Hollow silica nanosphere/polyimide composite films for enhanced transparency and atomic oxygen resistance
Graphical abstract
Introduction
Polyimide (PI) films are widely used on spacecraft, mainly as thermal blankets, space inflatable structure membrane materials, critical components in solar arrays and thermal control materials due to remarkable advantages of high thermal stability, low dielectric constants, good radiation resistance, and excellent mechanical properties [1]. In addition to many attractive characteristics, highly transparent and colorless PIs are especially pursued for applications in lightweight solar arrays in space [[2], [3], [4], [5]]. Many efforts have been made to develop highly transparent and colorless PIs by incorporation of fluorine, chlorine, and a selenophene unit [[6], [7], [8]]. However, like all hydrocarbon-based polymers, PIs are vulnerable to atomic oxygen (AO) attacks in the low Earth orbit (LEO, 200–700 km) environment, which leads to the mass loss and deteriorates the optical, mechanical, electrical, and chemical properties [[9], [10], [11]]. Thus, it is of great significance for the stable applications of PI films in the LEO environment to find a facile means to improve the resistance to AO and maintain the transparency.
To eliminate or reduce the AO erosion, inorganic materials, such as aluminum, germanium, silica, or metallic oxides, are coated on PI films for protection from AO attacks [[12], [13], [14]], but it is difficult to obtain a defect-free protective coating. Spalling, cracking, and exfoliation often occur in the coated PI films during thermal cycling, owing to the mismatch of thermal expansion coefficients and the poor wettability between inorganic coatings and PI films, which may allow for the entry of AO under PI films [15]. A more ambitious option is to introduce the functional compounds, such as phenylphosphine oxide groups, SiO2, or siloxane, into polyimide by physical blend or chemical bond [3,[16], [17], [18], [19], [20], [21], [22]]. For instance, Minton et al. prepared the polyhedral oligomeric silsesquioxane (POSS), a siloxane with a unique cage-like molecular structure containing an inorganic silica core surrounded by organic groups, which was incorporated into the PI backbone by copolymerizing [20]. They demonstrated that the POSS/PI copolymers possessed a one percent AO erosion yield of Kapton via laboratory and spaceflight experiments, while they did not pay much attention to transparency [20]. Lei et al. [3] gradually became aware of the importance of both AO resistance and transparency for potential applications in space electronic devices and the solar cells. They reported that the hyperbranched polysiloxane (HBPSi) was introduced into PI chains via copolycondensation reactions. As a result, the mass loss of HBPSi/PI copolymers was only 7.7% of the control PIs, equivalent to an AO erosion yield of 2.39 × 10−25 cm3/atom, after a fluence of 3.87 × 1020 atoms/cm2 AO exposure while the transparency was not affected by the addition of HBPSi. Moreover, antireflection coatings were also in development for even enhancing the transparency [5,23,24]. Wang et al. [23] prepared the periodic mesoporous organosilica coatings on two sides of flexible PI films. They showed that the maximum transmittance of PI was increased from 88.68% to 99.67%, while they did not investigate the AO resistance.
Hollow silica nanospheres (HSNs) have attracted much attention to improve the transparency of glass, exhibiting the satisfactory durability compared with conventional solid silica nanoparticles [[25], [26], [27], [28], [29]]. However, it is difficult to prepare HSNs on PI films due to a low glass transition temperature of PI substrates [29], since the HSNs are generally formed by a high-temperature treatment above 400 °C. Thus far, some approaches have been developed to circumvent the high-temperature treatment for developing versatile coatings on flexible polymer substrates, such as multiple-step centrifugal washes [30,31], chemical oxidation [[32], [33], [34]], and so forth. The multiple-step centrifugal washes are time consuming and have low productivity [34,35]. The chemical oxidation has been only applied to mesoporous silica coatings [[32], [33], [34], [35]]. It is found that a facile removal of organic templates from core-shell nanospheres (CSNs) at low temperature is still challenging for formation of HSNs on polymeric substrates.
In this work, we proposed an approach to enhance both AO resistance and transparency where the hollow silica nanospheres (HSNs) were infiltrated into PIs, forming the HSN/PI composite flexible films. The HSNs were first obtained on glass substrates by a solution-based template removal process at a moderate temperature and then transferred to the PI substrates to form the HSN/PI composite films. Herein, the HSNs were incorporated into the PIs, instead of coating them on PIs, which could circumvent the difficulties of adhesion strength, wettability, and thermal expansion coefficient matching in conventional coating techniques. The fabricated HSN/PI composite flexible films exhibited not only the excellent resistance to AO but also the improved transparency. This work would be of interest for highly transparent and AO resistant applications in lightweight solar arrays of spacecraft in the LEO space environment.
Section snippets
Materials
Tetraethyl orthosilicate (TEOS, 98.6%) and poly(acrylic acid) (PAA, Mw ∼ 3000) were purchased from Aladdin Chemicals. Ammonium hydroxide (25–28% NH3 in water), N,N-dimethylacetamide (DMAc, 99%), anhydrous ethanol (EtOH, 99.7%), hydrogen peroxide (30% H2O2 in water) and hydrochloric acid (36–38% HCl in water) were purchased from Sinopharm Chemical Reagent Co. The water was deionized. All chemicals were used without further purification.
Preparation of silica sols
The synthesis protocols of the PAA-silica CSNs were
Low-temperature removal of PAA templates by the H2O2 posttreatment
In our experiment, the effects of the oxidation agent, H2O2, were investigated on the CSN-coated glass, instead of CSN-coated PIs, because the strong oxidation of H2O2 might deteriorate the PIs [42]. Fig. 3(a) shows that the RI decreases rapidly from 1.29 to 1.23 after a 50 min treatment without remarkable influences on the thickness. Further treatment had few effects on the RI and the thickness. It might be associated with a gradual decrease in PAA core templates until complete removal thanks
Conclusions
In summary, we demonstrated a facile preparation of HSN/PI composite films by transferring the HSNs from glass to the drop-casted PI solution based on the low-temperature removal of core organic templates. The infiltration of HSNs circumvented the challenges of wettability and thermal expansion coefficient matching that traditional physical or chemical coating approaches usually faced. The prepared HSN/PI composite flexible films showed a combination of desirable properties, such as high
Acknowledgments
This work was supported by the National Key R&D Program of China (No. 2018YFA0209200), the National Science Foundation of China (Nos. 61574144, 61605224, and 51403225), the Frontier Science Research Project (Key Programs) of Chinese Academy of Sciences under Grant No. QYZDJ-SSW-SLH018, the Zhejiang Natural Science Foundation (Nos. LY17A040004 and LY15F050003), the Ningbo Natural Science Foundation (Nos. 2016A610053, 2017A610063 and 2017A610021), the program for Ningbo Municipal Science and
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2021, Materials Today CommunicationsCitation Excerpt :Yi et al. [28] used graphene as fillers to enhance the AO resistance of composites, and 1% doping reduced the mass loss of composites by 42%. Nano-particles such as SiO2, Al2O3 are widely used as fillers, which are directly doped or produced after reaction with AO [29]. We find that most of these studies were focused on a single type of filler, and there are rare composite of two types of fillers.