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Quantitative calculations of film thickness and friction in elastohydrodynamic lubrication will require that the low-shear viscosity, [mu], be described with .far greater accuracy than it is today. The free volume model has the advantage, over those currently used. of reproducing all of the trends that were known 80 years ago, although not necessarily to experimental accuracy. A scaling parameter, [phi][varies]T[V.sup.[gamma]], based on the repulsive intermolecular potential having exponent -3[gamma] allows the viscosity to be written as a function of temperature, T, and volume, V, only, as [mu] = F([phi]). The appropriate function for lubricants appears to be a Vogel-like form, [mu][varies]exp([B.sub.F[phi][infinity]]/([phi]-[[phi].sub.[infinity]])). Parameters are presented here for seven liquids. When the dynamic crossover is present, two such functions are required. A low molecular weight dimethyl silicone having high compressibility is an exception. Keywords: EHL, elastohydrodynamics, rheology, high pressure, viscosity

Flexible large bandgap dielectric materials exhibiting ultra-fast charging-discharging rates are key components for electrification under extremely high electric fields. A polyoxafluoronorbornene (m-POFNB) with fused five-membered rings separated by alkenes and flexible single bonds as the backbone, rather than conjugated aromatic structure typically for conventional high-temperature polymers, is designed to achieve simultaneously high thermal stability and large bandgap. In addition, an asymmetrically fluorinated aromatic pendant group extended from the fused bicyclic structure of the backbone imparts m-POFNB with enhanced dipolar relaxation and thus high dielectric constant without sacrificing the bandgap. m-POFNB thereby exhibits an unprecedentedly high discharged energy density of 7.44 J/cm³ and high efficiency at 150 °C. This work points to a strategy to break the paradox of mutually exclusive constraints between bandgap, dielectric constant, and thermal stability in the design of all-organic polymer dielectrics for harsh condition electrifications. [ABSTRACT FROM AUTHOR]

The influence of physical aging on the ß relaxation in La60Ni15Al25 bulk metallic glass has been investigated by mechanical spectroscopy. The amplitude of the ß relaxation (ΔG") decreases while its relaxation time ( τß) increases during aging. We find that, as in organic glasses, the changes of ln(τß) and ln(τGmax) are linearly correlated with ln(τ=b-a ln(G"max). This behavior is discussed in term of the asymmetric double-well potential (ADWP) model, with U and Δ the energies characterizing the ADWP. It is suggested that during aging the ratio U / Δ remains approximately constant, with a value close to the coefficient describing the linear correlation between ln(τß) and ln(G"max)(U/Δ ~ a). Moreover, the evolution versus aging time of ΔGmax can be described by a simple stretched exponential equation giving values of Taging consistent with tan(5) measurements during aging. The very similar behavior of the ß relaxation during aging in metallic glasses and organic material strongly suggests a common nature for this relaxation. [ABSTRACT FROM AUTHOR]

The preparation of highly aromatic elastomers from a bisphenol A‐based divinyl‐terminated resin and polymerization with various aromatic silane containing compounds utilizing a room temperature hydrosilylation reaction is demonstrated. The polymers exhibit high thermal and oxidative stability with 5% weight losses around 430 and 350°C and char yields ranging from 35% to 40%. The thermosets maintained their elastomeric properties with good hardness and mechanical properties as measured by elongation measurements. The toughness of the thermosets was not improved with the inclusion of aromatic moieties but the hardness did appear to increase with the addition of more aromatic groups. [ABSTRACT FROM AUTHOR]

The encapsulation of liquid crystals (LCs) within semiflexible, nanoscale media has potential use for developing flexible photodisplays, electronic devices, and sensors. Here, coaxial electrospinning is used to fabricate mats of nanofibers consisting of a polyvinylpyrrolidine polymer sheath and an azobenzene-doped, low-molecular-weight LC core. The addition of the azobenzene chromophore into the LC core allows for the nematic to isotropic phase transition to be initiated photochemically. Fiber morphology was investigated by polarized optical microscopy (POM), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM). Nanofibrous mats were analyzed using differential scanning calorimetry and polarized optical microscopy. Irradiation with UV light triggers the photoisomerization of the azobenzene dopant and the subsequent nematic to isotropic phase transition in the LC core. This was directly observed by the birefringence in the fibrous mats being turned "off" with UV light. Subsequently, the cis–trans isomerization triggered by visible light allows for the reformation of the nematic state and the birefringence being turned "on". Encapsulation of the liquid crystal within the nanofibers results in precise spatial control over the optical properties of the nanofibers, which is used to photochemically generate detailed images patterned into nanofibrous mats. [ABSTRACT FROM AUTHOR]

Structural dynamics and volume were measured as a function of both temperature and pressure for a propylene glycol and its oligomers (PPG), and the results compared with previous data on higher molecular weight polypropylene glycols. PPG is of special interest because the terminal groups form hydrogen bonds; thus, by studying different molecular weights, the manner in which hydrogen bonding influences the dynamics in the supercooled regime can be systematically investigated. The fragility (T[sub g]-normalized temperature dependence) of the dimer and trimer of PPG increases with pressure, similar to results for other H-bonded liquids, but different from van der Waals glass formers. This behavior is believed to be due to the effect of pressure in decreasing the extent of hydrogen bonding. From the combined temperature and volume dependences of the relaxation times, the relative degree to which thermal energy and volume govern the dynamics was quantified. With decreasing molecular weight, the relative contribution of thermal energy to the dynamics was found to strongly increase, reflecting the role of hydrogen bonding. By comparing the ionic conductivity and the dielectric relaxation times, a decoupling between rotational and translational motions was observed. Interestingly, this decoupling was independent of both pressure and molecular weight, indicating that hydrogen bonds have a negligible effect on the phenomenon. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Pressure-volume-temperature measurements in phenylphthalein-dimethylether (PDE) are reported, in combination with recently published dynamic light scattering data obtained under high pressure. We discuss the role that volume and thermal effects play in controlling the dynamics in the vicinity of the glass transition. Although volume is not the unique thermodynamical parameter characterizing the relaxation dynamics in PDE, its contribution to the super-Arrhenius behavior of the relaxation times in the vicinity of the glass transition is remarkable. The contribution of volume to the temperature dependence of the relaxation times has been characterized by means of the ratio of the activation energy at constant volume to the enthalpy of activation at constant pressure, E[sub V]/H[sub P]. We point out that this quantity is correlated with the temperature behavior of the nonexponentiallity parameter, β[sub KWW]. Moreover, the τ(T,P) values were analyzed in terms of an extension of the Adam-Gibbs model. It was shown that this model gives a satisfactory interpretation of the pressure sensitivity of the dynamics (d log[sub 10](τ)/dP)[sub T] in terms of thermodynamic properties of the material. [ABSTRACT FROM AUTHOR]

The physics underlying the dynamics of molecules near their glass temperature, T[sub g], comprises one of the major unsolved problems in condensed matter. Of particular interest are the striking changes in relaxation properties observed at temperatures ∼1.2 times higher than T[sub g]. Herein, we describe experiments in which these changes in dynamics are induced by pressure variations. For several supercooled liquids, the relaxation time associated with the change in dynamics is found to be invariant to both temperature and pressure. That is, the time scale of the molecular motions determines the onset of strong intermolecular cooperativity and the accompanying dynamical changes. While thermodynamic variables such as temperature and pressure influence the crossover in dynamics, the governing variable is the time scale. [ABSTRACT FROM AUTHOR]