| The extraordinary mechanical properties of ethylene/methacrylic acid (E/MAA) ionomers are a result of their fascinating and rich nanometer-scale morphology, which includes polyethylene crystals, amorphous polymer segments and ionic aggregates. The goal of this dissertation is to provide a framework for understanding the unique contribution of each structural motif to key nonlinear mechanical properties such as the yield stress (sigma y), ultimate stress/strain and toughness. We investigate these properties through a combination of complementary experimental techniques, including mechanical testing, calorimetry and X-ray scattering.;We begin by developing a model that relates polyethylene crystal thickness and incomplete amorphous phase relaxation mechanisms to sigma y, measured at various temperatures and strain rates. Taking advantage of the inverse correspondence between temperature and rate on the yield stress response, we devise an algorithm for the creation of sigma y master curves, thus expanding our predictive capabilities far beyond the typical experimentally accessible rate window.;Comparison of the master curves for the E/MAA copolymers shows that sigma y is highly dependent upon the glass transition temperature (Tg), which increases with MAA content. For high MAA-content copolymers, Tg can exceed room temperature. The resulting vitrification of the amorphous phase results in an increase in the yield stress, despite a reduction in the degree of crystallinity.;In E/MAA ionomers, partial neutralization of the MAA groups results in a several-fold increase in sigmay relative to the base resin. Here, nanoscale phase separation results in both ion-rich aggregates and ion-poor domains, with widely separated relaxation rates. The inability of the amorphous phase immediately surrounding the ionic aggregates to relax, except at extremely low strain rates, greatly increases the yield stress of the ionomers.;Moving to higher strains, we study the connection between the second yield shoulder---an anomalous hump in the plot of stress versus strain---and delayed permanent deformation. Mechanical testing and X-ray scattering show that this shoulder indicates the initiation of polyethylene crystal fracture, which results in crystalline fragments of reduced lateral extent. Connections formed between these fragments prevent the material from retracting to the initial sample dimensions once the load is removed, though greater recovery can be achieved by heating or melting the specimen. Only at strains beyond the end of the second yield hump does chain disentanglement begin, leading to truly unrecoverable deformation.;Finally, we review the effects of neutralization and test rate on the ultimate properties of E/MAA ionomers. We find that neutralization simultaneously increases the stress at break while decreasing the strain at break. The result is that the toughness, evaluated as the area under the tensile curve, does not change dramatically with ionomerization. This surprising result does not agree with high-speed tensile impact results, suggesting a strong rate-dependence of the toughness. |
