Atomic oxygen-resistant polyimide composite fibers based on wet spinning of polyamic acid-POSS ammonium salts

Polymer Degradation and Stability 168 (2019) 108959

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Atomic oxygen-resistant polyimide composite fibers based on wet spinning of polyamic acid-POSS ammonium salts Fangfang Liu a, b, Haiquan Guo a, Yong Zhao a, Xuepeng Qiu a, *, Lianxun Gao a, **, Yanna Zhang c a b c

Polymer Composites Engineering Laboratory, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, China University of Chinese Academy of Sciences, Beijing, 100049, China The 39th Research Institute of China Electronics Technology Group Corporation, Xi'an, 710065, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 29 April 2019 Received in revised form 30 July 2019 Accepted 4 September 2019 Available online 5 September 2019

As a vital component of flexible cables in aerospace vehicles, outstanding atomic oxygen (AO) resistance is in demand for polyimide (PI) fibers. Herein, a series of PI composite fibers with different polyhedral oligomeric silsesquioxane (POSS) ammonium salt contents (0e20 wt%) is prepared by adding diethylamino-POSS into polyamic acid (PAA) solution followed by spinning via wet process. The average silicon contents of the fibers by ICP (i.e., 2.99 wt%) are similar to the surface silicon content (2.93 wt%) measured by XPS. The interaction between positive charges from diethylamino-POSS and negative charges from PAA chains gives rise to the even dispersion of diethylamino-POSS in PI composite fibers. All the composite fibers exhibit Tg at ~396
C and T5% at ~553
C. Moreover, the fracture strength slightly increases from 0.57 GPa to 0.63 GPa with increasing POSS ammonium salt content from 0 wt% to 15 wt%. After AO erosion, SEM results exhibit lower roughness for composite fibers than pure PI fibers. As the ammonium salt content is increased from 0 wt% to 20 wt%, the lower mass loss with 0.15 mg cm2 is shown, and the retention rates of fracture strength and initial modulus are considerably improved from 59.65% to 89.09% and from 63.83% to 91.45%, respectively. Meanwhile, the decay rates of fracture strength of the composite fibers obviously decrease at the same AO fluence with increasing POSS ammonium salt content. XPS results of the fibers show that the contents of carbon atom decrease whereas the contents of silicon and oxygen atoms increase obviously after AO erosion. This result indicates that silicate passivation layers form on the fiber surfaces, and these layers prevent the fibers from further AO attack. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Polyimide fibers Diethylamino-POSS Atomic oxygen Polyamic acid-POSS ammonium salts Composite fibers

1. Introduction As a significant type of high-performance fibers, polyimide (PI) fibers and corresponding fabrics, as flexible construction elements, have been applied widely on the outer surface of spacecraft in low Earth orbit (LEO) [1e8]. Atomic oxygen (AO) is the dominant constituent in LEO with altitudes between 200 km and 700 km. Spacecraft in LEO is subjected to AO collision at a high energy of approximately 4.5 eV, which causes the rapid degradation of PI fiber materials [9e13]. Therefore, the AO resistance of PI materials must be improved to ensure safe in-orbit spacecraft operation. Inorganic coatings, such as SiO2, Al2O3, and SnO2, are generally

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (X. Qiu), [email protected] (L. Gao). https://doi.org/10.1016/j.polymdegradstab.2019.108959 0141-3910/© 2019 Elsevier Ltd. All rights reserved.

adopted to confer polymer materials with high AO resistance [10,14e16]. When polymers coated with inorganic substances experience high and low temperature cycle, folding, or bending, the cracks or the inorganic coatings peeling off from the polymer surface occur because of the difference in thermal expansion coefficients and the weak adhesion strength between the inorganic coatings and polymers. As a result, the polymer materials are damaged by AO erosion [10,14,17]. Polyhedral oligomeric silsesquioxane (POSS) derivatives are unique nanoscale inorganic/organic hybrids with cage-like structures that possess an inner inorganic silicon/oxygen core (SiO1.5)n and external organic substituents. The use of polar or nonpolar functional groups as organic substituents renders the POSS compatible or miscible with most polymers [18e23]. Consequently, many efforts have been devoted to incorporating POSS and their derivatives into polymers to improve their performance. Minton [16,19,24e27] synthesized two types of POSS monomer, POSS

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dianiline monomer with two 1-(4-aminophenyl)-pendant groups and eight cyclopentyl pendant groups, and N-[(heptaisobutyl POSS) propyl]-3,5-diaminobenzamide, and then prepared POSS/PI copolymers with the POSS in the main and side chains [19,24,25]. The two composite films showed excellent stability in a laboratory test environment with hyperthermal AO and in exposures on the International Space Station as part of the Materials Interactions Space Station Experiments (MISSE-1, MISSE-5, and MISSE-6) [19,24,25,28]. The results of the above two tests revealed that the POSS content determines the erosion yield of the POSS/PI materials rather than its position in the PI chain. With a hyperthermal AO fluence of 2.70  1020 O atoms cm2, the erosion yields for 7.0 wt% Si8O12 POSS in the side chain and for 7.0 wt% Si8O11 POSS in the main chain were 0.15 and 0.13  1024 cm3 atom1, respectively. In a space-flight AO test of 1.80  1020 O atoms cm2 on MISSE-5, the POSS/PI composites showed a similar erosion yield of 0.14  1024 cm3 atom1, indicating that the hyperthermal AO beam exposure can reflect the AO effects observed in LEO experiments. A much lower erosion yield of 0.026  1024 cm3 atom1 was obtained after longer exposure on MISSE-1 to an AO fluence of 8  1021 O atoms cm2 for 3.5 wt% Si8O11 MC POSS/PI. This result indicates that the growing SiO2 layer on the POSS/PI surface gives rise to lower erosion yields under higher AO fluence exposures. However, the above methods are highly complicated and difficult to apply in the large-scale fabrication of materials. Minton et al. used blends of trisilanolphenyl (TSP) POSS monomers in a pyromellitic dianhydride (PMDA)-4,40 -oxydianiline (ODA) PI to investigate the AO resistance of composites with different Si7O9 POSS cage loadings [29]. The erosion yield for 7.3 wt% Si7O9 TSP-POSS/PI showed a similar value of 0.10 ± 0.02  1024 cm3 atom1 to 7.0 wt% Si8O12 SC POSS/PI under AO exposure fluence in the range 4.56e5.28  1020 O atoms cm2. Verker found that adding 15 wt% TSP-POSS to PI increases AO erosion durability. AO exposure of hypervelocity impacted films induces a synergistic erosion effect, which causes an impacted pure PI film (0 wt% POSS) to erode one order of magnitude faster than impacted 15 wt% POSS-containing film [30e32]. Residual stress formed due to elevated temperatures is a key factor in the local immense erosion of impacted PI. Tensile tests showed that PI with 15 wt% TSP-POSS has weaker mechanical properties than pure PI both at room temperature and at 450
C. However, the 15 wt% POSS/PI sample has 33% higher resilience and 45% lower toughness than pure PI. This behavior reflects the ability of the 15 wt% POSS/PI sample to absorb less energy (lower toughness) while releasing higher percentage of its absorbed energy after fracture (higher resilience), making it less likely to accumulate residual stresses [33]. Furthermore, the reactive molecular dynamics simulation was adopted to investigate the effect of the surface modification of POSS and its concentration in a PI matrix, as well as the effects of nanoparticle type (POSS, graphene, and carbon nanotubes) and orientation on graphene and carbon nanotubes nanoparticles in the PI matrix [34]. The study showed that POSS is more resistant to AO attack damage than graphene or carbon nanotubes. Grafting of the POSS nanoparticles with PI and increasing the PI concentration can decrease the erosion yield of POSS/PI, and the former helps mitigate AO damage. Based on previous work, the dispersion of POSS in the PI matrix is a critical factor determining the AO resistance of POSS/PI composites. In the present study, diethylamino-POSS was first designed and synthesized, and then PI composite fibers with different POSS contents were fabricated via the addition of POSS salts into polyamic acid (PAA) followed by wet spinning. The dispersion behavior of POSS in the composite fiber was systematically investigated, and the AO resistance of the resultant composite fibers was discussed. The current work provides a strategy to improve the AO erosion resistance of POSS/PI composite materials.

2. Experimental 2.1. Materials ODA (purity >99.5%) and PMDA were purchased from Shanghai Research Institute of Synthetic Resins, and PMDA was dried in vacuum at 233
C for 2 h prior to use. N,N-Dimethylacetamide (DMAc, analytical purity 99.5%) was purchased from Tianjin Fine Chemical Co., Ltd. and distilled over CaH2 under reduced pressure. TSP-POSS was obtained from Hybrid Plastics, Inc. Trichloro [4(chloromethyl)-phenyl] silane with 97% purity was purchased from Alfa. All other commercially available reagent-grade chemicals were used without further purification. 2.2. Preparation of Cl-POSS As shown in Scheme 1, Cl-POSS was synthesized by reacting trichloro [4-(chloromethyl)-phenyl] silane (2.0 mL, 11.22 mmol) with phenyltrisilano-POSS (10.00 g, 10.22 mmol) in the presence of triethylamine (4.4 mL, 30.82 mmol) in 50.0 mL of dry THF. The reaction mixture was stirred at room temperature for 8 h and then filtered to remove the ammonium salt byproduct. The THF solution was poured into acetonitrile. The precipitate was collected by filtration and then dried in a vacuum (9.40 g, 85%). 1H NMR (300 MHz, CDCl3) d (ppm) 7.75 (d, J ¼ 7.5 Hz, 16H), 7.49e7.44 (m, 7H), 7.39e7.34 (m, 16H), 4.56 (s, 2H). 2.3. Preparation of diethylamino-POSS Cl-POSS (5.00 g, 4.62 mmol) was dissolved in dry CH2Cl2 (10.0 mL) and then added dropwise to a diethylamine solution (1.00 g, 13.8 mmol) in dry CH2Cl2 (50.0 mL) for 1 h. The reaction mixture was stirred at room temperature for 24 h and then filtered to remove the ammonium salt byproduct. The organic layer was washed with water (2  50.0 mL) and then concentrated in a vacuum to obtain diethylamino-POSS as a white powder (4.34 g, 84%). 1 H NMR (300 MHz, CDCl3) d (ppm) 7.75 (d, J ¼ 7.5 Hz, 16H), 7.47e7.42 (m, 7H), 7.40e7.36 (m, 16H), 3.57 (s, 2H), 2.53 (dd, 4H), 1.05 (t, 6H). 2.4. Preparation of PAA-POSS ammonium salt solutions A series of PAA-POSS ammonium salt solutions was initially prepared from mixtures of PAA solutions and diethylamino-POSS with different contents as shown in Scheme 2. For example, the PAA-POSS ammonium salt solution with 10 wt% diethylaminoPOSS was prepared as follows. ODA (56.80 g, 0.28 mol) and DMAc (672.0 g) were charged into a flask equipped with a stirrer under nitrogen atmosphere. Then, PMDA (61.87 g, 0.28 mol) was added into the above solution under stirring at 0
C. After stirring for 24 h at room temperature, the above PAA solutions were added with diethylamino-POSS (11.18 g, 0.01 mol) and then stirred again for another 8 h. Other PAA-POSS ammonium salt solutions containing 5, 15, and 20 wt% of diethylamino-POSS were also prepared through the same procedure. 2.5. Preparation of POSS/PI ammonium salt fibers POSS/PI ammonium salt fibers were spun by wet spinning with a homemade spinning machine. The viscous PAA-POSS ammonium salt solutions in DMAc were filtered, degassed, and then extruded through a spinneret (100 holes, 0.12 mm/hole) at room temperature. The fresh fluid filaments were entered into a coagulation bath (H2O/DMAc ¼ 1/1, v/v), and nascent fibers were obtained, washed thoroughly with water, and then dried to remove residual solvent.

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Scheme 1. Synthetic route of diethylamino-POSS.

The PAA-POSS ammonium salt fibers were subjected to thermal imidization at 350
C to produce POSS/PI ammonium salt fibers, followed by drawing at 480
C with the ratio of 2.0. The diameters of the fibers were in the range of 15e20 mm. 2.6. Morphological, structural, and chemical composition characterization Fourier-transform infrared (FTIR) spectroscopy was performed on a VERTEX 70 spectrometer at a resolution of 400e4000 cm1. The surface chemical composition of the fibers before and after AO exposure was analyzed by XPS using an ESCALAB 250Xi electron spectrometer from Thermo Scientific Corporation with monochromatic Al Ka radiations. The Si element content of the composite fibers was analyzed by ICP (ThermoScientific iCAP6300). The surface and cross-section morphologies of the fibers before and after AO exposure were examined by field-emission scanning electron microscopy (SEM, Hitachi S-4800). The samples were sprayed with Au before observation. 2.7. Thermal and mechanical characterization Dynamic mechanical analysis (DMA) was conducted by a Rheometric Scientific DMTA-V at 1 Hz with the heating rate of 3
C min1 in the temperature range of 100
C-450
C. The coefficient of thermal expansion of the fibers was characterized via thermal mechanical analysis (TMA) with a TAQ400 in nitrogen atmosphere from 50
C to 200
C at a heating rate of 5
C min1. Thermogravimetric analysis (TGA) was performed with a TA Q50 instrument at a heating rate of 10
C min1 from 50
C to 800
C under nitrogen and air atmosphere. The measurement of mechanical properties was carried out on an XQ-1 instrument at a strain rate of 20 mm min1, and the initial distance between grippers was 20 mm. Each sample was tested more than 20 times, and the average value was used for the final properties. 2.8. Ground-based simulated AO exposure experiment The AO exposure experiment was performed in a ground-based AO effects simulation facility, a type of filament discharging plasma-type AO simulation facility [2]. PI fibers with 20 cm length were placed on the platform in a vacuum chamber for AO exposure. In this work, different AO irradiation times were used (Table 1). The corresponding AO fluence of each irradiation test was calculated on the basis of the mass loss, irradiated area, density, and erosion yield of the Kapton® H film as described in our previous report [2]. The Kapton films were cut into 1  1 cm2 squares and placed on the platform randomly for the average AO fluence. All samples were

weighed at 5, 10, 15, 20, and 30 h. AO fluence was calculated from the mass loss of Kapton through Equation (1):

F ¼ DM=ðrAEÞ;

(1)

where F is the total AO fluence (atoms cm2), DM is the mass loss of Kapton (g), r is the density of Kapton (1.42 g cm3), A is the exposure area of Kapton (1 cm2), and E is the erosion constant of Kapton (3  1024 cm3 atom1). The POSS/PI composite film and POSS/PI composite fiber samples were weighed and analyzed after different AO exposure times [2,7]. 3. Results and discussion 3.1. Structural characterization of POSS/PI ammonium salt fibers Fig. 1 shows the FTIR spectra of the PI fiber and POSS/PI ammonium salt composite fibers with different diethylamino-POSS loading amounts. For the PI fiber, the peaks at 1774 cm-1 and 1722 cm1 belong to the C]O group of asymmetric and symmetric stretching vibrations of the aromatic imide ring, respectively. The peak at 1360 cm1 is attributed to the CeN stretching vibration of the imide ring. The peak at 734 cm1 corresponds to the bending vibration of the imide ring. The above-mentioned results indicated that PAA fibers were successfully converted into PI fibers. For POSS/ PI ammonium salt fibers, new bands obviously detected. The two peaks at 1428 cm-1 and 690 cm1 can be attributed to the AreSi and SieC stretching vibrations of diethylamino-POSS, respectively. The intensity of the above two peaks increased obviously as the diethylamino-POSS ammonium salt content was increased from 0 wt% to 20 wt%. This result indicates that diethylamino-POSS has been introduced into the PI fibers successfully. 3.2. Distribution of diethylamino-POSS in PI fibers XPS and ICP measurements were performed to evaluate the Si content of POSS/PI ammonium salt fibers. As shown in Table 2, the surface Si contents from XPS was almost consistent with the Si content measured by ICP. With the addition of diethylamino-POSS ammonium salts at 5, 10, 15, and 20 wt%, the surface and bulk Si contents showed the values of 0.97 wt%, 1.87 wt%, 2.25 wt%, 2.99 wt % and 0.81 wt%, 2.25 wt%, 2.46 wt%, 2.93 wt%, respectively. Moreover, the average Si contents of the fibers were calculated, and the corresponding values were 1.00 wt%, 2.01 wt%, 3.01 wt%, and 4.02 wt%. The measured Si contents were lower than the calculated Si contents. Therefore, the diethylamino-POSS nanoparticles were well distributed in the fibers. The PAA-POSS ammonium salt solutions formed via the interaction of diethylamino-POSS

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Scheme 2. Synthetic route of POSS/PI ammonium salt fibers.

Table 1 Experimental conditions of the simulated AO exposure. AO exposure time (h)

5

10

15

20

30

AO fluence (atoms cm2)

5.40  1019

1.01  1020

1.48  1020

1.94  1020

2.93  1020

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nanoparticles with negative charges and PAA with positive charges, which led to the uniform distribution of diethylamino-POSS nanoparticles in the fibers. 3.3. Thermal properties of POSS/PI ammonium salt fibers

Fig. 1. FTIR spectra of PI fiber and POSS/PI ammonium salt fibers, weight percent of diethylamino-POSS in fibers as indicated.

The glass-transition temperatures (Tg) and coefficient of linear expansion of the fibers were investigated by DMA and TMA. As shown in Table 3 and Fig. 2a, the Tg values of the POSS/PI ammonium salt fibers were maintained at 396
C, which revealed no change with the content of diethylamino-POSS nanoparticles. The coefficient of linear expansions of the fibers in the temperature range from 50
C to 200
C were 5.06 mm m-1 oC-1, 4.89 mm m-1 o -1 C , 6.37 mm m-1 oC-1, 5.84 mm m-1 oC-1, and 3.01 mm m1
C1, corresponding to PI fibers with 0 wt%, 5 wt%, 10 wt%, 15 wt% and 20 wt% POSS ammonium salts (Seen in Table 3 and Fig. 2b). Results indicated that the POSS/PI ammonium salt fibers showed good dimension stability in the wide temperature range. On the basis of the TGA curves in Fig. 3a, the T5% of the POSS/PI ammonium salt fibers almost showed the same value of 553
C under nitrogen atmosphere as the pure PI fiber. However, the T5% of the POSS/PI ammonium salt fibers under air atmosphere showed the average value of 524
C, higher than the T5% of PI fibers (Seen in Fig. 3b). The results indicated that the fibers showed good thermal

Table 2 Silicon content in the POSS/PI ammonium salt fibers. Sample

Diethylamino-POSS (wt%)

Calculated average Sia (wt%)

Si contentb (wt%)

Surface Si contentc (wt%)

5 wt% POSS/PI 10 wt% POSS/PI 15 wt% POSS/PI 20 wt% POSS/PI

5.0 10.0 15.0 20.0

1.00 2.01 3.01 4.02

0.97 1.87 2.25 2.99

0.81 2.25 2.46 2.93

a b c

The theoretical value calculated by the addition of diethylamino-POSS. The average silicon content by ICP. The surface silicon content by XPS.

Table 3 Thermal and mechanical properties of POSS/PI ammonium salt fibers. Sample

Thermal Properties Tg (
C)

PI 5 wt% POSS/PI 10 wt% POSS/PI 15 wt% POSS/PI 20 wt% POSS/PI

395 396 396 396 396

Coefficient of linear expansion (mm m1$
C1)

5.06 4.89 6.37 5.84 3.01

Mechanical Properties T5% (
C)

T5% (
C)

nitrogen

air

547 553 552 551 552

504 518 530 523 527

Fracture strength (GPa)

Initial modulus (GPa)

Elongation (%)

0.57 ± 0.03 0.60 ± 0.03 0.61 ± 0.04 0.63 ± 0.03 0.55 ± 0.03

9.15 ± 0.60 9.25 ± 0.45 9.09 ± 0.51 9.48 ± 0.49 9.21 ± 0.52

11.98 ± 0.55 9.58 ± 0.38 9.31 ± 0.49 9.45 ± 0.58 9.13 ± 0.45

Fig. 2. (a) DMA and (b) TMA curves of PI and POSS/PI ammonium salt fibers.

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F. Liu et al. / Polymer Degradation and Stability 168 (2019) 108959

Fig. 3. TGA curves of PI and POSS/PI ammonium salt fibers under (a) nitrogen and (b) air atmosphere.

Fig. 4. Fracture strength (a) and initial modulus (b) of POSS/PI ammonium salt fibers added with different diethylamino-POSS contents.

properties before and after the addition of diethylamino-POSS nanoparticles regardless of under nitrogen atmosphere or air atmosphere. 3.4. Mechanical properties of POSS/PI ammonium salt fibers The mechanical properties of the fibers for various applications are important parameters. Fig. 4 shows the variation in mechanical properties with different diethylamino-POSS ammonium salt contents. As the diethylamino-POSS ammonium salt content was increased from 0 wt% to 15 wt%, the fracture strength and initial modulus of the POSS/PI ammonium salt fibers only slightly increased, with values of 0.63 GPa and 9.48 GPa compared with the pristine PI fiber with a fracture strength of 0.57 GPa and initial modulus of 9.15 GPa. When the POSS ammonium salt content was increased to 20 wt%, the mechanical properties of the fibers slightly decreased, and the fracture strength and initial modulus became 0.55 GPa and 9.21 GPa, respectively. These data show that the addition of diethylamino-POSS nanoparticles exerted no effect on the mechanical properties of POSS/PI ammonium salt fibers. 3.5. Surface morphologies and mass loss of fibers after AO erosion SEM provides a visualized situation on the surface morphological evolution for the fibers before and after AO erosion in Fig. 5. Prior to AO exposure, all the fibers showed smooth and flat surfaces without visual defects (Fig. 5a0ee0). After the AO fluence was increased from 0 atoms cm2 to 2.93  1020 atoms cm2, the surface morphologies of all fibers changed obviously from smooth to gully-like and a number of densely distributed etching tunnels was revealed. The pure PI fibers displayed more etching tunnels with

larger sizes than the POSS/PI ammonium salt fibers. When the AO fluence was increased from 1.01  1020 atoms cm2 to 2.93  1020 atoms cm2, the “gully-like” morphologies of the pure PI fibers became broad and deep, as shown in Fig. 5a1ea3. The POSS/PI ammonium salt fibers showed better surface morphologies than the pure PI fibers after AO exposure. Although the surface roughness of the composite fibers increased with raising AO fluence from 0 atom cm2 to 2.93  1020 atoms cm2, the composite fibers exhibited lower roughness after exposure under the same AO fluence. The surface roughness of the fibers obviously reduced when the diethylamino-POSS nanoparticle content was increased from 5 wt% to 15 wt%. Notably, when the added POSS ammonium salt content reached 20 wt%, the surface roughness of the composite fibers only slightly changed without obvious defects even when the AO fluence was increased to 2.93  1020 atoms cm2 (Fig. 5e3). Mass loss occurred when the POSS/PI ammonium salt fibers were exposed to variable simulated AO fluences. In current work, the mass loss of the POSS/PI ammonium salt fibers was replaced by the corresponding films because the low weights of fibers can cause high errors during testing. The mass loss in the films is shown in Table 4 and Fig. 6. In the whole test, the Kapton® film was used as a standard. When the AO fluence was increased from 5.40  1019 atoms cm2 to 2.93  1020 atoms cm2, the mass loss of the Kapton film and pristine PI film clearly increased from 0.20 mg cm2 to 1.20 mg cm2 and 0.10 mg cm2 to 1.25 mg cm2, respectively. The mass loss of the POSS/PI films was lower than those of the Kapton and pristine PI films. As the AO fluence was increased from 5.40  1019 atoms cm2 to 2.93  1020 atoms cm2, the mass losses of 5 wt% POSS/PI, 10 wt% POSS/PI, 15 wt% POSS/PI, and 20 wt% POSS/PI slowly increased from 0.10 mg cm2 to 0.45 mg cm2, 0.05 mg cm2 to 0.30 mg cm2, 0.05 mg cm2 to 0.25 mg cm2, and 0.05 mg cm2 to 0.15 mg cm2,

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Fig. 5. Fiber surface morphologies of (a0ea3) pure PI, (b0eb3) 5 wt% POSS/PI, (c0ec3) 10 wt% POSS/PI, (d0ed3) 15 wt% POSS/PI, and (e0ee3) 20 wt% POSS/PI after exposure to AO with a fluence at (a0ee0) 0, (a1ee1) 1.01  1020 atoms cm2, (a2ee2) 1.94  1020 atoms cm2, and (a3ee3) 2.93  1020 atoms cm2.

Table 4 Mass loss of POSS/PI films under different AO fluences. Under different AO fluence

Sample

5.40  1019 atoms cm2

1.01  1020 atoms cm2

1.48  1020 atoms cm2

1.94  1020 atoms cm2

2.93  1020 atoms cm2

(mg cm2)

(mg cm2)

(mg cm2)

(mg cm2)

(mg cm2)

0.20 0.10 0.10 0.05 0.05 0.05

0.35 0.25 0.15 0.15 0.10 0.05

0.55 0.45 0.20 0.20 0.15 0.10

0.70 0.65 0.25 0.25 0.20 0.10

1.20 1.25 0.45 0.30 0.25 0.15

Mass lossa

Kapton film Pure PI 5 wt% POSS/PI 10 wt% POSS/PI 15 wt% POSS/PI 20 wt% POSS/PI a

Using Equation (1) to calculate the total AO fluence with Kapton® film in this Table.

fluence, from 0.45 mg cm2 to 0.10 mg cm2 at 1.48  1020 atoms cm2 AO fluence, from 0.65 mg cm2 to 0.10 mg cm2 at 1.94  1020 atoms cm2 AO fluence, and from 1.25 mg cm2 to 0.15 mg cm2 at 2.93  1020 atoms cm2 AO fluence. Notably, when the POSS ammonium salt contents were raised to 15 wt% and 20 wt%, the mass losses showed the same values at low AO fluence, i.e., 5.40  1019 atoms cm-2, 1.01  1020 atoms cm-2, and 1.48  1020 atoms cm2 AO fluence. The above results indicate that the AO resistance of the POSS/PI films significantly improved with increasing POSS ammonium salt content. As the POSS ammonium salt content was increased to a certain extent, i.e., 20 wt%, the AO resistance remained a constant at low AO fluence. 3.6. Mechanical properties of fibers after AO erosion Fig. 6. Mass loss of POSS/PI films versus AO fluence.

respectively. Under the same AO fluence, the mass loss of the POSS/PI films decreased with increasing POSS ammonium salt content. For example, as the POSS ammonium salt content was increased from 0 wt% to 20 wt%, the mass loss of the films decreased from 0.10 mg cm2 to 0.05 mg cm2 at 5.40  1019 mg cm2 AO fluence, from 0.25 mg cm2 to 0.05 mg cm2 at 1.01  1020 atoms cm2 AO

At present, the decrease in mechanical properties caused by AO erosion is a problem that needs be solved to widen the applications of high-performance fibers in LEO. The mechanical properties of POSS/PI composite fibers before and after AO exposure are shown in Fig. 7. The fracture strength and initial modulus of the pristine PI fibers almost linearly decreased with a constant decay rate with increasing AO fluence from 0 atoms cm2 to 2.93  1020 atoms cm2, and the values rapidly decreased from 0.57 GPa to 0.34 GPa and from 7.41 GPa to 4.73 GPa, respectively. Such substantial

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decrease in mechanical properties of the fibers under AO exposure is fatal to spacecraft in LEO. When the AO fluence was increased from 0 atom cm2 to 2.93  1020 atoms cm2, the fracture strengths of the composite fibers with POSS ammonium salt contents of 5 wt %, 10 wt%, 15 wt%, and 20 wt% decreased from 0.54 GPa to 0.39 GPa, 0.56 GPae0.45 GPa, 0.54 GPae0.48 GPa, and 0.55 GPae0.49 GPa, respectively. The initial moduli of the composite fibers with POSS ammonium salt contents of 5 wt%, 10 wt%, 15 wt%, and 20 wt% declined from 7.37 GPa to 5.15 GPa, 7.58 GPae6.89 GPa, 7.22 GPae6.12 GPa, and 6.78 GPae6.20 GPa with increasing AO fluence from 0 atom cm2 to 2.93  1020 atoms cm2. Definitely, the retentions of fracture strength and initial modulus of the fibers increased from 59.65% to 89.09% and from 63.83% to 91.45% with increasing POSS ammonium salt contents from 0 wt% to 20 wt%. Moreover, Fig. 8 showed the corresponding stress-strain curves of the fibers before and after AO exposure with the fluence of 2.93  1020 atoms cm2. These results demonstrate that the AO resistance of the POSS/PI composite fibers improved considerably with increasing POSS ammonium salt content. Thus, the improved AO resistance is likely sustainable even under high AO fluence conditions, which implies that the service time of the POSS/PI composite fibers applied in LEO can be effectively extended. To quantitatively investigate the AO resistance of these composite fibers, we considered the decay rate of mechanical performance of the fibers as a relevant parameter for evaluating and comparing the AO resistance of the fiber materials. The fracture strength decay rate is defined in Equation (2) as follows: D ¼ (SO  S)/F,

Fig. 7. Variation in (a) fracture strength, (b) initial modulus, and (c) fracture strength decay rate of POSS/PI composite fibers with AO fluences.

(2)

where D is the fracture strength decay rate (Pa cm2 atom1), So denotes the pristine value of fracture strength (Pa) before AO irradiance, S represents the retention value of fracture strength (Pa) after AO irradiance, and F is the AO fluence (atom cm2). The decay rate of these fibers was calculated, and the detailed results are listed in Fig. 7c. The strength decay rates of the PI fibers were similar under the different AO fluences, with an average decay rate of 9.51  1013 Pa cm2 atom1. As the POSS ammonium salt content was increased from 0 wt% to 20 wt%, the strength decay rates of the POSS/PI composite fibers dramatically decreased from 7.40  1013 Pa cm2 atom1 to 1.85  1013 Pa cm2 atom1, 6.93  1013 Pa cm2 atom1 to 1.98  1013 Pa cm2 atom1, 6.08  1013 Pa cm2 atom1 to 2.03  1013 Pa cm2 atom1, 6.18  1013 Pa cm2 atom1 to 2.06  1013 Pa cm2 atom1, and 6.14  1013 Pa cm2 atom1 to 2.04  1013 Pa cm2 atom1 under AO fluence at 5.4  1019 atoms cm-2, 1.01  1020 atoms cm-2, 1.48  1020 atoms cm-2, 1.94  1020 atoms cm-2, and 2.93  1020 atoms cm2. The results indicated that the AO surface erosion of

Fig. 8. The stress-strain curves of fibers (a) before and (b) after AO exposure with the fluence of 2.93  1020 atoms cm2.

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silicon dioxide formed on the fiber surface when attacked by AO irradiation and could protect the fibers from further AO erosion. As a result, the content of silicon atom combined with oxygen atom remarkably increased. Consequently, the SiO2 layer on the surface of the composite fibers under the AO erosion confers the fibers with excellent AO resistance. 4. Conclusion

Fig. 9. XPS survey spectra of 10 wt% POSS/PI and 20 wt% POSS/PI before and after AO exposure.

Table 5 Surface composition of the POSS/PI composite fibers before and after AO exposure. Elemental species

Elemental content (%) 5 wt% POSS/PI

10 wt% POSS/PI

20 wt% POSS/PI

Unexposed Exposed Unexposed Exposed Unexposed Exposed C O Si

76.39 21.45 1.18

74.72 18.54 5.09

75.40 20.89 2.48

48.64 36.42 13.29

71.39 22.99 3.88

31.89 42.02 25.08

POSS/PI composite fibers gradually decreased with increasing POSS ammonium salt content.

3.7. Surface elemental composition of fibers after AO erosion To study the AO resistance mechanism of the POSS/PI composite fibers, we investigated changes in the chemical component and elemental states on the surface of the composite fibers before and after AO exposure through XPS measurements. Fig. 9 shows the XPS survey spectra of the 10 wt% and 20 wt% POSS/PI composite fibers before and after AO exposure with an AO fluence of 2.93  1020 atoms cm2. Aside from C1s and O1s peaks, small Si 2p and Si 2s peaks near 100 eV were observed before AO erosion in the XPS curve of the composite fibers. After AO erosion, the peak intensities of Si 2p and Si 2s strengthened obviously. On the basis of the elemental contents of the composite fibers before and after AO exposure presented in Table 5, the carbon and oxygen atom contents of the 5 wt% POSS/PI composite fibers slightly reduced from 76.39% to 74.72% and from 21.45% to 18.54%, respectively. The content of silicon atoms obviously increased from 1.18% to 5.09%. However, the carbon atom contents of the 10 wt% and 20 wt% POSS/ PI composite fibers dramatically decreased from 75.40% to 48.64% and from 71.39% to 31.89%, respectively. The oxygen atom contents of the 10 wt% and 20 wt% POSS/PI composite fibers prominently increased from 20.89% to 36.42% and from 22.99% to 42.02%, respectively. A similar change pattern was observed for the silicon atom contents of the 10 wt% and 20 wt% POSS/PI fibers, which increased from 2.48% to 13.29% and from 3.88% to 25.08%, respectively. Volatile gases are produced and released after reactions of strong oxidizing AO with the atoms on the fiber surface of carbon, hydrogen, nitrogen. Thus, the content of carbon atoms decreased after exposure to AO beam. As for the silicon atom, the nonvolatile

PI composite fibers containing POSS ammonium salts were prepared via introduction of diethylamino-POSS into PAA solution, followed by wet spinning. The good thermal and mechanical properties of the composite fibers were maintained compared with the pure PI fibers. The average T5% and Tg were 552
C and 396
C, respectively. When the POSS ammonium salt content was increased from 0 wt% to 15 wt%, the fracture strength of the fibers slightly increased from 0.57 GPa to 0.63 GPa, whereas the initial modulus did not clearly change with the average value of 9.27 GPa. Moreover, the uniform distribution of POSS ammonium salts in the composite fibers was proven by comparing the Si content of surface (0.81 wt%, 2.25 wt%, 2.46 wt%, and 2.93 wt%) with that of average (0.97 wt%, 1.87 wt%, 2.25 wt%, and 2.99 wt%). SEM results indicated that the composite fibers displayed a compact surface, whereas the pure PI fiber possessed a loose surface with large number of gullies after AO erosion. When exposed to AO fluence with 2.93  1020 atoms cm2, mass loss obviously decreased from 1.25 mg cm2 to 0.15 mg cm2 as the POSS ammonium salt content was increased from 0 wt% to 20 wt%. The retentions of fracture strength and initial modulus of the fibers at the AO fluence of 2.93  1020 atoms cm2 dramatically improved from 59.65% to 89.09% and from 63.83% to 91.45% when the POSS ammonium salt contents were increased from 0 wt% to 20 wt%, respectively. Notably, the strength decay rates of the composite fibers dramatically reduced from 9.21  1013 Pa cm2 atom1 to 2.04  1013 Pa cm2 atom1 at the AO fluence of 2.93  1020 atoms cm2. Actually, the passivating layer of SiO2 formed on the fiber surface after AO exposure and can protect the fibers from further AO erosion. The introduction of diethylamino-POSS with positive charges improved the distribution of POSS nanoparticles in the PAA solutions with negative charges. Thus, the as-prepared composite fibers possess excellent AO resistance and are promising candidates for spacecraft applications. Acknowledgements This work was supported by the National Key R&D Program of China (2017YFB0308300). We also thank Beihang University for their helps in ground-based simulated AO exposure experiment. References [1] C. Yang, J. Dong, Y. Fang, L. Ma, X. Zhao, Q. Zhang, Preparation of novel low-k polyimide fibers with simultaneously excellent mechanical properties, UVresistance and surface activity using chemically bonded hyperbranched polysiloxane, J. Mater. Chem. C 6 (2018) 1229e1238. [2] F. Liu, H. Guo, Y. Zhao, X. Qiu, L. Gao, Eur. Polym. J. 105 (2018) 115e125. [3] X. Yi, Y. Gao, M. Zhang, C. Zhang, Q. Wang, G. Liu, X. Dong, D. Wu, Y. Men, D. Wang, Eur. Polym. J. 91 (2017) 232e241. [4] Q. Zhang, J. Dong, D. Wu, Advanced Polyimide Materials: Chapter 2-Advanced Polyimide Fibers, Elsevier, 2018, pp. 67e92. [5] J. Dong, C. Yang, Y. Cheng, T. Wu, X. Zhao, Q. Zhang, Facile method for fabricating low dielectric constant polyimide fibers with hyperbranched polysiloxane, J. Mater. Chem. C. 5 (2017) 2818e2825. [6] Y. Zhao, Z. Dong, G. Li, X. Dai, F. Liu, X. Ma, X. Qiu, Atomic oxygen resistance of polyimide fibers with phosphorus-containing side chains, RSC Adv. 7 (2017) 5437e5444. [7] Y. Zhao, G. Li, X. Dai, F. Liu, Z. Dong, X. Qiu, AO-resistant properties of polyimide fibers containing phosphorous groups in main chains, Chin. J. Polym. Sci. 34 (2016) 1469e1478.

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