ISSN:0032-3888    

DOI:10.1002/pen.760211402

出版商:Society of Plastics Engineers, Inc.    

年代:1981

数据来源: WILEY

ISSN:0032-3888    

DOI:10.1002/pen.760211403

出版商:Society of Plastics Engineers, Inc.    

年代:1981

数据来源: WILEY

摘要:

AbstractPhenomenological treatments of viscoelasticity and recent models proposed to explain the kinetic aspects of glass transition phenomena are shown to be based on the same underlying physical considerations. This realization suggests the possible unification of these two areas by a single model. This possibility is explored for the simple case of a glass subjected to a single temperature jump followed by a stress relaxation experiment. Presently, sufficient data on any single chemical system to support a critical test of the viability of this model does not exist. Nevertheless, data from several sources on various materials indicate that the qualitative aspects of viscoelastic behavior of glasses at temperatures well belowTgare in accord with the predictions of the unified model.

ISSN:0032-3888    

DOI:10.1002/pen.760211404

出版商:Society of Plastics Engineers, Inc.    

年代:1981

数据来源: WILEY

摘要:

AbstractSamples of melt‐quenched, amorphous poly(ethylene terephthalate), APET, annealed for up to four successive 30‐min intervals at 50°C, were analyzed by Fourier transform IR (FTIR) spectroscopy at room temperature. Results confirm earlier observations that the fraction of the gauche conformation of the ethylene glycol moiety increases upon annealing. This suggests a change toward a more random structure during the concomitant process of volume recovery. Spectra obtained after annealing for times to and beyond the onset of embrittlement, as judged from mechanical response, indicated that several distinct changes in band intensities occur during this time interval. Spectroscopic results for times longer than that required for embrittlement suggest an increase in motions associated with in‐plane vibrations located in and near the aromatic portions of the chains, as well as a decrease in out‐of‐plane vibrations. The overall results suggest that this method has potential for evaluating fast as well as slow recovery

ISSN:0032-3888    

DOI:10.1002/pen.760211405

出版商:Society of Plastics Engineers, Inc.    

年代:1981

数据来源: WILEY

摘要:

AbstractFree volume fraction of the Williams‐Landel‐Ferry(WLF)‐Doolittle type has been applied to the analysis of the nonequilibrium state for the glassy polymers. The free volumefractionis a two parameter variable since it depends on the free volume and the occupied volume. The excess entropy is derived from the mixing of vacant and occupied sites, and for polymers a factor is added which corresponds to disordering the molecular segments. A relationship between the excess entropy and enthalpy is derived. For a given level of fractional free volume, there is a unique rate constant associated with changing that level of the fractional free volume. The reciprocal of this rate constant, which depends on the exponential order of the fractional free volume, can be considered as a time constant for changing the molecular conformational probability, and its value is in the order of the average dielectric relaxation time in the corresponding state. When mechanical deformation is. imposed with a rate which is too fast as compared to this time constant, the deformation without a change in the conformational probability, i.e., the reversible elastic deformation, will ensue. In contrast, a sufficiently slow deformation will be accompanied by a change in fractional free volume and the excess entropy, and the above‐mentioned time constant will change with deformation. Since dilation will tend to shorten this time constant, tensile deformation will result in reducing the modulus accompanied by the increase in entropy. Shear deformation is considered as a mixture of compression and tension, where only tension contributes to a change in entropy, and the net result is the strain softening which can be predicted from the tensile behavior. Uniaxial compression is controlled by the shear behavior, and the excess entropy and the fractional free volume increase, while the occupied volume decreases with strain. A constitutive relationship has been proposed which accounts for the effects of temperature, pressure, strain rates and thermal history on nonlinear viscoelastic behavior of polymeri

ISSN:0032-3888    

DOI:10.1002/pen.760211406

出版商:Society of Plastics Engineers, Inc.    

年代:1981

数据来源: WILEY

摘要:

AbstractVolume relaxation of glassy polymers has been followed by diffusion of simple alkane permeants. The changes in diffusion rates allow direct monitoring of changes in segmental mobilities of the polymers. In general, the diffusion rates decrease linearly with annealing time on double‐logarithmic plots and then level off as equilibrium is reached. The reduction in diffusion rates, however, can be recovered by raising the temperature above the glass transition temperature and then cooling to the same annealing temperature. Such behavior indicates that the predominant molecular process responsible for changes in mechanical properties of glassy polymers that accompany volume relaxation is not “structure formation” but is the loss of segmental mobilities arising from a reduction of the total free volume in the pol

ISSN:0032-3888    

DOI:10.1002/pen.760211407

出版商:Society of Plastics Engineers, Inc.    

年代:1981

数据来源: WILEY

摘要:

AbstractThe conformational and enthalpic changes that occur in poly(vinyl chloride) (PVC) glasses that have been vitrified from the melt under pressure have been examined by Fourier transform infrared spectroscopy and quantitative differential scanning calorimetry. It is shown that these pressures freeze in the high energy states that are characteristic of the vitrification temperature and increase the apparent glass transition temperature of the polymer. In addition, pressures in excess of the vitrification pressure, cause intermolecular effects that can be relaxed out belowTg. Both of these processes create characteristic endothermic and exothermic changes in the apparent heat capacity of the glass that appear over a period of time and are sensitive functions of the glass formation. processes as well as the subsequent annealing history. The endothermic events are interpreted as the stress perturbed volumetric relaxation process while the exotherms are associated with the release of the frozen in stresses.

ISSN:0032-3888    

DOI:10.1002/pen.760211408

出版商:Society of Plastics Engineers, Inc.    

年代:1981

数据来源: WILEY

摘要:

AbstractPoly(methyl methacrylate) (PMMA) microspheres were swollen in methanol vapor, the swelling penetrant was quickly evacuated, and the subsequent relaxation of the polymer under vacuum was monitored by determination of the rate of settling of rapidly measured low pressure pseudoequilibrium CO2sorption isotherms. The decrease in CO2sorption capacity occurred very rapidly at short times, but the final stages of consolidation were protracted. In all cases, the microspheres were under vacuum between isotherm determinations. A single relaxation time is not sufficient to describe the relaxation processes characterized by the time‐dependent sorption measurements. The sorption relaxation curves are similar in shape to volume relaxation curves for glassy polymers following imposition of a large pressure or temperature step change. The observed similarity between the sorption relaxation curves and classical volume relaxation curves is consistent with the notion that the excess sorption capacity introduced by methanol preswelling results from excess unrelaxed volume introduced into the glass by the quickly removed alcohol. As the excess volume relaxes, the excess sorption capacity decreases. One can, therefore, monitor the subtle process of consolidation using CO2as a probe of the excess volume introduced by the swelling perturbation. Subatmospheric pressure CO2sorption isotherms measured between 20 and 40°C for two different diameter microsphere samples (5436 Å and 1453 Å) for pressures up to 700 mm Hg were concave to the pressure axis. Such general isotherm shapes can be described by a two‐term expression consisting of a Henry's law term and a Langmuir term. The Langmuir term, which arises due to unrelaxed volume in the nonequilibrium glass, is responsible for the observed concavity in the sorption isotherm. Annealing the sample, increasing the temperature of the sorption experiment closer to theTgof the PMMA or permitting the preswollen sample to relax tends to reduce the concavity of the isotherm relative to the corresponding case for sorption in the preswollen sample measured at 25°C. This trend is presumably a consequence of the reduction in unrelaxed volume in the glass which attends either annealing, approaching the glass transition temperature of the polymer, or consolidation of excess volume following exposure to a swelling pe

ISSN:0032-3888    

DOI:10.1002/pen.760211409

出版商:Society of Plastics Engineers, Inc.    

年代:1981

数据来源: WILEY

摘要:

AbstractThe water absorbed by poly(vinyl acetate), PVAc, at 23°C was found in two states. The first, which can account for up to 4 weight percent, was bound to the polymer. The second was in a freezable or clustered form. The latter type of water had no effect on PVAc's glass temperature, whereas, the former kind plasticizedTg. In annealing studies, the enthalpic and dielectric response of PVAc when held at a fixed temperature increment, ΔT, belowTg, was observed to be independent of the amount of bound water. The time dependence of the shift in the dielectric relaxation spectrum and the recovery of the enthalpy towards its equilibrium value as PVAc approached its equilibrium glassy state from a lower temperature as compared to a higher temperature was initially slower. This delayed response to expansion was of the order of the polymer's average relaxation time at the lower temperature. A model was proposed to explain this asymmetric behavior based upon changes in the polymer's free volume as well as its occupied volum

ISSN:0032-3888    

DOI:10.1002/pen.760211410

出版商:Society of Plastics Engineers, Inc.    

年代:1981

数据来源: WILEY

摘要:

AbstractThe permeability and diffusion coefficients (PandD) for gases in a biaxially oriented polystyrene film have been found to increase when a specimen is stretched in simple tension and to decrease with time when the strain is held constant. These effects are attributed, respectively, to an increase in free volume with strain and to the continuous volume recovery (densification) at constant strain. The strain dependence, at small strains, ofPandDfor Ar, Kr, N2, CO2, and Xe at 1 atm pressure and 50°C indicates that the size distribution of free‐volume elements is not distorted when a specimen is stretched. At a constant strain of 1.8 percent at 50°C,PandDfor xenon decrease about 13.8 and 11.8 percent, respectively, per decade of time—two to threefold faster than for carbon dioxide. These results and those obtained with argon, whose molecular diameter is significantly smaller than that for xenon, suggest that the larger free‐volume elements decrease in size faster than the smaller ones as volume recovery pro

ISSN:0032-3888    

DOI:10.1002/pen.760211411

出版商:Society of Plastics Engineers, Inc.    

年代:1981

数据来源: WILEY