SAXS measurements of the interface in polyacrylate and epoxy interpenetrating networks with fractal geometry

The interface thickness in two-component interpenetrating polymer networks (IPN) system based on polyacrylate and epoxy were determined using small-angle X-ray scattering (SAXS) in terms of the theory proposed by Ruland. The thickness was found to be nonexistent for the samples at various compositions and synthesized at variable conditions-temperature and initiator concentration. By viewing the system as a two-phase system with a sharp boundary, the roughness of the interface was described by fractal dimension, D, which slightly varies with composition and synthesis condition. Length scales in which surface fractals are proved to be correct exist for each sample and range from 0.02 to 0.4 Angstrom(-1). The interface in the present IPN system was treated as fractal, which reasonably explained the differences between Pored's law and experimental data, and gained an insight into the interaction between different segments on the interface. (C) 1997 Elsevier Science Ltd.

In situ scanning tunneling microscopy studies of nanometer size palladium particles on highly oriented pyrolytic graphite

A special electrodeposition process of palladium was studied by cyclic voltammetry, X-ray photoelectron spectroscopy (XPS) and in situ scanning tunneling microscopy (STM). A kind of palladium(IV) complex was attached to the highly oriented pyrolytic graphite (HOPG) electrode surface by electro-oxidation of palladium(II) complex first, and was then reduced to palladium particles. The surface complexes and particles of palladium were both characterized by in situ STM and XPS. The Pd particles are in the nanometer range of size and exhibit electrocatalytic activity towards the oxidation of hydrazine and hydroxylamine.

Tip-induced dynamic characterization of multilayers and variable morphologies of cytochrome c molecules on highly oriented pyrolytic graphite revealed by in-situ scanning tunnelling microscopy

The dynamic states of cytochrome c multilayers on electrochemically pretreated highly oriented pyrolytic graphite (HOPG) have been studied by in-situ scanning tunnelling microscopy (STM) under potential control of both the tip and the substrate in cytochrome c and phosphate buffer solution. The dynamic characterization of cytochrome c multilayers and relatively stable adsorbed single cytochrome c molecules scattered on HOPG imply that physically adsorbed multilayers were more easily influenced by the STM tip than those of chemically adsorbed single molecules. In-situ STM images of chemically adsorbed cytochrome c molecules with discernible internal structures on HOPG revealed that morphologies of cytochrome c molecules also suffered tip influence; possible tip-sample-substrate interactions have been discussed.

Use of SAXS to characterize the morphology of poly(polyethylene glycol diacrylate)/epoxy resin interpenetrating networks

Interpenetrating polymer networks (IPNs) have been synthesized from prepolymers that form miscible blends. All IPNs made from polyacrylate ((polyethylene glycol diacrylate), PEGDA) and epoxy (diglycidyl ether of bisphenol A, DGEBA) can be made in phase separated states by incorporating crosslinks. However, blends of these prepolymers, having a negative Flory-Huggins interaction parameter, are highly miscible. This indicates that formation of IPNs favours phase separation relative to blends. The microphase separation characteristics in the PEGDA/DGEBA IPNs were determined using smalt-angle X-ray scattering (SAXS). The Debye-Bueche and Guinier methods were used to calculate the correlation lengths of the segregated phases existing in the PEGDA/DGEBA IPNs. The results from SAXS showed that the size of the phase segregation zones changed with composition from about 50 to 100 Angstrom.

Dynamic mechanical properties of interpenetrating polymer networks based on polyacrylates and epoxy

Interpenetrating polymer networks (IPNs) based on polyacrylate (poly(polyethylene glycol diacrylate), PEGDA) and epoxy(diglycidyl ether of bisphenol A, DGEBA) were prepared simultaneously Dynamic mechanical properties of the SINs (simultaneous interpenetrating networks) with various compositions were studied. Enhanced mechanical properties were found in this case. From the point of view of pre-swollen networks, all of the PEGDA/DGEBA IPNs were composed of the individual pre-swollen networks. A micro-phase segregation system was produced in the SIN. Glass transition temperatures shifted inward, which was attributed to molecular packing effects or mutual-entanglements of molecular segments among the individual pre-swollen networks. In accordance with the additivity of properties, namely the parallel model, the entanglement density between the two polymer networks reached its maximum at 50/50 PEGDA/DGEBA IPN.

In situ electrochemical scanning tunneling microscopy investigation of renewing graphite surface accompanied by electrochemical reaction

In situ electrochemical scanning tunneling microscopy (ECSTM) has been employed to follow the renewal process of a graphite electrode accompanied by flavin adenine dinucleotide (FAD) electrochemical reaction which involves adsorption of the reduced form (FADH(2)) and desorption of the oxidized form (FAD). The renewal process initiates from steps or kinks on the electrode surface, which provide high active sites for adsorption. This renewal depends on the working electrode potential, especially in the range near the FAD redox potential. Our experiment suggests that delamination of the graphite surface is caused by interaction between the substrate and adsorbed molecules. A simple model is proposed to explain this phenomenon.

Synthesis and characterization of interpenetrating polymer networks based on polyacrylate and epoxy

The synthesis and properties of simultaneously interpenetrating networks (SINs) based on poly(polyethylene glycol diacrylate) (PEGDA) and epoxy (diglycidyl ether of bisphenol A, DGEBA) were studied. The effect of compositional variation on the morphology and properties of products was investigated. The swelling coefficient, densities, glass transition behavior, and thermal stability of these interpenetrating networks (IPNs) are discussed. Microphase separation morphological structures were found in all PEGDA/DGEBA IPNs. Decreased swelling ratios compared to the calculated swelling coefficients based on the weight additivity of the components were obtained after the formation of IPNs. Increased density and thermal stability were also obtained in these IPNs, implying the existence of interpenetration (topological entanglements) among the component networks.

A comparison of electrocatalytic ability of various mediators adsorbed onto paraffin impregnated graphite electrodes for oxidation of reduced nicotinamide coenzymes

Twelve mediators have been modified by adsorption onto the paraffin impregnated graphite electrodes (IGE). The resulting electrodes exhibit electrocatalytic activity of different degrees towards oxidation of 1,4-dihydronicotinamide adenine dinucleotide (NADH). The electrocatalytic ability of the chemically modified electrode (CME) depends mainly on the formal potential and molecular structure of mediator. The formation of the charge transfer complex between NADH and adsorbed mediator has been demonstrated by the experiments using a rotating disk electrode. An electrocatalytic scheme obeying Michaelis-Menten kinetics has been confirmed, and some kinetic parameters were estimated. The solution pH influences markedly the electrocatalytic activity of the modified electrode. Various possible reasons are discussed.

STM image of tungstosilicic acid molecular layer on highly oriented pyrolytic graphite

Molecular layer of tungstosilicic acid (H4SiW12O40) deposited on freshly-cleaved highly oriented pyrolytic graphite (HOPG) was observed by scanning tunneling microscopy (STM) in air at room temperature. The molecular dimension (11.5 Angstrom) of H4SiW12O40 measured by STM is consistent with known crystallographic parameter. We also imaged the boundary of H4SiW12O40 molecular layer on HOPG showing that molecular layer of H4SiW12O40 was formed. It has been proved that individual tungstosilicic acid species is imaged. The probable reason for the formation of the molecular layer is also discussed.

VOLTAMMETRY OF CYTOCHROME-C ENTRAPPED IN HYDROGEL MEMBRANE ON GRAPHITE ELECTRODE

The voltammetric behavior of cytochrome c entrapped in hydrogel membranes at paraffin wax-impregnated spectroscopic graphite electrodes (WISGE) was studied in this paper. A pair of well-defined peaks appeared at +70 mV (vs. Ag/AgCl). Beside these two peaks, another pair of peaks emerged at around +225 mV. Further investigations suggested that at least three states of cytochrome c existed in the membranes due to the special structure of the hydrogel. The native conformation of cytochrome c molecules was stabilized by the hydrophilic environment that was formed by the hydroxyl structure of the membranes and facilitated the cytochrome c electron transfer reaction at +70 mV. The molecules directly adsorbed on the surface of the graphite electrode were responsible for the redox peaks at around +225 mV. Whether the adsorption peaks were detectable or not was related to the thickness of membranes and the pre-retaining time before the formation of membranes.

IN-SITU SCANNING-TUNNELING-MICROSCOPY IMAGE OF A HIGHLY ORIENTED PYROLYTIC-GRAPHITE ELECTRODE MODIFIED WITH POTASSIUM BIS(11-MOLYBDOSILICATO) DYSPROSATE(III) IN SODIUM-SULFATE SOLUTION

The variation in molecule adsorption mode on pretreated highly oriented pyrolytic graphite electrodes, modified with the title complex K10H3[Dy(SiMo11O39)(2)] by cyclic voltammetry in the title complex solution, was observed in situ by electrochemical scanning tunnelling microscopy (ECSTM) with molecular resolution in sodium sulphate solution. According to the ECSTM images and the known molecular structure we conclude that the adsorption mode of the title complex modified electrode changed during potential cycling from ''vertical'' to ''inclined'' and then ''horizontal'' or ''flat'' mode, i.e. the title complex adsorbed on the surface of electrode by one ligand of the complex at first, then began to incline and was finally adsorbed by two ligands of the complex. This result indicates that the adsorption mode on the modified electrode surface changed during potential cycling in the sulphate solution and a much more stable molecular layer was formed. The change in adlattice of adsorbates on the modified electrode surface from hexagonal to rectangular was also observed by ECSTM. A plausible model was given to explain this process.

STM OF FOLDED AND UNFOLDED HEMOGLOBIN MOLECULES ELECTROCHEMICALLY DEPOSITED ON HIGHLY ORIENTED PYROLYTIC-GRAPHITE

The structural characterization of folded and unfolded haemoglobin has been performed by scanning tunnelling microscopy (STM) for the first time. STM images show an oval-shaped pattern for the folded structure of this protein, and moreover two dimers consisting of one haemoglobin molecule can be clearly discerned. The dimensions of a folded molecule were determined as 6.4 x 5.4 x 0.7 nm(3), which are in good agreement with the known size obtained from X-ray analysis. We have found that unfolding of haemoglobin molecules on the surface of highly oriented pyrolytic graphite (HOPG) can be achieved by electrochemical deposition. The STM analysis indicates clearly that the tertiary structure of the protein was lost by electrochemical deposition, and most of the haemoglobin molecules were almost fully extended and exhibited a twisted rope-like or a rod-like aggregated structure. Our investigation demonstrates the capability of the electrochemical method in denaturing this redox protein and in preparing stable biological samples for use in STM imaging.

CURE AND IONIC-CONDUCTIVITY OF A 2-COMPONENT EPOXY NETWORK POLYMER ELECTROLYTE

In order to raise the room temperature ionic conductivity and improve the mechanical strength of a PEO-based polymer electrolyte, a non-crystalline two-component epoxy network was synthesized by curing diglycidyl ether of polyethylene glycol (DGEPEG) with triglycidyl ether of glycerol (TGEG) in the presence of LiClO4 salt, which acts in this system as both a ring opening catalyst and a source of ionic carrier. The structure of the precursors, the curing process and the cured films have been characterized by C-13 NMR, IR, DSC and ionic conductivity measurement techniques. The electrolyte system exhibits an ionic conductivity as high as similar to 10(-5) S/cm at 25 degrees C and is mechanically self-supportable. The dependence of ionic conductivity was investigated as a function of temperature, salt content, MW of PEG segment in DGEPEG and the proportion of DGEPEG in DGEPEG/TGEG ratio.

AN ORGANIC-PHASE ENZYME ELECTRODE BASED ON AN APPARENT DIRECT ELECTRON-TRANSFER BETWEEN A GRAPHITE ELECTRODE AND IMMOBILIZED HORSERADISH-PEROXIDASE

A mediatorless horseradish peroxidase (HRP) enzyme electrode operated in nonaqueous media is constructed by cryohydrogel immobilization.