The Synthesis of Poly(Propylene Fumarate)

Poly(propylene fumarate) is a promising material in the field of bone tissue engineering. A recently described chain growth polymerization was developed for the synthesis of poly(propylene fumarate). This pathway uses the ring-opening polymerization of maleic anhydride and propylene oxide with magnesium ethoxide as an initiator to yield poly(propylene maleate). The poly(propylene maleate) was then isomerized to poly(propylene fumarate) using diethyl amine. In order to scale up the reaction to a 20L reactor, several optimizations of the purification schemes was required. The main optimizations were done to the purification of poly(propylene maleate). Several methods were tested including replacing the water washes with brine washes, distillation to remove the remaining maleic anhydride, and the precipitation in diethyl ether. The diethyl ether precipitation method was significantly faster than the other methods and able to successfully remove the maleic anhydride. For these reasons the precipitation method was chosen for the purification of the poly(propylene maleate). The next step will be to attempt to improve the purification of poly(propylene fumarate). Once this is completed, the reaction will be ready for implementation at the 20 L batch reactor scale

Hybrid gold single crystals incorporating amino acids

Composite hybrid gold crystals are of profound interest in various research areas ranging from materials science to biology. Their importance is due to their unique properties and potential implementation, for example in sensing or in bio-nanomedicine. Here we report on the formation of hybrid organic-metal composites via the incorporation of selected amino acids histidine, aspartic acid, serine, glutamine, alanine, cysteine, and selenocystine into the crystal lattice of single crystals of gold. We used electron microscopy, chemical analysis and high-resolution synchrotron powder X ray diffraction to examine these composites. Crystal shape, as well as atomic concentrations of occluded amino acids and their impact on the crystal structure of gold, were determined. Concentration of the incorporated amino acid was highest for cysteine, followed by serine and aspartic acid. Our results indicate that the incorporation process probably occurs through a complex interaction of their individual functional groups with gold atoms. Although various organic gold composites have been prepared, to the best of our knowledge this is the first reported finding of incorporation of organic molecules within the gold lattice. We present a versatile strategy for fabricating crystalline nanohybrid composite gold crystals of potential importance for a wide range of applications

Variable temperature study of the crystal and magnetic structures of the giant magnetoresistant materials LMnAsO (L=La, Nd)

Peer reviewedPublisher PD

Evidence of a structural anomaly at 14 K in polymerised CsC60

We report the results of a high-resolution synchrotron X-ray powder diffraction study of polymerised CsC$_{60}$ in the temperature range 4 to 40 K. Its crystal structure is monoclinic (space group I2/m), isostructural with RbC$_{60}$. Below 14 K, a spontaneous thermal contraction is observed along both the polymer chain axis, $a$ and the interchain separation along [111], $d_1$. This structural anomaly could trigger the occurrence of the spin-singlet ground state, observed by NMR at the same temperature.Comment: 8 pages, 5 figures, submitte

Spin-driven Phase Transitions in ZnCr2_2Se4_4 and ZnCr2_2S4_4 Probed by High Resolution Synchrotron X-ray and Neutron Powder Diffraction

The crystal and magnetic structures of the spinel compounds ZnCr$_2$S$_4$ and ZnCr$_2$Se$_4$ were investigated by high resolution powder synchrotron and neutron diffraction. ZnCr$_2$Se$_4$ exhibits a first order phase transition at $T_N=21$ K into an incommensurate helical magnetic structure. Magnetic fluctuations above $T_N$ are coupled to the crystal lattice as manifested by negative thermal expansion. Both, the complex magnetic structure and the anomalous structural behavior can be related to magnetic frustration. Application of an external magnetic field shifts the ordering temperature and the regime of negative thermal expansion towards lower temperatures. Thereby, the spin ordering changes into a conical structure. ZnCr$_2$S$_4$ shows two magnetic transitions at $T_{N1}=15$ K and $T_{N2}=8$ K that are accompanied by structural phase transitions. The crystal structure transforms from the cubic spinel-type (space group $Fd$\={3}$m$) at high temperatures in the paramagnetic state, via a tetragonally distorted intermediate phase (space group $I4_1$ / $amd$) for $T_{N2} < T < T_{N1}$ into a low temperature orthorhombic phase (space group $I m m a$) for $T < T_{N2}$. The cooperative displacement of sulfur ions by exchange striction is the origin of these structural phase transitions. The low temperature structure of ZnCr$_2$S$_4$ is identical to the orthorhombic structure of magnetite below the Verwey transition. When applying a magnetic field of 5 T the system shows an induced negative thermal expansion in the intermediate magnetic phase as observed in ZnCr$_2$Se$_4$.Comment: 11 pages, 13 figures, to be published in PR

Analysis and Implementation of Median Type Filters

Median filters are a special class of ranked order filters used for smoothing signals. These filters have achieved- success in speech processing, image processing, and other impulsive noise environments where linear filters have proven inadequate. Although the implementation of a median filter requires only a simple digital operation, its properties are not easily analyzed. Even so, a number of properties have been exhibited in the literature. In this thesis, a new tool, known as threshold decomposition is introduced for the analysis and implementation of median type filters. This decomposition of multi-level signals into sets of binary signals has led to significant theoretical and practical breakthroughs in the area of median filters. A preliminary discussion on using the threshold decomposition as an algorithm for a fast and parallel VLSI Circuit implementation of ranked filters is also presented* In addition, the theory is developed both for determining the number of signals which are invariant to arbitrary window width median filters when any number of quantization levels are allowed and for counting or estimating the number of passes required to produce a root- i.e. invariant signal, for binary signals. Finally, the analog median filter is defined and proposed for analysis of the standard discrete median filter in cases with a large sample size or when the associated statistics would be simpler in the continuu