Silver nanoparticles cross-linked polyimide aerogels with improved high temperature microstructure stabilities and high mechanical performances

Highlights

• Two kinds of silver nanoparticles are aminated and served as cross-linkers of PI aerogels.

• Aerogels added with 5 wt % cross-linker have the highest mechanical properties among aerogels with similar densities.

• Silver nanoparticles cross-linked aerogels have better high temperature microstructure stability than pure PI aerogels.

Abstract

Nanoparticles can be considered as effective cross-linkers for PI aerogels. In order to investigate the effect of different nanoparticle-cross-linkers on the performances of aerogels, two types of silver nanoparticles, AgNPs (Ag nanoparticles) and AgNShs (Ag nanosheets) were modified via PATP (p-aminothiophenol) as cross-linkers and then added into PAA (polyamic acid) solution derived from BPDA (3, 3′, 4, 4′-biphenyltetracarboxylic dianhydride) and ODA (4, 4′-diaminodiphenyl ether) to form wet gel. The wet gel was processed with supercritical CO₂ drying to fabricate cross-linked polyimide aerogels. Raman, XPS and TG-IR were carried out to prove successful modification of silver nanoparticles. The as-prepared aerogels had densities around 0.2 g cm⁻³. Nitrogen adsorption desorption tests and mechanical tests were conducted to characterize the performance of aerogels at room temperature and high temperature (above 150 °C). The results showed that comparing to pure PI aerogels, aerogels cross-linked with silver nanoparticles had higher compressive strength and Young's modulus, which comes to 3.4 MPa and 52.7 MPa, respectively. Besides, silver nanoparticles cross-linked aerogels also had better high temperature stability in both size and microstructure comparing to pure PI aerogels. With remarkable mechanical properties and high temperature microstructure stabilities, these aerogels show potential in many areas of application.

Introduction

Generally speaking, as a promising material, aerogels have meso-porous structure, large void space, low density, large specific surface area, low dielectric constant, low thermal conductivity and high acoustic impedance. Hence, aerogels have wide potential applications in thermal insulation material [1,2], gas separation and filtration, catalyst and drug carrier, low dielectric constant material for semiconductor devices, adsorbent and sound attenuator [3]. Conventional silica aerogels are extremely light but rather fragile [1,3,4]. As a comparison, polyimide aerogels show excellent advantage of flexibility [5], combined with outstanding thermostability and fire resistance, which excels other organic aerogels [6] such as polystyrene, polyurethane [[7], [8], [9], [10]], polyurea, cellulose [11], poly(vinyl alcohol) [12], polyamide [13,14], chitin [15] etc.

During the drying process of wet-gels, the evaporation of pore-filling solvent generates capillary pressure which will cause the shrinkage of wet-gels. For linear polyimide wet-gels, their molecules in polyamic acid cannot withstand the internal pressure, tending to aggregate and entangle with each other. As a result, large scale shrinkage occurs during the process of drying, which will increase the density and decrease the specific strength of aerogels. In order to decrease the shrinkage and promote the mechanical properties, there have been some pioneering studies on PI aerogel [16].

Chemical cross-linker was tried first, from NASA (National Aeronautics and Space Administration), Meador [[17], [18], [19]] et al. used TAB (1, 3, 5-triaminophenoxybenzene) and BTC (1, 3, 5-benzenetricarbonyl trichloride) as cross-linkers to prepare a series of PI aerogels with different dianhydride and diamine by supercritical CO₂ drying. The results showed that the density of the PI aerogel could be as low as 0.14 g cm⁻³ and the specific surface area could be as high as 512 m² g⁻¹. Besides, the glass transition temperature could be improved to 340 °C. However, the compressive strengths of these PI aerogels were under 1 MPa. Guo et al. [20,21] prepared polyimide aerogels with regular monomer BPDA and BAX (bis-aniline-p-xylidine) using a cross-linker OAPS (octa-(aminophenyl)-silsesquioxane). The produced PI aerogel had smaller shrinkage due to the eight crosslinking sites provided by OAPS, and could reduce the density to 0.1 g cm⁻³; nevertheless, with a decrease in specific surface area to 260 m² g⁻¹ and unimproved compressive moduli of 1.7–5.3 MPa. In the aforementioned studies, small molecules cross-linkers like TAB, OAPS, and TAPO (tri (3-aminophenyl) phosphine oxide) [22], were quite expensive owing to their complicated synthetic route.

Nanoparticle filler were taken into consideration next. Theoretically, nanoparticles can serve as physical cross-linking point in polymeric networks, adding nanoparticles into PI aerogels appears to be a reliable method for their modification. Particles such as montmorillonite [23], glass fibers [24] and cellulose nano-fibers [25] are showing promise to act as physical cross-linking points and improve the properties of PI aerogels remarkably. Yet without any impact of chemical bonding between the nanoparticles and PI aerogels, the results of densities and shrinkages are still not ideal. There were samples of nanoparticles of same substance but in different shape used as fillers in preparation of aerogels. GO (graphene oxide) [26], and CNTs (carbon nanotubes) [27,28] were both used as fillers in PI aerogels, and the result showed that CNTs had better effect on enhancing the strength, and GO could provide the aerogels with higher specific Young's moduli. Glass fiber and silica aerogels were also used in preparing polyimide aerogels as two kinds of silicious fillers which differ in shapes and sizes [24,29]. The results showed that shapes and sizes of nanoparticles or fillers will affect densities and mechanical properties of aerogels.

In our previous work, AgNWs (Ag nanowires) [[30], [31], [32]], as a kind of one-dimensional nano-material with excellent properties, have been synthesized and treated with amino-functionalization. Those aminated AgNWs provided a great number of chemical cross-linking points and serve as reinforcement for the PI aerogels, which enhanced the mechanical properties of aerogels to a great extent. However, silver nanoparticles of different shapes or sizes may have different influence on aerogels as cross-linkers. Hence, in this study, we extended the type of silver nanoparticles from one-dimensional AgNWs to zero-dimensional AgNPs (Ag nanoparticles) and two-dimensional AgNShs (Ag nanosheets). Amination modification was carried out via the introduction of PATP, as PATP had been proven to be an excellent amino containing modifier for silver nanoparticles [33,34]. As cross-linkers, aminated silver nanoparticles were then added into 10 wt % PAA oligomers solution end-capped by anhydrides to form covalent bond and finally prepared into polyimide aerogels. Mechanical properties of aerogels increase with the addition of cross-linkers. Aerogels added with 5 wt % cross-linker have the highest specific compressive strength and specific Young's modulus among aerogels with similar densities.

Polyimides are quite stable at high temperature environment given by their imide ring frameworks. It is worth noting that the DSC and TG are usually used to characterized thermal stability of PI aerogels, usually the 5% weight loss temperature of PI aerogels reach above 500 °C. However, there is an inevitable phenomenon that the porous structure of aerogels will collapse with the increase of temperature, and the aerogels will shrink and then lose its quality as good insulation materials. Therefore, the study on the microstructure stability of polyimide aerogels at high temperature is worth carrying out, yet few reports are found in latest studies.

In this study, the microstructure stability of aerogels at high temperature (above 120 °C) of cross-linked aerogels were characterized. It turned out that aerogels cross-linked with silver nanoparticles had more stable microstructure under high temperature than pure PI aerogels, as the results of specific surface area, average pore size and mechanical tests all supported this conclusion. This work develops a novel strategy to enhance the properties of polymer aerogels. Additionally, with remarkable specific strength and modulus at both room temperature and high temperature up to 160 °C, these aerogels show promise to become high-duty, lightweight and heat resistant porous materials in aerospace and construction applications.