Wednesday, 13 October 2004 - 4:45 PM

This presentation is part of : Mechanics and Thermodynamics of Polymers

Modeling the finite strain behavior of poly(ethylene terephthalate) and poly(ethylene terephthalate)-glycol near the glass transition

Rebecca B. Dupaix, The Ohio State University, The Ohio State University, Suite 244, 650 Ackerman Rd, Columbus, OH 43202

Poly(ethylene terephthalate) (PET) is widely used for products such as drawn fibers, stretched films, and soda bottles. Its commercial success is principally due to its ability to crystallize at large strains during warm deformation processing. The imparted crystallinity increases stiffness and strength, improves dimensional stability, and increases density. The crystallization process and the stress-strain behavior above the glass transition depend strongly on temperature, strain rate, and strain state. A robust constitutive model is therefore highly desirable to account for its stress-strain behavior in the processing regime.

To better understand the role of strain-induced crystallization in PET, its behavior is compared to a non-crystallizing copolymer, PETG. A constitutive model of the rate and temperature dependent stress-strain behavior is developed based on experimental data for PETG and is then extended to PET. The model represents the material's resistance to deformation with two parallel resistances: an intermolecular resistance to flow and a resistance due to molecular network interactions. A reptation model accounts for time dependent molecular relaxation. The model incorporates an internal state variable to monitor the evolution of molecular orientation with deformation. Results indicate that the large strain hardening behavior of both materials can only be captured by including a criterion to halt the molecular relaxation process once the network achieves a specific level of orientation. This suggests that the strain hardening observed in PET is due to molecular orientation rather than crystallization. The model successfully captures the temperature, strain rate, and strain state dependence for both polymers in the processing regime.

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