Spectroscopic study of the penetration depth of grafted polystyrene onto poly (tetrafluoroethylene-co-perfluoropropylvinylether) substrates. 1. Effect of grafting conditions

This study concerns the radiation grafting of styrene onto poly(tetrafluoroethylene-co-perfluoropropylvinylether) (PFA) substrates and the penetration depth of the graft. Grafting was obtained by the simultaneous irradiation method, and the spectroscopic analysis was made with the micro-Raman technique. Effects of grafting conditions such as the type of solvent, dose rate, and irradiation dose on the grafting yield were investigated. Of the different solvents used, the most efficient in terms of increasing grafting yield were dichloromethane, benzene, and methanol, respectively. A mixture of methanol and dichloromethane used as a solvent for styrene achieved a higher degree of grafting and concentration of grafted polystyrene onto the surface of PFA substrates than solutions of the monomer in the separate solvents. The degree of grafting increased with increasing radiation dose up to 500 kGy, stabilizing above this dose. However, the grafting yield decreased with an increase in the dose rate. The increase in the overall grafting yield was accompanied by a proportional increase in the penetration depth of the grafts into the substrate. (C) 2002 Wiley Periodicals, Inc.

NMR study of the gamma radiolysis of poly(dimethyl siloxane) under vacuum at 303 K

The structural changes which occur on the gamma -radiolysis of poly(dimethyl siloxane) (PDMS) under vacuum at 303 K have been investigated using Si-29 and C-13 NMR. New structural units consistent with main chain scission and crosslinking through both H-linking and Y-linking reactions have been identified. The results obtained at various absorbed doses have been used to calculate the G-values for scission and crosslinking. G-values for scission of G(S) = 1.3 +/- 0.2, for H-linking of G(D-CH2-R) = 0.34 +/- 0.02 and for Y-Linking of G(Y) = 1.70 +/- 0.09 were obtained for radiolysis under vacuum at 303 K. Thus crosslinking predominates over scission for radiolysis of PDMS under these conditions, and, by contrast with previous studies, Y-links have been shown to be the predominant form of crosslinks. (C) 2001 Elsevier Science Ltd. All rights reserved.

Characterization of redox centers in dimethyl sulfide dehydrogenase from Rhodovulum sulfidophilum

Dimethyl sulfide dehydrogenase from the purple phototrophic bacterium Rhodovulum sulfidophilum catalyzes the oxidation of dimethyl sulfide to dimethyl sulfoxide. Recent DNA sequence analysis of the ddh operon, encoding dimethyl sulfide dehydrogenase (ddhABC), and biochemical analysis (1) have revealed that it is a member of the DMSO reductase family of molybdenum enzymes and is closely related to respiratory nitrate reductase (NarGHI). Variable temperature X-band EPR spectra (120122 K) of purified heterotrimeric dimethyl sulfide dehydrogenase showed resonances arising from multiple redox centers, Mo(V), [3Fe-4S](+), [4Fe-4S](+), and a b-type heme. A pH-dependent EPR study of the Mo(V) center in (H2O)-H-1 and (H2O)-H-2 revealed the presence of three Mo(V) species in equilibrium, Mo(V)-OH2, Mo(v)-anion, and Mo(V)-OH. Above pH 8.2 the dominant species was Mo(V)-OH. The maximum specific activity occurred at pH 9.27. Comparison of the rhombicity and anisotropy parameters for the Mo(V) species in DMS dehydrogenase with other molybdenum enzymes of the DMSO reductase family showed that it was most similar to the low-pH nitrite spectrum of Escherichia coli nitrate reductase (NarGHI), consistent with previous sequence analysis of DdhA and NarG. A sequence comparison of DdhB and NarH has predicted the presence of four [Fe-S] clusters in DdhB. A [3Fe-4S](+) cluster was identified in dimethyl sulfide dehydrogenase whose properties resembled those of center 2 of NarH. A [4Fe-4S](+) cluster was also identified with unusual spin Hamiltonian parameters, suggesting that one of the iron atoms may have a fifth non-sulfur ligand. The g matrix for this cluster is very similar to that found for the minor conformation of center 1 in NarH [Guigliarelli, B., Asso, M., More, C., Augher, V., Blasco, F., Pommier, J., Giodano, G., and Bertrand, P. (1992) Eur. J. Biochem. 307,63-68]. Analysis of a ddhC mutant showed that this gene encodes the b-type cytochrome in dimethyl sulfide dehydrogenase. Magnetic circular dichroism studies revealed that the axial ligands to the iron in this cytochrome are a histidine and methionine, consistent with predictions from protein sequence analysis. Redox potentiometry showed that the b-type cytochrome has a high midpoint redox potential (E-o = +315 mV, pH 8).

gamma-alumina nanofibers prepared from aluminum hydrate with poly(ethylene oxide) surfactant

Introducing poly(ethylene oxide) surfactant to aluminum hydrate colloids can effectively direct the crystal growth of boehmite and the crystal morphology of final gamma-alumina crystallites. Fibrous crystallites of gamma-alumina about 3-4 nm thick and 30-60 nm long are obtained. They stack randomly, resulting in a structure with a low contact area between the fibers but with a very large porosity. Such a structure exhibits strong resistance to sintering when heated to high temperatures. A sample retains a BET surface area of 68 m(2)/g, after being heated to 1473 K. The surfactant molecules form micelles that interact with the colloid particles of aluminum hydroxide through hydrogen bonding. This interaction is not sufficient to change the intrinsic crystal structure of boehmite, but induces profound changes in the morphology of boehmite crystallites and their growth. The surfactant-induced fiber formation (SIFF) process has distinct features from templated synthesis but shows similarities in some respects to biomineralization processes in which inorganic crystals with complex morphological shapes can be formed in biological systems. SIFF offers an effective approach to create new nanostructures of inorganic oxide from aqueous media.

New structure formation on gamma-irradiation of poly(chlorotrifluoroethylene)

The radiation chemistry of PCTFE at different temperatures has been studied. The polymer was irradiated under vacuum to absorbed doses of up to 1500 kGy. Three irradiation temperatures were chosen. These included ambient temperature, a temperature well above the T, and a temperature above the crystalline melting temperature. These were 298, 423 and 493 K, respectively. The formation of new structures was identified by solid-state FTIR and F-19 NMR. No branching was observed below the melting temperature, but branches were observed above the melting temperature. G-values for chain-end formation were 1.5 and 2.4 at room temperature and 423 K, respectively and the G-value for the formation of double bonds was found to be < 0.1. For the irradiations at 493 K, the G-values for the formation of chain ends, double bonds and branching points were 3.6, 0.2 and 0.5, respectively. The presence of long-chain branches within the polymer structure could not be proven for radiolysis at 493 K, but scission predominates and network formation does not occur upon irradiation. DSC studies of the polymers irradiated at ambient temperature were consistent with chain scission leading to an increase in the percentage crystallinity, as observed for other fluoropolymers. (C) 2003 Elsevier Science Ltd. All rights reserved.

Water sorption into poly[(2-hydroxyethyl methacrylate)-co-(1-vinyl-2-pyrrolidone]at 310 K

Poly(2-hydroxyethyl methacrylate) and copolymers of 2-hydroxyethyl methacrylate (HEMA) and 1-vinyl-2-pyrrolidone (VP) in the form of cylindrical samples (approximate to8mm x 20mm) have been prepared and the sorption of water into these cylinders has been studied by the mass-uptake methods and by magnetic-resonance imaging. The equilibrium water contents for the cylinders were found to vary systematically with the copolymer composition. Diffusion of water into the cylinders was found to follow Fickian behaviour for cylinders with high HEMA contents, with the diffusion coefficients obtained from mass-uptake studies dependent on the copolymer composition, varying from 1.7 x 10(-11) m(2) s(-1) for poly(HEMA) to 2.0 x 10(-11) m(2) s(-1) for poly(HEMA-co-VP) with a composition of 1:1. However, NMR-imaging studies showed that, while the profiles of the water diffusion fronts for cylinders with high HEMA contents were Fickian, that for the 1:1 copolymer was not and indicated that the mechanism was Case III. The polymers which were rich in VP were characterized by a water-sorption process which follows Case-III behaviour. (C) 2003 Society of Chemical Industry.

Comparative study of the radiation-induced grafting of styrene onto poly(tetrafluoroethylene-co-perfluoropropylvinyl ether) and polypropylene substrates. I: Kinetics and structural investigation

A comparative study has been made of the radiation grafting of styrene onto poly(tetrafluoroethylene-co-perfluoropropyl vinyl ether) (PFA) and polypropylene (PP) substrates, using the simultaneous irradiation method. Effects of grafting conditions such as monomer concentrations, type of solvent, dose rate and irradiation dose on the grafting yield were investigated. Under the same grafting conditions it was found that a higher degree of grafting of styrene was obtained using a mixture of dichloromethane/methanol solvents for PFA and methanol for PP and the degree of grafting was higher in PP than in PFA at all doses. However, the micro-Raman spectroscopy analysis of the graft revealed that, for the same degree of grafting, the penetration depth of the grafted polystyrene into the substrate was higher in PFA than in PP substrates. In both polymers the crystallinity was hardly affected by the grafting process and the degree of crystallinity decreased slightly with grafting dose. The dependence of the initial rate of grafting on the dose rate and the monomer concentration was found to be 0.6 and 1.4 order for PFA and 0.15 and 2.2 for PP, respectively. The degree of grafting increased with increasing radiation dose in both polymers. However, the grafting yield decreased with an increase in the dose rate. The increase in the overall grafting yield for PFA and PP was accompanied by a proportional increase in the penetration depth of the graft into the substrates. (C) 2003 Society of Chemical Industry.

Radical formation on gamma-radiolysis of poly [(methacrylic acid)-co-acrylonitri

The radicals formed on gamma-radiolysis of a series of copolymers of methacrylic acid and acrylonitrile have been investigated by ESR spectroscopy. This series of copolymers spanned the full composition range and the study was carried out at 77 K and ambient temperature. The radicals formed in the copolymers at 77 and 303 K were found to be similar to those found in the two homopolymers, but in the intermediate composition range the presence of acrylonitrile propagation radicals was also detected. These radicals were not observed to be formed in significant quantities on the radiolysis of polyacrylonitrile. They are believed to result from a scission of the main chain at methacrylic acid/acrylonitrile diad sequences following loss of the methacrylic acid carboxyl group. At 77 K, the copolymers with high methacrylic acid contents were found to be more sensitive to radical formation than the methacrylic acid homopolymer, but this enhanced sensitivity was not evident at ambient temperature, where the G-values for radical formation for the copolymers were slightly less than the values for the homopolymers. (C) 2003 Society of Chemical Industry.

A study of the radiation chemistry of poly(chlorotrifluoroethylene) by ESR spectroscopy

The ESR spectra of poly(chlorotrifluoroethylene) were recorded following gamma-radiolysis under vacuum at room temperature and 77 K. The very broad spectrum at 77 K revealed little fine structure with which to identity the radicals formed upon irradiation, but subsequent photobleaching and annealing studies, together with radiolytic studies at higher temperatures, afforded scope for making radical assignments. Both main-chain radicals and a range of chain-end radicals have been identified. The G-values for radical formation were 1.55, 0.36 and 0.32 at 77 K, 273 K and room temperature, respectively. (C) 2003 Elsevier Science Ltd. All rights reserved.

Fiber average size and distribution dependence on the electrospinning parameters of poly(vinylidene fluoride trifluoroethylene)) membranes for biomedical applications

Poly(vinylidene fluoride-trifluoethylene) electrospun membranes were obtained from a blend of dimethylformamide (DMF) and methylethylketone (MEK) solvents. The inclusion of the MEK to the solvent system promotes a faster solvent evaporation allowing complete polymer crystallization during the jet travelling between the tip and the grounded collector. Several processing parameters were systematically changed to study their influence on fiber dimensions. Applied voltage and inner needle diameter do not have large influence on the electrospun fiber average diameter but in the fiber diameter distribution. On the other hand, the increase of the distance between the needle tip to collector results in fibers with larger average diameter. Independently on the processing conditions, all mats are produced in the electroactive phase of the polymer. Further, MC-3T3-E1cell adhesion was not inhibited by the fiber mats preparation, indicating their potential use for biomedical applications.

Local piezoelectric response of single poly(vinylidene fluoride) electrospun fibers

It is shown that electrospun poly(vynilidene fluoride) nanofibers are fully poled right after preparation and show b-phase contents of 70%, therefore being able to be implemented into electroactive devices without further processing steps. Further,the local piezoelectric properties of individual electrospun fibers have been studied by piezoresponse force microscopy. Piezoelectric response, polarization switching, and nanoscale patterning of the fibers have been demonstrated.

Osteoblast, fibroblast and in vivo biological response to poly(vinylidene fluoride) based composite materials

Electroactivematerials can be taken to advantage for the development of sensors and actuators as well as for novel tissue engineering strategies. Composites based on poly(vinylidenefluoride),PVDF,have been evaluated with respect to their biological response. Cell viability and proliferation were performed in vitro both with Mesenchymal Stem Cells differentiated to osteoblasts and Human Fibroblast Foreskin 1. In vivo tests were also performed using 6-week-old C57Bl/6 mice. It was concluded that zeolite and clay composites are biocompatible materials promoting cell response and not showing in vivo pro-inflammatory effects which renders both of them attractive for biological applications and tissue engineering, opening interesting perspectives to development of scaffolds from these composites. Ferrite and silver nanoparticle composites decrease osteoblast cell viability and carbon nanotubes decrease fibroblast viability. Further, carbon nanotube composites result in a significant increase in local vascularization accompanied an increase of inflammatory markers after implantation.

Influence of electrospinning parameters on Poly(hydroxybutyrate) electrospun membranes fiber size and distribution

Poly(hydroxybutyrate) (PHB) obtained from sugar cane was dissolved in a blend of chloroform and dimethylformamide (DMF) and electrospun at 40 ºC. By adding DMF to the solution, the electrospinning process for the PHB polymer becomes more stable, allowing complete polymer crystallization during the jet travelling between the tip and the grounded collector. The influence of processing parameters on fiber size and distribution was systematically studied. It was observed that an increase of tip inner diameter promotes a decrease of the fiber average size and a broader distribution. On the other hand, an increase of the electric field and flow rate produces an increase of fiber diameter until a maximum of ~2.0 m, but for electric fields higher than 1.5 kV.cm-1, a decrease of the fiber diameter was observed. Polymer crystalline phase seems to be independent of the processing conditions and a crystallinity degree of 53 % was found. Moreover, thermal degradation of the as-spun membrane occurs in single step degradation with activation energy of 91 kJ/mol. Furthermore, MC-3T3-E1 cell adhesion was not inhibited by the fiber mats preparation, indicating their potential use for biomedical applications.

Fatigue Prediction on Fibrin Poly-ε-caprolactone Macroporous Scaffolds

Tissue engineering applications rely on scaffolds that during its service life, either for in-vivo or in vitro applications, are under mechanical solicitations. The variation of the mechanical condition of the scaffold is strongly relevant for cell culture and has been scarcely addressed. Fatigue life cycle of poly-ε-caprolactone, PCL, scaffolds with and without fibrin as filler of the pore structure were characterized both dry and immersed in liquid water. It is observed that the there is a strong increase from 100 to 500 in the number of loading cycles before collapse in the samples tested in immersed conditions due to the more uniform stress distributions within the samples, the fibrin loading playing a minor role in the mechanical performance of the scaffolds

Effect of fiber alignment in gelled electrospun membranes based on poly(vinylidene fluoride) for battery applications

Battery separators based on electrospun membranes of poly(vinylidene fluoride) (PVDF) have been prepared in order to study the effect of fiber alignment on the performance and characteristics of the membrane. The prepared membranes show an average fiber diameter of 272 nm and a degree of porosity of 87 %. The gel polymer electrolytes are prepared by soaking the membranes in the electrolyte solution. The alignment of the fibers improves the mechanical properties for the electrospun membranes. Further, the microstructure of the membrane also plays an important role in the ionic conductivity, being higher for the random electrospun membrane due to the lower tortuosity value. Independently of the microstructure, both membranes show good electrochemical stability up to 5.0 V versus Li/Li+. These results show that electrospun membranes based on PVDF are appropriate for battery separators in lithium-ion battery applications, the random membranes showing a better overall performance.