Influence of temperature on the structure of liquid water

Molecular dynamics simulations have been carried out for liquid water at 7 different temperatures to understand the nature of hydrogen bonding at molecular level through the investigation of the effects of temperature on the geometry of water molecules. The changes in bond length and bond angle of water molecules from gaseous state to liquid state have been observed, and the change in the bond angle of water molecules in liquid against temperature has been revealed, which has not been seen in literature so far. The analysis of the radial distribution functions and the coordinate numbers shows that, on an average, each water molecule in liquid acts as both receptor and donor, and forms at least two hydrogen bonds with its neigbors. The analysis of the results also indicates that the water molecules form clusters in liquid.     

Influence of macroinitiator’s glass transition temperature on the response times of polymer dispersed liquid crystals

Glass transition temperature (T_g), an important parameter of polymer, was reported to have great influence on the electro-optical properties of polymer dispersed liquid crystals (PDLCs). In this study, macroinitiators with different T_g were synthesised by reversible addition fragmentation chain transfer polymerisation, and used to prepare PDLCs with different T_g block chains. The effect of different T_g of the block chains on response times was investigated. It was found that rise time decreased and decay time increased with the decrease of the block chain’s T_g. We proposed a possible mechanism by which T_g of the block chains influence response times.     

On the continuity of the diffusion behaviour at amorphous polymer–polymer interfaces on both sides of the bulk glass transition temperature

The diffusion behaviour at amorphous polystyrene (PS)–PS interfaces has been investigated over an interval of temperatures (T) from below to above the bulk glass transition temperature (T _g ^bulk) using the Arrhenius and Vogel-Fulcher approaches. No discontinuity in the variation of the logarithm of the diffusion coefficient versus 1/T has been observed when going through the PS T _g ^bulk over a broad interval of T, from T _g ^bulk???50 °C to T _g ^bulk?+?50 °C. The molecular mechanism of interdiffusion has been discussed.     

Twinkling fractal theory of the glass transition: Rate dependence and time–temperature superposition

The twinkling fractal theory (TFT) of the glass transition temperature T_g provides a new method of analyzing rate effects and time–temperature superposition in amorphous materials. The rate dependence of T_g was examined in the light of new experimental and theoretical evidence for the nature of the dynamic heterogeneity near T_g. As T_g is approached from above, dynamic solid fractal clusters begin to form and eventually percolate rigidity at T_g. The percolation cluster is a solid fractal and to the observer, appears to “twinkle” as solid and liquid clusters interchange in dynamic equilibrium with a vibrational density of states g(ω) ∼ ω. The solid-to-liquid twinkling frequencies ω_TF are controlled by the Boltzmann population of intermolecular oscillators in excited energy levels of their anharmonic potential energy functions U(x) such that ω_TF = ω exp −B(T*² − T²)/kT in which T* ≈ 1.2T_g. An oscillator changes from a solid to a liquid when a thermal fluctuation causes it to expand beyond its inflection point in the anharmonic potential. This leads to a continuous solid fraction P_s near T_g given by P_S ≈ 1−[(1 − p_c) T/T_g] where p_c ≈ 1/2 is the rigidity percolation threshold. Since g(ω) is continuous from very low to very high frequencies, the complex twinkling dynamics existing near T_g produces a continuous relaxation spectrum with many different length scales and times associated with the fractal clusters. The twinkling frequencies control the kinetics of T_g such that for a given observation time t when the rate γ > 1/t, only those parts of the twinkling spectrum with ω > γ can contribute to relaxation or percolation upto time t. The most important results in this article are as follows: The TFT describes the rate dependence of T_g, both for DSC thermal heating/cooling rates and DMA frequencies as the classic T_g − lnγ law as T_g(γ) = T_go + (k/2B) ln γ/γ_o in which the constant B = 0.3 cal/mol K². The constant B appears quite universal for the 17 thermoset polymers investigated in this study and 18 linear polymers investigated by others. Many other amorphous metal and ceramic glass materials exhibited the same rate law but required a new B value approximately half that for polymers. The same B = 0.3 value was also used to successfully describe the TTS shift factors using the twinkling fractal frequencies ω_TF = ωexp −B(T*² − T²)/kT, as ln a_T(TFT) = exp B(T_R² − T²)/kT, which gave comparable results with the classical WLF equation, log a_T = [−C₁(T − T_R)]/[C₂ + (T − T_R)]. The advantage of the TFT over the WLF is that C₁ and C₂ are not universal constants and must be determined for every material, whereas the TFT uses one known constant B which appears to be the same for all polymers. The TFT has also been found to describe the strong and fragile nature of the viscosity behavior of liquids and the rate and temperature dependence of the yield stress in polymers. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2578–2590, 2009     

Effect of copper addition on glass transition of silicate–phosphate glasses

Glass transformation effect of mixed SiO₂?CP₂O₅?CK₂O?CMgO?CCaO?CCuO glasses was studied by DSC, XRD, SEM, and Raman spectroscopy methods. The relationship between the parameters characterizing glass transformation effect and an amount of phosphorous and copper forming the glassy structure was discussed. It was shown that an increasing content of phosphorous increased solubility of copper in the structure of the studied glasses which was the result of P?CO?CCu bonds formation. Degree of changes of T _g, ?c _p, and time of relaxation values were higher in glasses with higher content of P₂O₅ and CuO. The observed relations were explained on the basis of the local atomic interactions in the structure of glass.     

Effect of composition and drying method on glass transition temperature,water sorption characteristics and surface morphology of newly designed β-lactoglobulin/retinyl palmitate/disaccharides systems

Epoxy-based nanocomposites containing different concentrations (0–3%) of surface-modified graphene nanosheets (GNS) with 3-aminopropyltriethoxysilane were prepared and their thermal and mechanical properties including dynamic mechanical analysis, tensile strength, hardness, and abrasion tests were evaluated in order to have a database for thermo-mechanical properties of epoxy nanocomposites. The main aim of this study was to understand the optimum percentage of GNS which would perform the best reinforcing influence on mechanical and physical performance of an epoxy nanocomposite. The results explain how applying the analysis of variance (ANOVA) method as a useful tool in optimization of GNS concentration in preparation of high-performance epoxy-based nanocomposites.     

On the scaling law of the evolution of lap-shear strength with healing temperature at amorphous polymer−polymer interfaces and surface glass transition

The evolution of lap-shear strength (σ) with healing temperature T _h at symmetric and asymmetric amorphous polymer−polymer interfaces formed of the samples with vitrified bulk has been investigated. It has been found that the square root of the lap-shear strength behaves with respect to healing temperature as σ ^1/2 ~ T _h both at symmetric and asymmetric interfaces. Basing on this scaling law between σ and T _h, the values of the surface glass transition temperature ( T_g^surface ) \left( {T_{\rm{g}}^{\rm{surface}}} \right) have been estimated for a number of amorphous polymers by the extrapolation of the experimental curves σ ^1/2 ~ T _h for symmetric polymer−polymer interfaces and, in some cases, for asymmetric, both compatible and incompatible, polymer−polymer interfaces, to zero strength. A significant reduction in surface glass transition temperature T_g^surface T_{\rm{g}}^{\rm{surface}} with respect to the glass transition temperature of the polymer bulk ( T_g^bulk ) \left( {T_{\rm{g}}^{\rm{bulk}}} \right) , reported earlier, has been confirmed by the use of the new proposed approach. The quasi-equilibrium surface glass transition temperature T_g^surface T_{\rm{g}}^{\rm{surface}} of amorphous polystyrene (PS) has been predicted in the framework of an Arrhenius approach using the plot “logarithm of healing time − reciprocal surface glass transition temperature T_g^surface￠￠ T_{\rm{g}}^{\rm{surface}}\prime \prime and the activation energy of the surface alpha-relaxation of PS has been calculated.     

Solid water at room temperature?

Previously, we have shown that water adjacent to many hydrophilic substances excludes colloidal and molecular solutes. It was labelled “exclusion zone” (EZ) or “fourth phase” water. A salient feature is its characteristic light absorbance at or near 270 nm. In this study, EZ water formed against three chemically distinct surfaces, Nafion, ghee, and Whatman-5 filter paper was extracted, characterized by UV–Visible absorbance spectroscopy, and solidified either by lyophilizing or evaporation in an oven. The resulting highly stable solid was dissolved in water and confirmed as EZ water by its characteristic absorbance at 270–280 nm. We used mass spectroscopy to verify the absence of ionizable contaminants that could reproduce the characteristic “signature EZ” spectra in the three liquid preparations, or in the solids formed from desiccated EZ water that had been reconstituted in deionized water. Hence, a solid form of EZ water may, indeed, exist at room temperature.     

The influence of the temperature on the liquid–liquid equilibrium of the ternary system 1-pentanol–ethanol–water

Liquid–liquid equilibrium data for the ternary system 1-pentanol–ethanol–water have been determined experimentally at 25, 50, 85 and 95°C. These results have been correlated simultaneously by the uniquac method obtaining two sets of interaction parameters: one of them independent of the temperature and the other with a linear dependence. Both sets of parameters fit the experimental results well.     

Is adhesion between amorphous polymers sensitive to the bulk glass transition?

Two bulk samples of one and the same or of different amorphous polymers were brought into contact and held for a chosen period of time at a constant healing temperature (T) over the interval of T from below the bulk glass transition temperature (T _g ^bulk) by ~50 °C to above T _g ^bulk by ~10 °C. As formed adhesive joints were shear-fractured in tension at room temperature, and lap-shear strength (σ) was measured as a function of T. It has been found that σ develops with T as logσ?~?1/T both at symmetric and asymmetric interfaces of polystyrene, poly (methyl methacrylate) and poly (2,6-dimethyl-1,4-phenylene oxide). This behaviour implies that there is no discontinuity in the evolution of σ when going through T _g ^bulk, and that this process is controlled by one and the same diffusion mechanism both below and above T _g ^bulk. The results obtained indicate that the contact layer of the polymers investigated is in the viscoelastic state at T well below T _g ^bulk and support the concept of a decrease in the T _g of a near-surface layer with respect to T _g ^bulk.     

The gel-like structure of polymer in thin films: an explanation of the thickness dependent glass transition?

The understanding of the origin of the thickness (h) dependent glass transition temperature, T_g(h), reported over the last decade for supported and freely standing thin polymer films, is still unclear. Indeed, the spin-coating process, the interfacial adsorption as well as the freezing-in of non-equilibrated chain conformations and orientations caused by fast solvent evaporation could result in partially disentangled chains which can be depicted as a gel-like structure. The effect of PMMA stereoregularity on the chain conformation and orientation and its persistence length in thin films is discussed. Moreover, striking evolutions of T_g(h) by changing the nature of the solvent support the assumption of a specific organisation of the chains in thin films which can hold over thickness far above Rg.     

Mechanism of the molecular mobility of water in the temperature range 298–359 K

The Raman spectra of liquid water in the region of O-H stretching vibrations were obtained in the temperature range 298–359 K. The Raman spectra were decomposed into the components using the XPSPEAK-4.1 program, and their temperature dependence was evaluated. The number of bifurcate hydrogen bonds and the percentage of rotational conformers containing bifurcate bonds were shown to increase with temperature. The defect mechanism of the molecular mobility of water on the hydrogen bond network in the temperature range 298–359 K was proposed.     

Thermal analysis of cadmium addition on the glass transition and crystallization kinetics of Se–Te–Sn glassy network

Journal of Thermal Analysis and Calorimetry - Calorimetric measurements have been performed in quaternary glassy system Se78−xTe20Sn2Cdx (x = 0, 2, 4, and 6) to study the...     

Isochronal temperature–pressure superpositioning of the α-relaxation in type-A glass formers

Dielectric spectra were obtained at ambient and elevated pressures for three ‘type-A’ glass formers, which exhibit excess intensity on the high frequency side of the structural relaxation peak. The response to pressure of the peak maximum and the excess wing suggests categorization of such glass formers into two groups: associated liquids, in which the α-relaxation and the excess wing have a different pressure dependence, and van der Waals liquids, which at fixed value of the α-relaxation time, conform to temperature–pressure superpositioning. This distinction is believed to arise from the change in the number of intermolecular bonds (non-dispersive interactions) with volume.     

Do theories of the glass transition, in which the structural relaxation time does not define the dispersion of the structural relaxation, need revision?

Upon decreasing temperature or increasing pressure, a noncrystallizing liquid will vitrify; that is, the structural relaxation time, taualpha, becomes so long that the system cannot attain an equilibrium configuration in the available time. Theories, including the well-known free volume and configurational entropy models, explain the glass transition by invoking a single quantity that governs the structural relaxation time. The dispersion of the structural relaxation (i.e., the structural relaxation function) is either not addressed or is derived as a parallel consequence (or afterthought) and thus is independent of taualpha. In these models the time dependence of the relaxation bears no fundamental relationship to the value of taualpha or other dynamic properties. Such approaches appear to be incompatible with a general experimental fact recently discovered in glass-formers: for a given material at a fixed value of taualpha, the dispersion is constant, independent of thermodynamic conditions (T and P); that is, the shape of the alpha-relaxation function depends only on the relaxation time. If derived independently of taualpha, it is an unlikely result that the dispersion of the structural relaxation would be uniquely defined by taualpha.     

Influence of the anisometry of magnetic particles on the isotropic–nematic phase transition

The influence of the shape anisotropy of magnetic particles on the isotropic–nematic phase transition was studied in ferronematics based on the nematic liquid crystal (LC) 4-(trans-4-n-hexylcyclohexyl)-isothiocyanato-benzene (6CHBT). The LC was doped with spherical or rod-like magnetic particles of different size and volume concentrations. The phase transition from isotropic to nematic phase was observed by polarising microscope as well as by capacitance measurements. The influence of the concentration and the shape anisotropy of the magnetic particles on the isotropic–nematic phase transition in LC are demonstrated here. The results are in a good agreement with recent theoretical predictions.     

Infrared spectroscopic studies of hydrogen-bonded complexes of water with oxygen bases—V. The effect of temperature on the OH-stretching bands of water molecules dissolved in some ketones

The infrared spectra, in the OH-stretching region, of H₂O molecules in dilute solution in cyclohexanone and in di-n-propyl ketone have been recorded at a series of temperatures between 15 and 85°C. The observed bands have been resolved into their component absorptions, based on the assignments established earlier, and the effects of temperature on the wavenumbers and their separations, the half-widths and the component intensities have been noted. The results have been interpreted in terms of a shift in the equilibrium between the 1 : 1 and 2 : 1 hydrogen-bonded solvent: water complexes which exist in such solutions.