Melting kinetics in polymers

In polymers, it is possible to obtain single chain forming single crystals. It is feasible to melt these crystals by simple consecutive detachment of chain segments from the crystalline substrate and its diffusion into the melt. However, complication in the melting process occurs when the chain in the process of detachment from the surface is shared between different crystals. Experimentally, a clear distinction in different melting processes is observed, by the differences in the activation energies required for the consecutive detachment of chain segments or of segments having topological constraints. The consecutive detachment of free chain segments starts at the melting temperature predicted from the Gibbs-Thomson equation, whereas higher temperature or time is required if the chain has to overcome the constraints.     

Phonon dispersion and specific heat in ω-helical poly-N ∊(p-bromobenzoyl-l-ornithine)

Poly-N ^?(p-bromobenzoyl-l-ornithine) (PBrBO) is one of the few biopolymers existing in the rare ω-helical (fourfold) conformation. It can be obtained from the α form through an irreversible heat treatment. A study of its normal modes and dispersive behavior shows several interesting features such as “crossing over,” “repulsion with character exchange,” and Von Hove type “singularities” which lead to a fuller interpretation of its IR spectra. The existence of alternately different side-chain conformations, which makes the phonon problem too large to solve directly, has been taken into account by considering the vibrations of the PBrBO chains with two independent conformations. Detailed study of skeletal, amide, and side-chain modes is reported, and the results have been used for the calculation of specific heat via density of states.     

Conformation and phonon dispersion in trans-1,4-polybutadiene

Phonon dispersion studies for trans-l,4-polybutadiene (TPBD) are reported. The change in profile of the dispersion curves as a function of the ═CH─CH₂─ dihedral angle φ has been obtained in order to find the geometry of the polymer that gives the best interpretation of the spectra. It is found for φ = 71°. The regions of high density of states obtained from the dispersion curves for an isolated chain have been used to explain the features of the spectra. Some of these, which were earlier attributed to interchain interactions, in fact arise due to intrachain interactions. Prominent features of the dispersion curves are discussed and heat capacity, derived therefrom, is reported. Calculations have also been performed for unsaturated C-deuterated and fully deuterated TPBD samples to check the validity of the force field and assignments.     

Biocatalysis for C–S bond formation: Porcine pancreatic lipase (PPL) catalysed thiolysis/hydrothiolation reactions in sole water

In this paper, a biocatalytic route is described wherein PPL, lipase from porcine pancreas, in conjunction with water on reaction with different thiophenols and styrene oxides undergo thiolysis with C-S bond formation without the use of any metal catalysts, oxidants, bases, additives or organic solvents towards formation of β-hydroxysulfides in good to excellent yields with high regioselectivity at room temperature. Furthermore, PPL also facilitates thiophenols to undergo hydrothiolation with styrenes or phenylacetylenes in sole water and thus forming linear thioethers or vinylsulfides respectively via C–S bond formation. In addition to the straightforward and atom-efficient protocol, a gram-scale synthesis of β-hydroxysulfide and recyclability for three consecutive cycles without decrease in efficiency of PPL make our biocatalytic protocol for constructing C–S bond highly valuable from both environmental and economic viewpoints than traditional chemical practices.     

Development of a certified reference material for lead in noodles powder

Accreditation and Quality Assurance - Food Safety and Standards Authority of India has set maximum permissible level of lead in noodle at 2.5 mg/kg. The standardized lead measurement is of high...     

Heat of fusion and excess thermodynamic functions of organic eutectics

Phase diagrams and heats of fusion of some organic eutectics have been studied. An empirical equation is proposed for the determination of heats of fusion of eutectics. Excess thermodynamic functions such as h^E, S^E and g^E have been calculated. The heats of fusion of various organic eutectics have been compared with values obtained from the mixture law. The results have been explained on the basis of the fact that clusters are formed during melting. The cluster formation tendency is greater in systems in which hydroxyl groups are present.     

Solubilization of 5-methoxy tryptamine molecular probes in CTAB and SDS micelles: a cmc and binding constant study

To understand the change in environmental conditions when a probe is incorporated in micelles, an absorption and fluorescence spectral study on the solubilization behaviour of 5-methoxy tryptamine (5-MT) and N,N-dimethyl-5-methoxy tryptamine (5-NMT) has been made in CTAB and SDS for their neutral and cationic forms. The blue shift in emission wavelength is accompanied by increase in intensity with increasing surfactant concentration for almost all the cases except for the cationic probe in CTAB surfactant. This exceptional behaviour can be attributed to the quenching of emission intensity caused by Br(-) ions. Spectral correlations with solvent polarity parameters indicate relatively less polar binding sites in CTAB and SDS as compared to water. The binding constant and cmc values have been determined for CTAB and SDS using both the neutral and the cationic probes, which are in good agreement with the literature values. Higher value of binding constant and lower polarity of the binding sites justify 5-NMT as a better binding probe as the two methyl groups make the molecule more hydrophobic and drag it to the interior of both the micelles.     

Role of superheated water in the dissolution and perturbation of hydrogen bonding in the crystalline lattice of polyamide 4,6

Here, we demonstrate that water, in the superheated state, is a solvent for polyamide 4,6 (PA4,6) and that the water molecules can strongly influence hydrogen bonding. In the presence of superheated water, the melting temperature of PA4,6 can be suppressed by nearly 100 degrees C. The depression in the melting temperature follows the Flory-Huggins principle. The instantaneous dissolution of the polymer hardly influences the molar mass of the polymer. However, if the polymer is retained in solution above the dissolution temperature for more than 10 min, hydrolysis occurs. These findings suggest that the dissolution of the aliphatic polymer in superheated water is mainly a physical process as opposed to a chemical process. Time resolved X-ray studies show that the dissolution occurs prior to the Brill transition temperature, as reported earlier. Crystals grown from the water solution show a lath-like morphology with interchain and intersheet distances that are similar to the distances obtained for crystals grown from other known solvents. Electron diffraction further confirmed that the crystals grown from superheated water are single crystals, where the chains are perpendicular to the ab-plane. SAXS performed on dried sedimented water grown single crystals showed a lamellar thickness of 6 nm. The lamellar thickness is in accordance with other reported studies on PA4,6, confirming that the single crystals incorporate four repeat units between re-entrant folds with an amide group incorporated in the tight fold. Solid state NMR studies performed on mats of these single crystals showed two different mobilities of the proton associated with the amide groups: a higher mobility linked to the amide protons in the fold and a reduced mobility of the hydrogen bonded amide protons within the crystal. Additionally, the solid state NMR studies on the dried water crystallized single crystals show the presence of water molecule(s) in the vicinity of the amide groups. This was confirmed by infrared studies that conclusively demonstrated the appearance of two new bands arising due to the binding of a water molecule in the vicinity of the amide group (i.e., NH3(+) and COO(-) bands that disappear upon heating at approximately 200 degrees C). Additionally, DSC traces of the water crystallized PA4,6 show an exothermic event in the same temperature region (i.e., in the vicinity of the Brill transition temperature, where the bound water exits from the lattice). Furthermore, this event was corroborated by TGA data.     

Biosorption of lead(II) from aqueous solutions by non-living algal biomass Oedogonium sp. and Nostoc sp.--a comparative study

Industrial wastewaters containing heavy metals pose a major environmental problem that needs to be remedied. The present study reports the ability of two non-living (dried) fresh water algae, Oedogonium sp. and Nostoc sp. to remove lead(II) from aqueous solutions in batch system under varying range of pH (2.99-7.04), contact time (5-300 min), biosorbent dose (0.1-0.8 g/L), and initial metal ion concentrations (100 and 200mg/L). The optimum conditions for lead biosorption are almost same for the two algal biomass Oedogonium sp. and Nostoc sp. (pH 5.0, contact time 90 and 70 min, biosorbent dose 0.5 g/L and initial Pb(II) concentration 200mg/L) however, the biomass of Oedogonium sp. was found to be more suitable than Nostoc sp. for the development of an efficient biosorbent for the removal of lead(II) from aqueous solutions, as it showed higher values of q(e) adsorption capacity (145.0mg/g for Oedogonium sp. and 93.5mg/g for Nostoc sp.). The equilibrium data fitted well in the Langmuir isotherms than the Freundlich isotherm, thus proving monolayer adsorption of lead on both the algal biomass. Analysis of data shows that the process involves second-order kinetics and thermodynamic treatment of equilibrium data shows endothermic nature of the adsorption process. The spectrum of FTIR confirms that the amino and carboxyl groups on the surface of algal biomass were the main adsorption sites for lead removal. Both the biosorbents could be regenerated using 0.1 mol/L HCl solution, with upto 90% recovery. The biosorbents were reused in five biosorption-desorption cycles without a significant loss in biosorption capacity. Thus, this study demonstrated that both the algal biomass could be used as an efficient biosorbents for the treatment of lead(II) bearing wastewater streams.