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Free volume theory and nonlinear thermoviscoelasticity
Author(s) -
Losi Giancarlo U.,
Knauss Wolfgang G.
Publication year - 1992
Publication title -
polymer engineering and science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.503
H-Index - 111
eISSN - 1548-2634
pISSN - 0032-3888
DOI - 10.1002/pen.760320806
Subject(s) - glass transition , materials science , viscoelasticity , rheology , volume (thermodynamics) , thermodynamics , isobaric process , polymer , relaxation (psychology) , fragility , thermal expansion , nonlinear system , dynamic mechanical analysis , composite material , psychology , social psychology , physics , quantum mechanics
The rheological behavior of polymers in the neighborhood of the glass transition is investigated in the framework of the free volume theory of nonlinear viscoelastic behavior. Free volume theory as normally applied above the glass transition is modified to account for the effect of the residual volume of vacancies below the glass transition; this modification is accomplished by modeling the changes in the state of the polymer as the sum of viscoelastic changes and a random disturbance deriving from the thermal collisions between molecule segments. The changes in mechanical properties in passing across the glass transition follow from the freezing‐in of relaxation mechanisms and of free volume; the model, which also incorporates a time‐dependent coefficient of thermal expansion under isobaric conditions, does not require additional parameters other than those characterizing the rubbery state. The pressure dependence of the glass transition is found to be in qualitiative agreement with measurements on PVAc, while the ratio of the glassy and rubbery heat capacities is found to coincide with the ratio of the equilibrium bulk compliances in the glassy and rubbery domains.