The field of viscous liquid and glassy solid dynamics is reviewed by a process of posing the key questions that need to be answered, and then providing the best answers available to the authors and their advisors at this time. The subject is divided into four parts, three of them dealing with behavior in different domains of temperature with respect to the glass transition temperature, Tg , and a fourth dealing with ‘‘short time processes.’’ The first part tackles the high temperature regime T.Tg ,i n which the system is ergodic and the evolution of the viscous liquid toward the condition at Tg is in focus. The second part deals with the regime T;Tg , where the system is nonergodic except for very long annealing times, hence has time-dependent properties ~aging and annealing!. The third part discusses behavior when the system is completely frozen with respect to the primary relaxation process but in which secondary processes, particularly those responsible for ‘‘superionic’’ conductivity, and dopart mobility in amorphous silicon, remain active. In the fourth part we focus on the behavior of the system at the crossover between the low frequency vibrational components of the molecular motion and its high frequency relaxational components, paying particular attention to very recent developments in the short time dielectric response and the high Q mechanical response. © 2000 American Institute of Physics.@S0021-8979~00!02213-1#

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1065&context=mse_pubs

Relaxation in glassforming liquids and amorphous solids

Filled elastomers exhibit a complex dependence of their viscoelastic modulus as a function of both temperature and frequency. Otherwise, recent observations on thin polymer films have shown that their glass transition temperature depends on their thickness. On the basis of these recent results and on a recent model, we propose that the mechanical behavior of the filled elastomer is strongly influenced by a gradient of the glass transition temperature in the vicinity of the particles. This allows us to suggest a specific temperature−frequency superposition law for filled rubbers. This law seems to apply very successfully on two systems with different dispersion qualities, revealing the existence of a glass transition temperature gradient in the vicinity of the particles.

/pdf/evidence-for-the-shift-of-the-glass-transition-near-the-30vgp2y724.pdf

Evidence for the Shift of the Glass Transition near the Particles in Silica-Filled Elastomers

Dynamics of supercooled liquids and glassy solids

The nuclear magnetic resonance (NMR) WEBLAB is described, providing an interactive tool for analyzing NMR spectra of solids and anisotropic liquids to elucidate their molecular dynamics. It is a collection of programs freely available over the Internet at http://www.mpip-mainz.mpg.de/weblab40/that permit the interpretation of one-dimensional NMR spectra in the case of motions occurring on a cone and of two-dimensional exchange NMR data for discrete jumps. The programs are described in detail and analytical formulae for the averaged interaction tensor components that govern the spectra in the fast motional limit are derived in an appendix. To facilitate the validation of the results obtained from the WEBLAB, we present examples for two- and three-site jumps and demonstrate how WEBLAB can be utilized for analyzing more complex motions around more than one axis as met, e.g., in poly(diethysiloxane), where experimental data are available.

The NMR-WEBLAB: An internet approach to NMR lineshape analysis

Relation of gas permeability with structure of aromatic polyimides II

Pulsed deuteron NMR line shapes have been analysed to characterize type and time scale of the phenylene group motion in glassy bisphenol-A polycarbonate. The motional mechanism involvesπ-flips about theC1C4 axis augmented by small angle fulctuations about the same axis, reaching a rms amplitude of ±35‡ at 380 K. The distribution of correlation times for theπ-flips is heterogeneous in nature and can be described either by a log-Gaussian or an asymmetric distribution with a more rapid decay at high correlation times comparable to the Williams-Watts distribution. From both distributions essentailly the same mean activation energy of 37 kJ/mol is obtained, whereas the temperature dependent width of the highly asymmetric distribution is somewhat smaller compared to the log-Gaussian distribution. Time scale and activation energy of theπ-flip motion are correlated to secondary mechanical relaxations. Low molecular mass additives, which suppress the mechanical relaxation, also hinder the phenylene motion for a substantial fraction of phenylene groups. The effect of additives is not only to shift the mean value of the distribution of correlation times to higher values but also to increase drastically the width of the distribution. The results of this work strongly suggest that the secondary mechanical relaxation and the large amplitude motions of the phenylene groups in polycarbonate are related.

Phenylene motion in polycarbonate and polycarbonate/additive mixtures

Two effects are of outstanding importance for the mechanical behaviour of polymer-additive mixtures, (a) the plasticizer effect and (b) an effect, which we shall refer to as the “β-suppression” effect: Additives can suppress the secondary (β-) relaxations of the polymer. The β-suppression effect is studied on a number of polymers and additives (dynamic mechanical and dielectric measurements), the molecular motions involved are characterized (2H-NMR spectroscopy) and the thermal density fluctuations are measured (SAXS experiments). A model to explain the effect in terms of the free volume fluctuations is presented.

M. Wehrle

Papers

Phenylene motion in polycarbonate and polycarbonate/additive mixtures

Mechanical properties, molecular motions and density fluctuations in polymer-additive mixtures