This document relates to epoxy compositions containing a sliding-ring polymer (polyrotaxane) additive and a thermally-curable epoxy resin. The epoxy compositions exhibit increased flexural toughness and impact resistance as compared to the same epoxy composition that does not contain the additive. This document also relates to 3D-printed epoxy compositions containing a sliding-ring polymer (polyrotaxane) additive and a thermally-curable epoxy resin.