Coexistence of Hardening and Softening Phenomena in Elastomeric Polymers under Nano-Impact Loading

This article reports the coexistence of hardening and softening phenomena when polyurea is submitted to repeated nano-impacts with various impact forces while controlling the strain rate. The manifestation of these phenomena is further elucidated by interrogating ultraviolet irradiated samples under ambient and nitrogen atmospheres, wherein artificial weathering accelerates hardening by reducing the nano-impact depths as a function of exposure duration while increasing the impact load, nano-impact repetitions and strain rate sensitivity favored softening. A 21% and 48% increase in indentation depth are recorded after 100 repetitions at a relatively higher force (10 mN) at a low strain rate and low force (2.5 mN) at a relatively higher rate for pristine and weathered polyurea, respectively. Electron microscopy evidences the induced, progressive damage at the nanoscale based on the agglomeration of hard segments, reduced free volume, and weathering-induced surface embrittlement.