Glass-forming liquids,anomalous liquids,and polyamorphism in liquids and biopolymers

Summary Glass formation in nature and materials science is reviewed and the recent recognition of polymorphism within the glassy state, polyamorphism, is discussed. The process by which the glassy state originates during the continuous cooling or viscous slowdown process, is examined and the three canonical characteristics of relaxing liquids are correlated through the fragility. The conversion of strong liquids to fragile liquids by pressure-induced coordination number increases is discussed, and then it is shown that for the same type of system it is possible to have the same conversion accomplished via a first-order transition within the liquid state. The systems in which this can happen are of the same type which exhibit polyamorphism, and the whole phenomenology can be accounted for by a recent simple modification of the van der Waals model for tetrahedrally bonded liquids. The concept of complex amorphous systems which can lose a significant number of degrees of freedom through weak first-order transitions is then used to discuss the relation between native and denatured hydrated proteins, since the latter have much in common with plasticized chain polymer systems. Finally, we close the circle by taking a short-time-scale phenomenon given much attention by protein physicists,viz., the onset of an anomaly in the Debye-Waller factor with increasing temperature, and showing that for a wide variety of liquids, including computer-simulated strong and fragile ionic liquids, this phenomenon is closely correlated with the experimental glass transition temperature. This implies that the latter owes its origin to the onset of strong anharmonicity in certain components of the vibrational density of states (evidently related to the boson peak) which then permits the system to gain access to its configurational degrees of freedom. The more anharmonic these vibrational components, the closer to the Kauzmann temperature will commence the exploration of configuration space and, for a given configurational microstate degeneracy, the more fragile the liquid will be. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

The concerted addition of HBr to aryl alkynes; orthogonal pi bond selectivity

In a weakly acidic solution, the addition of HBr to 1-phenylprop-1-yne produces predominantly the anti-Markovnikov product. In this paper, we consider five possible explanations for this behavior and conclude that the concerted addition is occurring on the acetylenic pi bond orthogonal to the extended aromatic pi system. The electronic effect of the distal methyl group and the steric hindrance of the coplanar phenyl ring combine to promote bromide attack at the beta carbon. Attack on this pi bond is insensitive to the electronic effect of meta and para substituents on the ring but is very (sterically) sensitive toward all ortho substituents.     

Enhanced ethanol sensing properties of WO3 modified TiO2 nanorods

Pristine and WO₃ decorated TiO₂ nanorods (NRs) were synthesised to investigate n-n-type heterojunction gas sensing properties. TiO₂ NRs were fabricated via hydrothermal method on fluorine-doped tin oxide coated glass (FTO) substrates. Then, tungsten was sputtered on the TiO₂ NRs and thermally oxidised to obtain WO₃ nanoparticles. The heterostructure was characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX) spectroscopy. Fabricated sensor devices were exposed to VOCs such as toluene, xylene, acetone and ethanol, and humidity at different operation temperatures. Experimental results demonstrated that the heterostructure has better sensor response toward ethanol at 200 °C. Enhanced sensing properties are attributed to the heterojunction formation by decorating TiO₂ NRs with WO₃.     

Glass-transition temperature of water: a simulation study

We report a computer simulation study of the glass transition for water using the extended simple point charge potential. To mimic the difference between standard and hyperquenched glass, we generate glassy configurations with different cooling rates, and we calculate the temperature dependence of the specific heat on heating. The absence of crystallization phenomena allows us, for properly annealed samples, to detect in the specific heat the simultaneous presence of a weak prepeak ("shadow transition") and an intense glass transition peak at higher temperature. Our results support the view-point that the glass transition temperature is higher than the conventionally accepted value 136 K. We also compare our simulation results with the Tool-Narayanaswamy-Moynihan phenomenological model.     

Relaxation dynamics and ionic conductivity in a fragile plastic crystal

We report a thorough characterization of the dielectric relaxation behavior and the ionic conductivity in the plastic-crystalline mixture of 60% succinonitrile and 40% glutaronitrile. The plastic phase can be easily supercooled and the relaxational behavior is investigated by broadband dielectric spectroscopy in the liquid, plastic crystalline, and glassy crystal phases. The α-relaxation found in the spectra is characterized in detail. A well-pronounced secondary and faint indications for a third relaxation process were found. The latter most likely is of Johari-Goldstein type. From the temperature dependence of the α-relaxation time, a fragility parameter of 62 was determined. Thus, together with Freon112, this material stands out among all other plastic crystals by being a relatively fragile glass former. This finding provides strong support for an energy-landscape related explanation of the fragility of glass formers. In addition, unusually strong conductivity contributions were detected in the spectra exhibiting the typical features of ionic charge transport making this material a good basis for solid-state electrolytes.     

Calorimetric studies of the energy landscapes of glassformers by hyperquenching methods

We consider some of the conditions associated with ergodicity-breaking and vitrification, in particular the equivalent, in quench vitrification, of the ωτ=1 condition that is well-known in relaxation spectroscopy. For a given quench rate, Q=dT/dt, strong liquids are trapped at much higher temperatures, relative to T _g, than are fragile liquids. We relate the trapping of the system during quenches to the multidimensional 'energy landscape' by means of which the configurational microstates of the system are defined. To characterize the energy landscape at energy levels that are usually associated with fluid materials, we use differential scanning calorimetry on hyperquenched glasses. This yields not only the excess potential energies of the states trapped-in during quench Q, but also the trap depths. The latter are found to be much smaller, relative to kT _g, for strong liquids than they are for fragile liquids. This revised version was published online in August 2006 with corrections to the Cover Date.     

Generalized exterior boundary-value problems and optimization for the Helmholtz equation

A class of radiation problems is considered for the Helmholtz equation in exterior domains bounded by a smooth surface on which Dirichlet, Neumann, or Robin boundary conditions are imposed. The problem of finding the boundary data which maximizes far field power in a restricted subset of far field directions is formulated as a constrained maximization problem. Existence of an optimal solution in a variety of control domains is established. The particular case when the boundary is circular and the control domain is the unit ball inL ² is treated in detail. An algorithm for constructing the optimal solution is derived and used to obtain explicit numerical results.This work was supported by the US Air Force under Grant No. AFOSR 81-0156. The work was completed while the first author was on leave to the Institut für Numerische und Angewandte Mathematik, Universität Göttingen, Göttingen, BRD.     

Boundary integral equations for the Helmholtz equation: The third boundary value problem

In this paper we study the application of boundary integral equation methods for the solution of the third, or Robin, boundary value problem for the exterior Helmholtz equation. In contrast to earlier work, the boundary value problem is interpreted here in a weak sense which allows data to be specified in L_∞ (?D), ?D being the boundary of the exterior domain which we assume to be Lyapunov of index 1. For this exterior boundary value problem, we employ Green's theorem to derive a pair of boundary integral equations which have a unique simultaneous solution. We then show that this solution yields a solution of the original exterior boundary value problem.