Relaxation Phenomena in Vitrifying Polymers and Molecular Liquids

Abstract

Recent experimental results on the dynamics of glass-forming materials, particularly polymers, are surveyed. The focus is on aspects of the behavior that are connected to or correlated with structural relaxation. These results include the invariance to thermodynamic conditions (temperature, pressure, volume) of a number of propertiesbreadth of the relaxation dispersion, number of dynamically correlating molecules, Johari−Goldstein secondary relaxation time, onset of the dynamic crossover, and the product of temperature and specific volume with the latter raised to a material constantprovided the structural relaxation time is maintained constant. Additional salient experimental findings include the correlation of various high-frequency processes, usually measured in the glassy state, with properties of the equilibrium material above T g. These correlations indicate that the glass transition, although conventionally defined by the relaxation time becoming larger than experimental time scales (>100 s), has its beginning many orders of magnitude sooner. Also described herein are effects of spatial confinement on the glass transition; these can be dramatic, yet taken in toto are rather discombobulating. Such generally observed phenomena must be included in a comprehensive theory or model of the glass transition, since properties intimately connected to structural relaxation cannot be derived separately and be expected to exhibit such correlations by coincidence.

Thermodynamic scaling of dynamics in polymer melts: Predictions from the generalized entropy theory

“…Moreover, an explanation of the origins of this * Electronic address: freed@uchicago.edu thermodynamic scaling is then crucial for deepening our understanding of universal characteristics of glass formation. [3][4][5][6]The exponent γ of the thermodynamic scaling also provides a measure of the relative importance of the density and temperature in controlling glassy dynamics. Hence, not surprisingly, a large body of experiments [7][8][9][10][11][12][13] and simulations [14][15][16][17][18][19][20] have probed the nature of thermodynamic scaling of glass-forming liquids since the first observation by Tölle et al for orthoterphenyl.…”

“…[21] The existence of thermodynamic scaling is well established for as diverse materials as van der Waals liquids, polymers, ionic liquids, weakly hydrogen-bonded systems, etc. [4][5][6] Thermodynamic scaling in glass-forming liquids is intriguing for both fundamental and practical reasons. Probing the origin of thermodynamic scaling assists attempts to establish a universal description of the dynamics of glassy materials and to improve significantly the existing models and theories.…”

“…[23][24][25][26][27][28] The isomorph theory indicates that the scaling exponent γ is not constant, but depends on density, [27][28][29][30] and the power-law form ρ γ is only a special case. However, the power-law density scaling is a useful approximation to the isomorph scaling since a number of experiments find a weak dependence of γ on density under certain thermodynamic conditions [4][5][6] and since using a constant value of γ leads to collapse onto a master curve of the dynamics in glass-forming liquids. Furthermore, the Avramov model, which is frequently employed in describing viscosity and relaxation data of glass-forming liquids in the T -P plane, [31] has been explored by Casalini et al [32] to rationalize the thermodynamic scaling and to show that the scaling exponent γ is related to the Grüneisen constant.…”

Comparison of the transient stress–strain response of rubber to its linear dynamic behavior

“…comparison to the DSC values, the T g defined as the temperature at which the inverse of the G 00 peak frequency (¼[2px max ] À1 ) equals 100 s. 6 All the loss tangent peaks in Figures 4-6 show deviations from time-temperature superpositioning. This is a wellknown effect of the increasing contribution of the segmental dynamics, as temperature is decreased.…”

“…5,6 However, material characterizations are usually limited to only one of these conditionsslow straining to the point of failure or rapid displacements of small amplitude. Consequently, the mechanical response for many applications is often predicted by extrapolating linear dynamic data, with the assumption that strain and rate effects are separable, [7][8][9][10][11][12][13][14] and that the time-superposition principle is valid.…”