Research featured in EurekAlert!, Science Daily, and ARL News:
- Elastomeric polymers are promising materials for ballistic impact protection both on the macroscale (for example, in armour panels or combat helmet) and on the microscale, for example as protective coatings for helicopter rotor blades and in powder blasting. Among elastomer candidates for enhanced ballistic protection, polyureas, polyurethanes, and poly(urethane urea)s have recently gained interest owing to their versatile dynamic response.
- In a recently published paper, the molecular influence in the dynamic deformation response of poly(urethane urea) elastomers (PUU) at high strain rates was investigated via an all-optical laser-induced projectile impact test (LIPIT).
- LIPIT measurements allowed the direct visualization of the impact of micro-projectiles (silica spheres) on substrates and in-situ characterization, including depth of penetration and the extent of rebound of the micro-projectiles with respect to the PUU molecular composition and projectile impact velocity.
FIG. 1. Schematic illustration of the laser-induced particle impact test. A single 300-picosecond duration, 800-nm wavelength laser pulse with variable energy (up to 0.30 mJ) is focused onto a laser absorbing polymer layer with a laser exposed area of 50 μm in diameter. Silica particles (D = 7.38 μm ± 0.24 μm) deposited as a sub-monolayer on top of the PDMS layer are accelerated upon rapid expansion of the gas produced by laser ablation of the gold film. The projectiles are ejected into free space with controllable speeds, depending on laser energy, after which they impact a target sample at near-normal incidence (± 5º).
- Materials: PUU elastomers composed of 4,4’-dicyclohexylmethane diisocyanate (HMDI), diethyltoluenediamine (DETA), and poly(tetramethylene oxide) (PTMO), with three different molecular weight (MW) of the PTMO soft segment (SS), 650, 1,000 and 2,000 g/mol, prepared via a two-step, pre-polymer synthesis, were chosen for this study. In the sample nomenclature, the numerals ‘xyz’ refer to the molar ratio of HMDI:DETA:PTMO, and the succeeding ‘650’, ‘1000’, and ‘2000’ refer to the MW of PTMO as 650, 1,000, and 2,000 g/mol, respectively. In this work, 211-650, 532-1000, 431-2000, 211-1000, 321-2000, and 211-2000 were chosen for study
- Impact of micro-particles upon the selected elastomers were recorded in real time. Fig. 2 displays representative sequences of images showing impacts of micro-spheres upon 211-650 and 321-2000 PUU samples, where a difference in material response was noted. On one hand, the 211-650 sample, the most rigid PUU at ambient temperature, exhibited a shallow particle penetration (about 4 μm) upon impact at 790 m/s and a fast projectile rebound of 195 m/s. On the other hand, the 321-2000 sample, a more flexible PUU at ambient temperature, showed a deeper penetration of particle to about 9 µm upon impact at 730 m/s and a slower particle rebound of 80 m/s. The strain-rates associated with these impacts were on the order of 108/s.
FIG. 2. Typical sequence of images recorded using a high-speed camera with 3-ns exposure time showing particle impact on (a) a 211-650 sample and (b) a 321-2000 sample. (a) A particle impacts a 211-650 sample with a speed of 790 m/s and almost instantaneously rebounds from the material surface with a speed of 195 m/s and with minimum penetration. (b) Unlike in 211-650, the particle impacting the 321-2000 sample at a speed of 730 m/s penetrates deeper in the sample and rebounds with a slower speed of 80 m/s.
- For comparison, impacts were also performed on a ductile glassy thermoplastic, polycarbonate (PC). Rebound of the micro-spheres from the PC sample surface occurs at relatively high velocities, similar to what was observed for the stiffest PUU. However, a drastic difference in the post-mortem surface morphologies was observed. No signs of post-mortem damage were observed after impacts on PUUs, including 211-650, 532-1000, 431-1000, 211-1000, 321-2000, and 211-2000 whereas plastic deformation is predominant in PC.
- We hypothesized that the dynamic strengthening and stiffening characteristics of PUUs could presumably be facilitated by intermolecular hydrogen bonding present throughout the physically-crosslinked network in PUUs. In contrast, the microsecond relaxation at ambient conditions is not present in polycarbonate nor are hydrogen bonding and the corresponding enabling molecular mechanism available at all in polycarbonate, despite its toughness and impact strength. Thus, PUUs or high performance elastomers with multiple relaxation times are greatly desired and key to enabling both the dynamic strengthening and dynamic stiffening over the temporal scale from microseconds to nanoseconds.
- In addition to combat helmets, other potential applications of robust high performance elastomers for Soldier protection include but are not limited to transparent face shields, mandible face shields, ballistic vests, extremity protective gear, and blast-resistant combat boots.
For more details, read the papers:
D. Veysset, A. J. Hsieh, S. E. Kooi, A. A. Maznev, K. A. Masser, K. A. Nelson, Dynamics of supersonic microparticle impact on elastomers revealed by real–time multi–frame imaging. Sci. Rep. 6, 25577 (2016).