Donald J. Plazek

Nonexponential relaxations in strong and fragile glass formers

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.

Volume and enthalpy recovery of polystyrene

Abstract Volume and enthalpy recovery measurements were used to study the physical aging behavior of a polystyrene. Isothermal aging at temperatures near Tg was studied with aging times ranging from several minutes to several days. The data are satisfactorily fit using the Tool–Narayanswamy–Moynihan model of structural recovery. The times required to reach equilibrium are the same for the two properties at temperatures where equilibrium was achieved. Extrapolation to lower temperatures using the model indicates that the times required to reach equilibrium may be different for the two properties at these lower temperatures. The recovery data were plotted as the departure from equilibrium and then were shifted horizontally to superpose at long times. The shift factors for volume and enthalpy recovery data agree with one another above 94°C but they diverge below this temperature. The shift factor data deviates from the WLF or VTFH equation for temperatures below Tg. In addition, we find that the normalized rate of approach is the same for volume and enthalpy recovery at 97°C in the limit of a linear temperature jump, but it differs between the two properties for a nonlinear jump even though the times required to reach equilibrium are the same.

Flow, diffusion and crystallization of supercooled liquids: Revisited

Within the last five years, investigators using NMR and forced Raleigh scattering techniques have found that the Stokes–Einstein (S–E) relation breaks down in supercooled liquids. It has been pointed out that the shear viscosity has a significantly stronger temperature dependence than either the self-diffusion coefficient, D(T), or the translational diffusion coefficient of tracer molecules of comparable size (not shape) to the host liquid. These observations confirm our results on trinaphthylbenzene (TNB) and 1,2 diphenylbenzene (OTP), published in a series of papers more than 30 years ago. An analysis of crystal growth rate measurements on these materials demonstrated that the transport-dominated crystal growth rate, G′(T), exhibited a weaker temperature dependence than the shear viscosity, η(T). Where the expression G(T)=f(T)/η(T) is often substituted for the more basic growth rate relationship G(T)=D(T)f(T). We showed that this practice (often used) is invalid. Here, f(T) is a nucleation/growth free e...

1995 BINGHAM MEDAL ADDRESS : OH, THERMORHEOLOGICAL SIMPLICITY, WHEREFORE ART THOU ?

Polyisobutylene (PIB) was the first polymer whose viscoelastic behavior was thoroughly studied. The measurements made worldwide on the PIB sample distributed from the National Bureau of Standards were compiled and analyzed by Bob Marvin. The superposition of the data substantiated the time–temperature reduction process, which was suggested by Herbert Leaderman, first put into practice by Arthur Tobolsky, and given a theoretical foundation by John Ferry. Polymers that behaved in a like manner were called thermorheologically simple by A. J. Staverman and F. Schwarzl. Subsequently the behavior of PIB has been found to be the exception rather than the rule. The various deviations for this simplicity are briefly reviewed and PIB is reexamined.

Physical aging of a polyetherimide: Volume recovery and its comparison to creep and enthalpy measurements

Volume recovery measurements have been used to study the physical aging behavior of a polyetherimide. Isothermal aging temperatures near Tg were studied with aging times ranging up to several days. The volume decreases during physical aging and levels off at equilibrium. For comparison purposes, the data are normalized to yield the departure from equilibrium which varies from unity at very short aging times to zero when equilibrium is reached. As the aging temperature decreases, the normalized curves are shifted to longer times without a significant change in shape. Hence, the data can be reduced by aging time—temperature superposition. The temperature dependence of the shift factors used to reduce the volume recovery data and the times to reach equilibrium for the volume recovery follow the WLF equation and agree within experimental error with the values from enthalpy and creep measurements obtained in previous work. However, the approach to equilibrium for volume appears to differ from that of enthalpy, with volume recovery being faster than the enthalpy recovery at short times.

Couplings between the cooperatively rearranging regions of the Adam–Gibbs theory of relaxations in glass‐forming liquids

The Adam–Gibbs theory of relaxation for glass‐forming liquids is based on an ensemble of independent and equivalent cooperatively rearranging regions (CRR’s). In this work we extend the Adam–Gibbs theory by incorporating the mutual interactions between the CRR’s. This is done by the method of the coupling scheme in which the transition rate for each independent CRR is slowed down by a time‐dependent factor. The new combined theory predicts a stretched exponential relaxation function and modifies the Adam–Gibbs parameters for the temperature dependence of the relaxation time making them functions of the stretched exponent β. The model is applied to analyze specific heat and dielectric data of supercooled liquids. It is able to explain a previously proposed correlation of the increasing fragility of glass forming liquids with the size of 1−β at Tg and the rapidity of the temperature variation of β. The model also explains, in hydrogen bonded liquids, the correlation of the increasing fragility with the size...

Enthalpy recovery, creep and creep–recovery measurements during physical aging of amorphous selenium

The physical aging behavior of amorphous selenium has been investigated using differential scanning calorimetry and conventional and interrupted creep experiments. As a result of physical aging, enthalpy decreases and the creep and recovery curves shift to longer times. The times required to reach equilibrium for enthalpy recovery and creep appear to have different temperature dependences resulting in enthalpy reaching equilibrium before creep at aging temperatures a few degrees below the nominal glass temperature. In the nominal glass transformation range, however, the times required to reach equilibrium are approximately the same. A general picture of aging behavior has emerged from our data on selenium coupled with past work on polyetherimide and polystyrene.

The recoverable compliance of amorphous materials

Abstract The viscosity, η, and the recoverable compliance, J r ( t ), of three super-cooled liquids have been determined in the neighborhood of the glass temperature, T g , and at high temperatures. The liquids were a chlorinated biphenyl, Monsanto Aroclor 1248, bis(m-(m-phenoxy phenoxy) phenyl)ether, and 1,2 diphenyl benzene, which is commonly called ortho terphenyl. Bona fide T g s for these liquids were determined from dilatometric cooling measurements. The recoverable compliances were a linear function of the cube root of the time at short times. This linearity is called Andrade creep. At longer times, steady-state recoverable compliances were achieved. Within the limits of confidence of the T g s, the viscosities and the Andrade coefficients at T g were the same. Other amorphous materials have similar Andrade coefficients, but polymers have differing viscosities at T g .

Some examples of possible descriptions of dynamic properties of polymers by means of the coupling model

Thanks to the research efforts of Prof. John D. Ferry and others over the last several decades, the viscoelastic properties of polymers have been extensively determined. From this accumulated wisdom, polymer viscoelasticity has become a mature field of research. This basic knowledge of polymer viscoelasticity has made it possible to discern the deviations from the apparently established general rules that one of us (DJP) have found, continuing the tradition of exhaustive experimental measurement started by Prof. Ferry. From many experimental studies on polymers carried out in different laboratories, it has also become clear that these viscoelastic anomalies are general and not exceptional features. Therefore, they pose significant problems in the quest of a truly satisfactory understanding of polymer viscoelasticity. The Coupling Model (CM) has been used to rationalize a number of deviations from thermorheological simplicity. In the realm of polymer viscoelastic behavior, we consider first the local segmental motion that is responsible for the glass temperature and show that the CM provides a consistent description in either the modulus or the compliance representation. Next, we elucidate several viscoelastic anomalies which originate from the different viscoelastic mechanisms being thermorheologically complex. Finally, we revisit the original formulation of the terminal relaxation of entangled polymer chains using the CM. The neglect of the lateral nature of the constraints imposed on one chain by other chains in the original formulation leads to failure in explaining the shape of the terminal relaxation, although it is successful in other aspects. A new formulation, which includes the lateral nature of the constraints and its subsequent mitigation when the terminal relaxation is reached, has restored consistency of the prediction with the terminal relaxation of a monodisperse polyisoprene melt probed dielectrically. The results can describe also the experimental data of dilute polyisoprene probes in polybutadiene matrices and in networks.

Reduction of the glass temperature of thin freely standing polymer films caused by the decrease of the coupling parameter in the coupling model

Abstract Dramatic reduction of the glass temperature, T g , in freely standing polymer thin films has been found when the thickness of the films become approximately smaller than the average end-to-end distance of the unperturbed polymer chains. The observation of a large molecular weight dependence of the T g reductions provides evidence for the importance of chain confinement effect on the glass temperature of thin polymer films. We propose that for freely standing polymer thin films of high molecular weights, there are: (a) induced orientations of the polymer chains when their average end-to-end distance becomes comparable to the film thickness and (b) decreases of the cooperative length scale for chain segments near the surfaces. As a consequence, the intermolecular constraints of local segmental motions for molecular units are reduced, particularly those located in the vicinity of the surfaces. The reduction of T g is attributed to an attendant decrease of the coupling parameter in the coupling model. This explanation is consistent with a recent photon correlation spectroscopy (PCS) and quartz crystal microbalance (QCM) measurement of the dynamics of local segmental motions in polymer thin films. Calculation of the T g of the thin film studied using the coupling model gives an order of magnitude estimate of the reduction of T g , which is in agreement with experiment.