Relating structure and chain dynamics to ballistic performance in transparent epoxy networks exhibiting nanometer scale heterogeneity

Abstract

The ballistic performance was examined for a series of broad glass transition temperature epoxy formulations consisting of a di-epoxy monomer crosslinked with bi-modal mixtures of both a rigid, low molecular weight diamine and a flexible, high molecular weight diamine. Interestingly, the resins did not macro-phase separate during cure, but exhibited structural and dynamic heterogeneity on a length scale of a few nanometers, as confirmed by X-ray scattering, dynamic mechanical analysis, modulus-mapped atomic force microscopy, and broadband dielectric spectroscopy. The nano-structured resins were optically transparent and demonstrated a nearly 300% increase in ballistic energy dissipation relative to the neat resins, as well as when compared to epoxy formulations composed of similar bi-modal blends that exhibited a macro-phase separated structure. The ballistic performance is found to be insensitive to sub-glass transition temperature (Tg) relaxations, but appears to be dependent on both the nano-structure and the Vogel temperature of the high Tg component. The study demonstrates a new class of transparent protective materials composed of rigid and flexible components with a fine scale heterogeneous structure.