Investigation of the (PEG)xLiCl system using conductivity, DSC and NMR techniques

Electrolytes based on polyethylene glycol (PEG, mol.wt.8000) and LiCl of compositions, (PEG)(x)LiCl, x=4, 6, 8, 10, 12, 40, 60, where x is the O/Li ratio, were prepared by solution casting from methanol solutions. FTIR studies indicate that the ether oxygens of the polymer chain participate in Li+ ion conduction. The presence of a salt-polymer complex that melts around 190 degrees C was evidenced by DSC measurements for the electrolytes with compositions x<12. The highest conductivity was obtained at the composition x=10 which was attributed to the presence of a mostly amorphous compound. NMR measurements indicated two regions of motional narrowing, one attributable to the glass transition and another to translational diffusion.

Orientational glassy phases of C60 and neopentane. A Monte Carlo study

Based on an isothermal, isobaric simulation the structure and properties of the plastic crystalline phases of C60 and neopentane have been examined. Instantaneous cooling of the plastic crystalline phases of both C60 and neopentane leads to orientational glassy phases. These are accompanied by significant slowing down of reorientational motion. Constant pressure quench experiments on C60 yield a glass transition temperature of around 80 K.

Spectroscopic investigations of phase transitions in complex solids

Spectroscopic methods have provided information of seminal importance in understanding phase transitions in solids. After briefly examining some fundamental concept related to phase transitions, we shall discuss several case studies particularly involving the use of vibrational (IR and Raman) spectroscopy. Examples will include both order-disorder and displacive transitions. Under the former are included transitions in nitrates, ammonium halides, alkylammonium salts, plastic state of C60 and superionic conductors (specially CsHSO4). In addition, we shall discuss some aspects of incommensurate phase transitions, the glass transition and electronic phase transitions. Transitions of phosphonitrilic halide tetramers and alkane dicarboxylic acids are also examined

Interfacial shear rheology of highly confined glassy polymers

We present results on interfacial shear rheology measurements on Langmuir monolayers of two different polymers, poly(vinyl acetate) and poly(methyl methacrylate) as a function of surface concentration and temperature. While for the high glass transition poly(methyl methacrylate) polymer we find a systematic transition from a viscous dominated regime to an elastic dominated regime as surface concentration is increased, monolayers of the low glass transition polymer, poly(vinyl acetate), remain viscous even at very high surface concentrations. We further interpret the results in terms of the soft glassy rheology model of Sollich et al. P. Sollich, F. C. Lequeux, P. Hebraud and M. E. Cates, Phys. Rev. Lett., 1997, 78, 2020-2023] and provide evidence of possible reduction in glass transition temperatures in both poly(methyl methacrylate) and poly(vinyl acetate) monolayers due to finite size effects.

Optical properties change in amorphous (As2S3)(0.87)Sb-0.13 thin films by photo and thermal induced process

Exposure with above band gap light and thermal annealing at a temperature near to glass transition temperature, of thermally evaporated amorphous (As2S3)(0.87)Sb-0.13 thin films of 1 mu m thickness, were found to be accompanied by structural effects, which in turn, lead to changes in the optical properties. The optical properties of thin films induced by illumination and annealing were studied by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and Raman spectroscopy. Photo darkening or photo bleaching was observed in the film depending upon the conditions of the light exposure or annealing. These changes of the optical properties are assigned to the change of homopolar bond densities. (C) 2010 Elsevier B.V. All rights reserved.

A Molecular Dynamics Study of Atomic Correlations in Glassy BzS3t

Glassy B&, the parent compound of the superionic conductor LiI-Li&B& has been studied by the molecular dynamics technique using a new potential model. The results suggest that the glass is made up of local units of four-membered B2S2 rings bridged by sulfur atoms, leading to a chainlike structure. Various pair correlation functions have been analyzed, and the B2Sz rings have been found to be planar. The calculated neutron structure factor shows a peak at 1.4 A-' which has been attributed to B-B correlations at 5.6 A. The glass transition temperature of the simulated system has been calculated to be around 800 K.

Electrical Relaxation and Transport in 0.5Cs(2)O-0.5Li(2)O-3B(2)O(3) Glasses

The frequency and temperature dependence of the dielectric constant and the electrical conductivity of the transparent glasses in the composition 0.5Cs(2)O-0.5Li(2)O-3B(2)O(3) (CLBO) were investigated in the 100 Hz - 10 MHz frequency range. The dielectric constant for the as-quenched glass increased with increasing temperature, exhibiting anomalies in the vicinity of the glass transition and crystallization temperatures. The temperature coefficient of dielectric constant was estimated (35 +/- 2 ppm. K-1) using Havinga's formula. The dielectric loss at 313 K is 0.005 +/- 0.0005 at all the frequencies understudy. The activation energy associated with the electrical relaxation determined from the electric modulus spectra was found to be 1.73 +/- 0.05 eV, close to that of the activation energy obtained for DC conductivity (1.6 +/- 0.06 eV). The frequency dependent electrical conductivity was analyzed using Jonscher's power law. The combination of these dielectric characteristics suggests that these are good candidates for electrical energy storage device applications.

Partial Discharge Resistant Characteristics of Epoxy Nanocomposites

Epoxy nanocomposite samples with a good dispersion of alumina nanoparticles in epoxy matrix were prepared and experiments were performed to measure their partial discharge resistant characteristics. Epoxy alumina nanocomposites with 0.1, 1, 5, 10 and 15 wt% nanofillers were prepared in the laboratory and partial discharge (PD) experiments were conducted at a voltage of 10 kV for different durations using IEC (b) type electrodes. The degradation of the sample surfaces were analyzed using SEM techniques, surface profile studies, FTIR spectroscopy as well as PD studies. An attempt was made to understand the interaction dynamics between the nanoparticle and the epoxy chain by measuring the glass transition temperature of the nanocomposites. The partial discharge resistance obtained for the nanocomposites are compared with those of unfilled epoxy and epoxy microcomposites. It was observed that even with 0.1 wt% of nanofiller added to the epoxy matrix, the partial discharge resistance to degradation gets improved considerably. It was also observed that the inter particle distance has a significant effect on the discharge resistance to degradation. The improvement in the degradation resistance is attributed to the interactions between the nanoparticle and the epoxy chain. A possible mechanism for the surface degradation of nanocomposites has been proposed.

Molecular Dynamics Investigation of Structure and Transport in the K2O-2SiO2 System Using a Partial Charge Based Model Potential

Potassium disilicate glass and melt have been investigated by using a new partial charge based potential model in which nonbridging oxygens are differentiated from bridging oxygens by their charges. The model reproduces the structural data pertaining to the coordination polyhedra around potassium and the various bond angle distributions excellently. The dynamics of the glass has been studied by using space and time correlation functions. It is found that K ions migrate by a diffusive mechanism in the melt and by hops below the glass transition temperature. They are also found to migrate largely through nonbridging oxygenrich sites in the silicate matrix, thus providing support to the predictions of the modified random network model.

Electrical switching studies on Ge-Te-Tl chalcogenide glasses: Effect of thallium on the composition dependence of switching voltages

Bulk Ge(17)Te83_,JI glasses (05x.5_13), have been found to exhibit memory type electrical switching. The switching voltages (also known as threshold voltage V-th) of Ge17Te83-xTlx glasses are found to decrease with increasing thallium content. The rate of decrease of Vtry is greater at lower concentrations and \textbackslashid, falls at a slower rate for higher thallium concentrations (x 6). The addition of thallium to the Ge-Te network fragments the covalent network and introduces ionic nature to it; the reduction in network connectivity leads to the decrease in switching voltages with thallium content. The decrease in the glass transition temperatures of Ge17Te83-xTlx glasses with increasing thallium concentration supports the idea of decrease in network connectivity with TI addition. The more metallic nature of TI also contributes to the observed reduction in the switching voltages of Ge17Te83-xTlx glasses with TI content. Further, there is an interesting correlation seen between the threshold voltage V-th and the average bond energy, as a function of TI content. In addition, the switching voltages of Ge17Te83-xTlx glasses have been found to decrease with sample thickness almost linearly. The set-reset studies indicate that the Ge17Te83-xTl2 sample can be switched for more than 10 cycles, whereas other glasses could not be reset beyond two switching cycles. (C) 2010 Elsevier B.V. All rights reserved.

Molecular dynamics Investigation of structure and transport in the K2O-2SiO2 system using a partial charge based model potential

Potassium disilicate glass and melt have been investigated by using anew partial charge based potential model in which nonbridging oxygens are differentiated from bridging oxygens by their charges. The model reproduces the structural data pertaining to the coordination polyhedra around potassium and the various bond angle distributions excellently. The dynamics of the glass has been studied by using space and time correlation functions. It is found that K ions migrate by a diffusive mechanism in the melt and by hops below the glass transition temperature. They are also found to migrate largely through nonbridging oxygen-rich sites in the silicate matrix, thus providing support to the predictions of the modified random network model.

Brownian dynamics simulation of dense binary colloidal mixtures. II. Translational and bond-orientational order

We report the Brownian dynamics simulation results on the translational and bond-angle-orientational correlations for charged colloidal binary suspensions as the interparticle interactions are increased to form a crystalline (for a volume fraction phi = 0.2) or a glassy (phi = 0.3) state. The translational order is quantified in terms of the two- and four-point density autocorrelation functions whose comparisons show that there is no growing correlation length near the glass transition. The nearest-neighbor orientational order is determined in terms of the quadratic rotational invariant Q(l) and the bond-orientational correlation functions g(l)(t). The l dependence of Q(l) indicates that icosahedral (l = 6) order predominates at the cost of the cubic order (l = 4) near the glass as well as the crystal transition. The density and orientational correlation functions for a supercooled liquid freezing towards a glass fit well to the streched-exponential form exp[-(t/tau)(beta)]. The average relaxation times extracted from the fitted stretched-exponential functions as a function of effective temperatures T* obey the Arrhenius law for liquids freezing to a crystal whereas these obey the Vogel-Tamman-Fulcher law exp[AT(0)*/(T* - T-0*)] for supercooled Liquids tending towards a glassy state. The value of the parameter A suggests that the colloidal suspensions are ''fragile'' glass formers like the organic and molecular liquids.

Influence of poling methods on the orientational dynamics of 2-methyl-4-nitro-aniline in poly(methyl methacrylate) studied by second harmonic generation

The magnitude and stability of the induced dipolar orientation of 2-methyl-4-nitroaniline (MNA)/poly(methyl methacrylate) (PMMA) guest/host system is investigated. The chromophores are aligned using both the corona discharge and contact electrode poling techniques. The magnitude of order parameter (also an indicator for the second order nonlinear susceptibility) is measured by recording absorbances of the poled (by the two different techniques) and unpoled PMMA films at different concentrations of MNA. Under the same conditions the corona poling technique creates a higher alignment of molecules along the field direction. The time dependence of the second harmonic intensity of the MNA/PMMA film prepared by the two techniques can be described by a Kohlrausch-Williams-Watts stretched exponential. The temperature dependence of the decay time constant is found to generally follow a modified Williams-Landel-Ferry (WLF) or Vogel-Tamann-Fulcher (VTF) equation. The glass transition temperature seems to be the single most important parameter for determining the relaxation time tau(T).

Collective Effects on Single Particle Orientational Relaxation in Slow Dipolar Liquids

Theoretical and computer simulation studies of orientational relaxation in dense molecular liquids are presented. The emphasis of the study is to understand the effects of collective orientational relaxation on the single-particle orientational dynamics. The theoretical analysis is based on a recently developed molecular hydrodynamic theory which allows a self-consistent description of both the collective and the single-particle orientational relaxation. The molecular hydrodynamic theory can be used to derive a relation between the memory function for the collective orientational correlation function and the frequency-dependent dielectric function. A novel feature of the present work is the demonstration that this collective memory function is significantly different from the single-particle rotational friction. However, a microscopic expression for the single-particle rotational friction can be derived from the molecular hydrodynamic theory where the collective memory function can be used to obtain the single-particle orientational friction. This procedure allows, us to calculate the single-particle orientational correlation function near the alpha-beta transition in the supercooled liquid. The calculated correlation function shows an interesting bimodal decay below the bifurcation temperature as the glass transition is approached from above. Brownian dynamics simulations have been carried out to check the validity of the above procedure of translating the memory function from the dielectric relaxation data. We have also investigated the following two issues important in understanding the orientational relaxation in slow liquids. First, we present an analysis of the ''orientational caging'' of translational motion. The value of the translational friction is found to be altered significantly by the orientational caging. Second, we address the question of the rank dependence of the dielectric friction using both simulation and the molecular hydrodynamic theory.

Structural dependence of density in high molecular weight esters

The effect of molecular structure on density has been examined in high molecular weight esters (molecular weight 300-900), having varying degrees of branching. Densities were calculated from an empirical equation, which agrees well with the experimental values (error +/-1.5%), irrespective of branching. Since density is related to molecular packing and hence to the molecular rotation, in n-alkanes, the glass transition temperature (T(g)) and density both increase with molecular weight, and hence T(g) is directly related to the density. The esters exhibit a complex behavior. In linear esters the T(g) decreases with molecular weight which is explained from group contribution and molecular interactions. In the +-branched esters, however, T(g) decreases with molecular weight until the molecular weight reaches 600 and increases sharply thereafter. The Y-branched esters show an intermediate behavior. The dependence of T(g) on molecular weight has been explained from the segmental motion.