X-ray photon correlation spectroscopy is employed to investigate the motion of dilute suspensions of gold nanoparticles in low-molecular-weight polystyrene melts. At high temperatures, the observed motion is diffusive, with a rate that follows a Vogel-Fulcher temperature dependence. Closer to the glass transition temperature Tg, diffusion is superseded by a hyperdiffusive process that first becomes observable near a crossover temperature Tc approximately 1.1Tg and is identified with heterogeneous strain in the melts. Following rapid cooling to temperatures sufficiently below Tc, but still above Tg, the hyperdiffusive dynamics displays a time dependence similar to aging in polymer glasses.

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We have studied, using dynamical Monte Carlo methods, a facilitated kinetic Ising model for structural glasses. We observe stretched-exponential decays (␣ relaxation͒ of the equilibrium spin autocorrelation function at late times and are able to estimate accurately the corresponding relaxation times . These are found to diverge at nonzero temperatures following a Vogel-Fulcher law. In addition, we observe early-time exponential relaxation analogous to ␤ relaxation of glass formers. We also examined the effective thermodynamics of systems quenched below this divergent temperature and subsequently heated. The result is a peak in the specific heat with properties matching those of the putative glass transition. We also find that these peaks, for different heating rates, can be rescaled to lie on a universal curve. Finally, we studied the evolving structure factor of a frozen glass following a rapid jump in temperature. The results are qualitatively the same as those for x-ray studies of heated glasses. We thus demonstrate that this purely dynamical microscopic model can reproduce much of the phenomenology of the glass transition and near-glass relaxation, with no tunable parameters. ͓S1063-651X͑97͒07301-7͔

The extrusion foaming process involves several critical steps, in which the physical foaming agent plays a significant role: plasticization, solubility, nucleation and bubble growth. Although these aspects can be studied by different techniques, a novel method based on ultrasonic sensors has proven to provide valuable information with respect to the thermoplastic foaming process. This technique can be either used off-line as a characterization tool to improve our understanding of the foaming agent characteristics, or it can be installed in-line, on the extrusion line, as a control device. Review of the different applications of this technique will be covered in this paper, with numerous examples given to the mixture of PS with CO2. The degree of plasticization of the polymer as a function of the blowing agent concentration will be addressed first, followed by the detection of the conditions prone to induce nucleation, in terms of pressure, temperature, type of nucleating agents and flow conditions. The evaluation of the kinetics of bubble growth will also be explored.

We introduce a phase-field model to describe the dynamics of a self-sustaining propagating combustion front within a medium of randomly distributed reactants. Numerical simulations of this model show that a flame front exists for reactant concentration c>c*>0, while its vanishing at c* is consistent with mean-field percolation theory. For c>c*, we find that the interface associated with the diffuse combustion zone exhibits kinetic roughening characteristic of the Kardar-Parisi-Zhang equation

The heats of mutarotation of D-glucose, D -X Y~O S~, lactose, maltose, and cellobiose have been determined sing a microcalori~r~eter of the Tian-Calvet type. Part I of this paper describes the experimental work and the results obtained. Discussion of these results is reserved for Part 11.Since the discovery of the phenomenon of mutarotation, much work has been done t o elucidate the kinetics and mechanism of the reaction as well as t o study the stability of the various forms postulated. Some attempt has also been made t o determine the heats of mutarotation of the sugars from the temperature dependence of the equilibrium constants for the mutarotations (I). The only direct measurement of a heat of mutarotation has been performed by S t~~r t e v a n t (2, 3) for D-glucose using an adiabatic calorimeter. I t seemed desirable, however, to have values for other sugars, particularly for those undergoing an opposing first-order mutarotation, in order to make a comparison of the appropriate thermodynamic quantities. MATERIALSThe sugars studied were all of reagent grade purity, and were obtained as almost pure a or p anomers by slow recrystallization from slightly supersaturated aqueous or alcoholic solutions according t o the method of Hudson and Dale (4).T h e "purity", i.e. the percentage of a or P anomer in the "a" or "P" sugar, may be determined polarinletrically using known optical rotations. These experiments were performed a t O°C. Isbell and Pigman ( 5 ) have determined the optical rotations a t 0" C. for both anomers of D-glucose and lactose and for a-D-xyl0se and p-maltose. They have also obtained a value a t this temperature for P-cellobiose (6). T h e optical rotations a t 20" C. for the other anomers required have been obtained by Hudson and Yanovsky ( 7 ) using an indirect method requiring the separation of only one of the anomers of the sugar. Their values have been used t o find the "purity" of the sugars a t O0 C. Since the "purities" of the sugars, save for cellobiose, were nearly loo%, this introduces a negligible uncertainty. In the case of cellobiose, the "purity" was sufficiently far from 100% t o warrant a polarimetric run a t a higher temperature.T h e water used in all experiments was ordinary distilled water, saturated with COz, and of p H approximately 5.5. Since the rate of mutarotation of D-glucose is constant (8) between p H 2 and 7 , buffering of the solution (8, 9) was deemed unnecessary for a determination of the heats of mutarotation. EXPERIMENTALT h e Tian-Calvet nlicrocalorimeter has been described in various p~iblications (10,11,12), but an outline of its more relevant characteristics will be given here. Although arranged differentially, it consists essentially of a cell in which the reaction takes place 'il/lanuscript received Nonenzber 12, 1957.

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