Composition dependent structural modulations in transparent poly(vinyl alcohol) hydrogels

Transparent and stable Poly(vinyl alcohol) hydrogels were synthesized from polymer aqueous solution without resorting to a mixed solvent such as dimethyl sulfoxide and water. Contrary to the reported methods involving hydrogen bond induced physical crosslinking by repeated freeze–thawing at −20 °C, the present process demonstrates the gelation taking place at relatively higher temperature, i.e. 0 °C. While maintaining transparency in all the synthesized hydrogels, the present paper reports systematic structural and morphological variations in the hydrogels as a function of polymer concentration.     

Spin-parity measurements in the neutron-rich N ∼ 20 34P and 36S nuclei

Yrast and near-yrast energy levels in the neutron-rich N ∼ 20 nuclei ³⁴P, ³⁶S were populated using transfer/deep-inelastic processes following the ³⁴S + ¹¹⁵In reaction at an incident energy of 140MeV. The use of a multi-clover array has facilitated polarization measurements of the observed γ-rays and necessitated some changes in the previously known level scheme. The observation of the negative-parity levels in these nuclei on the periphery of the “island of inversion” is indicative of the influence of the intruder orbitals on the level structure at low spins. Shell-model calculations indicate that the inclusion of the orbitals from the upper pf shell is important even for the low-lying positive-parity states.     

A novel and unusual method for C-N bond formation in 1-substituted-pyrrolidin-2-ones

A novel 5-C-N bond forming reaction is reported, which involves heating 1-substituted-pyrrolidin-2-ones with cyclic amines in the presence of a base. This reaction provides a convenient method for the synthesis of 1,5-disubstituted-pyrrolidin-2-ones.     

Combined neutron and synchrotron studies of magnetic films

We discuss specular reflectivity and off-specular scattering of neutrons and X-rays from magnetic films. Both these techniques are capable of providing information about the morphology of the chemical and magnetic roughness and the magnetic domain structure. The use of neutrons with polarization analysis enables the spatial distribution of different vector components of the magnetization to be determined, and the use of resonant magnetic X-ray scattering enables magnetization in a compound system to be determined element-selectively. Thus both these methods provide powerful and complementary new probes for studying magnetism at the nanoscopic level in a variety of systems such as those exhibiting exchange bias, giant magnetoresistance, spin injection, etc. We shall illustrate with an example of both techniques applied to an exchange bias system consisting of a single crystal of antiferromagnetic FeF₂ capped with a ferromagnetic Co film, and discuss what has been learned about how exchange bias works in such a system.     

Quantum chromodynamics phase transition in the early Universe and quark nuggets

A first-order quark hadron phase transition in the early Universe may lead to the formation of quark nuggets. The baryon number distribution of these quark nuggets have been calculated and it has been found that there are sizeable number of quark nuggets in the stable sector. The nuggets can clump and form bigger objects in the mass range of 0.0003M⊙ to 0.12M⊙. It has been discussed that these bigger objects can be possible candidates for cold dark matter.     

Effect of colour singletness of quark-gluon plasma in quark-hadron phase transition

Consequences of the constraint of SU(3) colour singletness of quark-gluon plasma are studied. This restriction increases the free energy barrier for the formation of hadronic bubble in supercooled phase and influences significantly the dynamics of the initial stage of quark-hadron phase transition. It also introduces terms dependent on the volume occupied by the plasma in the energy density and the pressure. These modifactions vanish in the limit of an infinite volume. The last stage of the hadronization of the QGP likely to be formed in relativistic heavy ion collisions is necessarily characterized by a decreasing volume containing the quark matter, and thus these corrections become important. The nucleation of plasma droplets at AGS energies is also seen to be strongly affected by the requirement of colour singletness, and the choice of prefactor. Received: 4 December 1997 / Published online: 15 May 1998     

Thermal characterization of lacustrine dolomites from the Sambhar Lake playa,Thar desert,India

The Sambhar Lake playa sediments in Thar desert, western India, host a variety of evaporite minerals. Carbonates form a major proportion of the evaporites in a 23 m deep core and several dolomite-rich horizons are recorded. Our XRD and DTA based investigations show that these dolomites are non-stoichiometric and disordered (proto-dolomites) except at one level (4.08 m) from where a well-ordered dolomite is reported. We also record frequent variation in the characteristics of dolomites with depth in terms of structural ordering, Ca/Mg ratio and Fe content. Dolomites reported form the Sambhar Lake playa are interpreted to be authigenic due to the absence of any carbonate rock in the catchment and seem to be derived from a precursor calcite under (sub-) evaporitic conditions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Mössbauer study on M1 and M2 sites of Fe2+ in natural hypersthene under dynamic crystal field potential approach

⁵⁷Fe Mössbauer spectra of hypersthene, a natural silicate mineral belonging to the orthopyroxene group, have been taken over the temperature range 77–292 K. At temperatures above 77 K, they show asymmetric quadrupole peaks. This asymmetry arises from the overlapping of two quadrupole doublets from Fe²⁺(3d⁶,⁵D) ions in two different sites (M1 and M2). The quadrupole splitting, isomer shift and their temperature dependence are appreciably different for Fe²⁺ ions in the two sites. The Fe²⁺ quadrupole splitting in the M1 site decreases linearly with temperature, which can be explained quite satisfactorily by using a very simple model of the orbit-lattice interaction.     

Ionization of metastable H(2S) atom by electron and positron impact

Summary Triple differential cross-section (TDCS) of hydrogen atom in its metastable (2S) state is studied theoretically by electron and positron impact for coplanar and asymmetric geometry. The final-state wave function in the present model satisfies the asymptotic boundary condition for asymmetric geometry (i.e. fork ₁≫k ₂). In the absence of experiment, present results are compared with other existing theoretical results. Apart from the double-peaked structure of the TDCS as in ground-state ionization, some interesting secondary structures are found, in conformity with other theories for ionization from 2S state.     

Ordering of a lateral crown ether and terminal short POE chains in some symmetrical nematogens by 13 C NMR

A lateral crown ether fragment can be introduced on symmetrical mesogens containing only three aromatic rings. The replacement of the terminal alkoxy chains by chains containing oxyethylene units decreases the melting and clearing temperatures allowing one to obtain nematic compounds near room temperature. These compounds dissolve LiBF 4 salt only to a small extent, but the nematic arrangement, is thereby destroyed. The carbon chemical shift anisotropy and the local C-H bond order parameters were obtained for the nematic phase for the crown ether fragment and the terminal chains. This study indicates that the crown ether average conformation changes insignificantly on decreasing the temperature. The lateral crown ether protrudes markedly from the core with the consequence that the molecular shape is far from rod-like in geometry.