Hierarchical elastomers such as segmented polyureas, poly(urethane ureas), and polyurethanes typically consist of a combination of hard and soft segments, which often vary considerably in chemical structure. The evolution through self-association of hard segments, facilitated via strong intermolecular hydrogen bonding, would result in microphase separation and a complex microstructure. This class of elastomeric materials could have potential to undergo a transient phase transition upon extreme dynamic loading conditions, from rubbery at ambient to become leathery or even glassy when the impulse approaches the respective segmental mobility.

The discovery of this novel molecular mechanism was coined as high-rate, deformation-induced glass transition, through which the potential toward enhanced energy absorption and dissipation was postulated. Meanwhile, it’s been further elucidated that intersegment mixing between the soft phase and hard domains was essential toward enhanced dynamic stiffening, where a poly(urethane urea), PUU 532-1000, revealed moderate improvement in the resistance against impact by a 20-μm steel particle at strain rates of approximately 108 s–1, than a polyurea, PU 1000, despite both having approximately the same hard segment contents.

The variation in the extent of dynamic stiffening upon impact corroborated well with the corresponding segmental dynamics. PUU 532-1000 exhibited a segmental α relaxation time associated with the soft phase, which was about four orders of magnitude slower than that of PU 1000, approximately 1.1 × 10–1 s versus approximately 2.2 × 10–5 s, determined at 25°C by broadband dielectric relaxation spectroscopy. Additionally, the segmental dynamics of the local relaxation of PUU 532-1000 appeared to be very close to that of the segmental relaxation of PU 1000, which was further indicative of greater intersegment mixing in PUU 532-1000 than PU 1000. Furthermore, recent experimental observations obtained from select model two-component polyurethanes clearly elucidated the essence of molecular attributes toward dynamic stiffening, where a predominantly amorphous polyurethane exhibited greater dynamic stiffening than the corresponding semicrystalline counterpart.

Meanwhile, advancements in high-performance fibers, inorganic fillers, nanoparticles, and carbon nanotubes have led to the development of lightweight polymer matrix composites for integration into the design of a broad range of engineered structures and components used in automobiles, aircraft, as well as lightweight military tactical vehicles. In practice, organosilanes are utilized in surface modification of fibers or fillers to yield better dispersion, thereby mitigating agglomeration of these reinforcement materials.

Additionally, surface modification can lead to proper interface interaction and subsequently adequate stress transfer between matrix and reinforcement in order to achieve the desired mechanical properties and performance characteristics of the composites. Zirconia (ZrO2) is known to exhibit high hardness, stiffness, flexural strength, and fracture toughness, as well as a low coefficient of friction.

Recent research also revealed that addition of dopants such as ceria and yttria to zirconia led to the development of unique characteristics such as shape memory or superelasticity, where stress-induced martensitic transformation was noted between tetragonal and monoclinic phases. This transformation was observed in micron-sized, ceria-doped zirconia particles and was shown to be reproducible upon deformation over hundreds of cycles to strains up to approximately 4.7%. There has been increased interest in the development of ZrO2-based and ZrO2-containing ceramics, particularly with respect to their potential toward enhanced fracture toughness.

The motivation for this research is to exploit the intrinsic hardness of ZrO2 along with the dynamic stiffening characteristics of hierarchical elastomers for integration into fabrication of hybrid composites for dynamic mechanical properties optimization. Recent progress has shown that incorporation of zirconia nanoparticles functionalized by 3aminopropyltriethoxysilane led to a strong covalent bond with the epoxy-based matrix and subsequently greater interlaminar strength of the resultant fiber-reinforced, zirconia-modified epoxy matrix composites.

Successful silanization of zirconia nanoparticles was hypothesized as a result of the reaction of organosilanes with surface hydroxyl groups, which was attributed to the presence of strongly absorbed water from the atmosphere. Meanwhile, it was also reported that the surface of zirconia contains a variety of catalytically active sites, including Brønsted acidic and basic hydroxyl groups and coordinatively unsaturated Lewis acidic-base Zr4+O2- pairs. The presence of Lewis acidic sites was shown to be more abundant on the monoclinic phase, and further, the nature of the zirconia phases strongly affected the adsorption of carbon monoxide (CO) and carbon dioxide (CO 2).

This work was done by Alex J. Hsieh, Victor K. Champagne, Steven E. Kooi, and Christopher A. Schuh of MIT for the Army Research Laboratory. For more information, download the Technical Support Package (free white paper) below. ARL-0233

This Brief includes a Technical Support Package (TSP).
Progress on Zirconia-Polyurea Matrix Hybrid Composites

(reference ARL-0233) is currently available for download from the TSP library.

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This article first appeared in the December, 2020 issue of Aerospace & Defense Technology Magazine.

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