Anomalous aging in two-phase systems: Creep and stress relaxation differences in rubber-toughened epoxies

Andre Lee, Gregory B. McKenna

Research output: Contribution to journalArticlepeer-review

25 Scopus citations


From time-aging time superposition principles, similar to time-temperature superposition, one would expect similar shifting or superposition behaviors for both creep and stress relaxation responses. In particular, for isotropic homogeneous systems, in the linear viscoelastic regime, consideration of superposition in rheology by Markowitz1 or the discussion by Ferry2 from the Kramers-Kronig relation would seem to demand that creep and stress relaxation shift in the same way. Here we report on results from creep and stress relaxation measurements in two-phase, rubber-toughened epoxies that exhibit Boltzman additivity of creep or relaxation behaviors and follow the time-aging time superposition behavior in creep, but not in stress relaxation. While the lack of superposition in stress relaxation is, perhaps, not surprising, the finding that the creep responses at different aging times superimpose while the stress relaxation responses do not, presents an anomalous behavior that has not been previously reported. In addition, our findings show that the stress relaxation responses show short time "softening" upon aging. Possible reasons for the anomalous behaviors are briefly considered.

Original languageEnglish
Pages (from-to)1167-1174
Number of pages8
JournalJournal of Polymer Science, Part B: Polymer Physics
Issue number8
StatePublished - Jun 1997


  • Creep compliance
  • Physical aging
  • Rubber-toughened epoxies
  • Stress relaxation
  • Superposition
  • Two-phase materials


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