Study of the effect of thermal history on the structural relaxation and thermoviscoelasticity of amorphous polymers
Abstract
During processing and use polymer components are subjected to a complex thermal and deformation history. Prediction of the polymer's response to process conditions and life-time performance requires constitutive models capable of addressing the coupling between thermal history and deformation. The predictive capabilities of a thermoviscoelastic model developed by Caruthers and coworkers (Lustig et al., 1995) via the rational thermodynamics framework which included a material timescale were investigated. The thermoviscoelastic model described quantitatively the Kovacs (1963) volume recovery data for poly(vinyl) acetate using a single set of model parameters, which is a significant improvement over the existing structural relaxation models. The key difference between the thermoviscoelastic model and other models is that the configurational entropy, which controls the viscoelastic timescale, depends not only on the current temperature and volume but also explicitly depends upon the thermal history, which was found to be critical in predicting the Kovacs' expansion gap. To investigate the coupling between thermal history and viscoelasticity, isothermal and nonisothermal tensile creep experiments were conducted using the epoxy resin Epon 1001F/DDS. The isothermal creep results were carried out to loading times much greater than the aging time during the thermal pre-treatment. Consequently, significant physical aging effects were observed during creep, which resulted in the failure of time-temperature superposition in the glass. Using the same thermal pre-treatment, the nonisothermal experiments consisted of simultaneously heating while measuring the creep compliance. The nonisothermal creep results were analyzed by a temperature-reduced time, which is the traditional method of analyzing nonisothermal viscoelastic data and implicitly assumes the nonisothermal response can be constructed from a sequence of isothermal viscoelastic responses. For annealing temperatures in the T$\sb{\rm g}$ region, a discrepancy of approximately half an order of magnitude between the nonisothermal response and the response predicted by the temperature-reduced time was observed. The thermoviscoelastic model successfully described both the isothermal and nonisothermal creep results by accounting for the effect of thermal expansion on the magnitude of the tensile compliance and the effect of evolution of the shift factor during isothermal annealing and heating respectively on the timescale of the compliance.
Degree
Ph.D.
Advisors
Caruthers, Purdue University.
Subject Area
Chemical engineering|Polymers|Plastics
Off-Campus Purdue Users:
To access this dissertation, please log in to our
proxy server.