Intercalated clays from pentaerythritol stearate for use in polymer nanocomposites

Smectite clays treated with quaternary ammonium salts have been utilized for decades in paints, greases, cosmetics, and personal care products as rheological modifiers. They have also been used in industrial wastewater treatment extensively. In more recent times these surface modified clays have demonstrated benefits in polymer/clay nanocomposites. The use of quaternary ammonium modifiers limits the usefulness of these composites in food packaging because they are not approved for direct food contact. It would be advantageous to have surface modifying chemicals acceptable for direct food contact in these composites. This article reports research conducted on a promising surface modifier pentaerythritol stearate (PS), which is approved by the FDA for inclusion in food as a preservative. The surface modification of montmorillonite with PS is reported in detail as well as the production of nanocomposites with selected polymers made with the modified clay. Molecular modeling and purification of commercial PS samples indicate that the mono‐ and diesters are the critical surface modifiers, although the as received commercial material works well in forming intercalated clay complexes. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Syndiotactic Polystyrene/Organoclay Nanocomposites: Synthesis via In Situ Coordination‐Insertion Polymerization and Preliminary Characterization

Summary: Syndiotactic polystyrene (sPS)/organophilic clay nanocomposites were obtained by in situ coordination‐insertion polymerization of styrene. Two cationic surfactants (alkylammonium and alkylphosphonium) were used for the intercalation of montmorillonite (MMT). For each organically modified clay, three protocols were performed using an MAO‐activated hemi‐metallocene catalyst, in order to compare the influence of experimental conditions on the composite microstructure and on its thermal stability. The microstructures of nanocomposites were investigated by wide angle X‐ray scattering and DSC. Partially exfoliated or intercalated materials were obtained in all cases and a decrease of crystallinity is observed. Thermal properties were also studied by DSC and thermogravimetric analysis. The presence of clay does not have a strong influence on the sPS thermal transitions but the thermal decomposition process of the material was slowed down in the presence of few organoclay percents, particularly in the degradation beginning. The influence of these two organically modified clays on the thermal stability of the material is discussed.

Gel and suspension formed from the combination of cloisite with toluene (left) and styrene (right), respectively.     

Formation of a Surficial Bifunctional Nanolayer on Nb2O5 for Ultrastable Electrodes for Lithium‐Ion Battery

Safe and long cycle life electrode materials for lithium‐ion batteries are significantly important to meet the increasing demands of rechargeable batteries. Niobium pentoxide (Nb₂O₅) is one of the highly promising candidates for stable electrodes due to its safety and minimal volume expansion. Nevertheless, pulverization and low conductivity of Nb₂O₅ have remained as inherent challenges for its practical use as viable electrodes. A highly facile method is proposed to improve the overall cycle retention of Nb₂O₅ microparticles by ammonia (NH₃) gas‐driven nitridation. After nitridation, an ultrathin surficial layer (2 nm) is formed on the Nb₂O₅, acting as a bifunctional nanolayer that allows facile lithium (Li)‐ion transport (10–100 times higher Li diffusivity compared with pristine Nb₂O₅ microparticles) and further prevents the pulverization of Nb₂O₅. With the subsequent decoration of silver (Ag) nanoparticles (NPs), the low electric conductivity of nitridated Nb₂O₅ is also significantly improved. Cycle retention is greatly improved for nitridated Nb₂O₅ (96.7%) compared with Nb₂O₅ (64.7%) for 500 cycles. Ag‐decorated, nitridated Nb₂O₅ microparticles and nitridated Nb₂O₅ microparticles exhibit ultrastable cycling for 3000 cycles at high current density (3000 mA g^?1), which highlights the importance of the surficial nanolayer in improving overall electrochemical performances, in addition to conductive NPs.     

Direct fluorination of the polyimide matrimid® 5218: The formation kinetics and physicochemical properties of the fluorinated layers

Fluorinated polymers have a set of unique properties, including improved chemical stability and thermal stability and good barrier and membrane parameters, which are mainly defined by their surface properties. This article presents systematic data on the direct fluorination of the polyimide Matrimid® 5218, a commercially available polymer suitable for the formation of gas‐separation hollow fibers. Changing the fluorination conditions (i.e., the fluorinated mixture composition, fluorine partial pressure, and treatment duration) allows the rate of formation of the surface‐fluorinated layer over the 0.1–10 μm range to be kept under control. The physicochemical properties of modified layers (i.e., the chemical composition, formation of radicals, refractive index, IR and UV spectra, density, and surface energy) are examined. The thickness of the fluorinated layer (δ_F) depends on the fluorination duration (t): δ_F ～ t^0.5. During fluorination, hydrogen atoms are replaced with fluorine, double bonds are saturated with fluorine, and at least one CN bond in the five‐member ring is disrupted. Fluorination results in a significant increase in the polymer density, transparency in the visible and ultraviolet regions of spectra, and a reduction of the refractive index. A high concentration of long‐living radicals (up to ～5 × 10¹⁹ radicals/cm³ of the fluorinated layer) is generated under fluorination. This can be used for subsequent grafting (e.g., with acrylonitrile). © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 6–17, 2004     

Development of antimicrobial stainless steel via surface modification with N‐halamines: Characterization of surface chemistry and N‐halamine chlorination

N‐halamine modification of materials enables the development of antimicrobial materials whose activity can be regenerated after exposure to halogenated sanitizers. Surface and bulk modification of polymers by N‐halamines has shown great success, however, modification of inorganic substrates (e.g., stainless steel) remains an area of research need. Herein, we report the covalent surface modification of stainless steel to possess rechargeably antimicrobial N‐halamine moieties. Multilayers of branched polyethyleneimine and poly(acrylic acid) were immobilized onto the surface of stainless steel and the number of N‐halamines available to complex chlorine was quantified. Samples were characterized through contact angle, Fourier transform infrared spectroscopy, ellipsometry, dye assay for amine quantification, and X‐ray photoelectron spectroscopy. Increasing the number of multilayers from one to six increased the number of N‐halamines available to complex chlorine from 0.30 ± 0.5 to 36.81 ± 5.0 nmol cm^?2. XPS and FTIR confirmed successful covalent layer‐by‐layer deposition of the N‐halamine multilayers. The reported layer‐by‐layer deposition technique resulted in a greater than seven‐fold increase of available N‐halamine compared to prior reports of N‐halamine surface modifications. The N‐halamine modified steel demonstrated antimicrobial activity (99.7% reduction) against the pathogen Listeria monocytogenes. Such surface modified stainless steel with increased N‐halamine functionality, and therefore potential for rechargeable antimicrobial activity, supports efforts to reduce cross‐contamination by pathogenic organisms in the food and biomedical industries. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Effects of physical confinement (< 125 nm) on the curing behavior of phenolic resin

How the physical confinement of phenolic resin in nano porous silica (8 nm ≤ pore diameter (D_p) ≤ 125 nm) affected the polymer's curing behavior was examined by conducting differential scanning calorimetry experiments at 320 K ≤ T ≤500 K. Our results suggested that upon incorporating the phenolic resin into the silica, its curing temperature was lowered. However, what was interesting was that there was an inverse linear dependence between the pore size and the curing temperature, i.e., the smaller the pore diameter the higher the curing temperature. There was evidence that phenolic resin was unable to penetrate into 8 nm‐sized pores. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci, 2006     

Self‐assembled honeycomb polyurethane nanofibers

Electrospinning uses a high voltage electric field to produce fine fibers. A new phenomenon of self‐assembly in the electrospinning of polyurethane nanofibers is observed. This report is the first known self‐assembling phenomenon in polyurethane electrospun nanofibers. Electrospun polyurethane nanofibers self‐assemble into unique honeycomb patterns on the collector surface. This novel observation opens up new and interesting opportunities for electrospun fibers in the areas of drug delivery devices, protective clothing, filters, and tissue scaffolds. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 3121–3124, 2006     

Preparation of a poly(vinyl chloride)/layered double hydroxide nanocomposite with a reduced heavy‐metal thermal stabilizer

A novel poly(vinyl chloride) nanocomposite with layered double hydroxide was successfully prepared via a melt‐compounding process in which the positively charged platelets were dramatically exfoliated and homogeneously dispersed. Because of the absorption of the released hydrochloric vapor by both layered double hydroxide platelets and an organomodifier (stearate anions), the added amount of the conventional toxic heavy‐metal heat stabilizer could be reduced. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Magnetization reversal in YIG/GGG(111) nanoheterostructures grown by laser molecular beam epitaxy

Abstract

Thin (4–20 nm) yttrium iron garnet (Y₃Fe₅O₁₂, YIG) layers have been grown on gadolinium gallium garnet (Gd₃Ga₅O₁₂, GGG) 111-oriented substrates by laser molecular beam epitaxy in 700–1000 °C growth temperature range. The layers were found to have atomically flat step-and-terrace surface morphology with step height of 1.8 Å characteristic for YIG(111) surface. As the growth temperature is increased from 700 to 1000 °C the terraces become wider and the growth gradually changes from layer by layer to step-flow regime. Crystal structure studied by electron and X-ray diffraction showed that YIG lattice is co-oriented and laterally pseudomorphic to GGG with small rhombohedral distortion present perpendicular to the surface. Measurements of magnetic moment, magneto-optical polar and longitudinal Kerr effect (MOKE), and X-ray magnetic circular dichroism (XMCD) were used for study of magnetization reversal for different orientations of magnetic field. These methods and ferromagnetic resonance studies have shown that in zero magnetic field magnetization lies in the film plane due to both shape and induced anisotropies. Vectorial MOKE studies have revealed the presence of an in-plane easy magnetization axis. In-plane magnetization reversal was shown to occur through combination of reversible rotation and abrupt irreversible magnetization jump, the latter caused by domain wall nucleation and propagation. The field at which the flip takes place depends on the angle between the applied magnetic field and the easy magnetization axis and can be described by the modified Stoner–Wohlfarth model taking into account magnetic field dependence of the domain wall energy. Magnetization curves of individual tetrahedral and octahedral magnetic Fe³⁺ sublattices were studied by XMCD.     

Synthesis and characterization of superhydrophobic wood surfaces

Superhydrophobic films were developed on wood substrates with a wet chemical approach. Growth of zinc oxide (ZnO) nanorods was found differentially in the cross‐sectional walls and inner lumenal surfaces. The surface roughness of the prepared films on the inner lumenal surface conformed to the Cassie–Baxter wetting model, whereas the roughness across the microsurface of the cell wall was in conformity with the hydrophobic porous wetting model. The space between the ZnO nanorods and the microstructure of the wood surface constituted the nanoscale and microscale roughness of the ZnO nanofilm, respectively. The water contact angle of the prepared wood surfaces was up to 153.5°. In the prepared films, monolayers of stearic acid molecules were self‐assembled on the ZnO nanorods, which in turn, were attached to the wood surface via dimeric bonds. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011