James W. Mitchell

Improve the Strength of PLA/HA Composite Through the Use of Surface Initiated Polymerization and Phosphonic Acid Coupling Agent

Bioresorbable composite made from degradable polymers, e.g., polylactide (PLA), and bioactive calcium phosphates, e.g., hydroxyapatite (HA), are clinically desirable for bone fixation, repair and tissue engineering because they do not need to be removed by surgery after the bone heals. However, preparation of PLA/HA composite from non-modified HA usually results in mechanical strength reductions due to a weak interface between PLA and HA. In this study, a calcium-phosphate/phosphonate hybrid shell was developed to introduce a greater amount of reactive hydroxyl groups onto the HA particles. Then, PLA was successfully grafted on HA by surface-initiated polymerization through the non-ionic surface hydroxyl groups. Thermogravimetric analysis indiated that the amount of grafted PLA on HA can be up to 7 %, which is about 50 % greater than that from the literature. PLA grafted HA shows significantly different pH dependent ζ-potential and particle size profiles from those of uncoated HA. By combining the phosphonic acid coupling agent and surface initiated polymerization, PLA could directly link to HA through covalent bond so that the interfacial interaction in the PLA/HA composite can be significantly improved. The diametral tensile strength of PLA/HA composite prepared from PLA-grafted HA was found to be over twice that of the composite prepared from the non-modified HA. Moreover, the tensile strength of the improved composite was 23 % higher than that of PLA alone. By varying additional variables, this approach has the potential to produce bioresorbable composites with improved mechanical properties that are in the range of natural bones, and can have wide applications for bone fixation and repair in load-bearing areas.

Effects of Inorganic Fillers on the Thermal and Mechanical Properties of Poly(lactic acid)

Addition of filler to polylactic acid (PLA) may affect its crystallization behavior and mechanical properties. The effects of talc and hydroxyapatite (HA) on the thermal and mechanical properties of two types of PLA (one amorphous and one semicrystalline) have been investigated. The composites were prepared by melt blending followed by injection molding. The molecular weight, morphology, mechanical properties, and thermal properties have been characterized by gel permeation chromatography (GPC), scanning electron microscope (SEM), instron tensile tester, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA). It was found that the melting blending led to homogeneous distribution of the inorganic filler within the PLA matrix but decreased the molecular weight of PLA. Regarding the filler, addition of talc increased the crystallinity of PLA, but HA decreased the crystallinity of PLA. The tensile strength of the composites depended on the crystallinity of PLA and the interfacial properties between PLA and the filler, but both talc and HA filler increased the toughness of PLA.

Synthesis of Carboxylic Block Copolymers via Reversible Addition Fragmentation Transfer Polymerization for Tooth Erosion Prevention

Dental professionals are seeing a growing population of patients with visible signs of dental erosion. The approach currently being used to address the problem typically leverages the enamel protection benefits of fluoride. In this report, an alternative new block copolymer with a hydrophilic polyacrylic acid (PAA) block and a hydrophobic poly(methyl methacrylate) (PMMA) block was developed to similarly reduce the mineral loss from enamel under acidic conditions. This series of PMMA-b-PAA block copolymers was synthesized by reversible addition fragmentation transfer (RAFT) polymerization. Their structures were characterized by gel permeation chromatography (GPC) and 1H nuclear magnetic resonance (NMR) spectra. The molar fractions of acrylic acid (AA) in the final block copolymer were finely controlled from 0.25 to 0.94, and the molecular weight (Mn) of PMMA-b-PAA was controlled from 10 kDa to 90 kDa. The binding capability of the block copolymer with hydroxyapatite (HAP) was investigated by ultraviolet–visible spectroscopy (UV-Vis) and Fourier transform infrared (FTIR) spectroscopy. FTIR spectra confirmed that the PMMA-b-PAA block copolymer could bind to HAP via bridging bidentate bonds. Both UV-Vis and FTIR spectra additionally indicated that a high polymer concentration and low solution pH favored the polymer binding to HAP. The erosion-preventing efficacy of the PMMA-b-PAA block copolymer in inhibiting HAP mineral loss was quantitatively evaluated by atomic absorption spectroscopy (AAS). Based on the results, polymer treatment reduced the amount of calcium released by 27% to 30% in comparison with the unprotected samples. Scanning electron microscope (SEM) observations indicated that PMMA-b-PAA polymer treatment protected enamel from acid erosion. This new amphiphilic block copolymer has significant potential to be integrated into dentifrices or mouthrinses as an alternative non-fluoride ingredient to reduce tooth erosion.

Determination of boron in glass by the nuclear track technique

SummaryDetermination of Boron in Glass by the Nuclear Track Technique The nuclear track technique using the10B(nα)7Li reaction, has been applied to determine boron in concentration ranges (1.59 to 7.75%) ordinarily considered high for nuclear determinations. Factors limiting the overall precision of the method and restricting the linear dependence of track density on boron concentration are examined. Boron alpha track densities are used to construct planar profiles of boron distribution within the surface region of bulk glass and CVD films. Calibration curves for quantitative lateral profiling of surfaces are constructed and show the selective corrosion of borosilicate glass surface resulting in boron depletion.

Selective Substoichiometric separation of fluoride by solvent extraction into chloroform with triphenyl-tin chloride

SummaryA unique method has been developed for the selective separation of fluoride from a 50% dimethylsulfoxide (DMSO) aqueous phase via Substoichiometric replacement reaction with triphenyl-tin chloride and extraction into chloroform. Precise (±2.5%) Substoichiometric separations of fluoride in the presence of bromide, chloride, iodide, nitrate, thiocyanate, or sulfate have been accomplished. Applicability of the method for radionuclide separations in activation analysis is demonstrated via the examples of irradiated ammonium fluoride, hydrofluoric acid, and18F isotope labeled sodium and magnesium fluorides. Results for quantitative radioisotope dilution determinations show an error of ±2.1% for a fluoride level of 0.761±0.044 mg.ZusammenfassungDie selektive Abtrennung von Fluorid aus 50% igem Dimethylsulfoxid in Wasser durch substöchiometrische Verdrängungsreaktion mit Triphenylzinnchlorid und Extraktion in Chloroform wurde ausgearbeitet. Auf ±2,5% genaue substöchiometrische Trennungen von Fluorid in Gegenwart von Bromid, Chlorid, Jodid, Nitrat, Rhodanid oder Sulfat wurden durchgeführt. Die Anwendbarkeit des Verfahrens für die radiochemische Abtrennung bei der Aktivierungsanalyse wurde an Beispielen von bestrahltem Ammoniumfluorid, Fluorwasserstoffsäure und mit18F markiertem Natrium-bzw. Magnesiumfluorid gezeigt. Die Ergebnisse der quantitativen Bestimmung der Radioisotop-Verdünnung zeigen einen Fehler von ±2,1% für Fluoridmengen von 0,76±0,044 mg.

Reliability Assessment of Lead-Free Universal Solder for Direct Bonding in Power Electronics Packaging

A novel, lead-free universal (rare earth) solder is investigated for high temperature power electronics packaging. The material is Sn-3.5Ag-0.5Cu solder with small additions of a rare earth element exhibiting superb bonding properties. It is capable of direct, reliable bonding of dissimilar materials without the use of flux and eliminates the need for multilayer metallization which is typically required to aid in wetting. The problem of surface oxidation is done away with because of the strong thermodynamic affinity of the rare earth metal in the solder to oxygen. [1] This paper offers a first investigation of the thermal performance and reliability of this universal solder joint for power switching applications.

Chemical etching of nanocomposite metal-semiconductor films monitored by Raman spectroscopy and surface probe microscopy

The complementary tools of atomic force microscopy (AFM) and Raman spectroscopy are used to extract information on the microstructural properties of nanocomposite n-doped Si (n-Si) and Ag/n-Si films deposited on Si(111) substrates at 400 °C and 550 °C. AFM measurements indicated that Ag/n-Si films had grain sizes and roughness values one order of magnitude higher than n-doped Si films. The onset of metal-mediated crystallization of a-Si in Ag/n-Si films at ~ 400 °C is confirmed by Raman spectroscopy. Spectral Raman red-shifts of the transverse optical phonon region compared to monocrystalline silicon originate from the interplay of phonon confinement and higher defect density caused by n-type doping. Two protocols using the etchants ammonium fluoride - HF (2%::4% ) and ammonium citrate- acetic acid-hydrogen peroxide (2.5%::2.5%::2%) solutions were investigated. A comparison between non-etched and etched films showed little variability in roughness indicating retention of the microstructure.

Part per billion determination of oxygen containing impurities in ammonia-nitrogen mixtures

Ammonia-nitrogen feedstock streams, free of oxygen containing impurities, are essential reagents for fabricating low oxygen contaminated nitride films. An analytical method is described for determining part per billion levels of oxygen containing impurities, which are converted to nitric oxide (NO) through reaction with microwave discharge produced active nitrogen. Quantitative determinations are then based on detecting the resulting chemiluminescence of NO at characteristic wavelengths. This metastable transfer spectrometric method (MTES) provides direct measurements of oxygen impurities over the 14 to 500 ppb region and has dynamic range extending to 200 ppm. The method, unparalleled in its measurement sensitivity for oxygen impurities in flowing nitrogen gas streams, is calibrated with an NBS standard and applied to the determination of the purification efficiency of an on-line resin system for removal of O2, CO and NO from ammonia-nitrogen mixtures. Gaseous streams, doped deliberately in the 0.2 to 100 ppm level with either O2, CO or NO following purification were found to contain less than 10 ppb of the residual impurity. A commercially available 1% ammonia in nitrogen semiconductor grade reagent (99.999%) is shown by MTES to be contaminated with 470 ppb of oxygen impurities.