Effect of temperature and deformation of the relaxation behavior in the glass transition region
Abstract
In order critically to assess the predictive capabilities of a variety of constitutive equations, the two step moderately nonlinear creep data for poly(vinyl acetate) in the glass transition T$\sb{\rm g}$ region were analyzed using: (i) a Modified form of the Boltzmann relationship (Findley and Lai, 1966), (ii) an Eyring activation model by Hasan Boyce (1993), (iii) the purely mechanical Green Rivlin (1957) theory, and the reduced time approaches by (iv) Schapery (1969), (v) Knauss and Emri (1981) and (vi) the thermoviscoelastic model developed by Caruthers and Shay (1990) and Song (1991). The Green Rivlin, Schapery, and Knauss and Emri models could not predict the two step behavior. The Modified Boltzmann and Hasan and Boyce formulation were able quantitatively to represent the two step creep behavior, but required nonlinear deformation data to determine model parameters. On the other hand, the thermoviscoelastic model using parameters obtained from linear viscoelastic experiments was able to predict the single and two step nonlinear behavior. The nonlinear viscoelastic behavior in the T$\sb{\rm g}$ region was studied using diglycidyl ether of bisphenol A epoxy resin cured with diamino diphenyl sulfone 1001F/DDS (T$\sb{\rm g}=127\sp\circ$C). Creep recovery experiments were performed at 118, 123, and 129$\sp\circ$C and the results indicate that stress alters the rate of viscoelastic relaxation. Specifically at temperatures below T$\sb{\rm g}$, the rate of recovery after application of a large stress was greatly retarded. At 129$\sp\circ$C just above T$\sb{\rm g}$, the recovery behavior was qualitatively different with complete recovery occurring rapidly. This highly nonlinear recovery behavior offers a significant challenge for a constitutive model to predict. The Modified Boltzmann and Hasan and Boyce approaches could not quantitatively or qualitatively represent the recovery behavior. The Schapery theory did a reasonable job at representing the behavior below T$\sb{\rm g}$, but above T$\sb{\rm g}$ the predictions were more consistent with a fluid response. The thermoviscoelastic constitutive model was able adequately to predict the recovery behavior below T$\sb{\rm g}$ and quantitatively predicted the above T$\sb{\rm g}$ relaxation responses and additional two step nonlinear deformation results.
Degree
Ph.D.
Advisors
Caruthers, Purdue University.
Subject Area
Chemical engineering
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