Multiscale Investigation of Mechanical Response and Identification of Molecular Source of Ratcheting in Polycarbonate

Transparent amorphous polymers including PC have been widely used in structural applications, such as impact-resistant aircraft window, helmet and body armor. Their thermo-mechanical characteristics have been of great interest and attracting extensive research for several decades. However, most characterization efforts have been focused on the responses under a limited number of loading conditions, and the molecular mechanism of macroscopic deformation under complex loading conditions, such as cyclic loading, is still unclear. This dissertation studies the mechanical responses (e.g., stress-strain relations) and characteristic features (e.g., equilibrium stress) of PC using MD simulation under complex loading conditions. These MD results not only complement existing experimental observations, give new insights into those thermo-mechanical characteristics of PC that cannot be easily observed in experimental testing, but also shed light on building better constitutive models. Furthermore, the molecular source of ratcheting was identified using MD simulation and MD-to-Continuum tools. This study may contribute to designing better ratcheting-resistant PC by changing its molecular distribution. To achieve these objectives, a physically realistic all-atom MD model, that has the same molecular weight distribution as a widely used PC (LEXAN 9034), was built and then thermally conditioned to produce characteristics of aged and unaged PC samples. This model was validated against a broad range of experimental results under different loading conditions and temperatures. Utilizing this realistic PC model, the stress-strain responses under a series of more complex loading scenarios were evaluated. In addition, the equilibrium stresses from monotonic loading, unloading, and saw-tooth cyclic loading were extracted using MD simulation. Finally, MD-to-Continuum tools were developed to reveal the molecular mechanism of ratcheting for PC systems with different molecular weights and distribution. It is shown that the short molecules in the polydisperse systems play an essential role in producing non-affine volume, which assists in increasing the ratcheting strain. These short molecules may be acting as lubricating agents to accelerate the ratcheting process