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PubMed1986

Bulk characterization and scanning electron microscopy of hydrogels of P(VA-co-NVP).

The bulk properties of uncrosslinked and crosslinked random copolymers of vinyl alcohol and N-vinyl-2-pyrrolidone [P(VA-co-NVP)] are reported. Analytical studies were performed by infrared and nuclear magnetic resonance spectroscopy, gel permeation chromatography, potentiometric titration, elemental analysis and scanning electron microscopy. The microscopic studies elucidated the effect of freeze-drying on the network surface topology. The results indicate macrosyneresis of the copolymer hydrogels upon freeze-drying even at -23 degrees C. Rapid collapse of the hydrogel structure is seen upon removal of the bulk and interfacial water in the frozen state.

Biocompatible Materials↗
PubMed2007

Recent progress in the determination of solid surface tensions from contact angles.

Advancing contact angles of different liquids measured on the same solid surface fall very close to a smooth curve when plotted as a function of liquid surface tension, i.e., gamma(lv)costheta versus gamma(lv). Changing the solid surface, and hence gamma(sv), shifts the curve in a regular manner. These patterns suggest that gamma(lv)costheta depends only on gamma(lv) and gamma(sv). Thus, an "equation of state for the interfacial tensions" was developed to facilitate the determination of solid surface tensions from contact angles in conjunction with Young's equation. However, a close examination of the smooth curves showed that contact angles typically show a scatter of 1-3 degrees around the curves. The existence of the deviations introduces an element of uncertainty in the determination of solid surface tensions. Establishing that (i) contact angles are exclusively a material property of the coating polymer and do not depend on experimental procedures and that (ii) contact angle measurements with a sophisticated methodology, axisymmetric drop shape analysis (ADSA), are highly reproducible guarantees that the deviations are not experimental errors and must have physical causes. The contact angles of a large number of liquids on the films of four different fluoropolymers were studied to identify the causes of the deviations. Specific molecular interactions at solid-vapor and/or solid-liquid interfaces account for the minor contact angle deviations. Such interactions take place in different ways. Adsorption of vapor of the test liquid onto the solid surface is apparently the only process that influences the solid-vapor interfacial tension (gamma(sv)). The molecular interactions taking place at the solid-liquid interface are more diverse and complicated. Parallel alignment of liquid molecules at the solid surface, reorganization of liquid molecules at the solid-liquid interface, change in the configuration of polymer chains due to contact with certain probe liquids, and intermolecular interactions between solid and liquid molecules cause the solid-liquid interfacial (gamma(sl)) tension to be different from that predicted by the equation of state, i.e., gamma(sl) is not a precise function of gamma(lv) and gamma(sv). In other words, the experimental contact angles deviate from the "ideal" contact angle pattern. Specific criteria are proposed to identify probe liquids which eliminate specific molecular interactions. Octamethylcyclotetrasiloxane (OMCTS) and decamethylcyclopentasiloxane (DMCPS) are shown to meet those criteria, and therefore are the most suitable liquids to characterize surface tensions of low energy fluoropolymer films with an accuracy of +/-0.2 mJ/m2.

Journal Article↗
PubMed1995

Drug transport across nylon 610 films: influence of synthesis variables.

Nylon 610 is a hydrophilic polymer with considerable potential as a membrane for drug microencapsulation. To better understand drug transport through such membrane, the influence of the solvents and monomers used in the synthesis of nylon films were examined using a full factorial study. Nylon 610 films were synthesized by an interfacial polycondensation reaction using hexamethylenediamine (HD) in the water phase and sebacoyl chloride (SC) in the organic phase, which was a solvent blend of chloroform and trichlorotrifluoroethane at ratios of 1:1, 1:4, and 4:1. Monomer concentrations studied were 0.2, 0.4, and 0.6 M with respect to their appropriate phase, while the monomer ratios were 1:1, 3:1, and 1:3. The molecular weight, porosity, thickness, and crystallinity of the films were characterized. The transport of potassium chloride, hydrocortisone, and m-cresol was studied at 25 degrees C as a function of the synthesis variables. Potassium chloride was selected to measure the porosity of the membrane. Hydrocortisone and m-cresol, a known solvent for nylon 610, were used to study pore and solution-diffusion transport, respectively. The molecular weight of the films was proportional to the chloroform concentration. As the molecular weight increased, film thickness, porosity, and hydrocortisone permeability increased. As the molecular weight decreased, film thickness and porosity decreased, while m-cresol permeability increased. These results can be explained on the basis of HD ability to readily partition into a good solvent such as chloroform permitting high molecular weight polymer to form before precipitation.

Cresols↗
PubMed2005

Chemical modification of poly(vinyl chloride) resin using poly(ethylene glycol) to improve blood compatibility.

Poly(vinyl chloride) (PVC) was aminated by treating the resin with a concentrated aqueous solution of ethylenediamine. The aminated PVC was then reacted with hexamethylene diisocyanate to incorporate the isocyanate group onto the polymer backbone. The isocyanated PVC was further reacted with poly(ethylene glycol) (PEG) of molecular weight 600 Da. The modified polymer was characterized using infrared and X-ray photoelectron spectroscopy (XPS) and thermal analysis. Infrared and XPS spectra showed the incorporation of PEG onto PVC. The thermal stability of the modified polymer was found to be lowered by the incorporation of PEG. Contact angle measurements on the surface of polymer films cast from a tetrahydrofuran solution of the polymer demonstrated that the modified polymer gave rise to a significantly hydrophilic surface compared to unmodified PVC. The solid/water interfacial free energy of the modified surface was 3.9 ergs/cm(2) as opposed to 18.4 ergs/cm(2) for bare PVC surface. Static platelet adhesion studies using platelet-rich plasma showed significantly reduced platelet adhesion on the surface of the modified polymer compared to control PVC. The surface hydrophilicity of the films was remarkably retained even in the presence of up to 30 wt% concentration of the plasticizer di-(2-ethylhexyl phthalate). The study showed that bulk modification of PVC with PEG using appropriate chemistry can give rise to a polymer that possesses the anti-fouling property of PEG and such bulk modifications are less cumbersome compared to surface modifications on the finished product to impart anti-fouling properties to the PVC surface.

Biocompatible Materials↗
PubMed2005

Liquid-vapor equilibrium and surface tension of nonconformal molecular fluids.

Molecular dynamics simulations at constant temperature have been performed on the liquid-vapor interface for fluids characterized by a recently introduced three-parameter potential. This potential is a modification of the well-known spherical Kihara interaction and is termed approximate nonconformal (ANC). It has been used successfully to describe many real molecules in the gaseous phase. Besides the usual molecular energy and size, the ANC potential introduces a third parameter s, called softness, to measure the form of the potential profile. Study of these systems shows that their critical and interfacial properties follow very closely those of four selected substances: argon, methane, propane, and hexane. Deviations of the properties predicted from the experimental values are analyzed and their probable causes are determined. The critical properties of ANC fluids and their dependence on s are also obtained via first-order perturbation theory in the form of an augmented van der Waals model. Analysis of the results shows that ANC potential functions can be used as reliable effective interactions for real dense fluids.

Journal Article↗
PubMed1993

Imaging of features on surfaces by condensation figures.

Condensation of a vapor to a liquid on a cold surface that is not wet completely by this liquid leads to the formation of an array of droplets. If the surface is heterogeneous in its physical properties (especially its interfacial free energy), the patterns of these arrays reflect this heterogeneity. The distribution of droplets of water (condensation figures or CFs) observed by optical microscopy on a surface can be correlated with the molecular structure of that surface. The substrates used to investigate the formation and morphology of the CFs were patterned, self-assembled monolayers of different alkanethiolates on gold and of alkyl siloxanes on glass. Analysis of CFs is a valuable nondestructive technique for characterizing heterogeneities in surfaces.

Chemical Phenomena↗
PubMedDate not supplied

Dynamic hairspray analysis. II. Effect of polymer, hair type, and solvent composition.

A texture analyzer was employed to study the properties of hairspray resins by measuring a number of mechanical parameters of hair modified by them. Hairset stiffness, stiffness retention at high humidity, and duration and magnitude of polymer film tackiness during drying were the properties employed to characterize fixative-treated hair. They were investigated for formulations containing ethyl ester of PVM/MA copolymer, butyl ester of PVM/MA copolymer, vinyl caprolactam/PVP/dimethylaminoethyl methacrylate copolymer, VA/butyl maleate/isobornyl acrylate copolymer, and polymer blends such as poly(methylvinylether) (PMVE) - ethyl ester of PVM/MA copolymer. It was demonstrated that the properties of hair treated with a fixative are affected by the concentration and molecular weight of a polymer, low-molecularweight additives that can modify the mechanical, surface, or interfacial properties of a polymer film, and the presence of water in the formulation. It was also shown, that the type of hair employed, such as virgin, untreated hair or hair damaged by bleaching, can also have an effect on the performance of a given hairspray formulation. For compositions based on blends of PMVE and ethyl ester of PVM/MA copolymer, the employed method demonstrated the sensitivity of the properties of polymer blends to the presence of a hydrophilic component such as PMVE.

Cosmetics↗
PubMed1991

An investigation of bioactive glass powders by sol-gel processing.

Bioactive glass powders, with a composition of SiO 2-CaO-P 2O 5, have been successfully synthesized via a sol-gel process at considerably lower temperatures than required for conventional melting methods. Bioactive glass powders made via conventional methods form an interfacial bond with bone when they are implanted. Bonding is correlated with the formation of a surface hydroxyapatite layer. This study examined the formation of a hydroxyapatite layer in Tris-buffered solution as a function of SiO 2 content of sol-gel derived powders. A FT-IRRS technique was used to monitor the formation of the hydroxyapatite on the surface of the powders. X-ray diffraction analysis and BET were also used to characterize the chemical and physical properties of the sol-gel derived bioactive powders. It was discovered that: (a) the rate of hydroxyapatite formation decreased with increasing SiO 2 content for powders whose SiO 2 content was less than 90 mol%; (b) a hydroxyapatite film does not form for the powders whose SiO 2 content is more than 90 mol%; (c) the SiO 2 limit, beyond which the powders lost their bioactivity, was much higher for bioactive glass powders made through sol-gel process (90%) than those made by conventional melting methods (60%). These results indicate that it is possible to significantly expand the bioactive composition range through microstructural control made possible by sol-gel processing techniques.

Glass↗
PubMed2002

Mapping the mesoscale interface structure in polycrystalline materials.

A new experimental approach to the quantitative characterization of polycrystalline microstructure by scanning electron microscopy is described. Combining automated electron backscattering diffraction with conventional scanning contrast imaging and with calibrated serial sectioning, the new method (mesoscale interface mapping system) recovers precision estimates of the 3D idealized aggregate function G(x). This function embodies a description of lattice phase and orientation (limiting resolution approximately 1 degree) at each point x (limiting spatial resolution approximately 100 nm), and, therefore, contains a complete mesoscale description of the interfacial network. The principal challenges of the method, achieving precise spatial registry between adjacent images and adequate distortion correction, are described. A description algorithm for control of the various components of the system is also provided.

Journal Article↗
PubMed2003

Adhesion and friction of PDMS networks: molecular weight effects.

The objective of this work is to find relations between adherence and friction behaviors of elastomer networks. The chosen approach is based on the parallel study of the initial molecular weight (i.e., the degree of cross-linking) dependence of both adherence and friction. The polymers used are cross-linked polydimethylsiloxane (PDMS) and the substrate is a smooth glass plate. The experimental procedure uses both friction (pin on disk tribometer) and adhesion (tack test) measurements, associated with surface analysis and mechanical and rheological characterizations. Tack results show that high molecular weight PDMS exhibits the greater adherence energy. This can be explained by the role of both chain length and free and pendant chains: more numerous and longer free chains favor the substrate wetting (at a molecular scale) and increases the energy dissipation during separation (extraction and reptation mechanisms). However, friction results indicate a higher friction resistance for low molecular weight PDMS. This result could be quite surprising. An explanation based on interfacial sliding properties of free and pendant chains can be proposed. Elsewhere, for the lower molecular weight polymer, elastic contact present during friction is able to act as a forced wetting, constraining the network and consequently leading to a greater energy dissipation.

Journal Article↗
PubMed2006

Covalent molecular assembly in supercritical carbon dioxide: a comparative study between amine- and anhydride-derivatized surfaces.

Layer-by-layer covalent assembly of an oligoimide on an anhydride- derivatized silicon dioxide surface is investigated using supercritical carbon dioxide (SCCO2) as the depositing medium. The deposited films were characterized by X-ray photoelectron spectroscopy (XPS), ellipsometry (VASE), UV-visible spectroscopy, electrochemical impedance spectroscopy, nano indentation, and atomic force microscopy (AFM) and the properties of the films compared with those of an oligoimide deposited on amine-derivatized surfaces. Films formed on the anhydride surface are more uniform and stable possibly because the silane precursor for the anhydride is anchored to the surface through two (-Si-O-Si-) tripods. XPS results indicate that the interfacial reaction resulting in amide formation is almost complete in the case of the anhydride, but not in the case of the amine. We infer that the twin tripods linking the anhydride group to the surface may have improved the accessibility of the functional groups for immobilization of the next layer, thereby contributing to the better quality.

Journal Article↗
PubMed2007

Interphase effects in dental nanocomposites investigated by small-angle neutron scattering.

Small-angle and ultrasmall-angle neutron scattering (SANS and USANS) were used to characterize silica nanoparticle dispersion morphologies and the interphase in thermoset dimethacrylate polymer nanocomposites. Silica nanoparticle fillers were silanized with varying mass ratios of 3-methacryloxypropyltrimethoxysilane (MPTMS), a silane that interacts with the matrix through covalent and H-bonding, and n-octyltrimethoxysilane (OTMS), a silane that interacts through weak dispersion forces. Interphases with high OTMS mass fractions were found to be fractally rough with fractal dimensions, D(s), between 2.19 and 2.49. This roughness was associated with poor interfacial adhesion and inferior mechanical properties. Mean interparticle distances calculated for composites containing 10 mass % and 25 mass % silica suggest that the nanoparticles treated with more MPTMS than OTMS may be better dispersed than OTMS-rich nanoparticles. The results indicate that the covalent bonding and H-bonding of MPTMS-rich nanoparticles with the matrix are necessary for preparing well-dispersed nanocomposites. In addition, interphases containing equal masses of MPTMS and OTMS may yield composites with overall optimal properties. Finally, the combined SANS/USANS data could distinguish the differences, as a function of silane chemistry, in the nanoparticle/silane and silane/matrix interfaces that affect the overall mechanical properties of the composites.

Dental Materials↗
PubMed2005

Low-temperature activation and deactivation of high-Curie-temperature ferromagnetism in a new diluted magnetic semiconductor: Ni2+-Doped SnO2.

We report the synthesis of colloidal Ni(2+)-doped SnO(2) (Ni(2+):SnO(2)) nanocrystals and their characterization by electronic absorption, magnetic circular dichroism, X-ray absorption, magnetic susceptibility, scanning electron microscopy, and X-ray diffraction measurements. The Ni(2+) dopants are found to occupy pseudooctahedral Sn(4+) cation sites of rutile SnO(2) without local charge compensation. The paramagnetic nanocrystals exhibit robust high-Curie-temperature (T(C)) ferromagnetism (M(s)(300 K) = 0.8 mu(B)/Ni(2+), T(C) >> 300 K) when spin-coated into films, attributed to the formation of interfacial fusion defects. Facile reversibility of the paramagnetic-ferromagnetic phase transition is also observed. This magnetic phase transition is studied as a function of temperature, time, and atmospheric composition, from which the barrier to ferromagnetic activation (E(a)) is estimated to be 1200 cm(-1). This energy is associated with ligand mobility on the surfaces of the Ni(2+):SnO(2) nanocrystals. The phase transition is reversed under air but not under N(2), from which the microscopic identity of the activating defect is proposed to be interfacial oxygen vacancies.

Colloids↗
PubMed2005

An extracellular lipase from the dimorphic yeast Arxula adeninivorans: molecular cloning of the ALIP1 gene and characterization of the purified recombinant enzyme.

The lipase-encoding Arxula adeninivorans ALIP1 gene was isolated using fragments of lipase isolates obtained by trypsin digestion for the definition of oligonucleotide primers in a PCR screening approach. The gene harbours an ORF of 1347 bp encoding a 420 amino acid protein of some 50 kDa preceded by an N-terminal 28 prepro-secretion sequence. The deduced amino acid sequence was found to be similar to the lipases from Candida albicans and C. parapsilosis (34-38% identity) and more distantly related to other lipases. The sequence contains the consensus pentapeptide motif (-Gly-X-Ser-X-Gly-) that forms a part of the interfacial lipid recognition site in lipases. The expression of the gene is regulated by carbon source. In media supplemented with Tween 20, induction of the ALIP1 gene and accumulation of the encoded lipase in the medium is observed, thus demonstrating gene regulation by lipophilic compounds. The enzyme characteristics are analysed from isolates of native strains as well as from those of recombinant strains expressing the ALIP1 gene under control of the strong A. adeninivorans-derived TEF1 promoter. For both proteins a molecular mass of 100 kDa was determined, indicating a dimeric structure, a pH optimum at pH 7.5 and a temperature optimum at 30 degrees C. The enzyme hydrolyses all ester bonds in all triglyceride substrates tested. Middle-sized chain fatty acids are more efficiently hydrolysed than short- and long-chain fatty acids, with the highest activity on C8/C10 fatty acid esters pNP-caprylate, pNP-caprate and tricaprylin.

Amino Acid Sequence↗
PubMedDate not supplied

Isolation and characterization of the human pulmonary surfactant apoprotein gene.

Pulmonary surfactant is a phospholipid-protein complex which serves to lower the surface tension at the air-liquid interface in the alveoli of the mammalian lung and is essential for normal respiration. Inadequate levels of surfactant at birth, a frequent situation in premature infants, results in respiratory failure. In all species examined, surfactant is composed primarily of dipalmitoylphosphatidylcholine and two major protein species of relative molecular mass (Mr) 32,000 (32K) and 10K (refs 2-5). Reconstitution in vitro of purified 32K pulmonary surfactant apoprotein (PSAP) with synthetic lipids forms a lipoprotein complex that lowers surface tension by spreading to create a thin interfacial film. Here we describe the cloning of the human PSAP gene and complementary DNA, and discuss features of the unusual encoded protein.

Amino Acid Sequence↗
PubMed2004

Characterization of surfactin from Bacillus subtilis for application as an agent for enhanced oil recovery.

Surfactin produced by Bacillus subtilis (ATCC 21332) was used to examine the effect of altering salt concentration, pH, and temperature on surfactin activity (as measured by reductions in surface tension). These parameters are some of the conditions that define oil reservoir characteristics and can affect the application of surfactants. The Biotechnology for Oilfield Operations research program at the Idaho National Engineering and Environmental Laboratory (INEEL) has successfully produced surfactin from potato process effluents for possible use as an economical alternative to chemical surfactants for improved oil recovery. Surfactants enhance the recovery of oil through a reduction of the interfacial tension between the oil and water interfaces, or by mediating changes in the wettability index of the system. We investigated changes in surfactin activity under a range of conditions by measuring surface tension. Surface tension was determined using video image analysis of inverted pendant drops. Experimental variables included NaCl (0-10%), pH (3.0-10.0), and temperature (21-70 degrees C). Each of these parameters, as well as selected combinations, resulted in discrete changes in surfactin activity. It is therefore important to consider the exploration of the studied surfactin as an enhanced oil recovery agent.

Bacillus subtilis↗
PubMed2001

Improved oil solubilization in oil/water food grade microemulsions in the presence of polyols and ethanol.

Microemulsions based on five-component mixtures for food applications and improved oil solubilization have been studied. The compositions included water, oil phase [such as R(+)-limonene and medium-chain triglycerides (MCT)], short-chain alcohols (such as ethanol), polyols (propylene glycol and glycerol), and several surfactants and their corresponding mixtures (nonionic, such as ethoxylated sorbitan esters, polyglycerol esters, sugar ester, and anionic, such as phosphatidylcholine). The phase behavior of these systems is discussed with respect to the influence of polyols and short-chain alcohols on the degree of solubilization of oils in the aqueous phase. The alcohol and polyols modify the interfacial spontaneous curvature and the flexibility of the surfactant film, enhancing the oil solubilization capacity of the microemulsions. The solubilization of R(+)-limonene was dramatically improved in the presence of the alcohol and polyols, whereas the improvement of solubilization for triglycerides containing MCT was less pronounced. In some systems high oil solubilization was achieved, and some of them can be easily diluted to infinity both with the aqueous phase and with the oil phase. Viscosity measurements along selected dilution lines [characterized by a single continuous microemulsion region starting from a pseudo binary solution (surfactant/oil phase) to the microemulsion (water/polyol corner)] indicate that at a certain composition the system inverts from a W/O to an O/W microemulsion.

Emulsions↗
PubMed2000

A conserved membrane-spanning amino acid motif drives homomeric and supports heteromeric assembly of presynaptic SNARE proteins.

Assembly of the SNARE proteins synaptobrevin/VAMP, syntaxin, and SNAP-25 to binary and ternary complexes is important for docking and/or fusion of presynaptic vesicles to the neuronal plasma membrane prior to regulated neurotransmitter release. Despite the well characterized structure of their cytoplasmic assembly domains, little is known about the role of the transmembrane segments in SNARE protein assembly and function. Here, we identified conserved amino acid motifs within the transmembrane segments that are required for homodimerization of synaptobrevin II and syntaxin 1A. Minimal motifs of 6-8 residues grafted onto an otherwise monomeric oligoalanine host sequence were sufficient for self-interaction of both transmembrane segments in detergent solution or membranes. These motifs constitute contiguous areas of interfacial residues assuming alpha-helical secondary structures. Since the motifs are conserved, they also contributed to heterodimerization of synaptobrevin II and syntaxin 1A and therefore appear to constitute interaction domains independent of the cytoplasmic coiled coil regions. Interactions between the transmembrane segments may stabilize the SNARE complex, cause its multimerization to previously observed multimeric superstructures, and/or be required for the fusogenic activity of SNARE proteins.

Amino Acid Sequence↗