摘要:

AbstractWe review first recent results concerning the relation between the relaxation of extensive thermodynamic and of viscoelastic functions. In the underlying theory a central role is assigned to a particular excess‐free volume functionh. It was originally introduced in a theory of the equilibrium melt. The time dependence ofhcan then be derived from volume (or enthalpy) recovery data and serves to predict other quantities, such as mean‐square density fluctuations and viscoelastic‐temperature shift factors during the aging process. Next the physical basis and results of two kinetic theories of volume relaxation are summarized. In both approaches theh‐function is employed as an expression of the molecular dynamics in the drive to equilibrium. The first describes a gradual elimination of free‐volume gradients through a diffusion process, characterized by a diffusion parameter, varying with the local free volume in accord with a Doolittle relation. The second, a stochastic theory, derives expressions for the matrix of transition rates between different free‐volume states, the resulting spectrum of retardation times, and the size distribution of free volume in the relaxing system. Satisfactory agreement between observed and predicted volume recovery of poly(vinyl acetate) ensues. In the limit of an infinitesimal temperature jump, both theories yield a Williams‐Watts correlation function as an approximate interpolation expression. The exponent β, however, varies in time with a constant value of approximately 0.60 over a limited

ISSN:0032-3888    

DOI:10.1002/pen.760241402

出版商:Society of Plastics Engineers, Inc.    

年代:1984

数据来源: WILEY

摘要:

AbstractAn overview of a new molecular kinetic theory of glass‐transition phenomena is presented and experimental comparisons of its prediction for a variety of thermal and stress histories reviewed. The theory, which was developed in accordance with the balance of nonequilibrium statistical entropy, is shown to provide a unified interpretation of some recent models. The volume‐relaxation process in amorphous polymers over the glass‐transition region is regarded as the result of the collapse of a series of free volumes having different levels of energies of hole formation. An applied stress is shown to contribute to the variation of the entropy. An activation volume is introduced as a new tensorial extensive variable. The theory is applied to the phenomenon of physical aging in polymer glasses and shown to provide good quantitative agreement with the results of a well‐known experiment on volume recovery of poly(vinyl acetate). This supports the underlying postulate of a fundamental link between the apparent relaxation time and the mean energy of hole formation, the distribution of relaxation times and the free‐volume fractions. In contrast to the prevalent thinking toward free volume theories, an explicit expression betweenTgand stress is presented and reveals thatTgdoes not continue to increase at all pressures but levels off to a semi universal asymptote at very high pressure. The calculated effect of stress rate is found to be in good agreement with dynamic viscoelastic mea

ISSN:0032-3888    

DOI:10.1002/pen.760241403

出版商:Society of Plastics Engineers, Inc.    

年代:1984

数据来源: WILEY

摘要:

AbstractThe response of glass‐forming systems to isobaric three‐step thermal cycles involving cooling, isothermal annealing, and subsequent reheating has been investigated comprehensively using numerous combinations of the experimental and material parameters. The latter include the retardation spectrum or response function and the parameterxwhich determines the relative contributions of temperature and structure to the retardation times. The results show that, on heating, multiparameter systems can display three kinds of peak in the expansion coefficient α or the heat capacityCporiginating from the interactions of the elementary retardation processes with the thermal history of the glass. The conditions under which these peaks occur, their shifts withx, and the experimental variables have been investigated in detail. In particular, it has been shown that for a thoroughly stabilized glass reheated rapidly, the temperature at which the main peak occurs is strongly dependent on the experimental variables and onx, and that this dependence can lead to an estimate of the value ofx. For poorly stabilized glasses reheated slowly, on the other hand, the main peak apparently vanishes and allows an upper peak to appear, which is small and insensitive toxand the experimental variables. Intermediate situations in which the main and the upper peaks occur separately give rise to a range of effects which may become quite complicated as the two peaks approach each other. The occurrence of such multiple peaks, which has usually been overlooked or misinterpreted in the past, is discussed and explained in detail and is compared with some experimental observations reported in the litera

ISSN:0032-3888    

DOI:10.1002/pen.760241404

出版商:Society of Plastics Engineers, Inc.    

年代:1984

数据来源: WILEY

摘要:

AbstractA new model of enthalpy recovery is presented which is based on a microscopic‐relaxation theory. The microscopic basis and manner of construction of the enthalpy‐recovery model are discussed in detail. The qualitative features of the model are illustrated using numerical simulations. In particular, the way in which the effects of physical aging are produced is discussed. The qualitative features observed in differential‐scanning‐calorimetry experiments are reproduced, and a program of extensive simulations is proposed and outlined. The present microscopically based model is also used to suggest interpretations and possible modifications of currently used phenomenological enthalpy recovery

ISSN:0032-3888    

DOI:10.1002/pen.760241405

出版商:Society of Plastics Engineers, Inc.    

年代:1984

数据来源: WILEY

摘要:

AbstractTorsional creep measurements of physical aging in high‐molecular‐weight glassy polystyrene are presented. The data are analyzed using a relaxation model which has been successful in quantitatively describing a wide range of relaxation phenomena. The model predicts the time‐functional dependences of the creep compliance and, upon physical aging, a possible systematic change of the functional dependence as creep is shifted to longer time scales. These features are established by experimental measurements. Good quantitative fits are found using a restricted application of the model in which all but one of the model parameters are held constant with annealing. Relations of the present study to other polymer relaxation phenomena are disc

ISSN:0032-3888    

DOI:10.1002/pen.760241406

出版商:Society of Plastics Engineers, Inc.    

年代:1984

数据来源: WILEY

摘要:

AbstractThe enthalpy changes during structural relaxation or physical aging of a 58ZrF4‐33BaF2‐5LaF3‐4AlF3(ZBLA) glass during annealing well below the glass‐transition temperature have been studied using differential‐scanning calorimetry at several sub‐Tgtemperatures. Substantial relaxation within the course of several hours was detected at temperatures as low as sixty degrees belowTg(585K). The relaxation process is extremely nonlinear and self‐retarding. The time dependence of the enthalpy during the initial stages of annealing was modeled approximately using the Narayanaswamy‐Tool approach. The structural‐relaxation parameters obtained from this fit were used to predict rates of physical aging for rapidly cooled ZBLA glass at temperatures

ISSN:0032-3888    

DOI:10.1002/pen.760241407

出版商:Society of Plastics Engineers, Inc.    

年代:1984

数据来源: WILEY

摘要:

AbstractEnthalpy relaxations in glassy poly(vinyl chloride) following varied pre‐aging treatments and under varied aging conditions have been compared through observations of sub‐Tgendothermal DSC (differential scanning calorimetry) aging peaks. The extent of enthalpy relaxation for a fixed time and temperature of aging is progressively enhanced by the imposition and release of increasing mechanical stress before aging. The same effect is produced by sorption and desorption of increasing amounts of CO2or CH3Cl vapor before aging. In contrast, the continued application of mechanical stress, or the presence of vapor, during the aging period suppresses enthalpy relaxation. The extent of suppression increases with increasing vapor pressure and solubility or increasing stress. These effects are interpreted as consequences of an increase in the enthalpy of the polymer under mechanical or sorptive stress and an enthalpy relaxation following the release of this stress. In addition to these effects on the DSC endotherm, a pronounced exotherm between the aging peak andTgis observed for samples which have undergone shear yielding or orientation either before or during aging. This exotherm may be the result of release of stored strain energy during the DSC s

ISSN:0032-3888    

DOI:10.1002/pen.760241408

出版商:Society of Plastics Engineers, Inc.    

年代:1984

数据来源: WILEY

摘要:

AbstractThe relationship of valence‐coordinate deformation to the temperature dependence of some infrared peak‐absorption frequencies in Poly(vinyl chloride) (PVC) and polystyrene (PS) is stated. A skeletal band and a CH2rocking band in PVC and a ring‐mode band in PS were studied in two kinds of experiments: steady heating and cooling of a quenched (nonequilibrium, glassy) sample through its glass‐transition temperature,Tg, and long‐term annealing of quenched samples belowTg, followed by steady heating and cooling. The results, a slope discontinuity, ΔM, in thev(T) relation atTgand a frequency shift, Δviso, during isothermal annealing belowTg, are analyzed in two theoretical approaches. Interchain and intrachain contributions to the observed frequency shifts are expected to occur with a differing relative significance in different kinds of molecular vibrations, leading to one possible method of distinguishing valence‐coordinate deformation (chain strain) from

ISSN:0032-3888    

DOI:10.1002/pen.760241409

出版商:Society of Plastics Engineers, Inc.    

年代:1984

数据来源: WILEY

摘要:

AbstractCylindrical specimens of poly(methyl methacrylate) (PMMA) were quenched from above the glass transition and subsequently tested in torsion. Torque and normal‐force relaxation responses were recorded simultaneously. The experiments were performed at 40, 60, and 80°C, at increasing aging times and at deformations ranging from γ = 0.0027 to γ = 0.074. It was found at 40 and 60°C, that, contrary to the classical picture of aging, the small‐deformation torque‐relaxation curves could not be superposed by any combination of vertical and horizontal shifts. On the other hand, at 80°C, and in the nonlinear deformation range even at lower temperatures, the relaxation curves are superimposable simply by horizontal shifts, but as γ increases they involve decreasing values of the double‐logarithmic shift rate. At the same deformation level, however, the shift rate for the normal force is significantly higher than is that for the torque over most of the γ rang

ISSN:0032-3888    

DOI:10.1002/pen.760241410

出版商:Society of Plastics Engineers, Inc.    

年代:1984

数据来源: WILEY

摘要:

AbstractChange in yield and deformation associated with physical aging was studied on a series of polycarbonates (bisphenol‐A polycarbonate, polyestercarbonates, and phenolphthalein‐bisphenol‐A copolycarbonates) and a polysulfone. A combination of tensile and biaxial yield measurements, dynamical mechanical analysis, and high‐resolution density measurements were performed. These data indicate that both free volume arguments and molecular entanglement arguments are inadequate for explaining the differences in large‐scale deformation associated with physical aging for the polymers and the superiority of polyestercarbonates over the other resins. The local intramolecular and intermolecular conformation in the glassy state, as predicated by molecular structure, is apparently an important key to understanding physical aging in polycarbonate‐ty

ISSN:0032-3888    

DOI:10.1002/pen.760241411

出版商:Society of Plastics Engineers, Inc.    

年代:1984

数据来源: WILEY