摘要:

The tensile stress relaxation and creep behavior of thermally quenched and aged polymer glasses have been investigated. Theoretical and experimental results for the effect of residual thermal stresses and strains on the creep and stress relaxation behavior of polymer glasses are presented. Novel optical and mechanical techniques were used to determine if either the residual strains and/or the residual stresses were changing at room temperature in quenched samples of bisphenol‐A polycarbonate. Relaxation of the residual internal stresses at room temperature was observed and this relaxation is proposed to occur by a conversion of the residual elastic strains into anelastic and persistent strains. The effects of these residual strains and stresses on the tensile stress relaxation and creep behavior have been modeled by dividing up the sample intoncontiguous slices. The dimensions of these slices are assumed to be dependent upon the rate of quenching of the slice and upon a mechanical equilibration of the slices. Incorporation of these effects into constitutive equations for tensile stress relaxation and creep yields equations which predict how residual volume, strains, and stresses can alter the mechanical response. Simulations with the model indicate that relaxation of the internal stresses will reduce the long term compliance, while recovery of the internal strains will increase the long term modulus of the glass. It is suggested that similar effects may be observed in other types of mechanical testing and in solvent cast samples.

ISSN:0148-6055    

DOI:10.1122/1.550706

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

The effects of physical aging on the mechanical and thermal behavior of a high performance engineering resin, poly(arylene etherimide) (PEI), were characterized as a function of aging time and temperature using dynamic mechanical spectrometry (dms), room temperature tensile tests, and differential scanning calorimetry (dsc). The aging temperature range investigated extended fromTg−140 °C toTg−20 °C. dms and tensile measurements demonstrated that aging for 1000 and 10 000 min at temperatures as low asTg‐140 °C had a significant influence on the mechanical behavior of PEI. Shift rates (μ), as defined by isothermal increases in the dynamic storage modulus (E’) as a function of frequency, ranged from 0.4 to 3.0 with increasing aging temperature. Dynamic loss modulus measurements (E‘) demonstrated that PEI has a pronounced β relaxation which exhibits a significant decrease in magnitude with aging. Suppression of this relaxation paralleled dramatic changes in the room temperature stress‐elongation behavior of PEI. Increases in yield strength after aging for 10 000 min were as large as 16%, while elongations at break decreased by as much as 64%. While mechanical measurements demonstrated aging effects for all temperatures studied, changes in thermodynamic behavior by dsc, occurred only after aging at temperatures greater thanTg−55 °C.

ISSN:0148-6055    

DOI:10.1122/1.550707

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

Following a quench from above the glass transition temperatureTgto below, it is known that the mechanical response evolves (physical aging) and can be represented by a time‐aging time superposition principle. The value of the aging time shift factorateis known to depend on the magnitude of the applied stress or strain. Here the viscoelastic response of a material undergoing physical aging and subjected to a single large strain ‘‘probe’’ (step deformation followed by a step to zero deformation) followed by small strain probes is analyzed. It was found that a Bernstein, Kearsley, and Zapas (BKZ) constitutive equation which includes an aging clock that depends upon the deformation magnitude describes the response. The results are discussed and related to the apparent ‘‘rejuvenation’’ described by Struik in similar thermomechanical histories.

ISSN:0148-6055    

DOI:10.1122/1.550708

出版商:The Society of Rheology    

年代:1995

数据来源: AIP