Molecular dynamics simulation of the glass transition and the deformation behavior of amorphous polymers.

This dissertation examines two of the most important behaviors of amorphous polymers: the glass transition and plastic deformation. These phenomena are examined using atomic-scale molecular dynamics simulations, in which the atomic interactions are described through empirical bonded and non-bonded interatomic potentials representing amorphous polyethylene. Molecular dynamics simulations were performed to elucidate the effects of temperature and thermodynamic constraints on the glass transition. The simulation results are found to be in excellent agreement with experimentally established trends in glass transition behavior. Analyses of these data show that the glass transition is primarily associated with the freezing of the torsional degrees of freedom of the polymer chains and that these degrees of freedom are strongly coupled to the non-bonded potential. This study also indicated the mechanisms by which the non-equilibrium, glassy structure is produced during cooling through the glass transition. A rigorous molecular dynamics method for simulating strain controlled, uniaxial deformation tests at constant temperature and a numerical integration procedure for applying it were developed. Application of this approach to tension and compression tests yielded stress-strain curves, and the density and conformational state dependence on strain, strain rate and temperature. Detailed correlations between the stress-strain curve and density and torsional conformational changes during deformation were established. While the model exhibited nearly linear elastic behavior at small strains (and temperature), torsional conformation changes led to micro-yielding, macro-yielding, strain softening and strain hardening behavior at higher strains. At elevated temperatures, thermal fluctuations play an important role in inducing these conformational changes. The atomic motion/structure changes that occurred during deformation were identified with conformational change events that were spatially localized prior to the onset of macroscopic yield, while the entire structure undergoes important structural modifications during post-yield deformation. The present study clearly shows that the most important feature for understanding both glass transition and deformation behavior is rotational conformation changes, whether induced by the input of mechanical or thermal energy. The glass transition and elastic and viscoplastic results described herein were obtained without the need to resort to any assumptions other than the general form of the atomic interactions.Ph.D.Applied SciencesMaterials scienceMolecular physicsPolymer chemistryPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/130874/2/9811227.pd