Crystallization kinetics of linear polyethylene: The maximum in crystal growth rate-temperature dependence

This rapid communication reports a summary of the key findings of crystallization kinetics studies of unfractionated high density (linear) polyethylene at extremely large supercoolings. We report, for the first time, the maximum in crystal growth rate-crystallization temperature data for linear polyethylene, which has been sought by many researchers since the 1950s. The maximum growth rate was found to occur in the range of 70-75 °C with two separate methods. The kinetics studies were performed using a newly developed quench-crystallization technique based on depolarized reflection light microscopy that is capable of achieving enormously higher quench rates than existing methods. Typical onset crystallization temperatures accessed with this technique range from 40 to 90 °C. Bulk growth rates of crystals were obtained as the reciprocal of crystallization half times measured from the change in the depolarized light intensity upon direct crystallization from the melt. Separately, radial growth rates of spherulites were measured over a wide range of supercoolings. Secondary nucleation analysis of the crystal growth rates resulted in single linear fits extending into deep regime III, suggesting no change in mechanism of formation of the crystals at the largest supercoolings. The deeply quenched films, crystallized at temperatures below the maximum, contain non-impinged spherulites, capable of further crystallization.     

Kaluza-Klein Type Robertson Walker Cosmological Model with Dynamical Cosmological Term Λ

In this paper we have analyzed the Kaluza-Klein type Robertson Walker (RW) cosmological models by considering three different forms of variable Λ: , and Λ∼ρ. It is found that, the connecting free parameters of the models with cosmic matter and vacuum energy density parameters are equivalent, in the context of higher dimensional space time. The expression for the look back time, luminosity distance and angular diameter distance are also derived. This work has thus generalized to higher dimensions the well-known results in four dimensional space time. It is found that there may be significant difference in principle at least, from the analogous situation in four dimensional space time.     

Electroluminescence in lead titanate single crystals

Electroluminescence in PbTiO₃ single crystals is studied with variation in applied electric field, frequency (20 Hz to 5 kHz) and temperature. The EL onset depends on the rate at which the dipole switches. Extremely sharp upward rising nature of the pulses of micro second duration suggest that there is a self maintained discharge in the dielectric due to secondaryγ _p mechanism. Frequency dependence of EL suggests that both the secondary mechanisms, viz. theγ _p andγ _i are active after the application of a high field and the critical field at which this occurs decreases with increase in the frequency of the applied voltage. Similarly the onset voltage decreases with increase in frequency. The temperature dependence of EL at the applied frequency of 50 Hz shows that the onset voltage is intimately connected with the coercive field of the crystal and it is minimum at the Curie point. The study suggests that EL occurs in the bulk and there is a breakdown in the dielectric due to an avalanche formation.     

X-ray spectroscopic study of zirconium and molybdenum diselenides

The K absorption spectra of zirconium, molybdenum and selenium in ZrSe₂ and MoSe₂ have been recorded photographically using a Cauchois type bent crystal (mica) spectrograph. The absorption edge shifts are used along with the data for NbSe₂ (Bhide and Bahl 1971J. Phys. Chem. Solids 32 1001) to propose bond schemes for these compounds.     

One-pot synthesis of multicomponent pyrazole-4-carbonitrile derivatives under solvent-free condition by using engineered polyvinyl alcohol catalyst

Heterocyclic chemistry has fascinated the researchers owing to its wide range of applications in various chemical fields. With this perspective, herein we present an environmentally benign procedure for the synthesis of pyrazole and its derivatives through multicomponent reaction by using SPVA as a heterogeneous acid catalyst. The synthesis protocol of SPVA catalyst includes functionalization of polyvinyl alcohol by sulfonic acid groups. The synthesized SPVA catalyst was then subjected to several characterization techniques to confirm its formation and study its physicochemical properties. The SPVA catalyst was then tested for its activity toward a multicomponent reaction of aromatic aldehyde, malononitrile and phenyl hydrazine. The SPVA catalyst with sufficient acidic sites displayed appreciable catalytic performance yielding 89% of the desired pyrazole product under ambient reaction conditions. The SPVA catalyst showed recyclability up to the sixth cycle without considerable loss in its activity. Furthermore, we made an effort to demonstrate the plausible mechanistic pathway for the SPVA-catalyzed pyrazole synthesis reaction. Interestingly, the present synthetic approach could effectively produce pyrazole products with high yields in the absence of base and solvent and in short reaction time making it a green and sustainable process.

Graphic abstract