Polyimide Layered

Article Cite This: ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

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Thermally Robust Bendable Silicon Dioxide/Polyimide Layered Composite Film Through Catalytic Fluorination Chan Jiang, Zheng Cheng, Xin Li, Cao Li,* and Xiangyang Liu* State Key Laboratory of Polymer Material and Engineering, College of Polymer Science and Engineering, Sichuan University, Chengdu 610065, P.R. China

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S Supporting Information *

ABSTRACT: The poor atomic oxygen (AO) resistant property of polyimide (PI) film is the bottom problem for its application in aerospace. Coating SiO2 layer on PI is expected to solve this problem efficiently since SiO2 is inherently resistant to AO. However, the adhesion between PI and SiO2 is weak by their mismatch in polarity, leading to failure of layered SiO2/PI composite film as a result of interlaminar cracking in application. Here, by complexing Fe3+ onto benzimidazole units, polyimide film was modified by catalytic fluorination. It was found that the CF bond on phenyl group produced in catalytic fluorination was three times as much as that in direct fluorination of original PI. Further, γ-aminopropyl triethoxysilane (APTES) was grafted onto PI through nucleophilic substitution between aromatic CF and amine on APTES, introducing numerous SiOH groups on PI surface. Finally, a flat and condense SiO2 layer covalently bonded to PI was obtained with condensation of Si OH from PI surface and Si sol. As a result, peeling strength between PI and SiO2 layer increased from 3 to 210 cN/cm, which is 70 times higher after modification. Moreover, the SiO2/PI composite film can endure 1000 times of bending and 50 times of thermal cycles (−100 °C to 100 °C) without peeling off or cracking. KEYWORDS: polyimide, SiO2, interface, fluorination, covalent bonding

1. INTRODUCTION Polyimide (PI) is a kind of high-performance polymer that is widely used in aerospace as solar cell substrates and insulation layers, owning to its high strength-to-weight ratio, dimensional stability.1−6 However, when used in satellites and space shuttles, which are placed in low earth orbit, almost all polymers face severe environmental conditions such as ultraviolet radiation and orbital debris.7 Among them, atomic oxygen (AO), which is the main component in low earth orbit, is one of the most degrading effects that polymers have to withstand.8−10 As reported by previous studies, the energy of AO reaches nearly 5 eV, which exceeds the bonding energy of PI. As a result, the pristine PI will be eroded to small molecules as a result of chain scission by atomic oxygen, and these invasive circumstances will overwhelmingly accelerate degradation of PI.11−13 Therefore, it is crucial to improve the antiatom oxygen property of PI. Oxides such as SiO2, Al2O3, and TiO2 possess excellent oxidation resistance and thermal stability.14−20 Among them, SiO2 is most commonly used to improve the AO resistance of polymer due to its high bonding energy (8 eV) and convenient preparation process. There are numerous reports on SiO2/PI composite films with improved AO or corona resistance, and they generally fall into two categories. A common strategy is matrix strengthening by forming SiO2/PI hybrid film or © XXXX American Chemical Society

introducing Si element, like POSS, into PI molecular structure.10,11,21−27 However, the surface of those hybrid films still contains PI macromolecules, and they will be inevitably eroded by AO. A large number of holes will be produced afterward, and these holes will become new path for further erosion. Therefore, a SiO2 layer completely covered on PI surface is in urgent need. In order to achieve this, researchers proposed to fabricate composite SiO2/PI layered film by introducing dense SiO2 layer on PI surface,14,28,29 which can protect the underlying PI from AO by close coating. Nevertheless, the adhesion between pure SiO2 and inert PI surface is comparatively weak, so these inorganic oxides would be easily peeled off. Surface modifications, such as alkali treatment, thermal solvent permeation, plasma or radiation modification, and surface topography design, can hopefully enhance the interaction between SiO2 and PI by introducing hydrogen bond and other polar groups at the interface. Nevertheless, the interaction provided by hydrogen bond or van der Waals force is far from enough for the long-term application.30−33 In order to enhance the adhesion between PI substrate and SiO2 coating layer Received: January 6, 2019 Accepted: March 5, 2019 Published: March 5, 2019 A

DOI: 10.1021/acsapm.9b00012 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

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ACS Applied Polymer Materials

Figure 1. Preparation of SiO2/PI layered composite film (1, preparation of PI films coordinated with Fe3+; 2, preparation of fluorinated PI films; 3, preparation of PI Films containing OH on the surface through CF graft reaction; 4, preparation of SiO2/PI composite film by spin coating).

generated on PI surface owing to the in situ catalytic fluorination of the benzene ring. The whole process is presented in Figure 1. First, benzimidazole-containing PI film is fabricated according to our previous reports, and Fe3+ is then coordinated on PI film surface by the reaction with benzimidazole unit. Second, traditional fluorination of original PI film and catalytic fluorination of Fe3+-coordinated PI film are both conducted, and their surface structures are compared to acquire higher aromatic CF content on PI surface. Third, utilizing nucleophilic substitution reaction of CF with amine of APTES, SiOH functionalized PI can be obtained. Lastly, the SiOH groups on PI surface further act as linking sites for the SiO2/PI composite film with covalent bond at the interface. Owing to the inherent tolerance of SiO2 to AO and the lightweight of the composite film, the as-prepared flexible SiO2/PI composite film is expected to meet the requirement of application in aerospace.

further, introducing stronger covalent bonding through chemical grafting is proposed. Since PI owns an inert surface, generating active groups for grafting is highly desired. However, for common surface functionalization strategies like oxidation, hydrolysis, and permeation,32,34−36 the content of functional groups introduced through these methods is always limited, which will result in low grafting density of SiO2. Moreover, the macromolecular chains on the surface may fracture severely as a result of oxidation and hydrolysis, and small molecules generated from chain scission could bring about weak interface.37,38 Consequently, interlaminar cracking or stripping happens as a result of weak interlaminar adhesion. Therefore, developing a new strategy to introducing SiO2 on PI surface with strong adhesion is highly required. Recently, direct fluorination becomes a new surface treatment approach with versatile superiorities.39−42 Compared with conventional surface modification methods, more polar groups such as CF can be introduced on polymer surface due to the high reactivity of fluorine gas.43 The fluorination is easy to perform with no solvent, and the uniformity of it is promised by gas−solid reaction process as well. Also, researchers have found that, the CF bond in fluorine-containing aromatic, fluorinated graphene and CNT can act as an active grafting site for nucleophilic reagents such as amine and thiol.44−46 So direct fluorination of PI may be the potential method for the grafting of SiO2 layer on PI surface through interfacial molecular design. However, the high reactivity of F2 also brings about negative effect for direct fluorination. It is reported that direct fluorination usually resulted in fracture of macromolecules, thus weak interface with low bonding strength would still be formed,47 and the content of CF bond on the benzene ring of macromolecule chain would not be enhanced as well. In this paper, polyimide films containing benzimidazole moieties were used for the preparation of SiO2/PI laminated film through chemical grafting with the assist of direct fluorination. Inspired by the catalytic action of Fe3+ ion in halogenation of benzene ring,48,49 catalytic fluorination of PI was realized by utilizing coordination effect of Fe3+ with benzimidazole units, where fluorine reacts more with benzene ring of macromolecule chain. As a result, more CF bonds are

2. EXPERIMENTAL SECTION 2.1. Materials. 3,3′,4,4′-Biphenyl tetracarboxylic dianhydride (BPDA, 99%) and 2-(4-aminophenyl)-5-aminobenzimidazole (PABZ, 99%) were purchased from Sunlight Pharmaceutical Co., Ltd. (Chang Zhou, China). The F2/N2 (10 vol % for F2) with purity up to 99.99% was obtained from Chengdu Kemeite Fluorine Industry Plastic Co., Ltd. 3-Aminopropyltriethoxysilane (APTES), tetraethoxysilane (TEOS), N-methyl pyrrolidone (NMP), hydrochloric acid (HCl), ferric chloride hexahydrate (FeCl3·6H2O), and acetone were all reagent grade and used without further purification. 2.2. Preparation of PI Film. The PI film was prepared through a conventional two-stage solution polymerization and thermal imidization process. First, viscous poly(amic acid) solution was synthesized from PABZ and BPDA with equivalent molar ratio in nitrogen atmosphere. The polycondensation was carried out in NMP at room temperature for 16 h, and viscous PAA solution with content of 12 wt % was obtained. Then PAA solution was cast on clean glass plates, followed by subsequent imidization at 80 °C for 1 h, 120 °C for 2 h, 220 °C for 1 h, and 330 °C for 0.5 h in a vacuum oven to get PI films. The PI films were cleaned and dried before further treatment. 2.3. Coordination of Fe3+ onto PI Film. FeCl3·6H2O were added into the mixed solvent of ethanol and water (1 vt % for water) to get a solution with Fe3+ concentration of 0.08 g/mL. Then PI film was immersed in this solution at 50 °C for 6 h. Afterward, the film was B


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ACS Applied Polymer Materials taken out and washed with deionized water. Finally, PI films coordinated with Fe3+ was obtained after dried in a vacuum oven and it is named as PI−Fe. 2.4. Catalytic Fluorination of PI−Fe. Before fluorination, the PI−Fe film was dried completely. Then it was placed in a closed stainless-steel chamber with a vacuum line. The air in the closed chamber was removed after three cycles of nitrogen replacement. Then F2/N2 mixed gas was introduced into the chamber at room temperature (22 °C) with a duration of 20 min. Then the residual F2 and byproducts in the chamber were pumped out thoroughly. After fluorination, the Fe3+ on PI−Fe was removed in hydrochloric acid solution, and then washed with acetone in Soxhlet extractor. Finally, this fluorinated PI film by Fe3+ induced catalysis was obtained after dried in vacuum and labeled as PI−CF (catalytic fluorination). For comparison, the fluorination of PI without coordination of Fe3+ was carried out in the same fluorination process as PI−Fe film, and the obtained film is denoted as PI−F. 2.5. Graft of APTES onto PI Film. PI−CF film was soaked in APTES and kept at 80 °C for 3 h. Then the film was taken out and washed with Soxhlet extraction in acetone for 6 h. Finally, it was hydrolyzed in pure water for 1 h, and it is named as PI−CF−A. For comparison, PI−F−A and PI−A were also prepared through the same method based on PI−F and PI. 2.6. Preparation of SiO2 Film on the PI Surface. The SiO2 film was prepared through sol−gel process and spin coating from TEOS. The silica sol was prepared in such way: First, 25 mL of TEOS and 50 mL of ethanol were added into a flask and stirred for 10 min to disperse. Then HCl with pH value of 1 was added into the flask and the solution was stirred for 4 h at 40 °C to promote hydrolysis of TEOS. After aging for 2 h, clear and transparent silica sol was obtained. The as-prepared silica sol was spin-coated on the surface of PI− CF−A. First, 0.5 mL of silica sol was deposited on PI−CF−A with a size of 4 cm × 4 cm. Then the substrate spun up followed with twoperiod rotation. After drying at 60 °C for 10 h, SiO2/PI composite film labeled as PI−CF−A−SiO2 was obtained. For comparison, SiO2 layer was deposited on PI, PI−A, and PI−F−A in the same way as PI−CF−A, and they are correspondingly denoted as PI−SiO2, PI− A−SiO2 and PI−F−A−SiO2. 2.7. Characterization. The amount of Fe3+ coordinated onto PI film was characterized by colorimetric measurement of ferricsulfosalicylate complex. First, PI−Fe film was statically immersed in HCl solution for the decomplexation of Fe3+. Two hours later, the film was taken out and the solution was transferred to a volumetric flask into which an excessive dose of sulfosalicylic acid was added. Afterward, the pH value of the solution in the volumetric flask was adjusted to 11 with the addition of ammonia to form ferric− sulfosalicylate complex. After diluting with water to volume, UV−vis spectra of the solution were measured by UV-1800PC spectrophotometer at the wavelength of 420 nm, where the absorption maximum of ferric−sulfosalicylate complex is observed. The specific content of Fe3+ was calculated on the basis of Lambert−Beer law. First, the absorption of standard solutions with incremental Fe3+ concentration (0, 0.1, 0.2, 0.3, 0.4 μg/mL) was measured at 420 nm. Then the standard calibration curve was obtained by plotting a graph of absorption at 420 nm versus ferric ion concentration. X-ray photoelectron spectroscopy (XPS) was applied to characterize the chemical composition of these samples with Kratos ASAM 800 spectrometer (Kratos Analytical Ltd., U.K.), using a nonmonochromatic Al Ka (1486.6 eV) X-ray source (a voltage of 15 kV, a wattage of 250 W) radiation. Nicolet 560 FTIR spectrometer (Thermo Electron) using attenuated total reflection (ATR) mode was applied to measure the functional groups on film surface. The wavenumber range of the spectra was set between 4000 and 500 cm−1, and the frequency scale was internally calibrated with a reference helium−neon laser to an accuracy of 4 cm−1.

Scanning electron microscope (SEM) was carried out on a FEI Inspect F (FEI Company, EU/USA) at 20 kV with magnification of 20,000 and 80,000. Atomic force microscopy (AFM, HE002-H, AIST-NT, USA) was utilized to characterize the surface roughness of untreated PI and modified PI films. The surface with area of 5 um*5 um was scanned in tapping mode and the roughness was calculated by instrument software. The mechanical properties of PI films were tested on an electronic universal material testing machine (Instron 5967, U.S.A.). The gauge during measurement is 20 mm and extension rate was 10 mm min−1.

3. RESULTS AND DISCUSSION 3.1. Preparation of PI Film and Its Coordination with Fe3+. The ATR-FTIR spectrum of the as-prepared PI can be found in Figure 2a. The characteristic asymmetric and

Figure 2. (a) The FTIR spectrum of PI; (b) chemical structure of PI.

symmetric stretching vibrations of CO in imide ring are observed at 1772 and 1704 cm−1. Moreover, a peak at 1356 cm−1 appears, which is attributed to the CN in imide ring. The absorption peaks at 1612 and 1494 cm−1 for CC in benzene ring can be observed as well. Wide absorption peak between 3000 and 3700 cm−1 is attributed to NH in benzimidazole. Therefore, PI film with benzimidazole units is successfully prepared and its chemical structure is shown in Figure 2b. The as-prepared PI film with benzimidazole units was further coordinated with Fe3+ ion. To ascertain the amount of Fe3+ on PI film, UV−vis colorimetric measurement of ferric− sulfosalicylate complex was employed. First, the absorption of standard solution and sample solution was measured at 420 nm (SI Supplement 1), where the maximum absorption for ferric− sulfosalicylate complex appears. Then, the standard calibration curve for the content of Fe3+ was obtained by plotting a graph of absorption at 420 nm versus ferric ion concentration. As has been shown in Figure 3a, the equation (y = 0.00096 + 0.00355x) with correlation coefficient of 0.999 is calculated out of the curve. When substituting absorption of sample solution at 420 nm into the equation, the specific concentration of ferric−sulfosalicylate complex can be obtained, which is equal to the concentration of Fe3+ since the ratio of ferric− sulfosalicylate complex to Fe3+ is one. As a result, the Fe3+ concentration on PI film is 2.7 μg/cm2. C


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Figure 3. (a) standard calibration curve of 420 nm absorption versus Fe3+ concentration; (b) XPS spectra of PI−Fe and PI, inserted image is the Fe 2p spectrum of PI−Fe; (c) N 1s spectra of PI; (d) N 1s spectra of PI−Fe

Further XPS was utilized to identify the location of Fe3+ on PI structure. From the XPS spectra of PI−Fe and PI (Figure 3b), characteristic peaks for Fe around 712 eV are observed, confirming the existence of Fe3+ on PI surface. For the N 1s spectrum of untreated PI film (Figure 3c), there exist two peaks at 398.4 and 400.2 eV, which are correspondingly attributed to NCN in the benzimidazole structure and NCO in the imide ring. After the coordination of Fe3+, a new peak is observed at 399.5 eV, which is attributed to the new generated CNFe bond (Figure 3d). The change in N 1s spectrum after coordination indicates that Fe 3+ is coordinated onto CN in benzimidazole structure. Overall, the results confirm the coordination of Fe3+ onto benzimidazole structure of PI surface successfully. 3.2. Catalytic Fluorination of PI Induced by Fe3+ Coordination. PI and PI−Fe films were surface treated with fluorination. To explore the effect of Fe3+ on fluorination, the surface chemical structure of PI−CF and PI−F is analyzed by XPS. Strong F 1s signal around the binding energy of 687 eV can be detected for both PI−CF and PI−F as shown Figure 4a. According to the surface elemental compositions listed in SI Supplement 2, F content in PI−CF (11.5%) is increased by 49.4% compared with that in PI−F (7.7%). Furthermore, for the F 1s spectra of them (Figure 4b,c), there exist two peaks at binding energy of 684.9 and 687.1 eV, which are correspondingly attributed to HF and CF covalent bond.50−52 For PI−CF, the proportion of CF to total F content is 0.975. By contrast, that is 0.778 for PI−F. As a result, effective F contents from CF bond for PI−CF and PI−F are actually 11.2% (0.975 × 11.5%) and 6.0% (0.778 × 7.7%) respectively, which is increased by 86.7%. Therefore, in situ catalytic fluorination occurs on the benzene ring unit of PI’s macromolecules, by complexing Fe3+ ion onto its

structure, which is similar to the organic catalytic reaction of benzene chlorination by FeCl3. To further evaluate the changed fluorination reaction by coordination of Fe3+, the C 1s spectra of PI, PI−F, and PI−CF are analyzed to assess the attribute of CF bond. For the C 1s spectrum of PI as shown in Figure 4d, there exist three peaks at the binding energy of 284.7, 285.9, and 288.2 eV, which are correspondingly attributed to CC, CN, and CO. After fluorination as shown in Figure 4e,f, two new peaks assigned as aromatic and aliphatic CF bond appear around 288.7 and 290.6 eV, which confirm the success of fluorination modification on PI surface. Moreover, it could be observed that the content of aromatic CF bond in PI−CF (9.4%) is two times larger than that of PI−F (3.1%). Therefore, PI−Fe can introduce more reactive aromatic CF bond from fluorination than original PI, which is beneficial for subsequent high-density grafting of SiO2 layer. Consequently, the results above illustrate that this Fe3+ coordination induced catalytic fluorination of PI macromolecules, with more aromatic CF bond generated. Moreover, the F content of fluorinated PI films with different Fe3+ content coordinated on PI surface was further investigated (SI Supplement 3). As a result, the F content increases slightly from 10.4% to 11.5% with the increasement of Fe3+ content from 1.8% to 3.8%. In addition, the Fe content obtained from XPS result shows that, the Fe3+ content on PI surface almost remains the same after fluorination. 3.3. PI Films Containing OH on the Surface through CF Graft Reaction. As has been stated above, the aromatic CF bond can be used as grafting site for nucleophilic reagent like amine and thiol through nucleophilic substitution. So, APTES was grafted onto PI−CF surface by CF bond to introduce OH on the surface. The surface of PI−CF−A obtained from PI−CF was analyzed by XPS (Figure 5b) and its D


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Figure 4. (a) XPS survey spectra of PI, PI−Fe, PI−CF, and PI−F; F 1s spectra of (b) PI−F and (c) PI−CF; C 1s spectra of (d) PI, (e) PI−F and (f) PI−CF.

After the grafting of SiO2 onto PI−CF−A surface, the absorption for PI structure is dramatically weakened and strong absorbance bands appear at 1000∼1100, 910∼960, and 792 cm−1, which are assigned to the SiOSi and SiOH in the SiO2 layer. The XPS analysis was further used to analyze the surface chemical structure (Figure 5b). The O content and Si content on PI−CF−A−SiO2 surface are, respectively, 63.2% and 21.1%, while that on PI−CF−A surface is 28.9% and 7.8%. After subtracting the amount of silicon and oxygen on PI− CF−A, the ratio of O to Si in SiO2 layer is calculated to be 2.58, which is consistent with the values obtained in other reported SiO2 layer.36,53,54 In high-resolution Si spectra as shown in Figure 5c,d, there exists one peak at 103.0 eV for PI− CF−A, which is attributed to SiOH from APTES. By contrast, two peaks at 102.7 and 103.7 eV are observed in that of PI−CF−A−SiO2, which are correspondingly assigned to SiOH and SiOSi in SiO2 layer. 3.5. Properties of the SiO2/PI Composite Film. 3.5.1. Surface Structure. The surface structure of PI−CF− A−SiO2 was measured by AFM. The AFM images of PI films are shown in Figure 6. The surface pattern of untreated PI is flat and smooth with few defects as shown in Figure 6a, and the RMS roughness of it is 0.62 nm. For PI−CF and PI−CF−A (Figure 6b,c), their surfaces both receive slightly increased

elemental composition can be found in SI Supplement 2. As can be seen, F content on PI surface decreases from 11.5% to 4.0% after grafting APTES onto PI−CF, indicating the consuming of CF bond in the reaction. Also, a massive increase of Si content from 0.3% to 7.6% is observed after the grafting of APTES onto PI−CF. Besides, the addition of oxygen-rich APTES also increases O content from 18.1% to 28.9%. Therefore, APTES is successfully grafed onto PI−CF film. To understand the effect of the content of reactive CF bonds on grafting quantity of APTES, PI−F−A was prepared from PI−F in the same way as PI−CF−A. Then it was also analyzed by XPS and its surface elemental composition result can be found in Supplement 2. The Si content of PI−F−A is 4.0%, while that of PI−CF−A increases to 7.6%. The results indicate that the grafting density increases is by 95.0% by this catalytic fluorination of PI−Fe. 3.4. SiO2/PI Composite Film Prepared by Sol−Gel Method. On the surface of PI−CF−A, SiO2 layer is constructed by sol−gel method.The formation of SiO2 on PI film was first characterized by ATR-FTIR. As can be seen in Figure 5a, strong imide absorbance bands from PI backbone are observed at 1772 cm−1 and 1704 cm−1 before the deposit of SiO2, which are correspondingly attributed to the asymmetric and symmetric stretching vibration of CO. E


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Figure 5. (a) ATR-FTIR spectra of PI films; (b) XPS spectra of PI−CF−A and PI−CF−A−SiO2; (c) Si 2p spectrum of PI−CF−A; (d) Si 2p spectrum of PI−CF−A−SiO2.

Figure 6. AFM height images of (a) PI, (b) PI−CF, (c) PI−CF−A, and (d) PI−CF−A−SiO2.

The SEM image and Si distribution of PI−CF−A−SiO2 surface are shown in Figure 7. As shown in Figure 7a, the SiO2 layer is flat and condense, which is consistent with the AFM result. The deposit of SiO2 layer on PI surface enriches the surface with rich Si content as shown in Figure 7b. To characterize the thickness of this SiO2 layer, the cross-section of PI−CF−A−SiO2 was also studied by SEM. According to the cross-section image shown in Figure 7c, different phases of SiO2 and PI can be observed clearly. As a result, the thickness

roughness with their RMS roughnesses turning to 1.65 and 3.32 nm respectively, and this is owing to the grafting of single molecules on PI surface. After the deposit of SiO2 layer by spin-coating, the surface morphology of PI−CF−A−SiO2 is shown in Figure 6d. It is observed that the surface of PI−CF− A−SiO2 is even and smooth with RMS roughness of 0.36 nm, which is even smaller than that of initial PI. AFM results show that the surface morphology of SiO2 layer is smooth without obvious defects. F


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Figure 7. (a) SEM surface image of PI−CF−A−SiO2; (b) Si element distribution on PI−CF−A−SiO2 surface; (c) SEM cross-section image of PI−CF−A−SiO2.

Figure 8. (a) Interlayer diagram of SiO2/PI composite films; (b) peeling strength test method; (c) peeling strength results of SiO2/PI composite films

of SiO2 layer can be directly obtained and it turns out to be around 460 nm. Moreover, many other regions were checked and the SiO2 layer was proved to be uniform in thickness. The uniform SiO2 surface can also be revealed by the cross-section image. 3.5.2. Mechanical Properties of the SiO2/PI Composite Film. To discuss whether the treatment processes especially fluorination will affect the mechanical properties of PI films, tensile strengths of them were measured. As shown in SI Supplement 4, the tensile strengths of PI, PI−CF, PI−CF−A, and PI−CF−A−SiO2 are 205, 211, 226, and 205 MPa, respectively. The tensile strength of PI substrate is maintained during complexation, fluorination, and grafting process, indicating that modification process will not have negative effect on mechanical properties of PI film. 3.5.3. The Adhesion of PI with SiO2 Layer. As has been stated above, the weak adhesion of SiO2 with PI is a big problem for common method to fabricate SiO2/PI laminate film, such as coating and chemical oxidation. However, the stability of SiO2 layer grafted onto PI film is a matter of great concern, which is vital for the SiO2/PI composite film to maintain durable AO resistance. The adhesion stability of SiO2 layer on PI film was first evaluated by ultrasonication. As shown in SI Supplement 5, the SiO2 layer on PI−CF−A−SiO2

is preserved with no cracking or peeling off after 5 min of ultrasonication (DOVES, 240W), while the SiO2 layer on PI− SiO2 and PI−A−SiO2 is cracked and peeled off severely. The results confirm the great adhesion stability of SiO2 layer on PI−CF−A−SiO2 film through covalent bonding. The adhesion properties of SiO2 layer onto PI film was also characterized by the universal tensile test machine. To characterize the interfacial adhesion, the samples made of PI−SiO2, PI−A−SiO2, PI−F−A−SiO2, and PI−CF−A−SiO2 were bonded with epoxy resin. Because the interaction of SiO2 and epoxy is greater than that of PI and SiO2, the interface between PI and SiO2 will be destroyed first under pulling force (Figure 8a,b). Under the same test conditions, the peeling strength for PI−SiO2 is 3 cN/cm, which is attributed to the absence of chemical bonding at the interface. The peeling strength for PI−A−SiO2 is 14 cN/cm and this slight increasement is attributed to the interaction brought about by APTES, which is physically absorbed onto PI film. By comparison, the peeling strengths for PI−CF−A−SiO2 and PI−F-A-SiO2 increase to 210 and 112 cN/cm respectively, which are attributed to the SiOSi chemical bond at the interface. Moreover, comparing PI−F−A−SiO2 and PI−CF− A−SiO2, stronger adhesion for PI−CF−A−SiO2 is achieved with higher density of grafting at the SiO2/PI interface. G


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fluorination. First, the coordination of Fe3+ on benzimidaozle unit of PI surface induces changed reaction mechanism of direct fluoirnation, and more aromatic CF bond is generated, which benefits the further higher grafting density of APTES molecules. As a result, the high-density grafting derived from this catalytic fluorination process provides the interface with strong adhesion between the final grafted SiO2 layer and PI film, which is superior to the common methods, such as direct coating and chemical grafting via traditonal fluoriantion. Moreover, it endows the SiO2 layer with flexibility. This flexible SiO2 layer can endure at least 50 thermal cycles and 1000 times of bending to 90° without cracking or peeling off. It is expected that this SiO2/PI layered composite film integrates with excellent comprehensive performance and is hopeful to be used in space field.

The correlation between the content of APTES on PI surface and peeling strength was further studied. Since Si content on PI−CF−A and PI−F-A surface is only related with APTES, the content of APTES is represented by Si content on PI films. To begin with, a curve of peeling strength vs Si content on PI films was plotted. As shown in Figure 8c, an euqation (y = 30.80x-19.99) with correlation coefficient of 0.995 is caculated out of the curve. The excellent correlation between Si content and peeling strength demonstrates that the covalent bonding derived from APTES is the dominating factor for the improvement of interfacial adhesion. 3.5.4. Flexbility of SiO2/PI Composites Film. Because of the uniform and dense SiO2 layer that is tightly bonded on PI surface, the SiO2/PI composite film is promising in space field. However, when applied in practice, there will be disturbances such as thermal and mechanical stress that may cause the SiO2 layer to crack or peel off. Therefore, the flexibility of PI−CF− A−SiO2 was characterized under thermal cycles (Figure 9a)

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsapm.9b00012. The absorptions at 420 nm of standard solutions and sample solution can be found in Supplement 1. The elemental composition results of PI films obtained from XPS results are provided in Supplement 2. The F and Fe contents of PI films before and after fluorination can be found in Supplement 3. Also, the stress−strain curve as well as tensile strengths of PI, PI−CF, PI−CF−A, and PI−CF−A−SiO2 are all shown in Supplement 4. Finally, the SEM images of PI−CF−A−SiO2, PI-SiO2, and PI-ASiO2 after 5 min of ultrasonication are provided in Supplement 5 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xiangyang Liu: 0000-0002-9246-3437 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grants 51873113 and 51573105), State Key Laboratory of Polymer Materials Engineering (Grant sklpme2017-2-03), the National Key Research and Development Plan (2017YFB0404701), and Sichuan Science and Technology Support Project (2018GZ0153). The authors acknowledge Analytical and Testing Centre of Sichuan University, College of Polymer Science and Engineering of Sichuan University, State Key Laboratory of Polymer Materials Engineering (Sichuan University), and National Demonstration Center for Experimental Materials Science and Engineering Education of Sichuan University for characterizations.

Figure 9. (a) The diagram for thermal cycles; SEM of (b) surface and (d) cross-section image of PI−CF−A−SiO2 after 50 thermal cycles; SEM of (c) surface and (e) cross-section image of PI−CF−A−SiO2 after 1000 times of bending.

and bending. As a result, the SiO2 layer can endure 50 thermal cycles (−100 °C to 100 °C) and 1000 times of bending (0− 90°) without peeling off or cracking as shown in Figure 9b−e, exhibiting excellent thermostability and flexibility.

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REFERENCES

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4. CONCLUSION In this paper, completely covered SiO2 layer was introduced on PI film by chemical grafting via Fe3+ induced catalytic H


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