Molecular influence on dynamic stiffening of synthetic polyurethanes through laser-induced supersonic microscale impact tests

High-strain-rate response is an important characteristic of protective elastomers. To discern the molecular influences on dynamic stiffening in polyurethane (PU), we synthesized two-component PUs from poly(tetramethylene oxide) (PTMO) and either hexamethylenediisocyanate (HDI) or 4,4’-methylenediphenyldiisocyanate (MDI). From dynamic mechanical analysis, HDI-PU displays semicrystallinity while MDI-PU appears amorphous. We also synthesized three-component PUs with the chain extender butanediol (BD) to introduce segmented hard domains. We performed supersonic microscale impacts with a laser-induced particle impact test in which a silica microsphere is accelerated up to ~1000 m/s. The impact is recorded with micron spatial resolution and nanosecond temporal resolution on an ultra-high-speed camera. In a velocity range of 50 to 1000 m/s, we show that MDI-PU exhibits greater dynamic stiffening than HDI-PU. We hypothesize that greater intermolecular hydrogen bonding in MDI-PU plays an important role. Finally, we discuss the role of segmented hard domains in dynamic stiffening of polyurethanes.