Intelligent dual-responsive cellulose surfaces via surface-initiated ATRP

Novel thermo-responsive cellulose (filter paper) surfaces of N-isopropylacrylamide (NIPAAm) and pH-responsive cellulose surfaces of 4-vinylpyridine (4VP) have been achieved via surface-initiated ATRP. Dual-responsive (pH and temperature) cellulose surfaces were also obtained through the synthesis of block-copolymer brushes of PNIPAAm and P4VP. With changes in pH and temperature, these "intelligent" surfaces showed a reversible response to both individual triggers, as indicated by the changes in wettability from highly hydrophilic to highly hydrophobic observed by water contact angle measurements. Adjusting the composition of the grafted block-copolymer brushes allowed for further tuning of the wettability of these "intelligent" cellulose surfaces.     

Mean-field model of immobilized enzymes embedded in a grafted polymer layer

Two-dimensional mean-field lattice theory is used to model immobilization and stabilization of an enzyme on a hydrophobic surface using grafted polymers. Although the enzyme affords biofunctionality, the grafted polymers stabilize the enzyme and impart biocompatibility. The protein is modeled as a compact hydrophobic-polar polymer, designed to have a specific bulk conformation reproducing the catalytic cleft of natural enzymes. Three scenarios are modeled that have medical or industrial importance: 1), It is shown that short hydrophilic grafted polymers, such as polyethylene glycol, which are often used to provide biocompatibility, can also serve to protect a surface-immobilized enzyme from adsorption and denaturation on a hydrophobic surface. 2), Screening of the enzyme from the surface and nonspecific interactions with biomaterial in bulk solution requires a grafted layer composed of short hydrophilic polymers and long triblock copolymers. 3), Hydrophilic polymers grafted on a hydrophobic surface in contact with an organic solvent form a dense hydrophilic nanoenvironment near the surface that effectively shields and stabilizes the enzyme against both surface and solvent.     

Comb polymers prepared by ATRP from hydroxypropyl cellulose

Hydroxypropyl cellulose (HPC) was used as a core molecule for controlled grafting of monomers by ATRP, the aim being to produce densely grafted comb polymers. HPC was either allowed to react with an ATRP initiator or the first generation initiator-functionalized 2,2-bis(methylol)propionic acid dendron to create macroinitiators having high degrees of functionality. The macroinitiators were then "grafted from" using ATRP of methyl methacrylate (MMA) or hexadecyl methacrylate. Block copolymers were obtained by chain extending PMMA-grafted HPCs via the ATRP of tert-butyl acrylate. Subsequent selective acidolysis of the tert-butyl ester moieties was performed to form a block of poly(acrylic acid) resulting in amphiphilic block copolymer grafts. The graft copolymers were characterized by 1H NMR and FT-IR spectroscopies, DSC, TGA, rheological measurements, DLS, and tapping mode AFM on samples spin coated upon mica. It was found that the comb (co)polymers were in the nanometer size range and that the dendronization had an interesting effect on the rheological properties.     

Effective separation of peptides using highly dense thermo-responsive polymer brush-grafted porous polystyrene beads

For the development of well-defined highly dense thermo-responsive polymer grafted surface as an improved stationary phase for thermo-responsive chromatography, poly(N-isopropylacrylamide) (PIPAAm) brush-grafted porous polystyrene beads were prepared by surface-initiated atom transfer radical polymerization (ATRP). The PIPAAm grafted region of polystyrene beads was adjusted by the addition of isooctane as a poor solvent for polystyrene upon the reaction of ATRP initiator immobilization. Using a thermo-responsive HPLC column containing the prepared beads with PIPAAm brush grafted on the inside pores nearby the outer surfaces, angiotensin subtypes were effectively separated with aqueous mobile phase, because the densely grafted PIPAAm on nearby the outer surface effectively interacted with the peptides hydrophobically. Retention of basic peptide was achieved by the beads with basic mobile phase. These results indicated that the prepared beads with grafted PIPAAm nearby the outer surface became an effective chromatographic stationary phase for retaining basic peptides using wide pH range of mobile phase.     

Cellular response to magnetic nanoparticles "PEGylated" via surface-initiated atom transfer radical polymerization

A new method to PEGylate magnetic nanoparticles with a dense layer of poly(poly(ethylene glycol) monomethacrylate) (P(PEGMA)) by surface-initiated atom transfer radical polymerization (ATRP) is reported. In this approach, an initiator for ATRP was first immobilized onto the magnetic nanoparticle surface, and then P(PEGMA) was grafted onto the surface of magnetic nanoparticle via copper-mediated ATRP. The modified nanoparticles were subjected to detailed characterization using FTIR, XPS, and TGA. The P(PEGMA)-immobilized nanoparticles dispersed well in aqueous media. The saturation magnetization values of the P(PEGMA)-immobilized nanoparticles were 19 emu/g and 11 emu/g after 2 and 4 h polymerization respectively, compared to 52 emu/g for the pristine magnetic nanoparticles. The response of macrophage cells to pristine and P(PEGMA)-immobilized nanoparticles was compared. The results showed that the macrophage cells are very effective in cleaning up the pristine magnetic nanoparticles. With the P(PEGMA)-immobilized nanoparticles, the amount of nanoparticles internalized into the cells is greatly reduced to <2 pg/cell over a 5 day period. With this amount of nanoparticles uptake, no significant cytotoxicity effects were observed.     

Control of nanobiointerfaces generated from well-defined biomimetic polymer brushes for protein and cell manipulations

To better understand protein/material and cell/material interactions at the submolecular level, well-defined polymer brushes consisting of poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) on silicon wafers were prepared by atom transfer radical polymerization (ATRP). Silicon wafers were treated with 3-(2-bromoisobutyryl)propyl dimethylchlorosilane (BDCS) to form a monolayer that acts as initiators for ATRP. Silicon-supported BDCS monolayers were soaked in a methanol/water mixture solution containing Cu(I)Br, bipyridine, and a sacrificial initiator. After MPC was added to the solution, ATRP was carried out for 18 h. The molecular weight and thickness of the PMPC brush layer on the silicon surface increased with an increase in the polymerization time. The dense polymer brushes were obtained by the "grafting from" system. By selective decomposition of the BDCS monolayer by UV light-irradiation, the PMPC brush region and the sizes were well controlled, resulting in fabricating micropatterns of the PMPC brushes. When the thickness of the PMPC brush layer was greater than 5.5 +/- 1.0 nm (3 h polymerization), serum protein adsorption and fibroblast adhesion were effectively reduced, i.e., proteins and cells could recognize such thin polymer brushes on the surface. In addition, the density of the adherent cells on the patterned PMPC brush surface could be controlled by changing the size of the pattern.     

The influence of grafted polymer architecture and fluid hydrodynamics on protein separation by entropic interaction chromatography

Entropic interaction chromatography (EIC) provides efficient size-based separation of protein mixtures through the entropy change associated with solute partitioning into a layer of hydrophilic homopolymer that has been end-grafted within the pores of a macroporous chromatography support. In this work, surface-initiated atom-transfer radical polymerization (ATRP) is used to prepare a library of EIC stationary phases covering a wide range of grafted-chain densities and molecular weights. Exhaustive chain cleavage and analysis by saponification and GPC-MALLS, respectively, show that the new ATRP synthesis procedure allows for excellent control over graft molecular weight and polydispersity. The method is used to prepare high-density grafts (up to 0.164 +/- 0.005 chains/nm(2)) that extend the range of EIC applications to include efficient buffer-exchange and desalting of protein preparations. Reducing the graft density allows for greater partitioning of high molecular weight solutes, extending the linear range of the selectivity curve. Increasing graft molecular weight also alters selectivity, but more directly affects column capacity by increasing the volume of the grafted layer. Protein partitioning in high-density EIC columns is found to decrease with mobile-phase velocity (u). Although solute mass transfer resistances leading to an increase in plate height can explain this effect, pressure drop data across the column are indicative of weak convective flow through at least a fraction of the grafted architecture. Modeling of the grafted brush properties in the presence of solvent flow by subjecting a self-consistent-field theory representation of the brush to a viscous shear force predicts that the grafted chains will tilt and elongate in the direction of flow. The shear force may therefore act to reduce the number of conformations available to chains, increasing their rigidity without significantly altering the thickness of the grafted layer. A reduction in protein partitioning is then predicted when the dependence on u of the solute entropy loss is stronger than that of the grafted polymer, a condition met at high graft densities.     

Amphiphilic amylose-g-poly(meth)acrylate copolymers through "click" onto grafting method

Periodate oxidation and subsequent reductive amination with propargylamine was adopted for the controlled functionalization of amylose with alkyne groups, whereas ATRP polymerization was exploited to obtain end-(α)- or end-(ω)-azide functionalized poly(meth)acrylates to be used as "click" reagents in Cu(I) catalyzed azide-alkyne [3 + 2] dipolar cycloaddition. Amylose was effectively grafted with poly(n-butyl acrylate), poly(n-butyl methacrylate), poly(n-hexyl methacrylate), and poly(dimethylaminoethyl methacrylate) with this strategy. Their structure and composition were confirmed by FT-IR, NMR spectroscopies, and thermogravimetric analysis (TGA). Dynamic and static light scattering analyses, as well as TEM microscopy showed that the most amphiphilic among these hybrid graft copolymers self-assembled in water, yielding nanoparticles with ca. 30 nm diameter.     

Cellulose microfibrils grafted with PBA via surface-initiated atom transfer radical polymerization for biocomposite reinforcement

Immobilizing poly(butyl acrylate) (PBA) on cellulose microfibrils (CMFs) by atom transfer radical polymerization (ATRP) of butyl acrylate (BA) on the surface of 2-bromoisobutyryl-functionalized CMF generated highly hydrophobic microfibrils (CMF-PBA) with a hard core and a soft-shell structure. TGA and static water contact angle results suggested that the surfaces of the modified CMF samples were not completely covered by PBA chains until the molecular weight of grafts became sufficiently long. The GPC results indicated that the grafts with low molecular weight showed controlled/"living" characteristics of the surface-initiated ATRP; however, there existed more side reactions with the increase in molecular weights. Biocomposites consisting of polypropylene (PP) and CMF-PBA samples exhibited significantly improved compatibility, interface adhesion, and mechanical properties with the increase in PBA graft length. The findings confirmed that the longer grafts facilitated the better entanglement of PBA grafts with PP macromolecules and thus further improved the mechanical properties.     

Preparation of cellulose graft poly(methyl methacrylate) copolymers by atom transfer radical polymerization in an ionic liquid

Cellulose graft poly(methyl methacrylate) copolymers were prepared by atom transfer radical polymerization (ATRP) in an ionic liquid. Cellulose chloroacetate, as a macroinitiator, was first synthesized by direct acylation of cellulose with chloroacetyl chloride without any catalysts under mild conditions in an ionic liquid, 1-allyl-3-methylimidazolium chloride (BMIMCl). Then, the macroinitiator was used for the ATRP of MMA mediated by the CuBr and 2,2′-bipyridine (bpy) catalysis system. The copolymerization was carried out in BMIMCl without homopolymer byproduct. The polymers were easily separated from the catalyst when the ionic liquid was used as reaction medium. The grafting copolymers were characterized by means of ¹H NMR, AFM and GPC. The results showed that the obtained copolymers had grafted polymer chains with well-controlled molecular weight and polydispersity, and the polymerization was a “living/controlled” system. Further, through AFM observation, it was found that the cellulose graft copolymer in solution could aggregate and self-assembly into sphere-like polymeric structure.     

Synthesis of multiarm star poly(glycerol)-block-poly(2-hydroxyethyl methacrylate)

Well-defined multiarm star block copolymers poly(glycerol)-b-poly(2-hydroxyethyl methacrylate) (PG-b-PHEMA) with an average of 56, 66, and 90 PHEMA arms, respectively, have been prepared by atom transfer radical polymerization (ATRP) of HEMA in methanol by a core-first strategy. The hyperbranched macroinitiators employed were prepared on the basis of well-defined hyperbranched polyglycerol by esterification with 2-bromoisobutyryl bromide. Polydispersites M(w)/M(n) of the new multiarm stars were in the range of 1.11-1.82. Unexpectedly, with the combination of CuCl/CuBr(2)/2,2'-bipyridyl as catalyst, the polymerization conversion can be driven to maximum values of 79%. The control of CuCl catalyst concentration is also very important to achieve high conversion and narrow polydispersity. The absolute M(n) values of the obtained multiarm star polymers were in good agreement with the calculated ones, and the highest M(n) values of the multiarm star copolymer is around 10(6) g/mol. Kinetic analysis shows that an induction period exists in the polymerization of HEMA. After this induction period, a linear dependence of ln ([M](0)/[M](t)()) on time was observed. Due to the star architecture, the viscosity of the obtained multiarm star PHEMA is much lower than that of linear PHEMA.     

Versatile grafting of polysaccharides in homogeneous mild conditions by using atom transfer radical polymerization

A versatile atom transfer radical polymerization (ATRP) method for polysaccharide grafting in homogeneous mild conditions without using protecting group chemistry is presented. Water/DMF mixtures with different compositions were used as the solvent. The "grafting-from" approach was used in order to prepare suitable pullulan and dextran ATRP macroinitiators with a well controlled degree of functionalization. Methacrylate and acrylamide monomers were grafted obtaining good control over the number, molecular weight and polydispersity of the grafted chains without homopolymer formation and polysaccharide degradation. The versatility of this method allowed us to prepare comblike derivatives with a wide range of properties (amphiphilic, ionic, and thermoresponsive) by simply changing the solvent composition and the catalyst. This could make possible the synthesis of new interesting biomaterials starting from a wide range of polysaccharides.     

Conformational change of the hexagonally packed intermediate layer of Deinococcus radiodurans monitored by atomic force microscopy.

Both surfaces of the hexagonally packed intermediate (HPI) layer of Deinococcus radiodurans were imaged in buffer solution by atomic force microscopy. When adsorbed to freshly cleaved mica, the hydrophilic outer surface of the HPI layer was attached to the substrate and the hydrophobic inner surface was exposed to the stylus. The height of a single HPI layer was 7.0 nm, while overlapping edges of adjacent single layers adsorbed to mica had a height of 14.7 nm. However, double-layered stacks with inner surfaces facing each other exhibited a height of 17.4 nm. These stacks exposed the outer surface to the stylus. The different heights of overlapping layers and stacks are attributed to differences in the interaction between inner and outer surfaces. At high resolution, the inner surface revealed a protruding core with a central pore connected by six emanating arms. The pores exhibited two conformations, one with and the other without a central plug. Individual pores were observed to switch from one state to the other.     

PCL film surfaces conjugated with P(DMAEMA)/gelatin complexes for improving cell immobilization and gene transfection

Successful gene transfection on a tissue scaffold is of crucial importance in facilitating tissue repair and regeneration by enabling the localized production of therapeutic drugs. Polycaprolactone (PCL) has been widely adopted as a scaffold biomaterial, but its unfavorable cell-adhesion property needs to be improved. In this work, the PCL film surface was conjugated with poly((2-dimethyl amino)ethyl methacrylate) (P(DMAEMA))/gelatin complexes via surface-initiated atom transfer radical polymerization (ATRP) for improving cell immobilization and subsequent gene transfection. A simple aminolysis-based method was first used for the covalent immobilization of ATRP initiators on the PCL film. Well-defined P(DMAEMA) brushes were subsequently prepared via surface-initiated ATRP from the initiator-functionalized PCL surfaces. The P(DMAEMA) chains with a pK(a) of 7.0-7.3 were used for conjugating gelatin with a pI of 4.7 via electrostatic interaction. The amount of complexed gelatin increased as that of the grafted P(DMAEMA) layer. The cell-adhesion property on the functionalized PCL surface could be controlled by adjusting the ratio of P(DMAEMA)/gelatin. It was found that the gene transfection property on the immobilized cells was dependent on the density of the immobilized cells on the functionalized PCL film. With the good cell-adhesive nature of gelatin and the efficient gene transfection on the dense immobilized cells, the incorporating the suitable of P(DMAEMA)/gelatin complexes onto PCL surfaces could endow the PCL substrates new and interesting properties for potential tissue engineering applications.     

Modification of jute fibers with polystyrene via atom transfer radical polymerization

Atom transfer radical polymerization (ATRP) was investigated as a method of covalently bonding polystyrene to jute (Corchorus capsularis) and as a possible approach to fiber composites with enhanced properties. Jute fibers were modified with a brominated initiator and subsequently ATRP modified to attach polystyrene and then examined using SEM, DSC, TGA, FTIR, XPS, elemental analysis, and Py-GC-MS. These techniques confirmed that polystyrene had been covalently bound to the fibers and consequently ATRP-modified jute fiber mats were used to prepare hot-pressed polystyrene composites. Composite specimens were tensile tested and fracture surfaces examined using SEM. Although SEM examination suggested different fracture modes between unmodified fiber and ATRP-modified samples, the tensile strength of modified samples was slightly lower on average than that of unmodified samples. For fiber composite applications, we conclude that further optimization of the ATRP method is required, possibly targeting higher and more uniform loading of polystyrene on the fibers.     

Chemical character and origin of organic acids in streams and seepage lakes of central Maine

Organic acids and inorganic chemistry were examined in seventeen seepage lakes, seven streams, and one seep in central Maine. The objectives of this analysis were to determine the quantity and quality of dissolved organic carbon (DOC), and to assess the relationship between organic and inorganic surface water chemistry. Lakes and streams sampled were dilute (average conductivity of 20.3 μS cm^?1) with a wide range of DOC (125–2593 μmol C L^?1). Organic acids in DOC were evaluated by:

DOC fractionation (hydrophobic acids and neutrals, and hydrophilic acids, bases, and neutrals);

DOC isolation followed by FT-IR, base titration, and chemical analyses;

adsorption on solid phase extraction columns; and charge balance studies.

All lakes and streams were dominated by hydrophobic and hydrophilic acids (60 to 92% of DOC). Lakes and streams with low DOC had low hydrophobic to hydrophilic acid ratios (ca 1.2–1.3), regardless of pH and acid neutralizing capacities (ANC), compared to lakes and streams with moderate to high DOC concentrations (ca 1.9–2.4). Based on FT-IR spectroscopy and chemical analysis, organic acids were found to be dominated by a strong carboxylic character. Titration data of isolated DOC allowed accurate prediction of organic anions, which were strongly pH dependent (organic anions ranged from 14 to 198 μeq L^?1). Exchange acidity averaged 11.3, 13.6, and 8.7 μeq mg C^?1 for lake hydrophobic acids, lake hydrophilic acids, and stream hydrophobic acids, respectively. Overall evidence suggested that DOC and organic acid characteristics were related to their carboxylic functional group content and that the nature of these constituents was similar despite the source of origin (upland soils, wetlands, or Sphagnum deposits). Also, contact of soil leachates with B horizons seemed to be a controlling factor in DOC quantity and quality in the lakes and streams studied.     

Influence of Substratum Characteristics on the Attachment of a Marine Pseudomonad to Solid Surfaces

The attachment of a marine Pseudomonas sp. to a variety of surfaces was investigated, and the number of bacteria which became attached was related to the surface charge and degree of hydrophobicity of the substratum. Large numbers of bacteria attached to hydrophobic plastics with little or no surface charge [Teflon, polyethylene, polystyrene, poly(ethylene terephthalate)]; moderate numbers attached to hydrophilic metals with a positive (platinum) or neutral (germanium) surface charge; and very few attached to hydrophilic, negatively charged substrata (glass, mica, oxidized plastics). The results suggest that both electrostatic and hydrophobic interactions are involved in bacterial attachment.     

Mild and modular surface modification of cellulose via hetero Diels-Alder (HDA) cycloaddition

A combination of reversible addition-fragmentation chain transfer (RAFT) polymerization and hetero Diels-Alder (HDA) cycloaddition was used to effect, under mild (T ≈ 20 °C), fast, and modular conditions, the grafting of poly(isobornyl acrylate) (M(n) = 9800 g mol(-1), PDI = 1.19) onto a solid cellulose substrate. The active hydroxyl groups expressed on the cellulose fibers were converted to tosylate leaving groups, which were subsequently substituted by a highly reactive cyclopentadienyl functionality (Cp). By employing the reactive Cp-functionality as a diene, thiocarbonyl thio-capped poly(isobornyl acrylate) synthesized via RAFT polymerization (mediated by benzyl pyridine-2-yldithioformiate (BPDF)) was attached to the surface under ambient conditions by an HDA cycloaddition (reaction time: 15 h). The surface-modified cellulose samples were analyzed in-depth by X-ray photoelectron spectroscopy, scanning electron microscopy, elemental analysis, Fourier transform infrared (FT-IR) spectroscopy as well as Fourier transform infrared microscopy employing a focal plane array detector for imaging purposes. The analytical results provide strong evidence that the reaction of suitable dienophiles with Cp-functional cellulose proceeds under mild reaction conditions (T ≈ 20 °C) in an efficient fashion. In particular, the visualization of individual modified cellulose fibers via high-resolution FT-IR microscopy corroborates the homogeneous distribution of the polymer film on the cellulose fibers.     

Selective separation of human serum albumin with copper(II) chelated poly(hydroxyethyl methacrylate) based nanoparticles

Poly(hydroxyethyl methacrylate) (PHEMA) nanoparticles with an average size of 300 nm in diameter and with a polydispersity index of 1.156 were produced by surfactant free emulsion polymerization. Specific surface area of the PHEMA nanoparticles was found to be 996 m²/g. Metal-chelating ligand 3-(2-imidazoline-1-yl)propyl(triethoxysilane) (IMEO) was covalently attached to the PHEMA nanoparticles. IMEO content was 0.97 mmol IEMO/g. The morphology and properties of these nanoparticles were characterized with scanning electron microscopy, Fourier transform infrared spectroscopy and atomic force microscopy. The Cu²⁺-chelated PHEMA–IMEO nanoparticles were used in the adsorption-elution studies of human serum albumin (HSA) in a batch system. Maximum HSA adsorption amount of the Cu²⁺ chelated nanoparticles was 680 mg HSA/g. The PHEMA–IMEO–Cu²⁺ nanoparticles exhibited a quite high adsorption capacity and fast adsorption rate due to their high specific surface area and the absence of internal diffusion resistance.     

Antibody purification with protein A attached supermacroporous poly(hydroxyethyl methacrylate) cryogel

Immunoglobulin G (IgG) purification from human plasma with protein A attached supermacroporous poly(hydroxyethyl methacrylate) [PHEMA] cryogel has been studied. PHEMA cryogel was prepared by bulk polymerization which proceeds in aqueous solution of monomer frozen inside a plastic syringe (cryo-polymerization). After thawing, the PHEMA cryogel contains a continuous matrix having interconnected pores of 10–200 μm size. Protein was covalently attached onto the PHEMA cryogel via cyanogen bromide (CNBr) activation. The maximum IgG adsorption on the PHEMA/protein A cryogel was found to be 83.2 mg/g at pH 7.4 from aqueous solutions. The non-specific IgG adsorption onto the PHEMA cryogel was about 0.38 mg/g. The macropore size of the cryogel makes it possible to process blood cells without blocking the column. Higher adsorption capacity was observed from human plasma (up to 88.1 mg/g). Adsorbed IgG was eluted using 0.1 M glycine–HCl buffer (pH 3.5) with a purity of 85%. PHEMA–protein A cryogel was used for repetitive adsorption/desorption of IgG without noticeable loss in IgG adsorption capacity after 10 cycles. PHEMA–protein A cryogel showed several advantages such as simpler preparation procedure, good selectivity for IgG purification from human plasma and good stability throughout repeated adsorption–desorption cycles.