Scalable, Hydrophobic and Highly-Stretchable Poly(isocyanurate-Urethane) Aerogels

Scalable, low-density and flexible aerogels offer a unique combination of excellent mechanical properties and scalable manufacturability. Herein, we report the fabrication of a family of low-density, ambient-dried and hydrophobic poly(isocyanurate-urethane) aerogels derived from a triisocyanate precursor. The bulk densities ranged from 0.28 to 0.37 g cm-3 with porosities above 70% v/v. The aerogels exhibit a highly stretchable behavior with a rapid increase in the Young\u27s modulus with bulk density (slope of log-log plot \u3e 6.0). In addition, the aerogels are very compressible (more than 80% compressive strain) with high shape recovery rate (more than 80% recovery in 30 s). Under tension even at high strains (e.g., more than 100% tensile strain), the aerogels at lower densities do not display a significant lateral contraction and have a Poisson\u27s ratio of only 0.22. Under dynamic conditions, the properties (e.g., complex moduli and dynamic stress-strain curves) are highly frequency- and rate-dependent, particularly in the Hopkinson pressure bar experiment where in comparison with quasi-static compression results, the properties such as mechanical strength were three orders of magnitude stiffer. The attained outcome of this work supports a basis on the understanding of the fundamental mechanical behavior of a scalable organic aerogel with potential in engineering applications including damping, energy absorption, and substrates for flexible devices

Mechanical Characterization Of Aerogels

As multifunctional porous nanostructured materials (e.g., thermally/acoustically insulating), aerogels are derived from their vast porosity and their high specific surface area and may also hold exceptional specific mechanical properties under certain conditions as well. In this chapter, the mechanical characteristics of aerogels are discussed in detail. First, the mechanical characterization of traditional aerogels is summarized, and then, the mechanical behavior of polymer crosslinked silica and vanadia (X-aerogels), as well as organic aerogels, is presented. Finally, the acoustic attenuation property is briefly discussed for polyurea aerogel. In polymer crosslinked aerogels, a few-nanometer-thick conformal polymer is coating on secondary particles, while the pores is not clogging, which thus preserves the multifunctionality of the native framework and improves the mechanical strength. The mechanical properties were characterized under both quasi-static loading conditions (dynamic mechanical analysis, compression, and flexural bending testing) and high-strain-rate loading conditions using a split Hopkinson pressure bar. We evaluated the effects of strain rate, mass density, loading–unloading, moisture concentration, and low temperature on the mechanical properties of aerogels. Digital image correlation was used to analyze the surface strains through ultrahigh-speed images for calculation of properties such as dynamic Poisson\u27s ratio. A remarkable result is that crosslinked vanadia aerogels remain ductile even at −180 °C, indicating a property derived from interlocking and sintering-like fusion of skeletal nanoworms during compression. Due to the substantial improvement in the mechanical properties of X-aerogels with a small amount of polymeric crosslinking agent, purely polymeric aerogels with similar X-aerogel nanostructures were investigated. Therefore, in this chapter, the mechanical properties of organic aerogels including polyurea and polyurethane aerogels were also studied. Furthermore, a special attention has been carried out on the acoustic attenuation of polyurea aerogels by means of normal incidence sound transmission loss measurements. Polyurea aerogels showed unprecedented high sound transmission losses over a broad range of frequencies, a trend that clearly breaks the empirical Mass Law nature of the conventional acoustic materials

High Thermo-Mechanical Stability in Polybenzoxazine Aerogels

Aerogels are three-dimensional networks of nanoparticles with high specific surface area and high porosity. Following the significant improvement on the mechanical strengths and ductility of traditional aerogels with polymer cross-linking (i.e., X-aerogels), the emergence of pure polymeric aerogels has enabled unprecedented aerogel applications such as ballistic armor protection, which is quite surprising for such low-density materials. However, generally low glass transition temperatures (Tg) of polymeric aerogels hinder their structural applicability at service temperatures above their Tg temperatures. Thereby, developing novel polymeric aerogels with high Tg temperatures is crucial for high-temperature structural applications. As phenolic resins, polybenzoxazines are heat-resistant and mechanically strong with high glass transition temperatures. In this study, polybenzoxazine aerogels have been successfully synthesized, and their mechanical properties at different densities and elevated temperatures have been investigated. High thermo-mechanical stability has been observed over the entire temperature range of interest (i.e., below 250 °C) for their quasi-static compressive properties such as Young\u27s modulus and compressive strength. Moreover, the storage and loss moduli in shear of the aerogels have been studied at different temperatures and frequencies. The strong mechanical performance of these aerogels at elevated temperatures makes them an important, inexpensive, and advanced material for high-temperature applications, competitive with significantly more expensive polyimides

Acoustic Properties of Aerogels: Current Status and Prospects

International audienceNoise pollution has been aptly described as one of the modern plagues. [1] Due to many adverse health effects of loud environments, ranging from sleep disturbances to cardiovascular diseases, reducing the exposure of humans to excess noise is essential to the public health of large populations living in the cities. Regarding sound absorption materials, the optimal choice depends on the intended sound frequency range; solutions of damping high-frequency sound waves rely on totally different absorption mechanism than the solutions for very low-frequency noise. Indoors, the most commonly used sound absorption materials are porous by nature due to their ability to efficiently absorb sound at mid to high frequencies with relatively thin layers. Common porous absorption materials in the market, targeting to over 90% absorption above 350 Hz, are glass and mineral wools and acoustic foams made, e.g., from melamine or polyurethane. Here, we review the acoustic properties of aerogels and demonstrate their high potential to challenge and exceed the absorption properties of the current market standards, whether we talk about the performance i

Sound Insulation Properties in Low-Density, Mechanically Strong and Ductile Nanoporous Polyurea Aerogels

Aerogels are quasi-stable, nanoporous, low-density, three-dimensional assemblies of nanoparticles. In this paper, an extremely high sound transmission loss for a family of ductile polyurea aerogels (e.g., over 30 dB within 1 to 4 kHz at bulk density 0.25 g/cm3 and 5 mm thickness) is reported. The fundamental mechanisms behind the aerogel acoustic attenuations are investigated. Sharing striking similarities with acoustic metamaterials, initially, aerogels are studied via a one-dimensional multi degree-of-freedom mass-spring system. Different effects such as spring constant disparity are investigated in regards to the structural vibration wave transmission loss. Results are given for different configurations consistent with the aerogel nano/microstructures. A significant wave attenuation is observed by considering a random spring distribution. In the next step towards modeling such a complex hierarchical and random structural material, the continuum Biot\u27s dynamic theory of poroelasticity is implemented to analyze the experimental sound transmission loss results. In this framework, a two-dimensional plane strain analysis is considered for the interaction of a steady state time-harmonic plane acoustic wave with an infinite aerogel layer immersed in and saturated with air. The effects of bulk density and thickness on the aerogel sound transmission loss are elucidated. By comparing the theoretical results with the experimental observations, this study develops a qualitative/quantitative basis for the dynamics of the aerogel nanoparticle network as well as the air flow and solid vibroacoustic interactions. This basis provides a better understanding on the overall acoustic properties of the aerogels that might also be helpful in the design of the future hierarchical materials

Synthesis of Aerogel Foams through a Pressurized Sol-Gel Method

We report monolithic aerogel foams as solid materials with hierarchical porosity created by a foam-like structure embedded in the skeletal framework of a regular aerogel. The foam-like structure is prepared without chemical foaming agents or templates, resulting in a less expensive, more efficient, and more readily adaptable process. Specifically, pressurized air (7 bar) is injected into a suitable sol, which is allowed to gel under pressure, followed by slow depressurization. Voids are created from the air bubbles formed during depressurization. The model material used for validation of the technique is based on poly(isocyanurate-urethane) aerogels (PIR-PUR) and selected material properties of the resulted aerogel foams are compared with those of their pristine aerogel counterparts. With an eye on scalability, all wet-gels were dried under ambient conditions. Aerogel foams exhibit lower bulk densities by about 25%, and higher porosities by about 10% in comparison with their pristine PIR-PUR aerogel counterparts. Interestingly, the thermal conductivities of aerogel foams were found reduced significantly (by 25%) from 0.104 to 0.077 Wm-1K-1 compared to the corresponding pristine aerogels. In addition, aerogel foams absorb 36% w/w more oil and show better oil retention in comparison with regular PIR-PUR aerogel samples made from the same sols. As this technique does not alter the chemical composition of the aerogel, it is anticipated that it can be used for a variety of different types of aerogels and formulations in order to lower their bulk density and improve desired physical properties such as thermal conductivity

Metamaterial-Like Aerogels for Broadband Vibration Mitigation

We report a mechanical metamaterial-like behavior as a function of the micro/nanostructure of otherwise chemically identical aliphatic polyurea aerogels. Transmissibility varies dramatically with frequency in these aerogels. Broadband vibration mitigation is provided at low frequencies (500-1000 Hz) through self-assembly of locally resonant metastructures wherein polyurea microspheres are embedded in a polyurea web-like network. A micromechanical constitutive model based on a discrete element method is established to explain the vibration mitigation mechanism. Simulations confirm the metamaterial-like behavior with a negative dynamic material stiffness for the micro-metastructured aerogels in a much wider frequency range than the majority of previously reported locally resonant metamaterials