Mechanics of amorphous polymers and polymer nanocomposites during high rate deformation

Abstract

It has been suggested that a polymer's macroscopic mechanical response to a general loading case is governed by its ability to access various primary and secondary molecular mobilities. Specifically, under conditions of high strain rate, restricted secondary molecular motions are thought to bring about enhanced stiffness and strength. In accordance with this theory, an experimental protocol and associated analytical techniques were established to better understand the rate- and temperature-dependent mechanical behavior of two exemplary amorphous polymers, PC and PMMA. The experiments included dynamic mechanical thermal analysis (DMTA), as well as uniaxial compression tests over a wide range of strain rates. In both cases, the polymer exhibited a distinct transition in the rate-dependent yield behavior, under the same temperature/strain rate conditions as the observed viscoelastic 0-transition. Drawing off of previous research in the field of polymer mechanics, a new continuum-level constitutive model framework is proposed to account for the contributions of different molecular motions which become operational in different frequency/rate regimes. This model is shown to capture well the unique rate-dependent yield behavior of PC and PMMA, as well as the compressive stress-strain response under isothermal conditions.

(cont.) Through the rest of the thesis, additional features are integrated into the model to allow for more accurate predictions of mechanical response under high-rate, impact loading. Adiabatic conditions are captured by considering the heat evolved during dissipative plastic deformation. The corresponding temperature rise predicted by the model is corroborated by experimental measurements obtained via infra-red techniques during the split-Hopkinson bar test. In conjunction with the implementation of adiabatic heating, the model's kinematic framework is altered in order to also capture the effects of thermal expansion. Finally, drawing off of existing experimental data in the literature, the implementation of pressure-dependence in the model is revised. In the final portion of this thesis, the generality of the experimental and theoretical methods is explored. The techniques are applied in the study of the rate-dependent mechanical behavior of a variety of polymer-based material systems, including a PC-POSS nanocomposite, homopolymer PVC, a plasticized PVC, and a PC-triptycene co-polymer. In every case, the methods garnered important insight into both macroscopic phenomena and the molecular mechanisms of deformation resistance.