The response of polyurea, a block copolymer with elastomer behavior at standard temperature and pressure,  has led to significant research into its potential use as a coating in the defense industry, with applications such as attenuation of shock waves, and increasing projectile penetration resistance in metal substrates, among others. While existing studies have confirmed polyurea’s feasibility as a protective coating for protection systems, further investigation of the atomic structure of polyurea is needed to understand how the hard and soft domains in polyurea’s structure interact and evolve as a function of stress and temperature, particularly under high pressures. This will help to improve our understanding of the mechanisms that drive polyurea’s high pressure behavior and to validate models that can be used to design polyurea coatings. While several studies have already been performed to characterize polyurea’s microstructure, those performed using analytical X-ray techniques tend to be at low pressures, while the response at higher pressures is needed to understand the mechanisms at play in polyurea coatings used in protection systems. The objective of this work is to conduct in situ energy dispersive x-ray diffraction (EDXD) measurements in polyurea samples undergoing hydrostatic stresses, obtain the structure factors and pair distribution functions (PDF) from data analysis, and use the results to determine the response of polyurea to these pressures at the atomic level. The in-situ EDXD measurements were performed using the polychromatic x-ray beam produced at the bending-magnet station 16-BM-B at the Advanced Photon Source (APS) synchrotron, using a combination of the wavelength bandwidth of the beam and variations of the scan angle to cover a range of 1.05 \< q \< 11.31 Å^-1^. Samples were compressed in a Paris–Edinburgh cell (PEC), where a 3-stage hydraulic pump system was used to raise the PEC’s bottom piston, thus compressing the sample between two tungsten carbide anvils. To obtain an in-situ pressure measurement, EDXD was performed on an MgO ring surrounding the sample each time the pressure was increased. An equation of state (EOS) for MgO was used to deduce pressure based on the position of the MgO diffraction peaks. Temperature was also varied using a high current power supply to apply a voltage to a graphite heater surrounding the sample.

Results indicate that the first sharp diffraction peak (FSDP) of the structure factor, which is a measure of the intermediate range order between backbone features, has a q position at ambient pressure that is consistent with the spacing of hard segments in polyurea, and it shifts to higher values of q as pressure increases. This corresponds to a reduced distance between hard segment backbones. This compression also leads to a greater disorder of the backbone segments, indicated by the drop in intensity with increasing pressure, as well as a broadening of the peak. Analysis of the pair distribution function results indicate that there is a significant response from the soft segments as pressure increases, but that response of atoms in aromatic rings is stiffer. A good agreement was found between experimental results and molecular dynamic simulations. This work is bets suited for the session on mechanical characterization.


Effects of High Pressure on Structural Evolution of Polyurea: An In-Situ Energy Dispersive X-Ray Diffraction Study

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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