Persistence length control of the polyelectrolyte layer-by-layer self-assembly on carbon nanotubes

We have studied layer-by-layer polyelectrolyte self-assembly on pristine individual single-wall carbon nanotubes as a function of solution ionic strength. We report the existence of an ionic strength threshold for the deposition, below which the majority of nanotubes remain uncoated. Once the ionic strength reaches the threshold value, the majority of the individual nanotubes become coated with polyelectrolytes. Our results indicate that the self-assembly process likely involves wrapping of polymer chains around nanotubes and that the polymer chain's ability to bend in order to accommodate the nanotube curvature is one of the critical parameters controlling layer-by-layer electrostatic self-assembly on these one-dimensional templates.     

Effect of dipping solution pH values on electrostatic layer-by-layer self-assembly of side-chain azo polyelectrolyte

The effect of pH value on the electrostatic layer-by-layer self-assembly and the photo-responsive behavior of Poly{2-[4-(4-ethoxyphenylazo)phenoxy]ethyl acrylate-co-acrylic acid} (PEAPE) was studied. Results show that in the studied pH value range, the lower the pH value is, the higher is the UV-vis absorbance and the larger is the thickness of the multilayer films. FTIR studies indicate that the azo polyelectrolyte exhibits a different ionization degree in solutions with different pH values. The higher absorbance and the larger thickness of the layer-by-layer films can be attributed to the low ionization degree and the shrinkage conformation of PEAPE in the solution with low pH values. FTIR analysis also confirms that the driving force for layer-by-layer self-assembly of PEAPE and PDAC is the electrostatic interaction. __________ Translated from Acta Polymerica Sinica, 2007, 5: 440–445 [译自: 高 分子学报]     

Interaction between microfibrillar cellulose fines and fibers: influence on pulp qualities and paper sheet properties

Due to the high potential of cellulose nanoparticles in composite materials and for both fundamental and technological considerations, we investigated the interaction between microfibrillar cellulose and fibers. The contribution to the paper properties of fines added to a pulp suspension was determined. The impact of various proportions of fines added to a softwood kraft pulp on the paper strength and how they affected porosity and density was evaluated. The respective effects of dried fines (dead fines), originating from paper or board degradation, and the newly formed secondary fines (fresh fines) generated during refining were examined. The nature of the bonding between the fines and the fibers versus physical retention was characterized in the pulp suspension. For the first time the respective parts in the interaction of hydrogen bonds and mechanical associations were demonstrated and quantified. The amount of H-bonded fresh fines exceeded that of dead fines by more than 30 %. The results revealed that, for both types, the amount of H-bonded fines reached a threshold, independently of the proportion of fines added to the fibers. Addition of fines significantly affected the porosity of papers, fresh fines decreasing porosity more readily than dead fines. All the results are convergent to indicate that fresh fines penetrate more evenly and more deeply into the fiber network and induce better bonding that produces a closure of the fiber mat structure. They also demonstrate that incorporating an optimal proportion of fresh cellulose fines in fiber networks can bring significant improvement to the final composite material.     

Surface characteristics and wetting properties of sol–gel coated base paper

Two hybrid coatings synthesized by using alkoxysilanes as precursors in a sol–gel process, differing from each other in terms of the organic components in alkoxysilanes, have been developed to improve the water repellent properties of base paper. The sol–gel‐coated base paper samples were characterized by scanning electron microscopy, atomic force microscopy, confocal laser scanning microscopy, X‐ray photoelectron spectroscopy, time‐of‐flight secondary ion mass spectrometry, and contact angle measurements. The sol–gel coatings were found to clearly change the surface properties of base paper. Thin coating layers were formed on base paper surfaces. The topographical data indicated the formation of discontinuous thin films; the time‐of‐flight secondary ion mass spectrometry analyses confirmed that the coatings were covering the fibres but only partially covered the fibre–fibre intersections. Water and the subsequent heat treatment used as a reference treatment reduced the surface roughness and porosity and slightly changed the surface chemistry of the base paper. The wettability and absorptivity of base paper was clearly reduced by the applied coatings. Copyright © 2011 John Wiley & Sons, Ltd.     

Surface modification of poly(tetramethylene adipate-co-terephthalate) membrane via layer-by-layer assembly of chitosan and dextran sulfate polyelectrolyte multiplayer

The improvement of hydrophilicity and hemocompatibility of poly(tetramethylene adipate-co-terephthalate) (PTAT) membrane was developed via polyelectrolyte multilayers (PEMs) immobilization. The polysaccharide PEMs included chitosan (CS, as a positive-charged and antibacterial agent) and dextran sulfate (DS, as a negative-charged and anti-adhesive agent) were successfully prepared using the aminolyzed PTAT membrane in a layer-by-layer (LBL) self-assembly manner. The obtained results showed that the contact angle of as-modified PTAT membranes reached to the steady value after four bilayers of coating, hence suggesting that the full coverage was achieved. It could be found that the PTAT–PEMs membranes with DS as the outmost layer could resist the platelet adhesion and human plasma fibrinogen (HPF) adsorption, thereby prolonging effectively the blood coagulation times. According to L929 fibroblast cell growth inhibition index, the as-prepared PTAT membranes exhibited non-cytotoxic. Overall results demonstrated that such an easy, valid and shape-independent processing should be potential for surface modification of PTAT membrane in the application of hemodialysis devices.     

基于层层组装钴铝水滑石/聚电解质包裹碳纳米管的无酶葡萄糖传感器

采用共沉淀法合成了钴铝水滑石(CoAl-LDH),将CoAl-LDH与PSS包裹的CNTs(CNTs@PSS)通过层层自组装法构筑CNTs@PSS/CoAl-LDH多层膜电极,并将其应用于葡萄糖的分析测定。X射线衍射光谱、红外光谱和SEM表明:共沉淀法合成的CoAl-LDH具有典型的水滑石特征峰及形貌。电化学阻抗谱表明:CoAl-LDH可与CNTs@PSS均匀有效地组装构筑多层膜。电化学研究表明:CNTs的引入很好地提高了CoAl-LDH修饰电极的灵敏度。研究结果表明该传感器对葡萄糖在3.0×10-6～4.98×10-4mol/L范围内呈良好的线性响应,灵敏度为1.03×10-3A.L/mol。     

Understanding the effects of lignosulfonate on enzymatic saccharification of pure cellulose

The effects of lignosulfonate (LS) on enzymatic saccharification of pure cellulose were studied. Four fractions of LS with different molecular weight (MW) prepared by ultrafiltration of a commercial LS were applied at different loadings to enzymatic hydrolysis of Whatman paper under different pH. Using LS fractions with low MW and high degree of sulfonation can enhance enzymatic cellulose saccharification despite LS can bind to cellulase nonproductively. The enhancing effect varies with LS properties, its loading, and hydrolysis pH. Inhibitive effect on cellulose saccharification was also observed using LS with large MW and low degree of sulfonation. The concept of “LS-cellulase aggregate stabilized and enhanced cellulase binding” was proposed to explain the observed enhancement of cellulose saccharification. The concept was demonstrated by the linear correlation between the measured amount of bound cellulase and saccharification efficiency with and without LS of different MW in a range of pH.     

Surface chemical modification of microfibrillated cellulose: improvement of barrier properties for packaging applications

Heterogeneous acetylation of microfibrillated cellulose (MFC) was carried out to modify its physical properties and at the same time to preserve the morphology of cellulose fibrils. The overall reaction success was assessed by FTIR together with the degree of substitution (DS) defined by titration and the degree of surface substitution (DSS) evaluated by means of XPS. Dynamic contact angle measurements confirmed the hydrophobicity improvement relative to non-modified samples. The increase of contact angle upon reaching a certain reaction time and some decrease following the further acetylation was confirmed. Mechanical properties of MFC films made from chemically modified material were evaluated using tensile strength tests which showed no significant reduction of tensile strength. According to SEM images, dimension analysis and tensile strength data, the acetylation seemed not to affect the morphology of cellulose fibrils.     

Preparation of Au/Ag multilayers via layer-by-layer self-assembly in spherical polyelectrolyte brushes and their catalytic activity

Spherical polyelectrolyte brushes(SPBs) consisting of polystyrene(PS) core and poly(2-aminoethyl methacrylate hydrochloride)(PAEMH) shell were prepared by photo-emulsion polymerization. Au nanoparticles(Au-NPs) with controlled size and size distribution were synthesized in situ using SPBs as nanoreactors. Via layer-by-layer deposition technique on the surface of SPBs, nano-composite particles with Au/Ag-NPs bilayer and Au/Ag/Au-NPs trilayer were prepared. The structures of the as-prepared Au/Ag multilayer SPBs were characterized by UV-Vis spectroscopy, TEM, ICP-AES and DLS. The charge reversal of the nano-composite particles observed by zeta potential confirmed the success of layer-by-layer assembly. The Au/Ag-NPs bilayer nano-composite particles showed high catalytic efficiency with an apparent activation energy of about 41.2 k J/mol in the reduction reaction of 4-nitrophenol to 4-aminophenol in the existence of sodium borohydride monitored. The catalytic activity of Au/Ag-NPs multilayer SPBs close to that of Au-NPs SPBs and much higher than that of Ag-NPs SPBs reveals its potential applications in cost-effective catalysts with high-performance.     

Surface modification of cellulose with plant triglycerides for hydrophobicity

Hydrophobic cotton was achieved by surface modification of the cellulose with triglycerides from several plant oils including soybean, rapeseed, olive and coconut oils. These oils were delivered to the cellulose substrates in homogeneous solutions of ethanol or acetone as well as aqueous emulsions. Surface modification was facilitated by solvent evaporation followed by heating between 110 and 120 °C for 60 min. All oils, except for coconut, produced hydrophobic and less water-absorbing cotton, supporting the desirable role of higher unsaturation in the fatty acids to achieve crosslinked network. The most hydrophobic surfaces were obtained by the reaction with 1% soybean oil in acetone. On both bleached and scoured cotton, a water contact angle of 80° and water absorption value of 0.82 μL/mg were achieved. The acquired hydrophobicity was not only retained after water washing but also improved with subsequent exposures to elevated temperatures. The surface tension of scoured cotton cellulose was lowered from 63.81 mJ/m² to 25.74 mJ/m² when modified by soybean oil delivered in acetone, which is lower than that of poly(ethylene terephthalate). An aqueous emulsion of soybean oil also rendered the scoured cotton hydrophobic, which shows promise for a green chemistry and bio-based approach to achieve water repellency on cellulosic materials.     

NRET from naphthalene labels in multilayer shell wall on melamine formaldehyde microparticles fabricated with layer-by-layer self-assembly to pyrene-labeled polyelectrolyte in solution

Core-shell colloidal particles were prepared with the core of monodisperse melamine formaldehyde particles (MF) with a diameter of 3.5 μm. The shell deposited on the core by the layer-by-layer (LbL) self-assembly was made with a copolymer ANp3 of 2-acrylamido-2-methylpropanesulfonate sodium (AMPS) and 3 mol% naphthalene label monomer and poly(diallyldimethylammonium chloride) (PD). Nonradiative energy transfer (NRET) from the naphthalene labels deposited on the MF particles to pyrene labels at a polyelectrolyte APy3, a copolymer of AMPS and 3 mol% pyrene label monomer, or to an ionic pyrene probe 1-pyrenemethylamine hydrochloride (PyMeA · HCl) in water was observed. The NRET efficiency was expressed as the emission intensity ratio I/I₀ of naphthalene with and without existence of pyrene in the surrounding solution. With increasing pyrene concentration, I/I₀ decreased down to about 0.2 and the mechanism for this NRET from the inner naphthalene label to the pyrene labels in solution is still ambiguous.     

Effect of electric wetting on the surface properties of cellulose and chitosan

Journal of Solid State Electrochemistry - Cellulose and chitosan have similar structure both with the β(1→4) linked functional groups, e.g., the former linked the D-glucose units and the...     

Surface modification of hafnia with polyelectrolytes based on the spin-coating electrostatic self-assembly method

In this paper, we describe organic surface modification and functionalization of a hafnia substrate, which has been extensively investigated as a replacement of the gate insulting SiO₂ layer in field effect transistors. The surface state of the hafnia was assessed by water contact angle (θ_water) measurement with comparison to that of the silicone during the layer-by-layer (LBL) deposition of poly(allyamine hydrochloride) (PAH)/poly(styrene sulfonate) (PSS) bilayers by means of the spin-coating electrostatic self-assembly, SCESA, method. The surface state of virgin hafnia (θ_water = 73 ± 1°) turned hydrophilic (θ_water = 8 ± 2°) after submission to the standard RCA cleaning process of silicon. The thickness of the multilayer films on the cleaned hafnia surface was found to grow linearly with an increase in the number of PAH/PSS bilayers (d = 2.2 ± 0.1 nm), indicating the consistency in the formation of uniform films. The average water contact angle of the PAH and PSS layers on hafnia alternately switched between 36.0 ± 0.7° and 29.7 ± 0.4° during the nine deposition cycles. The analysis of the surface topography by means of atomic force microscopy (AFM) indicated that the surface roughness of the first PAH layer deposited on the hafnia was strongly smoothed from 1.54 to 0.44 nm with increasing the LBL deposition of polyelectrolytes.     

Surface only modification of bacterial cellulose nanofibres with organic acids

Bacterial cellulose (BC) nanofibres were modified only on their surface using an esterification reaction with acetic acid, hexanoic acid or dodecanoic acid. This reaction rendered the extremely hydrophilic surfaces of BC nanofibres hydrophobic. The hydrophobicity of BC increased with increasing carbon chain length of the organic acids used for the esterification reaction. Streaming (zeta-) potential measurements showed a slight shift in the isoelectric point and a decrease in ζ_plateau was also observed after the esterification reactions. This was attributed to the loss of acidic functional groups and increase in hydrophobicity due to esterification of BC with organic acids. A method based on hydrogen/deuterium exchange was developed to evaluate the availability of surface hydroxyl groups of neat and modified BC. The thermal degradation temperature of modified BC sheets decreased with increasing carbon chain length of the organic acids used. This is thought to be a direct result of the esterification reaction, which significantly reduces the packing efficiency of the nanofibres because of a reduction in the number of effective hydrogen bonds between them.     

Surface methacrylation and graft copolymerization of ultrafine cellulose fibers

Ultrafine fibrous (? from 100 to 450 nm) cellulose membranes were generated by electrospinning of cellulose acetate [degree of substitution (DS): 2.45, weight‐average molecular weight: 30,000 Da], followed by alkaline deacetylation. Reaction of these ultrahigh surface‐area cellulose fibers with methacrylate chloride (MACl) produced activated surfaces without altering the fiber morphology. Surface methacrylation of these fibers was confirmed by the acquired hydrophobicity (θ_water = 84°) as compared to the originally hydrophilic (θ_water = 56°) cellulose. Changing the MACl:OH molar ratios could vary the overall DS of methacrylation. The very low overall DS values indicate the surface nature of the methacrylation reaction. At a DS of 0.17, the thermal properties of the surface methacrylated cellulose resemble those of cellulose derivatives at much higher DS values, an unusual behavior of the ultrafine fibers. The methacrylated cellulose could be further copolymerized with vinyl monomers (methyl methacrylate, acrylamide, and N‐isopropylacrylamide) as linear grafts or three‐dimensional (3D) networks. The morphology of cellulose fibers and the interfiber pore structure were not altered at 15–33% graft levels. This study demonstrates that either linear or 3D networks of vinyl polymers could be efficiently supported on ultrafine cellulose fibrous membranes via surface methacrylation. Through these surface reactions the chemical, thermal, and liquid wetting and absorbent properties of these ultrafine fibrous membranes were significantly altered with no change to the fiber dimensions or interfiber pore morphology. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 953–964, 2003     

Fabrication of ultrathin polyelectrolyte fibers and their controlled release properties

Ultrathin fibers comprising 2-weak polyelectrolytes, poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH) were fabricated using the electrospinning technique. Methylene blue (MB) was used as a model drug to evaluate the potential application of the fibers for drug delivery. The release of MB was controlled in a nonbuffered medium by changing the pH of the solution. The sustained release of MB in a phosphate buffered saline (PBS) solution was achieved by constructing perfluorosilane networks on the fiber surfaces as capping layers. Temperature controlled release of MB was obtained by depositing temperature sensitive PAA/poly(N-isopropylacrylamide) (PNIPAAM) multilayers onto the fiber surfaces. The controlled release of drugs from electrospun fibers have potential applications as drug carriers in biomedical science.     

Short branch effects on the creep properties of the ultra-high strength polyethylene fibers

Short branch (methyl branch) effects on the creep properties of the ultra-high strength polyethylene fibers were investigated. The temperature and the stress dependence of creep rates of several high-strength polyethylene fibers having different branch contents, which were prepared by blending of two polymers of highly and less branching, was evaluated according to a model described by Ward and Wilding, and the activation energies and the activation volumes were calculated in terms of their methyl branch contents and tensile moduli. The creep rates of ultra high strength fibers are strongly influenced by their methyl branch contents. The typical branching sample with ca.6 CH₃ units per 1000 CH₂ units shows ca. 1/20 lower creep rate than that of the less branching (1.0 CH₃ per 1000 CH₂) fiber sample at the room temperature. The activation energies of creep rate obtained by those highly branching samples are higher than those of lesser branching samples; the difference is nearly proportioal to their branch contents. Wide-angle x-ray diffraction results showed that the dimension of a-axis of unit cell increases in proportion to their branch contents. These results imply that the creep mechanism of ultra-high strength polyethylene fibers is dominated by chain slippage in the crystalline part, and also imply that some amount of methyl branch sites can be incorporated in the crystalline part in proportion to the branch content and those incorporated branch sites hinder the slippage motion of molecular chains in crystalline part, which results in the extreme lower creep rate. © 1994 John Wiley & Sons, Inc.     

The mechanism of chemical modification of artificial fibers based on cellulose derivatives

A fabric based on cellulose derivatives has been hydrophobized via coating with oligochloromethylethoxysiloxane and then treated with butyl alcohol, benzyl alcohol, and higher fatty alcohols. With the use of X-ray photoelectron spectroscopy, it has been shown that the reaction of alcohols with the siloxane coating proceeds through the exchange of ethoxysilane groups for higher alkoxy groups, whereas the expected reaction of chloromethyl groups with alcohols (the Williamson reaction) does not occur under the chosen conditions.     

Surface modification of natural cellulose substances:toward functional materials and applications

Combining various synthetic chemical processes and biological assemblies provides a promising strategy for the design and fabrication of functional materials with tailored structures and properties.The unique multilevel structures and morphologies of natural cellulose substances such as ordinary commercial laboratory filter paper make them ideal platforms for the self-assemblies of various functional guest molecules that are to be deposited on the surfaces of their fine structures,and the resulting composite matters show significant potentials for various applications.The surface sol-gel process was employed to deposit ultrathin metal-oxide(e.g.,titania and zirconia)gel films to coat the cellulose nanofibers in bulk filter papers;thereafter,monolayers of specific guest substrates were immobilized onto the surfaces of the metal-oxide gel films.Highly selective,sensitive,and reversible chemosensors based on the surface modification of filter paper were obtained toward the fluorescence and colorimetric detection of various analytes such as heavy-metal ions,inorganic anions,amino acids,and gases.Cellulosebased composite materials with superhydrophobic,antibacterial,or luminescent properties were fabricated by self-assembly approaches toward practical applications.