Analysis of the nonisothermal and nonlinear viscoelastic properties of amorphous polymers using a thermodynamic equation of state
Abstract
The nonisothermal and nonlinear viscoelastic properties of amorphous polymers have been measured and analyzed using the concept of a material time. The material time t* is a natural extension to nonisothermal and nonlinear deformations of time-temperature superposition, which has been successfully used to describe isothermal linear viscoelastic behavior. Specifically when the state of the polymer varies as a result of deformation or a temperature change, the change in the rate of viscoelastic relaxation is reflected in the material time t*. This study is a critical investigation of the applicability of a material time in analyzing nonisothermal and nonlinear mechanical behavior. The nonisothermal creep compliance of a styrene-butadiene random copolymer and poly(vinyl acetate) were measured, while being heated at a constant rate. The nonisothermal creep compliance was analyzed with a temperature convoluted time t$\sbsp{\rm T}{\*}$ computed from a temperature dependent shift function. When the polymer is annealed above and well below the glass transition region, the nonisothermal and isothermal compliance are the same function of t$\sbsp{\rm T}{\*};$ however, there is a significant difference between the nonisothermal and the isothermal compliance when the polymer is annealed in the transition region. The specific volume of the styrene-butadiene random copolymer was measured for the same thermal histories used in the nonisothermal creep experiments. The equilibrium behavior was well described by the Simha-Somcynsky equation of state. The reduced time t*, which depends upon both the thermal and volumetric history, was calculated using the Adam-Gibbs model, where the configurational entropy in the shift model was calculated from the Simha-Somcynsky equation of state with experimental volume-temperature data. The difference between the isothermal and nonisothermal compliance analyzed with t* is slightly reduced, but a significant discrepancy still exists. A potential resolution to this discrepancy is discussed. A nonlinear creep behavior of poly(vinyl acetate) was measured in the glass transition region. The results were analyzed using the Schapery constitutive equation which contains a stress reduced time. The constitutive equation does not describe the experimental results in the glass transition region, where volume changes that occur during deformation are important.
Degree
Ph.D.
Advisors
Caruthers, Purdue University.
Subject Area
Chemical engineering
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