The thermodynamics of deformation for thermoplastic polymers

• Main Author: Adams, Gary William
• Language: ENG
• Published: ScholarWorks@UMass Amherst 1987
• Subjects: Materials science
• Online Access: https://scholarworks.umass.edu/dissertations/AAI8805888

The post-yielding behavior of some common thermoplastics was examined in uniaxial tension to determine if these materials were ideally plastic from a thermodynamic viewpoint. Various polyethylenes, poly(methyl methacrylate) and polycarbonate, polyarylate and polysulfone based on bisphenol A were studied. Thermodynamic measurements were made during deformation using a novel isothermal deformation calorimeter capable of measuring the work and heat of deformation. Thermodynamically ideal plasticity was not observed for any of the polymers examined. The polyethylenes stored approximately 30% of the input work as a latent internal energy change while this value was 40-50% for the amorphous glasses. Differential scanning calorimetry results for the deformed polyethylenes indicated that the heats of transition were less for the drawn samples than for the isotropic samples. This result was primarily due to the stored deformation energy and was not necessarily indicative of a change in crystallinity. The energy stored during drawing was explained using some commonly accepted models for the deformation of polyethylene. Additional experiments were performed to determine the mechanism of deformation energy storage and to ascertain the implications of this stored energy in engineering applications. Relaxations at temperatures much less than the glass transition temperature (T$\sb{\rm g}$) were observed for the drawn amorphous glasses using dynamic mechanical and differential scanning calorimetry measurements. Substantial thermal shrinkage was found in unconstrained drawn glassy samples exposed to thermal cycles never exceeding T$\sb{\rm g}$. Considerable stress buildup was also observed for uniaxially constrained glassy samples cycled at temperatures much less than T$\sb{\rm g}$. The values of these stresses were typically greater than 50% of the yield stress. These thermally induced events were attributed to a partial release of the energy stored during deformation. These experimental observations were explained in terms of a proposed deformation model which involves chain deformation with breakage and re-formation of intermolecular secondary bonds.