Microstructural evolution of polyurea under hydrostatic pressure

Abstract

Polyurea (PU) is a versatile high strain rate elastomeric polymer that has attracted significant interest due to its utility as a blast and ballistic resistant coating. Predictive design of PU materials, particularly with a focus on structure, is complicated by the occurrence of both high pressure and high strain rate effects in response to blast and ballistic impact. To help elucidate the behavior of PU under such conditions, we sought to characterize the microstructural evolution of PU under hydrostatic compression. To this end, we utilized a diamond anvil cell with in-situ X-ray scattering to monitor the compression-induced microstructural changes of two PU materials differing in their soft segment length. We found that regardless of soft segment length, the initially phase separated structure becomes increasingly mixed during hydrostatic compression up to ~1 GPa. Despite this increase in phase mixing, the interchain network between urea linkages formed through hydrogen bonds remains apparent at pressures well above 1 GPa. The mean spacing associated with the interchain hydrogen-bonded network within the hard segment domains contracts under hydrostatic compression and in some cases recovers upon load removal. Furthermore, the extent of both deformation and recovery depends upon the degree of microphase separation as dictated by the length of the soft segment, wherein PU with a longer soft segment deforms less and recovers more.