Influence of nano-scale morphology on impact toughness of epoxy blends

Abstract

The microstructure and ballistic impact toughness was investigated for a series of dynamically heterogeneous epoxy blends composed of diglycidyl ether of bisphenol A (DGEBA) that is cross-linked with stoichiometric mixtures of a rigid cycloaliphatic diamine and flexible polypropylene oxide based diamines. The ballistic impact resistance of the blends correlates with the presence of nano-scale structure, where blends that contain 2–5 nm domains exhibit nearly a doubling of impact toughness when compared to blends that exhibit large scale phase separation. The length scale of phase separation was manipulated by controlling the ratio of short chain to long chain propylene oxide diamines. At low long chain content, the materials nanoscale phase separate into 2–5 nm domains, resulting in a transparent epoxy that exhibits relatively good impact toughness. At high long chain volume fractions, the mixtures undergo macroscale phase separation and lose both optical clarity and impact performance. Interestingly, some of the macro-phase separated blends with intermediate long chain polypropylene oxide diamine content still exhibit high impact resistance if a nanoscale structure is still present in the rigid continuous phase. Small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) were used to characterize the phase separated structure in these blends. When the SAXS peak associated with nanoscale phase separation shifts to lower scattering vector and its intensity is reduced, the impact performance of the mixtures is likewise reduced. We hypothesize that the small scale structure in the blends facilitates rapid deformation during high rate ballistic impact events, which allows the material to absorb more energy before failure.