摘要:

We review a variety of measurements on model systems in the medium viscosity range which seem consistent with both thermodynamical (entropy vanishing) and dynamical (mode coupling) origins of glassy behavior and then examine behavior near and below Tgto seek relations between liquid fragility and the non‐exponential and non‐linear aspects of liquid relaxation processes. We include the model ionic system Ca(NO3)2–KNO3and analogs, van der Waals systems, and the covalently‐bonded system Ge‐As‐Se in which the relation of liquid properties to the vector percolation concepts of Phillips and Thorpe can be conveniently studied.With some basic phenomenology in the liquid state itself thereby established, we turn attention to longer length‐scale processes occurring in viscous liquid media. Among these will be the kinetics of nucleation of crystals, the freezing of microemulsion droplet sizes during continuous cooling of temperature sensitive microemulsions, and the freezing of chemical reactions during continuous cooling or continuous evaporation of solvent. The latter freezings can occur at temperatures which are far above the solvent glass transition temperature depending on solvent fragility, which may be a consideration in the strategies adopted by nature in preservation of plant and insect integrity in cold and arid climates.Finally we consider the slowing down which occurs in liquids with density maxima like water and SiO2which appear to have, as their low temperature metastable limits, spinodal instabilities (with associated divergences in physical properties) in place of the usual ideal glass transitions. So far little studied for lack of tractable slow systems, these offer a new and challenging arena for relaxation studies.

ISSN:0094-243X    

DOI:10.1063/1.42338

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

This article reviews slow dynamics of relaxation processes of glassy crystals in which the reorientational motion of the constituent molecules is frozen out, keeping the translational periodicity with respect to the centers of molecular mass. The glassy crystals around their glass transitions exhibit relaxational behavior which resembles closely those of glass‐forming liquids. Sub‐glass transitions occasionally appear below and above the primary Tg. In this way, the glassy crystals show universal features characteristic of the glassy liquids in all respects. Glassy crystals are thus promizing model substance for deeper understanding of the glasses in the sense that the translational invariance of the molecular arrangements renders their study more tractable.

ISSN:0094-243X    

DOI:10.1063/1.42441

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Translational diffusion coefficients D of photochromic dye molecules have been measured by forced Rayleigh scattering in glass forming liquids and polymers at temperatures ranging to below the glass transition Tg. In mixtures where the polymer and diluent component have the same Tg, D is investigated over the whole composition range from pure polymer to pure liquid matrices. In the supercooled liquids, the size of the tracers is similar to that of the surrounding liquid molecules. Thus, we mimic self diffusion which is found to become much faster than predicted by the Stokes‐Einstein relation as the temperature is reduced below a characteristic temperature Tc≳Tg. The crossover temperature Tcagrees with the corresponding temperature determined from neutron scattering experiments and by a power law plot of the shear viscosity as suggested by mode coupling theory. Our finding that translational diffusion behaves different from rotational diffusion which follows the Debye relation down to Tgis discussed in terms of cooperative molecular motion.

ISSN:0094-243X    

DOI:10.1063/1.42374

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The liquid‐glass transition in calcium potassium nitrage (CKN), salol, and aqueous LiCl solution was investigated by laser light scattering spectroscopy. Combining data obtained with a Sandercock tandem Fabry Perot interferometer and a Spex tandem Raman spectrometer provided spectra spanning more than four decades in frequency. The data was analyzed to test both qualitative and quantitative predictions of the mode coupling theory of the glass transition, and produced generally excellent agreement. For CKN, we found that the MCT crossover temperature Tcis ∼378K, 45° above Tg.

ISSN:0094-243X    

DOI:10.1063/1.42341

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The dynamic structure factor in the supercooled liquid state just above the glass transition temperature reveals a number of characteristic universal features in the so‐called ‘‘fragile’’ class of glassy systems. A theory independent, unbiased analysis shows that, compared to the behavior expected in the ordinary liquid state, one can observe: (a) a two‐step structural relaxation process in time,the slower of which scaling with the bulk viscosity, (b) a relatively small, oscillatory wavenumber dependence of the relaxation dynamics, and (c) approximate master functions of unconventional shapes for both relaxation steps. These qualitative features are in agreement with predictions of recent mode coupling theories and, in addition, the quantitative behavior is also compatible with the theoretically predicted scaling properties.

ISSN:0094-243X    

DOI:10.1063/1.42382

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Diffusion‐controlled reaction kinetics of a molecular tracer was investigated in the miscible region of binary polymer mixtures. The reaction kinetics does not follow the Kohlrausch‐Williams‐Watts (KWW) mechanism and is controlled by polymer segmental free volumes in the context of classical free‐volume theory. Upon approaching the phase boundary, it was found that the reaction rates were affected by the concentration fluctuations with the wavelengths comparable to the size of the tracer in accord with the results obtained by small‐angle neutron‐scattering experiments.

ISSN:0094-243X    

DOI:10.1063/1.42402

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Recently, the mode‐coupling theory (MCTh) of Go¨tzeet al. has gained much importance in the interpretation of experimental data from glassy systems. Here, we present inelastic and quasi‐elastic neutron scattering experiments viewed under the aspect of this theory. The experiments have been carried out on polybutadiene with random microstructure. A main result emerging from different experiments is the existence of a transition temperatureTcin the range 213...220 K wellabovethe conventionally defined glass‐temperatureTg=181 K. Around this temperature power‐law singularities of the type predicted by MCTh appear in the non‐ergodicity parameter of the glass, the minimum of the susceptibility, and the viscosity.

ISSN:0094-243X    

DOI:10.1063/1.42403

出版商:AIP    

年代:1992

数据来源: AIP

ISSN:0094-243X    

DOI:10.1063/1.42404

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

We report the first calorimetric characterization of isothermal glass transitions on a so called ‘‘fragile’’ liquid and introduce the notion of the entropic relaxation consecutive to annealing effects in the compressed glass at P≳Pg. The piezothermal method involves measurements of the heat of compression of a sample enclosed in a hydrostatic pressure cell under isothermal and reversible conditions. Approximate relaxation times and relaxation functions near the glass transition are reported.

ISSN:0094-243X    

DOI:10.1063/1.42405

出版商:AIP    

年代:1992

数据来源: AIP

ISSN:0094-243X    

DOI:10.1063/1.42406

出版商:AIP    

年代:1992

数据来源: AIP