Deviations from thermally activated and from exponential response are typical features of the vitrification phenomenon and previously have been studied using viscoelastic, dielectric, calorimetric, optical, and other techniques. Linear response data from literature on about 70 covalent glass formers, ionic melts, supercooled liquids, amorphous polymers, and glassy crystals are surveyed. Except for orientational glasses and monohydric aliphatic alcohols a distinct but broad correlation of non-Debye behavior with non-Arrhenius relaxations is found. Within the broad trend several groups of materials, distinguished by their respective molecular complexity, can be identified and are shown to exhibit narrow correlations. At a given degree of deviation from Arrhenius behavior externally imposed stresses are relaxed with a departure from exponential behavior which is stronger the more the molecular or atomic subunits of the glassforming material are interconnected with each other.

The viscoelastic properties of amorphous polymers are reviewed with emphasis on the glass to rubber dispersion (often referred to as the transition zone). Deviations from thermorheologieal simplicity (where molecular retardation and relaxation mechanisms have the same temperature dependence) are identified. Most theories and models of polymer chain dynamics do not address or acknowledge thermorheological complexities and correlations, such as that between the temperature dependence and the breadth of viscoelastic and dielectric dispersions of the local segmental motion. Without successful theories of these phenomena the understanding of polymer chain dynamics must be considered incomplete. In this review, old and new experimental data are used to identify the different modes of molecular motions and the domains of their contributions to the time and frequency dependence of the mechanical response of amorphous polymers. The different modes are then shown generally to have their own dependence on temperature. Thus the viscoelastic spectrum, including local segmental motions which dominate the onset of glassy behavior and largely determine the glass temperature, Tg, the glass to rubber softening dispersion, the rubbery plateau and the terminal zone, is thermorheologically complex. A coupling theory, with the physics of intermolecular interactions and cooperativity built into it, describes well the many-body dynamics of densely packed molecular systems such as polymers. The many predictions of the coupling theory are applied to the different viscoeleatic modes to explain the observed anomalous experimental facts and established correlations. The theoretical understanding has been improved to the extent that now a connection can be made between the chemical structure of the monomer and the viscoelastic properties of the polymer.

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synopsisThe shear creep and creep recovery behavior of narrow molecular weight distribution polystyrene samples of low molecular weight, 1.1 X 108, 3.4 X loz, and 1.57 X lo4 are reported as a function of temperature, near and above the glass temperature. Timetemperature equivalence for the total creep compliance is found to be nonapplicable, and in fact the steady-state recoverable compliance, J,, is a strong function of temperature. The time-scale shift factors for the recoverable compliance are analyzed in the light of free volume theory. Viscosity data are presented for samples with molecular weights between 1.1 X lo3 and 6.0 X 106. The temperature dependence of the characteristic time constant q J , can be explained in terms of free volume concepts whereas that of viscosity 7 cannot. Effects of residual molecular weight heterogeneity are demonstrated.

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The viscoelastic behavior of a high molecular weight polyisobutylene has been revisited to investigate the unique high-frequency peak (or shoulder) seen in the loss tangent behavior in the glass to rubber softening dispersion. Creep measurements made in the temperature range -74 to f 2 7 "C were initially reduced to a curve, which, upon transformation from the time to the frequency domains, yielded a loss tangent peak with no hint of any shoulder. The original creep compliance curves were rescrutinized independently, and slight variations in the derivatives of the curves revealed the elusive high-frequency loss peak. Additional dynamic measurements were made to connect the transformed creep data to some of the original Fitzgerald, Grandine, and Ferry data [JAppl.Phys. 1953, 24, 9111. The combined results cover over 9 decades of frequency. This extensive range revealed that the mechanisms contributing to the high-frequency peak have a different temperature dependence than that of those contributing to the main loss peak. Historical BackgroundThe development of methods for studying the viscoelastic behavior of polymers began during the 1940s and were pursued rather intensively into the 1950s. Polyisobutylene, a readily available saturated hydrocarbon with good stability, was the most studied polymer during this time. A problem in the early days was comparing data obtained by different laboratories. It was unclear how results might vary from sample to sample, not due to experimental uncertainties, but because of a dependence on molecular weight and its distribution and on chain architecture. To establish a benchmark for the comparison of results extending over a time and frequency range not available in any single laboratory, Robert Marvin of the US. National Bureau of Standards obtained a 100-lb sample of a high molecular weight (M, = 1.56 x 109 regular production polyisobutylene (PIB) from the Esso Standard Oil Co. Tests were made to check the uniformity of the material, and in 1949 samples were sent to 27 laboratories in several countries. These labs where equipped to cover different ranges of time and frequency, yielding results which covered 15 decades of the time/frequency sca1e.l Times are equivalent to the reciprocal circular frequency w (radh), = 2nv, where v is the frequency in hertz (cycles/s).The majority of the measurements were made in an oscillatory mode, yielding complex dynamic moduli or compliances in the frequency domain. There were also two sets of stress relaxation measurements and one creep study. The results from seven of the laboratories were equal within a 10% spread. These were combined by Marvin to show for the first time how the components of the dynamic moduli and compliances of a high molecular linear polymer varied from the glassy level of response a t high frequencies to the approach to steady-state behavior a t low frequency.At the time it was not completely known how the features of the modulus and compliance curves pre-@ Abstract published in Advance ACS Abstracts, Augus...

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