The crystal structure of hydrated barium copper oxalate

BaCu(C2O4)(2) . 6H2O is triclinic, P (1) over bar, with a = 6.5405(9), b = 9.202(3), c = 10.939(1) Angstrom, alpha = 85.46(2), beta = 79.22(1), gamma = 80.45(2), V = 636.99(1) Angstrom(3), Z = 2, D-0 = 2.14, D-c = 2.465 g . cm(-3), R = 0.074, wR = 0.0746 for 2219 significant reflections \F-0\ greater than or equal to 6.0 sigma F-0. The barium has eleven coordinations and the coordination polyhedra is a capped antiprism. Six water oxygen atoms are coordinated whereas the other five are coming from the oxalate group. In the unit cell the molecule's form a polymeric network. One lattice water molecule belongs to the coordinating water. The barium oxygen distances vary from 2.75 Angstrom to 3.15 Angstrom.

Some observations on the ignition of composite solid propellants

Heat-up times derived from studies on the ignition characteristics of a few model composite solid propellants, containing polystyrene, carboxy-terminated polybutadiene, plasticised polyvinyl chloride and polyphenol formaldehyde as binders, show that they are directly proportional to the mass of the sample and inversely proportional to the hear flux. Propellant weight-loss prior to ignition and high pressure ignition temperature data on the propellants, ammonium per chlorate, and binders show that the ignition is governed by the gasification of the binder pyrolysis products. The activation energy for the gasification of the pyrolysed polymer products corresponds to their ignition behaviour suggesting that propellant ignition is controlled by the binder.

Spontaneously Igniting Hybrid Fuel-Oxidizer Systems

After briefly outlining the recent developments in hybrid rockets, the work carried out by the author on self-igniting (hypergolic) solid fuel-liquid oxidiser systems has been reviewed. A major aspect relates to the solid derivatives of hydrazines, which have been conceived as fuels for hybrid rockets. Many of these N-N bonded compounds ignite readily, with very short ignition delays, on coming into contact with liquid oxidisers, like HNO3 and N2O4. The ignition characteristics have been examined as a function of the nature of the functional group in the fuel molecule, in an attempt to establish a basis for the hypergolic ignition in terms of chemical reactivity of the fuel-oxidiser combination. Important chemical reactions occurring in the pre-ignition stage have been identified by examining the quenched reaction products. Hybrid systems exhibiting synergistic hypergolicity in the presence of metal powders have been investigated. An estimation of the rocket performance parameters, experimental determination of the heats of combustion in HNO3, thermal decomposition characteristics, temperature profile by thin film thermometry and and product identification by the rapid scan FT-IR, are among the other relevant studies made on these systems. A significant recent development has been the synthesis of new N-N bonded viscous binders, capable of retaining the hypergolicity of the fuel powders embedded therein as well as providing the required mechanical strength to the grain. Several of these resins have been characterised. Metallised fuel composites of these resins having high loading of magnesium are found to have short ignition delays and high performance parameters.

Dynamics of Fractional Brownian Walks

We investigate the dynamics of polymers whose solution configurations are represented by fractional Brownian walks. The calculation of the two dynamical quantities considered here, the longest relaxation time tau(r) and the intrinsic viscosity [eta], is formulated in terms of Langevin equations and is carried out within the continuum approach developed in an earlier paper. Our results for tau(r) and [eta] reproduce known scaling relations and provide reasonable numerical estimates of scaling amplitudes. The possible relevance of the work to the study of globular proteins and other compact polymeric phases is discussed.

Highly Efficient and Thermally Stable Second-Order Nonlinear Optical Chromophores and Electrooptic Polymers

Organic polymeric electro-optic (E-O) materials have attracted significant attention because of their potential use as fast and efficient components of integrated photonic devices (1,2). However, the practical application of these materials in optical devices is somewhat limited by the stringent material requirements imposed by the device design, fabrication processes and operating environments. Among the various material requirements, the most notable ones are large electro-optic coefficients (r(33)) and high thermal stability (3). The design of poled polymeric materials with high electro-optic activity (r(33)) involves the optimization of the percent incorporation of efficient (large beta mu) second order nonlinear optical (NLO) chromophores into the polymer matrices and the effective creation of poling-induced non-centrosymmetric structures. The factors that affect the material stability are a) the inherent thermal stability of the NLO chromophores, b) the chemical stability of the NLO chromophores during the polymer processing conditions, and c) the long-term dipolar alignment stability at high temperatures. Although considerable progress has been made in achieving these properties (4), organic polymeric materials suitable for practical E-O device applications are yet to be developed. This chapter highlights some of our approaches in the optimization of molecular and material nonlinear optical and thermal properties.

Covalent Grafting of Polydimethylsiloxane over Surface-Modified Alumina Nanoparticles

The synthesis of ``smart structured'' conducting polymers and the fabrication of devices using them are important areas of research. However, conducting polymeric materials that are used in devices are susceptible to degradation due to oxygen and moisture. Thus, protection of such devices to ensure long-term stability is always desirable. Polymer nanocomposites are promising materials for the encapsulation of such devices. Therefore, it is important to develop suitable polymer nanocomposites as encapsulation materials to protect such devices. This work presents a technique based on grafting between surface-decorated gamma-alumina nanoparticles and polymer to make nanocomposites that can be used for the encapsulation of devices. Alumina was functionalized with allyltrimethoxysilane and used to conjugate polymer molecules (hydride-terminated polydimethylsiloxane) through a platinum-catalyzed hydrosilylation reaction. Fourier transform infrared spectroscopy, X-ray-photoelectron spectroscopy, and Raman spectroscopy were used to characterize the surface chemistry of the nanoparticles after surface modification. The grafting density of alkene groups on the surface of the modified nanoparticles was calculated using CHN and thermogravimetric analyses. The thermal stability of the composites was also evaluated using thermogravimetric analysis. The nanoindentation technique was used to analyze the mechanical characteristics of the composites. The densities of the composites were evaluated using a density gradient column, and the morphology of the composites was evaluated by scanning electron microscopy. All of our studies reveal that the composites have good thermal stability and mechanical flexibility and, thus, can potentially be used for the encapsulation of organic photovoltaic devices.

Schiff base oxovanadium(IV) complexes of phenanthroline bases showing DNA photocleavage activity at near-IR light and photocytotoxicity

Oxovanadium(IV) complexes VO(L)(B)](ClO4) (1-3) of N-2-pyridylmethylidine-2-hydroxyphenylamine (HL) Schiff base and phenanthroline bases (B), viz. 1,10-phenanthroline (phen in 1), dipyrido3,2-d: 2',3'-f] quinoxaline (dpq in 2) or dipyrido3,2-a: 2',3'-c] phenazine (dppz in 3), were prepared, characterized and their DNA binding property, photo-induced DNA cleavage activity and photocytotoxicity in HeLa cells studied. The crystal structure of 1 shows the presence of a VO2+ moiety in VO2N4 coordination geometry. The complexes show a d-d band at similar to 830 nm in DMF. The complexes display an oxidative V(V)-V(IV) response near 0.5 V versus SCE and a reductive V(IV)/V(III) response near -0.65 V in DMF -0.1 M TBAP. The complexes that are avid binders to CT DNA giving K-b values within 7.1 x 10(4) to 3.2 x 10(5) M-1, do not show any significant chemical nuclease activity in presence of 3-mercaptopropionic acid or glutathione. The dpq and dppz complexes are photocleavers of pUC19 DNA in UV-A light of 365 nm forming both O-1(2) and (OH)-O-center dot radicals and in near-IR light of 785 nm forming (OH)-O-center dot radicals. The dppz complex exhibits photocytotoxicity in visible light in HeLa cells (IC50 = 6.8 mu M). Flow-cytometric study on this complex shows a high sub-G1 phase in light compared to dark indicating PDT effect. (C) 2011 Elsevier B. V. All rights reserved.

Polyethylene stibinite phosphate esters: Novel flame-retardant plasticizers for PVC

A new series of multielement flame-retardant plasticizers containing polyethylene stibinite phosphate esters have been prepared by bulk polymerization from ethylene glycol with various antimony (III) aryloxydichlorides and arylphosphorodichloridates possessing various combinations of substituent [Cl,Br,NO2]. All the polymers are pink-coloured viscous fluids. They were characterized by inherent viscosity, density, IR, H-1, C-13 and P-31 NMR spectroscopy. The thermal behaviour of the polymers was compared by thermogravimetric analysis and correlated with their structures. The flammability studies were carried out by the limiting oxygen index test. The polymers containing P, Sb, N and Pr elements in their backbone show superior thermal-and flame-retardant characteristics than the other polymers. A comparative study was carried out with one of the synthesized polymers as a polymeric flame-retardant additive to plasticized PVC. The results showed improved LOI and mechanical properties to that of the conventional flame-retardant additive composition. (C) 1997 Elsevier Science Ltd.

Bis{2-4-(methylsulfanyl)phenyl]-1H-benzimidazol-3-ium} tetrabromidocuprate(II) dihydrate

The asymmetric unit of the title compound, (C14H13N2S)(2)CuBr4]center dot 2H(2)O, contains two cations, one anion and two solvent water molecules that are connected via O-H center dot center dot center dot Br, N-H center dot center dot center dot Br and N-H center dot center dot center dot O hydrogen bonds into a two-dimensional polymeric structure. The cations are arranged in a head-to-tail fashion and form stacks along 100]. The central Cu-II atom of the anion is in a distorted tetrahedral environment.

Synthesis and characterization of epoxy resins based on N,N'-aliphatic dicarboxyl bis(hydrazones)

A new class of epoxy resins having N-N bonds in their structure has been synthesized by reacting N,N'-aliphatic dicarboxyl bis(hydrazones) (the aldehyde/ketone derivatives of malonic, adipic, and sebacic dihydrazides) with epichlorohydrin. The reactivity of the[GRAPHICS] protons as a function of the substituent group and the number of methylene spacer groups present in the hydrazone has been examined. The resins obtained have been characterized by elemental and epoxy equivalent analyses and IR and NMR spectra. All these resins are found to have adequate viscosity and cure easily with amine curatives at elevated temperatures. Relevant properties for their use as binders in propellant formulations, such as thermal stability, heat of combustion, density, temperature dependence of viscosity, and mechanical strength of the composites, have been evaluated. (C) 1997 John Wiley & Sons, Inc.

Transient absorption studies on 3,6-dibromo polyvinylcarbazole and its model compounds

3,6-Dibromo-N-ethylcarbazole (DBNEC) and its polymeric analogue poly-3,6-dibromovinylcarbazole (PDBVCz) were studied by transient absorption spectroscopy. The transient absorption spectrum of the 3,6-dibromo-N-ethylcarbazole radical cation and decay rate constants of radical cations of 3,6-dibromo-N-ethylcarbazole and its polymeric analogue are presented. In the case of unsubstituted carbazole, the ratio of the yield of radical cation of monomer to polymer is 2.0, whereas in the case of PDBVCz, under the same experimental conditions, the yield of the radical cation is an order of magnitude less in comparison with the monomer model compound DBNEC. This drastic difference in yield has been correlated to the difference in the conformational structure of the polymer as evidenced by nuclear magnetic resonance spectroscopy. (C) 1997 Elsevier Science S.A.

A perspective of biological supramolecular electron transfer

Electron transfer is an essential activity in biological systems. The migrating electron originates from water-oxygen in photosynthesis and reverts to dioxygen in respiration. In this cycle two metal porphyrin complexes possessing circular conjugated system and macrocyclic pi-clouds, chlorophyll and hems, play a decisive role in mobilising electrons for travel over biological structures as extraneous electrons. Transport of electrons within proteins (as in cytochromes) and within DNA (during oxidative damage and repair) is known to occur. Initial evaluations did not favour formation of semiconducting pathways of delocalized electrons of the peptide bonds in proteins and of the bases in nucleic acids. Direct measurement of conductivity of bulk material and quantum chemical calculations of their polymeric structures also did not support electron transfer in both proteins and nucleic acids. New experimental approaches have revived interest in the process of charge transfer through DNA duplex. The fluorescence on photoexcitation of Ru-complex was found to be quenched by Rh-complex, when both were tethered to DNA and intercalated in the base stack. Similar experiments showed that damage to G-bases and repair of T-T dimers in DNA can occur by possible long range electron transfer through the base stack. The novelty of this phenomenon prompted the apt name, chemistry at a distance. Based on experiments with ruthenium modified proteins, intramolecular electron transfer in proteins is now proposed to use pathways that include C-C sigma-bonds and surprisingly hydrogen bonds which remained out of favour for a long time. In support of this, some experimental evidence is now available showing that hydrogen bond-bridges facilitate transfer of electrons between metal-porphyrin complexes. By molecular orbital calculations over 20 years ago. we found that "delocalization of an extraneous electron is pronounced when it enters low-lying virtual orbitals of the electronic structures of peptide units linked by hydrogen bonds". This review focuses on supramolecular electron transfer pathways that can emerge on interlinking by hydrogen bonds and metal coordination of some unnoticed structures with pi-clouds in proteins and nucleic acids, potentially useful in catalysis and energy missions.

Probing the limits of steric constraints in the oligomerization of copper(I) complexes: structures of two sterically congested oligomers

Oligomeric copper(I) clusters are formed by the insertion reaction of copper(I) aryloxides into heterocumulenes. The effect of varying the steric demands of the heterocumulene and the aryloxy group on the nuclearity of the oligomers formed has been probed. Reactions with copper(I)2-methoxyphenoxide and copper(I)2-methylphenoxide with PhNCS result in the formation of hexameric complexes hexakis[N-phenylimino(aryloxy)methanethiolato copper(I)] 3 and 4 respectively. Single crystal X-ray data confirmed the structure of 3. Similar insertion reactions of CS2 with the copper(I) aryloxides formed by 2,6-di-tert-butyl-4-methylphenol and 2,6-dimethylphenol result in oligomeric copper(I) complexes 7 and 8 having the (aryloxy)thioxanthate ligand. Complex 7 was confirmed to be a tetramer from single crystal X-ray crystallography. Reactions carried out with 2-mercaptopyrimidine, which has ligating properties similar to N-alkylimino(aryloxy)methanethiolate, result in the formation of an insoluble polymeric complex 11. The fluorescence spectra of oligomeric complexes are helpful in determining their nuclearity. Ir has been shown that a decrease in the steric requirements of either the heterocumulene or aryloxy parts of the ligand can compensate for steric constraints acid facilitate oligomerization. (C) 1999 Elsevier Science Ltd. All rights reserved.

Pronounced hydrogel formation by the self-assembled aggregates of N-alkyl disaccharide amphiphiles

Six disaccharide amphiphiles were synthesized and their hydrogel-forming behavior was extensively studied. These amphiphiles were based on maltose and lactose. Since the gels formed from some of these systems showed the ability to "trap" water molecules upon gelation, these gels were described as "hydrogels". When these gels were heated to similar to 70 degrees C, the samples turned into clear, isotropic fluids, and upon gradual cooling, the hydrogels could be reproduced. Thus these systems were also "thermoreversible". The low molecular mass (MW 565) of the gelators compared to that of a typical polymeric gelator forming substance implies pronounced aggregation of the disaccharide amphiphiles into larger microstructures during gelation. To discern the aggregate textures and morphologies, the specimen hydrogel samples were examined by high-resolution scanning electron microscopy (SEM). A possible reason for the exceptionally high water gelating capacities (>6000 molecules of water per gelator molecule) exhibited by these N-alkyl disaccharide amphiphiles is the presence of large interlamellar spaces into which the water molecules get entrapped due to surface tension. In contrast to their single-chain counterparts, the double-chain lactosyl and maltosylamine amphiphiles upon solubilization in EtOH-H2O afforded hydrogels with reduced mechanical strengths. Interestingly, the corresponding microstructures were found to be quite different from the corresponding hydrogels of their single-chain counterparts. Rheological studies provided further insights into the behavior of these hydrogels. Varying the chain length of the alcohol cosolvent could modulate the gelation capacities, melting temperatures, and the mechanical properties of these hydrogels. To explain the possible reasons of gelation, the results of molecular modeling and energy minimization studies were also included.

Sensing of delaminations in composite laminates using embedded magnetostrictive particle layers

This is an exploratory study to illustrate the feasibility of detecting delamination type of damage in polymeric laminates with one layer of magnetostrictive particles. One such beam encircled with excitation and sensing coils is used for this study. The change in stress gradient of the magnetostrictive layer in the vicinity of delamination shows up as a change in induced voltage in the sensing coil, and therefore provides a means to sense the presence of delamination. Recognizing the constitutive behavior of the Terfenol-D material is highly nonlinear, analytical expressions for the constitutive relations are developed by using curve fitting techniques to the experimental data. Analytical expressions that relate the applied excitation field with the stress and magnetic flux densities induced in the magnetostrictive layer are developed. Numerical methods are used to find the relative change in the induced voltage in the sensing coil due to the presence of delamination. A typical example of unidirectional laminate, with embedded delaminations, is used for the simulation purposes. This exploratory study illustrates that the open-circuit voltage induced in the sensing coil changes significantly (as large of 68 millivolts) with the occurrence of delamination. This feature can be exploited for device off-line inspection techniques and/or linking monitoring procedures for practical applications.