In 2017, researchers from the Army Research Laboratory and MIT reported that PUUs are capable of demonstrating hyperelastic properties, meaning that the material becomes extremely hardened upon being deformed within a very short time. As a result, the material may withstand ballistic impacts at exceptionally high speeds.[4]
For the study, the researchers investigated the performance of different PUU variants where 4,4’-dicyclohexylmethane diisocyanate (HMDI) was chosen as the diisocyanate compound, diethyltoluenediamine (DETA) was chosen as the short-chain diamine compound, and poly(tetramethyleneoxide) (PTMO) was chosen as the long-chain polyol compound. Despite consisting of the same chemical compounds with the same stoichiometric ratio of 2:1:1 of [HDMI]:[DETA]:[PTMO], the samples differed regarding the molecular weight of their respective PTMO component, namely 650 g/mol (10,400 oz/lbmol), 1,000 g/mol (16,000 oz/lbmol), and 2,000 g/mol (32,000 oz/lbmol), for the soft segments of the elastomers.[5]
Each of the three samples were subjected to a laser-induced projectile impact test (LIPIT), which tested the dynamic response of the material by using a pulsed laser to shoot it with microparticles made of silica at speeds ranging from 200 to 800 m/s (660 to 2,620 ft/s).[5][6] The researchers found that the sample with the 650 g/mol (10,400 oz/lbmol) PTMO was the most rigid variant with the particle exhibiting a shallow penetration of about 4 μm (0.00016 in) upon impact despite travelling at 790 m/s (2,600 ft/s) before rebounding at 195 m/s (640 ft/s). In contrast, the sample with the 2,000 g/mol (32,000 oz/lbmol) PTMO displayed a deeper penetration of about 9 μm (0.00035 in), but had a slower particle rebound of 80 m/s (260 ft/s), making it the most rubber-like among the PUU samples. The strain-rates associated with these impacts were on the order of 2.0 x 10^8/s for the former and 8.1 x 10^7/s for the latter.[5]
However, all three PUU variants demonstrated rebound capabilities with no signs of post-mortem damage after impact from the microparticles. In contrast, when the LIPIT was performed on a ductile, glassy polycarbonate at similar speeds to that of the 650 g/mol (10,400 oz/lbmol) PTMO PUU variant, the polycarbonate displayed predominant deformation upon impact, despite its high fracture toughness and ballistic strength.[5][7] According to the researchers, the effectiveness of the PUUs may come from how the molecules “resonate” similar to chain-mail upon impact with each oscillations at specific frequencies dissipate the absorbed energy. In comparison, the polycarbonate lacked the broad range of relaxation times, a characteristic that reflects how efficiently the molecules in the polymer chains respond to an external impulse, that PUUs are known to have.[7] As a result, the researchers concluded that even the most rubber-like variant of the PUU, specifically the 2,000 g/mol (32,000 oz/lbmol) PTMO sample, demonstrated greater robustness and dynamic stiffening than the glassy polycarbonate.[5]
ARL researchers have stated that the PUU’s primary benefit comes not from its extra strength but its fabric-like flexibility, which demonstrates its potential as a replacement material for the rigid ceramic and metal plates generally found in military battle armor. However, as of 2018, the PUU is still under development in the testing phase.[8]