean force (PMF) to go from purely repulsive to attractive with a well depth on the order of kBT. These results are purely entropic in origin and arise from a competition between brush-brush repulsion and an attractive inter-nanoparticle interaction caused by matrix depletion from the inter-nanoparticle zone. The matrix-induced Asakura-Oosawa type inter-nanoparticle attraction, which dominates at intermediate nanoparticle separations especially in the case of long matrix chains, is thus implicated as the essential player in the autophobic dewetting phenomenon, which drives phase separation in these situations.

Synthesis of cellulose nanocrystals bearing photocleavable grafts by ATRP

Cellulose nanocrystals (CNC) were grafted with photocleavable polymeric chains using Atom Transfer Radical Polymerisation (ATRP). The nanoparticle synthesis relies on two main steps: first the grafting of a photosensitive moiety bearing an ATRP initiating site onto the surface of CNC, followed by surface initiated ATRP from the modified surface outwards. An original UV-sensitive linker bearing one ATRP initiating site and one anchoring hydroxyl functionality, which was grafted to the CNC surface using a diisocyanate linker, was designed to achieve this goal. Polystyrene (PS) was then polymerized from the modified CNC, resulting in CNC-g-photolinker-PS nanoparticles. The possibility to degraft the PS chains using UV-irradiation was further investigated, and PS grafts with a narrow polydispersity index (1.08) and molecular weights close to those of formed homopolymers through sacrificial initiator addition were obtained. This is the first example where it has been proven that good control of SI-ATRP from CNC can be achieved. It also confirms that homopolymers initiated by sacrificial initiator in the polymerization medium can be an indication of the grafted polymer brush length. These materials further offer potential as smart delivery vehicles and smart materials.

Controlling the Location of Nanoparticles in Polymer Blends by Tuning the Length and End Group of Polymer Brushes

This paper investigates controlling the location of nanoparticles (NPs) in a phase-separated polymer blend of deuterated poly(methyl methyl methacrylate) (dPMMA) and poly(styrene-ran-acrylonitrile) (SAN). Silica NPs are grafted with PMMA brushes having molecular weights of 1800, 21000, and 160000 at fixed grafting density. Using ion beam milling combined with SEM imaging, NP location and morphology are investigated for blends containing 10 wt % NP. With increasing brush length, the NPs are found to segregate to the dPMMA/SAN interface, partition between the interface and dPMMA phase, or locate in the dPMMA phase, respectively.

Thermally Modulated Retention of Lymphoctytes on Polymer‐Brush‐Grafted Glass Beads

Thermoresponsive PIPAAm-brush-grafted glass beads are prepared through siATRP and their physicochemical properties are characterized by micro-nitrogen analysis, XPS, and contact angle measurements. The amount of grafted PIPAAm on glass bead surfaces can be controlled by varying the ATRP reaction time, leading to a modulation of the temperature-dependent wettability of the prepared surfaces. To evaluate a possible use of the beads as cell separation matrices, loading with rat lymphocytes from mesenteric lymph nodes is studied. The results show that the interaction between PIPAAm brushes and lymphocytes can be controlled by modulating PIPAAm brush length and temperature. The PIPAAm-brush-grafted beads might therefore be useful as effective cell separation matrices.

Effect of Brush Thickness and Solvent Composition on the Friction Force Response of Poly(2-(methacryloyloxy)ethylphosphorylcholine) Brushes

The frictional properties of poly(2-(methacryloyloxy)ethylphosphorylcholine) (PMPC) brushes grown from planar silicon surfaces by atom transfer radical polymerization (ATRP) have been characterized using in situ friction force microscopy (FFM). The dry thicknesses of the PMPC brushes ranged from 20 to 421 nm. For brush layers with dry thicknesses greater than ca. 100 nm, the coefficient of friction decreased with increasing film thickness. For shorter brushes, the coefficient of friction varied little with brush thickness. We hypothesize that the amount of bound solvent increases as the brush length increases, causing the osmotic pressure to increase and yielding a reduced tendency for the brush layer to deform under applied load. A comparison of the force-displacement plots acquired for various PMPC brushes under water supports this hypothesis, since a greater repulsive force is measured for thicker brushes. FFM was also used to investigate the well-known co-nonsolvency behavior exhibited by PMPC chains. For a PMPC brush layer of 307 nm dry thickness, the friction force was determined as a function of the volume fraction of alcohol in alcohol/water mixtures. Unlike a previous macroscopic study, a significant increase in the coefficient of friction was observed for ethanol/water mixtures at a volume fraction of 90%. This is attributed to brush collapse due to co-nonsolvency, leading to loss of hydration of the brush chains and hence substantially reduced lubrication. Force measurements normal to the surface indicate much greater hysteresis between approaching and retraction curves under co-nonsolvency conditions. However, no such effect was observed for 2-propanol/water and methanol/water mixtures over a wide range of volume fractions, in agreement with recent ellipsometric studies of PMPC brushes.

콩.옥수수 육묘용 파종기 개발에 관한 연구

Soybean (Glycine max Merr.) and corn (Zea mays L.) transplanting has increased because soybean and corn crops cultivated by the direct seeding method were often damaged by wild birds. The purpose of this study is to develop a seeder to sow soybean (Glycine max Merr.) and corn (Zea mays L.) in a plug tray. In order to find out design factors for a metering device of the seeder, metering characteristics on metering hole size and roller speed were experimentally investigated. Soybean (cv. `Daewon`) and corn (cv. `Mibaekchal`) were used as a materials for testing the seeder in this experiment. The metering hole size of roller suitable for Daewonkong and Mibaekchal was determined. Daewonkong was suitable for hole diameter of 10 mm and hole depth of 5.5 mm, and Mibaekcal was suitable for hole diameter of 9 mm and hole depth of 5.5 mm. At a brush length of 4 mm, one grain seeding rates of Daewonkong and Mibaekchal was 99% and 93% respectively. By inducing Mibaekchal to the hole by swing, one grain seeding rate of that increased from 91.9% to 97.7%. When roller speed is 4 m per minut, seeding efficiency of prototype was 110 sheets per hour.

Atomic force microscopy study of immunosensor surface to scale down the size ofELISA-type sensors

Here we describe the use of atomic force microscopy (AFM) to study the nanoscalemechanics of the molecular layers of a popular immunosensor, ELISA (enzyme-linkedimmunosorbent assay) type. We characterize the sensor surface in terms of brush lengthand grafting density of the molecular layers. The obtained data demonstrated that areliable reading of the immunosignal (a suggested dimensionless combinationof brush length and grafting density) can be attained from an area as small as ∼ 3 µm2. This is approximately 4 million times smaller compared to typical ELISA sensors. Theimmunosensor described is composed of a molecular mix of two different antigens.Intriguingly, we find that AFM can reliably distinguish between having the immunosignalfrom either antibody and from both antibodies together. This was impossible to get byusing standard optical detection methods.

Polymer templated nanocrystalline titania network for solid state dye sensitized solar cells

We report a novel preparation method for nanocrystalline TiO2 networks with controlled pore sizes using spherical polyelectrolyte brushes (SPB) as templates. The SPB consists of a solid polystyrene core from which anionic polyelectrolytes are densely grafted. The SPB templates are synthesized via conventional photoemulsion polymerization with efficient control of core size and brush length. Subsequently, the TiO2 precursor is hydrolyzed at room temperature within the anionic brush to obtain anatase nanocrystals of 12–20 nm size. These stable and form-persistent composite particles of SPB decorated with anatase nanocrystals are then assembled on a conductive substrate. The subsequent calcination of this composite layer leads to a robust nanocrystalline TiO2 network, in which the pores and the wall thickness are directly correlated to the polystyrene core size and the amount of TiO2 hydrolyzed within the brush respectively. In this study, we optimized different thin-film preparation methods and characterized the resulting nanocrystalline TiO2 networks using SEM and XRD. Moreover, the applicability of these nanocrystalline networks as electron transport layers are tested in solid-state dye-sensitized solar cells (SDSCs). The first test devices exhibited efficiencies up to 0.8%. The precise and individual control of parameters such as porosity, thickness and crystallinity makes this concept highly attractive for the realization of efficient solid-state hybrid devices.

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.

Control of Morphology and Its Effects on the Optical Properties of Polymer Nanocomposites

Chain-grafted Au nanoparticles were synthesized and incorporated into a fluorescent polymer, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), host. We show that control of the Au nanoparticle distribution within MEH-PPV is achieved by manipulating the enthalpic and entropic interactions between the grafted brush layers and the host chains. Further, we show that the fluorescence of these Au/MEH-PPV nanocomposite thin films may be "tailored" by as much as an order of magnitude through changes in the nanoparticle distribution, brush length, and nanoparticle size. The ideas presented herein represent reliable strategies for materials design for devices.

Synthesis and self‐assembly behavior of amphiphilic polypeptide‐based brush‐coil block copolymers

AbstractSynthesis and self‐assembly behavior of a novel amphiphilic brush‐coil block copolymer bearing hydrophilic poly(ethylene glycol) segment and hydrophobic polypeptide brush segment were presented in this work. The poly(γ‐benzyl‐L‐glutamate) (PBLG) brush is synthesized through “grafting from” strategy by ring‐opening polymerization of γ‐benzyl‐L‐glutamate‐N‐carboxyanhydride (BLG‐NCA) initiated by the flanking terminal primary amino group of macroinitiator. The copolymers were characterized by 1H NMR, gel permeation chromatography, Fourier transform infrared, circular dichroism spectrum, and differential scanning calorimetry. The self‐assembly behavior of the brush‐coil block copolymers in aqueous solution was investigated by means of transmission electron microscopy, scanning electron microscopy, atomic force microscopy, and laser light scattering. Spherical micelles were observed when the length of PBLG brush is shorter. The aggregate morphology transforms to spindle‐like micelles and then to rod‐like micelles, as the length of polypeptide brush increases. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5967–5978, 2009

Active Protein-Functionalized Poly(poly(ethylene glycol) monomethacrylate)-Si(100) Hybrids from Surface-Initiated Atom Transfer Radical Polymerization for Potential Biological Applications

Protein-resistant poly(poly(ethylene glycol)monomethacrylate)-graft-Si(100), or Si-g-P(PEGMA) hybrids, were prepared via surface-initiated atom transfer radical polymerization (ATRP) of the poly(ethylene glycol)monomethacrylate (PEGMA) macromonomer from the hydrogen-terminated Si(100) surface (Si-H surface). The resultant robust Si-C bonded P(PEGMA) brushes can be further functionalized by the immobilization of human immunoglobulin (IgG) protein via different strategies, namely, the direct use of the alkyl halide chain ends preserved throughout the ATRP process and the postmodification of the hydroxyl side chains with by 1,1'-carbonyldiimidazole (CDI) or succinic anhydride (SA). The CDI exhibited a higher efficiency in activating the hydroxyl groups for coupling proteins. The surface density of the immobilized protein above 2.5 microg/cm(2) could be readily achieved. The distribution of active protein-docking sites on the Si-C bonded P(PEGMA) brushes can be also controlled by controlling the brush length. The resulting IgG-coupled Si-g-P(PEGMA) hybrid surface interacts only and specifically with the anti-IgG protein, while the dense P(PEGMA) brushes effectively prevent nonspecific protein binding and fouling. The simple concomitant incorporation of protein-resistant P(PEGMA) brushes and highly specific and active protein onto silicon surfaces via robust Si-C bonding should readily endow the silicon substrates with new and interesting properties for applications in silicon-based protein sensors or microarrays.

Cellulose Nanocrystals Grafted with Polystyrene Chains through Surface-Initiated Atom Transfer Radical Polymerization (SI-ATRP)

This paper reports the synthesis of cellulose nanocrystals grafted by polystyrene chains via surface-initiated ATRP. Naturally occurring cellulose was first hydrolyzed to obtain cellulose nanocrystals. Their surface was then chemically modified using 2-bromoisobutyryl bromide to introduce initiating sites for ATRP. A varying extent of surface modification was achieved by changing reaction conditions. Further initiation of styrene polymerization from these modified nanocrystals with a CuBr/PMDETA (N,N,N',N',N''-pentamethyldiethylenetriamine) catalytic system and in the presence of a sacrificial initiator produced polysaccharide nanocrystals grafted by polystyrene chains. A range of nanocrystals-g-polystyrene with different graft lengths (theoretical polymerization degree = 27-171) was synthesized through this method and characterized by elemental analysis, XPS, FT-IR, TEM, and contact angle measurements. We are thus able to produce cellulose nanoparticles with varying grafting densities (by altering extent of initiator surface modification) and varying polymer brush length (through polymerization control). The nanocrystals-g-polystyrene (NC-g-PS) particles were tested for their capacity to absorb 1,2,4-trichlorobenzene from water. The results obtained show that they can absorb the equivalent of 50% of their weight in pollutant compared to 30 wt % adsorption for nonmodified nanocrystals, while also displaying faster absorption kinetics.

レーザプリンタの2成分磁気ブラシ現像系におけるキャリアブラシの摩擦力(機械力学,計測,自動制御)

We have studied friction force of magnetic carrier brushes to the photoconductor drum in two-component magnetic development system of electrophotography by a model experiment and the numerical simulation with the Distinct Element Method. Axial and lateral forces of chain-like brushes applied to the photoconductor were measured to clarify the relationship between the lateral friction force and the diameter of carrier particles, magnetic flux density, and the brush length. It was observed that the force was increased in accordance with the increase of the magnetic flux density and decreased with the brush length but the particle diameter did not affect the total force. On the other hand, the numerical calculation deduced that although the total force is not affected by the particle diameter, the individual differential force of the chain was small and the density of contact points of the brush was high for small particles.

Conformation of Poly(styrene sulfonate) Layers Physisorbed from High Salt Solution Studied by Force Measurements on Two Different Length Scales

The conformation of poly(styrene sulfonate) (PSS) layers physisorbed from 1 M NaCl is determined by force measurements and imaging on two length scales. With colloidal probe technique steric forces as predicted for neutral grafted brushes are observed. On decrease and increase of the NaCl concentration, the grafting density remains constant, yet the brush thickness swells and shrinks reversibly with the salt concentration with an exponent of -0.3. At low salt conditions, the brush length amounts to 30% of the contour length, a behavior known for polyelectrolyte brushes and attributed to the entropy of the counterions trapped in the brush. Between a PSS layer and a pure colloidal silica sphere, the same steric forces are observed, and additionally at large separations (beyond the range of the steric repulsion) an electrostatic force is found. A negatively charged AFM tip penetrates the brush--a repulsive electrostatic force between the tip and surface is found, and single chains can be imaged. Thus, with the nanometer-sized AFM tip, the flatly adsorbed fraction of the PSS chains is seen, whereas the micrometer-sized colloidal probe interacts with the fraction of the chains penetrating into solution.

Dispersion of polymer-grafted magnetic nanoparticles in homopolymers and block copolymers

The dispersion of magnetic nanoparticles (NPs) in homopolymer poly(methyl methacrylate) (PMMA) and block copolymer poly(styrene-b-methyl methacrylate) (PS-b-PMMA) films is investigated by TEM and AFM. The magnetite (Fe3O4) NPs are grafted with PMMA brushes with molecular weights from M=2.7 to 35.7kg/mol. Whereas a uniform dispersion of NPs with the longest brush is obtained in a PMMA matrix (P=37 and 77kg/mol), NPs with shorter brushes are found to aggregate. This behavior is attributed to wet and dry brush theory, respectively. Upon mixing NPs with the shortest brush in PS-b-PMMA, as-cast and annealed films show a uniform dispersion at 1wt%. However, at 10wt%, PS-b-PMMA remains disordered upon annealing and the NPs aggregate into 22nm domains, which is greater than the domain size of the PMMA lamellae, 18nm. For the longest brush length, the NPs aggregate into domains that are much larger than the lamellae and are encapsulated by PS-b-PMMA which form an onion-ring morphology. Using a multi-component Flory–Huggins theory, the concentrations at which the NPs are expected to phase separate in solution are calculated and found to be in good agreement with experimental observations of aggregation.