Substantially enhanced durability of polyhedral oligomeric silsequioxane-polyimide nanocomposites against atomic oxygen erosion

Accepted Manuscript Substantially enhanced durability of polyhedral oligomeric silsequioxane-polyimide nanocomposites against atomic oxygen erosion Xiaobing Li, Ahmed Al-Ostaz, Mohemmed Jaradat, Farzin Rahmani, Sasan Nouranian, Grace Rushing, Alharith Manasrah, Hunain Alkhateb, Miria Finckenor, Joseph Lichtenhan PII: DOI: Reference:

S0014-3057(16)31728-1 http://dx.doi.org/10.1016/j.eurpolymj.2017.05.004 EPJ 7859

To appear in:

European Polymer Journal

Received Date: Revised Date: Accepted Date:

21 December 2016 22 April 2017 2 May 2017

Please cite this article as: Li, X., Al-Ostaz, A., Jaradat, M., Rahmani, F., Nouranian, S., Rushing, G., Manasrah, A., Alkhateb, H., Finckenor, M., Lichtenhan, J., Substantially enhanced durability of polyhedral oligomeric silsequioxane-polyimide nanocomposites against atomic oxygen erosion, European Polymer Journal (2017), doi: http://dx.doi.org/10.1016/j.eurpolymj.2017.05.004

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Substantially enhanced durability of polyhedral oligomeric silsequioxanepolyimide nanocomposites against atomic oxygen erosion

Xiaobing Lia*, Ahmed Al-Ostaz a, Mohemmed Jaradata,b, Farzin Rahmanic, Sasan Nouranianc, Grace Rushinga, Alharith Manasraha, Hunain Alkhateba, Miria Finckenord, Joseph Lichtenhane

ABSTRACT Several groups of polyimide (PI)-based nanomaterials reinforced with polyhedral oligomeric silsequioxane (POSS) nanoparticles were subjected to atomic oxygen (AO) exposure to investigate

the

effects

of POSS

and

glassification

plasma

pre-treatment.

Various

characterizations revealed the clear effects of AO degradation, such as decreased transmissions of all tested films. POSS significantly enhanced the stability of PI in terms of mass loss under an AO environment. One polyimide film sample containing 10 wt.% POSS also exhibited excellent stability in mechanical properties measured by dynamic mechanical analyzer (DMA). Surface topography and roughness of all films were qualitatively and quantitatively analyzed using the atomic force microscope (AFM). After AO irradiation the POSS-filled films showed a much smoother surface than that of neat PI film. The results of mass loss, mechanical property and AFM topography collectively indicate the exceptional self-healing capability of the POSS nanoparticles upon AO erosion that enables it to protect polyimide due to the formation of a thin glass layer. A further molecular dynamics simulation of the neat PI and PI-POSS system revealed reduced mass loss, damage propagation depth, and erosion yield of the nanocomposites with increasing POSS concentration, which are consistent with the experimental findings.

1

Keywords: POSS; Atomic Oxygen; Self-healing; Polyimide; Molecular Dynamics Simulation.

a

Department of Civil Engineering, University of Mississippi, University, MS

b

Department of Civil Engineering, University of New Mexico, Albuquerque, NM

c

Department of Chemical Engineering, University of Mississippi, University, MS

d

Environmental Effects Branch, Marshall Space Flight Center, AL

e

Hybrid Plastics Inc., Hattiesburg, MS

* Corresponding Author. Department of Civil Engineering, 106 Carrier Hall, University of Mississippi, University, MS 38677, USA Tel: 1 6629151975 Email address: [email protected]

2

1. Introduction

Satellites and space shuttles face severe environmental conditions such as ultra-violet radiation and orbital debris when placed in low earth orbit, which ranges from 200 to 1000 km above the earth [1,2]. The concentrations of near earth energetic species (e.g., photons from sunlight, particles from solar flares and galactic cosmic rays, high velocity neutral gases, the Van Allen Belts, and the ionosphere that originate from the Earth or from interactions between the Earth’s upper atmosphere and other energetic species) vary with altitude and are summarized in terms of importance to the success of space missions in Table 1 for the most common orbits – low Earth orbit (low and high inclination), medium Earth orbit, and geostationary Earth orbit [3]. Films of polymers such as fluorinated ethylene propylene (FEP), polyethylene, polyether ether ketone (PEEK), and Kapton polyimide (PI), which are commonly used on exterior spacecraft surfaces for thermal control blankets, structural composites, potting materials and adhesively bonded radiator surfaces, receive an unshielded dose of space radiation [3,4]. The space environment can be extremely damaging to these polymers. The damage caused by the space service missions after 3.6 and 6.8 years was evidenced by the cracking of the Hubble Space Telescope’s Teflon-FEP thermal blankets due to severe environmental degradation. The telescope is in LEO at an altitude of ~600 km, and the damage was observed by the servicing astronauts and from retrieval of token samples evaluated in ground tests. A taskforce conducted a comprehensive investigation into why FEP was degrading in LEO. Many of its findings can also be applied to polyvinylidene fluoride (PVDF) and copolymers [3].

3

Table 1 Importance of environmental effect on spacecraft in LEO, MEO and GEO. LEO(1) LEO Spacecraft Environment MEO(2) GEO(3) Low inclination High inclination Direct Sunlight 4(4) 4 4 4 Gravity Field 3 3 3 0 Magnetic Field 3 3 3 0 Van Allen Belts 0-5 2-5 5-8 1 Solar Flare Particles 0 4 3 5 Galactic Cosmic Rays 0 4 3 5 Debris Objects 7 7 0-3 3 Micrometeoroids 3 3 3 3 Ionosphere 3 3 1 0 Hot Plasma 0 3 0 5 Neutral Gases 7-9 7-9 0-3 0 (1) Low Earth Orbit (LEO) extends up to 1000km. (2) Mid Earth Orbit (MEO) is above 1000 km and extends up to 35,000 km. (3) Geosynchronous Orbit (GEO) is 35,000 km and higher. (4) Impacts are ranked on a scale from 0 (the effects can be ignored) to 10 (the effects will negate the mission).

Atomic oxygen, which is the main component in low earth orbit, is one of the most degrading effects that polymers have to withstand. It is formed when molecular oxygen is exposed to X-ray and UV radiation from the sun and experiences dissociation (ionization) of O2 into single O atoms [5]. Due to its very reactive nature, AO is not found on earth’s surface. At least it has not been for a long period of time [6]. The atmosphere in LEO is comprised of 96% atomic oxygen. In LEO the high flux of atomic oxygen (approximately 10 15 atoms/cm2-s with an orbital speed of 8 km/s) formed by photo-dissociation of a small concentration of molecular oxygen in the upper atmosphere, causes surface pitting and erosion, with reported rates of 0.35 x 10-24 cm3/atom for FEP and 3.0 x 10-24 cm3/atom for Kapton PI [3]. It is worth noting that the radiation effects are often synergistic with thermal effects. The two commonly used techniques for simulating the atomic oxygen found in LEO are: (a) hyperthermal or fast atomic oxygen, and (b) oxygen plasma. 4

Silica offers outstanding resistance to oxygen and has been used to strengthen a number of polymeric coatings [7-13]. Polyhedral oligomeric silsesquioxane (POSS), a class of silica-based nano chemicals, could be designed to fulfill various mechanical functions. Previous tests of polymers containing nanostructured POSS chemicals reveal that they are radiation insensitive and provide at least a ten-fold improvement in the resistance of AO erosion over the control materials, such as Kapton PI [14,15]. This order of magnitude of change is promising and could enable the development of a new generation of unique space survivable materials. The homogeneous dispersion of POSS cages within polyimides provides enhanced control over chain dynamics which increases the toughness and damage tolerance over conventional polyimides. The chemical composition of POSS provides increased oxidation resistance by enabling the insitu formation of a nanoscopically thin surface glass upon exposure to oxidants. POSS-enhanced polymers can self-heal when subjected to AO. Upon exposure of the POSSalloyed polymer to an oxidizing source (e.g. AO), the silicon–R bonds (R: functional groups attached to the corners of POSS cage) on the cage are broken and the R group is lost as a volatile byproduct [5,16,17]. The valence to the silicon in the cage is maintained through the fusing of cages by bridging oxygen atoms thus rendering the equivalent of glass on the surface. This process is thermodynamically favored and nonreversible (Fig. 1). This process has even been identified as self-healing in the space environment [17,18]. Surface glassification enables both increased durability and bonding to the surface. Glassified POSS coatings have been demonstrated by National Aeronautics and Space Administration (NASA) Glenn Research Center to be stable to vacuum ultraviolet (VUV) for 1024 equivalent sun hours of exposure [19].

5

(a)

(b)

Fig. 1. (a) In situ oxidative fusing of cages. (b) The formation of optically clear glass/ceramic surface layer.

2. Experimental

2.1 Materials

The POSS materials were from Hybrid Plastics Inc., MS, USA. The polyimide was thermally cured from polyamic acid 15 wt.% in N-Methyl-2-pyrrolidone (NMP) (Pyre-ML RC5019, Industrial Summit Technology Corporation, NJ, USA). The viscous polyamic acid solution was brushed onto a glass plate followed by curing in an oven in a hood. The curing procedure was stepwise heating at ~5 oC/min but the oven chamber was kept isothermal at 120, 180, 240 and 300 oC for 30 min, respectively. The cured PI film was peeled off after soaking in water for a few minutes. The PM1215 thin films were produced in the same way using the pre-mixed solution of NMP, polyamic acid and trisilanol phenyl POSS, while the rest of the materials, Thermalbright

(containing

trisilanol

phenyl

POSS)

and

Corin

XLS

(containing

aminopropylisobutyl POSS), were used as received. These two are commercial POSS-polyimide films produced by Nexolve Corporation, AL, USA, a division of ManTech International, VA. 6

Corin XLS is the trade name for Nexolves aerospace grade POSS polyimide which, in addition to being clear and colorless, provides on-orbit self-healing glassification capability. Thermalbright is a polyimide filled with 50 wt.% TiO2 in addition to POSS. Table 2 shows the different combinations of materials used in this research. Various degrees of glassification, namely, 1, 2, 3, 4 and 5 minutes, were conducted but no clear difference was found in the prescreening test of mechanical properties (modulus, strength, etc.). The 3-minute glassified samples were used in subsequent AO experiment.

Table 2 Materials used in the experiment of atomic oxygen exposure. Material Time of exposure to plasma (min) PI 0 Thermalbright 0, 3 PM1215 0, 3 Corin XLS 0, 3

POSS (wt.%) 0 5 10 30

2.2 Glassification process

Exposure to oxygen plasma causes oxidation, chain scission along with cross-linking between the different chains and the formation of a silica-like surface [13,19]. This silica-like surface is a thin glassy layer that reduces any additional surface damage to the underlying polyimide, and it is an inherent property of POSS-filled polyimides. The purpose of surface glassification was to complete the oxidation process before the surface was placed into an AO environment. If it is pre-glassified it may experience no, or very slight, mass loss when exposed to AO. Specimens were glassified using an oxygen plasma cleaner (Harrick Plasma, NY, USA) to modify thin films surfaces. A high setting of 29.6 Watts was used to produce a pressure of about 13.3-24.0 Pa.

7

2.3 Atomic oxygen experiment Seven samples (Table 2) were subjected to AO radiation at NASA Marshal Space Flight Center (MSFC), AL, USA. Specimens of 2.54 cm diameter were punched out of the test sample films. The specimens to be exposed were weighed before and after AO exposure. In addition, solar absorptance and infrared emittance measurements were made on the Thermalbright samples because they appeared to be non-transmissive. Photographs were taken at each step during testing. Many polymer films are hygroscopic. Therefore, these were weighed by the normal method for hygroscopic materials. One sample at a time was placed in a small vacuum chamber and pumped down to 6.7 Pa. Vacuum was broken, and a timer was started. Weight was noted at 30-second intervals up to 3 minutes, and then the weight at time zero was calculated by regression analysis. The Corin XLS and Thermalbright samples were not hygroscopic. Solar absorptance for the 250 nm to 2800 nm wavelength band was measured using an AZ Technology Laboratory Portable Spectroreflectometer (LPSR) (AZ Technology Corporation, AL, USA). Integrated infrared emittance was measured using an AZ Technology TEMP 2000 Infrared Reflectometer (AZ Technology Corporation, AL, USA). The detector for this instrument is sensitive between 2 and 35 micron wavelengths. Stray light was minimized as much as possible to reduce measurement error. The samples were placed in the MSFC Atomic Oxygen Beam Facility (AOBF) (Fig. 2). In this facility, magnetically-confined AO plasma interacts with an electrically biased metal plate. Following collision with the neutralizer plate, the atoms are reflected towards the test specimens. Periodic beam current measurements were used to monitor AO fluence, backed up by mass loss measurements of a 2.54 cm diameter sample of uncoated Kapton polyimide film in the center of 8

the sample holder. The O2 dissociation and ionization during the plasma production generates electromagnetic radiation primarily at 130 nm, the AO resonant peak in the vacuum ultraviolet region. Fig. 3(a) shows the films in the sample holder prior to AO exposure. Note that Corin XLS samples are clear and the aluminum shims are visible. A Kapton PI film for monitoring AO fluence was placed in the center. An image of the test specimens after AO exposure is shown in Fig. 3(b).

Fig. 2. MSFC Atomic Oxygen Beam Facility.

9

Fig. 3. (a) Before AO-exposure, samples were placed clockwise from 1:00 position: polyimide, PM1215, PM1215 with 3 min O2 plasma, Corin XLS, Corin XLS with 3 min O2 plasma, Thermalbright and Thermalbright with 3 min O2 plasma. A Kapton PI film was in the center. Each sample had an exposure area of 5.06 cm2. (b) After AO exposure: annulus was unexposed to AO and was particularly noticeable for the Kapton PI film (center) and the test samples.

2.4 Characterization

Transmission was measured at MSFC from 200 to 2400 nm using a Lambda 1050 dualbeam, ratio recording spectrophotometer (Perkin Elmer, MA, USA) with 150 mm diameter sphere. The effects of plasma treatment and AO exposure on the mechanical properties of the thin films were evaluated by a dynamic mechanical analyzer (DMA) (DMA Q800, TA Instruments, DE, USA). Two types of measurements were performed: (1) static stress-strain response and (2) dynamic response. The thin films were tested under the controlled force mode with constant force ramp (0.1 N/min). In the tensile tests, all samples were equilibrated at 35 oC and were kept isothermally for 5 minutes. A temperature sweep (1 Hz, 20 µm amplitude and a heating rate of 10 oC/ min) was employed to obtain the values of storage modulus, tan δ and glass transition temperature (Tg) for the thin films. 10

An easyScan 2 Flex Atomic Force Microscope (AFM) by Nanosurf (Boston, MA, USA) was used to examine the effect of AO exposure on the surfaces of thin films. The AFM was operated in tapping mode at ambient temperature. Silicon AFM probes with reflective aluminum coated tip, a 5 N/m spring constant and a resonance frequency of around 150 kHz were used. The tapping force was varied by controlling the amplitude set point for each scan and was dependent on the specimen conditions.

3. Computational A series of molecular dynamics (MD) simulations were run for representative PI, which contains characteristic imide groups [20,21], and PI-POSS systems to elucidate the material damage and its propagation as a function of POSS concentration when exposed to atomic oxygen attack. The methodology used here is adapted from the work of Rahnamoun and van Duin [22]. Models of the PI building blocks and POSS were created in BIOVIA Materials Studio (v8.0) (Fig. 4). Three different systems composed of 1) neat polyimide (designated as Neat PI), 2) polyimide/POSS at low POSS concentration (15 wt.%) (PI-POSS-15), and 3) polyimide/POSS at high POSS concentration (30 wt.%) (PI-POSS-30) were subsequently packed in a 3D-periodic simulation box and energy-minimized using the Conjugate Gradient method [23]. For each system, an initial NPT (constant number of atoms, N; constant pressure, P; constant temperature, T) simulation was run for 2 ns at room temperature (298 K) and atmospheric pressure using the COMPASS force field [24]. The system temperature and pressure were controlled with the NoséHoover thermostat and barostat [25]. The simulations were run until the system densities were equilibrated at ~1.4 g/cm3 for the neat PI and ~1.9-2.0 g/cm3 for the PI-POSS composites, respectively. The final atomic coordinates were then exported to the LAMMPS software package

11

[26] into a 2D-periodic simulation box with periodicity in the x and y directions. A vacuum slab was then placed on top of the material in the z direction. Next an NVE (constant number of atoms, N; constant volume, V; constant energy, E) simulation was run for each system for 10 ps using the reactive force field (ReaxFF) [27,28] until the system temperature was stabilized at 298 K. In these simulations, the time step and cut-off distance were 0.1 fs and 9 Å, respectively. Then another NVE simulation was run for a total simulation time of 35 ps, during which the material surface was bombarded with oxygen atoms in 200 fs intervals from a distance of 70 Å above the material surface and with the velocity of 0.08 Å/fs (=8 km/s) in the z direction. The trajectory files were saved every 200 fs and analyzed to generate mass loss, mass density, and temperature profiles for the different systems.

Fig. 4. Models of the polyimide building block and POSS.

12

4. Results and discussions 4.1 AFM analysis

AFM tapping mode imaging can produce both height and phase images of a sample by oscillating the cantilever at or near its resonant frequency while it scans the sample surface. The height, or topographic, images are obtained by measuring the changes in the oscillation amplitude and the deflection of the cantilever. The phase images are generated by measuring the difference between the input cantilever oscillation signal and the actual cantilever oscillation as it interacts with the surface. The greater the lag between the two signals, the greater the contrast in the phase image. Height and phase images are generally obtained simultaneously to show surface variations. The thin films were scanned in tapping mode and both the height and phase images were acquired. The images of a 3-D height scan on an area of 9.6 µm x 9.6 µm for different films are shown in Fig. 5. In general, for all of the samples, exposure to atomic oxygen greatly increased the roughness of the sample surface and erased any of the original features on the surface. However, polyimide was affected the most by AO attack, as indicated by the large and deep craters/valleys. Another trend that should be noted is that the plasma glassification pretreated samples were affected less than the non-plasma treated samples in the AO exposure experiment. Quantification of the roughness in terms of root mean square height, which is an indicator of the roughness of scanned area, is summarized in Table 3. Apparently the roughness caused by AO was reduced progressively with the amount of POSS incorporated due to the passivation upon AO exposure. As discussed in Section 1, the attack by AO on the surface of POSS-filled polyimide formed a thin layer of SiO2, which effectively hindered further AO erosion and protected underlying PI 13

matrix [15,29]. The AO-induced increased roughness is attributed to the fact that groups on the polyimide chains have different reaction rates under AO sputtering, which causes non-uniform mass losses throughout the surface [30]. Phase images and their corresponding Abbott-Firestone curves were collected to characterize the effects of POSS and AO irradiation on the stiffness. The curve gives an indication of distribution of the fractions with different hardness values on the surface of a specimen detected by the AFM tip. In the figure of Abbott-Firestone curve, the y-axis is the phase angle (o). The top x-axis is the cumulative probability density function (red line). The bottom x-axis is the percentage of the surface which falls in a certain phase angle range (blue bar graph). Seen in Fig. 6, neat polyimide clearly had very uniform distribution in surface hardness. After AO treatment, relatively softer fractions arose, which might be attributed to the voids/valleys created by AO attack or smaller molecules from chain scission. The results for POSS-filled nanocomposite materials are displayed in Fig. 7-9. In general, AO exposure produced higher percentages of more rigid components because certain soft polymer domains were etched and stricken off. In Fig. 7, the AO attack resulted in higher heterogeneity on the surface of the Thermalbright films which contained 5 wt.% POSS and 50 wt.% TiO 2. For PM1215 and CorinXLS (Fig. 8 and 9, respectively), the higher fraction of rigid phases in AO-exposed films should be ascribed to the removal of the soft PI matrix and the formation of SiO2. Compared with Thermalbright, the latter two exhibited more rigid content after AO treatment as a higher POSS loading could generate more silica. Oxygen plasma treatment reduced the relatively soft domain’s percentage on the surface for all the three POSS filled samples.

14

Fig. 5. AFM 3-D images (9.6 x 9.6 µm) of films: (a) polyimide; (b) Thermalbright; (c) plasmatreated Thermalbright; (d) PM1215; (e) plasma-treated PM1215; (f) CorinXLS and (g) plasmatreated CorinXLS. 15

Table 3 Effect of AO exposure on the roughness (root mean square height) based on AFM height scans of 9.6 x 9.6 µm. Roughness (nm) Before exposure After exposure PI 1.2 293.0 Thermalbright 19.0 223.0 Thermalbright plasma 52.7 198.0 PM1215 5.8 77.3 PM1215 plasma 2.5 55.5 CorinXLS 1.6 25.5 CorinXLS plasma 1.2 15.6

(a)

(b)

Fig. 6. Phase scans and corresponding Abbott-Firestone curves of PI and AO-exposed PI: (a) polyimide; (b) AO-exposed polyimide.

16

(a)

(b)

(c)

(d)

Fig. 7. Phase scans of Thermalbright and corresponding Abbott-Firestone curves: (a) Thermalbright; (b) AO-exposed Thermalbright; (c) plasma-treated Thermalbright; (d) AO-exposed plasma-treated Thermalbright. 17

(a)

(b)

(c)

(d)

Fig. 8. Phase scans of PM1215 and corresponding Abbott-Firestone curves: (a) PM1215; (b) AO-exposed PM1215; (c) plasma-treated PM1215; (d) AO-exposed plasma-treated PM1215.

18

(a)

(b)

(c)

(d)

Fig. 9. Phase scans of CorinXLS and corresponding Abbott-Firestone curves: (a) CorinXLS; (b) AO-exposed CorinXLS; (c) plasma-treated CorinXLS; (d) AO-exposed plasma-treated CorinXLS. 19

4.2 Mass loss and erosion yield

The AO fluence by beam current measurements on Kapton PI witness mass loss was 7.77 x 1020 atoms/cm2. The concurrent vacuum ultraviolet radiation exposure was approximately 600 equivalent sun-hours. Mass losses are given in Table 4. The densities in Table 4 were actually measured except that for PI. Compared with neat PI, all the other samples lost much less mass after AO exposure. It appeared that 50 wt.% TiO2 in Thermalbright did not improve the resistance to AO erosion. Plasma treatment slightly enhanced the stability in the AO environment for the Thermalbright and PM1215 samples. Plasma-treated CorinXLS showed a small increase in mass loss, which might be due to measurement error since the values were already very small. According to the ASTM standard E2089, erosion yield (Ey) it is defined as the ratio of the volume or mass lost per each incident oxygen atom. There is a wide range of factors that may affect the erosion yield value of a material (e.g. AO flux, AO fluence, impact angle, and material temperature). Due to the limited amount of in-space testing, a comprehensive understanding of the influence that these factors have on erosion yield has not been well established yet.

???? =

(

Where: ΔM = mass loss of the sample (g) ρ = density of the sample (g/cm3) A = surface area of the sample exposed to atomic oxygen (cm2) F = effective fluence (atoms/cm2), or actual fluence if measured in space

20

Table 4 Mass change due to AO exposure. Pre-test Material Mass (mg) 17.20 Polyimide 18.81 Thermalbright 19.23 Thermalbright plasma 47.08 PM 1215 47.33 PM 1215 plasma 8.10 Corin XLS 16.51 Corin XLS plasma

Post-test Mass (mg) 10.12 17.27 17.80 46.25 46.63 7.96 16.34

Mass Loss (mg) 7.08 1.54 1.43 0.83 0.70 0.14 0.17

Mass Loss (mg/cm2) 1.399 0.304 0.283 0.164 0.138 0.028 0.034

Erosion yield calculations were made for the samples with known densities. Using 1.24 g/cm3 for Corin XLS, the erosion yield of the non-plasma sample was calculated to be 2.87 x 1026

cm3/atom. Flight data from MISSE-7B ram-facing side indicated 3.05 x 10-26 cm3/atom for

Corin XLS, which is very close. The erosion yield for the Thermalbright samples using a density of 2.07 g/cm3 was 1.89 x 10-25 cm3/atom, which is somewhat higher than the MISSE-6A flight data of 9.0 x 10-26 cm3/atom. It is not unusual for fluorinated polymers to react more strongly in the AOBF than in orbit. The Thermalbright samples appeared to be slightly bleached by the AO exposure, which was confirmed by solar absorptance measurements. The results of erosion yield for all samples are summarized in Table 5. Similar to the mass losses, the erosion yield and erosion depth results reveal that the incorporation of POSS has significantly reduced the erosion by AO. PI generated the largest erosion yield due to the lack of protection from POSS. The plasma pre-treatment slightly enhanced the stability of both Thermalbright and PM1215. The mass loss was attributed to the breakage of various chemical bonds in polyimide producing a number of volatile products of small molecules such as CO, CO2, H2O and NOx when AO is sputtered onto film surface [8]. The C-N bond was reported to be predominantly

21

broken by AO-irradiation [31]. The fly-away of those volatiles is attributed to the mass loss of PI and the mass loss is linearly proportional to AO fluence [1,10,14,15] leading to continuous mass loss [30]. Large valleys on the surface of the PI film revealed by AFM images demonstrated the loss in polymer mass. However, the incorporated POSS nanoparticles might be converted to a thin layer of silica upon AO attack. Silica is very resistant to AO and thus stops further erosion. The mass loss, in this case, mainly occurs at the very beginning [7,10,17] when POSS nano particles lose the organic functional groups [13,17] and polyimide chemical structure at the surface is damaged [14]. The formation of a new SiO2 layer was called passivation [8,14,15], self-repairing or self-healing [17,32].

Table 5 Erosion yield after AO exposure of the tested samples, assuming the plasma surface treatment did not change the densities. Mass Loss Density Erosion yield (Ey) erosion depth Material (mg) (g/cm3) (cm3/atom) (μm) 7.08 1.42* 1.27E-24 9.85 Polyimide 1.54 2.07 1.89E-25 1.47 Thermalbright 1.43 2.07 1.76E-25 1.37 Thermalbright plasma 0.83 1.41 1.50E-25 1.16 PM 1215 0.70 1.41 1.26E-25 0.98 PM 1215 plasma 0.14 1.24 2.87E-26 0.22 Corin XLS 0.17 1.25 3.46E-26 0.27 Corin XLS plasma * This value was provided by the vendor.

4.3 Transmission Fig. 10 (a) shows that AO exposure of PI reduces transmission by about 10%. The same was observed for PM1215 [Fig. 10(b)]. Fig. 10(b) also shows that plasma treatment reduces the degradation of transmission after AO exposure. Transmission for Thermalbright is much lower than other films [Fig. 10(d)] with very little reduction due to AO exposure. The interference patterns observed for Corin XLS films (Fig. 10c) could be due to the optical quality of films used 22

in this study. Similar behavior to that observed in Fig. 11(a) and (b) for Corin XLS was observed by NASA researchers. As it can be seen, there is some interference pattern [Fig. 11(b)]. However, it is not as notable as that shown in Fig. 10(c).

100

80 60

Control AO-exp.

40 20

Transmission (%)

Transmission (%)

100

0 0

80 60

Control AO-exp. Plasma control Plasma AO-exp.

40 20 0

0

500 1000 1500 2000 2500 Wavelength (nm)

500

1500

2000

2500

Wavelength (nm)

(b)

(a) 100

80 60

Control AO-exp. Plasma control Plasma AO-exp.

40

20 0

0

500

1000 1500 2000 2500 Wavelength (nm)

Transmission (%)

100 Transmission (%)

1000

Control AO-exp. Plasma control Plasma AO-exp.

80 60 40 20 0 0

500

1000 1500 2000 Wavelength (nm)

2500

(d) (c) Fig. 10. Transmission of control and exposed thin films made from (a) Polyimide, (b) PM 1215, (c) Corin XLS, and (d) Thermalbright.

23

(a)

(b)

Fig. 11. Transmission measurements from control and flight samples of Corin XLS for (a) MISSE-6 and (b) MISSE-7.

4.4 Tensile and thermal mechanical properties

DMA was used to measure the stress-strain properties since the size of each sample was very small (diameter: 2.54 cm). A typical stress-strain result is shown in Fig. 12. Values of strength at break, strain at break and toughness (energy density) were tabulated (Table 6). The plasma glassification appeared to be effective for Thermalbright, in which the POSS content was at a low level (5 wt.%). The plasma treated Thermalbright lost less tensile properties compared with the untreated after AO experiment. In Table 6, either PM1215 or glassified PM1215 offered tensile properties favorably comparable to those of the pre-AO-treated samples. In comparison, neat polyimide exhibited the largest loss in toughness caused by AO attack. This result is generally consistent with the erosion yield discussed above. However, Corin XLS of high POSS loading was not helpful in retaining the mechanical properties probably because AO erosion caused POSS to generate too much volatile product which weakened the film. The volatiles might create voids or become impurities that then become the initiation points of rupture [31].

24

Stress (MPa)

150 120 90 60 30 0 0

20

40

60 80 Strain (%)

100

120

Fig. 12. Stress-strain response of PM1215 film.

Table 6 Tensile properties of films before and after AO erosion. Strength at break Strain at break Tensile Toughness (MPa) (%) (MJ/m3) Change Change Sample (%) (%) AO AO AO control control Control exposed exposed exposed PI 173.8 129.6 -25.4 91.6 61.8 -32.5 108.9 60.1 TB0 63.7 56.0 -12.1 7.8 6.1 -21.8 4.1 2.8 TB3 62.4 57.7 -7.5 7.1 6.9 -2.8 3.65 3.25 PM0 118.6 143.6 21.1 105.6 102.3 -3.1 100.4 117.8 PM3 125.3 118.3 -5.6 102.1 105.4 3.2 103.5 99.7 Corin0 62.3 67.5 8.3 20.5 13.9 -32.2 10.3 9.7 Corin3 62.1 56.6 -8.9 12.9 10.4 -19.4 6.1 4.0 PI: Polyimide. TB0: Thermalbright 0-minute plasma-treated. TB3: Thermalbright 3-minute plasma-treated. PM0: PM1215 0-minute plasma-treated. PM3: PM1215 3-minute plasma-treated. Corin0: Corin XLS 0-minute plasma-treated. Corin3: Corin XLS 3-minute plasma-treated.

Change (%) -44.8 -31.7 -11.0 17.3 -3.7 -34.0 -34.4

Fig. 13 is a typical plot of the storage modulus and tan delta vs. temperature for a PI thin film. Table 7 summarizes the results. In general, the AO erosion resulted in some decrease in the storage modulus and some increase in the glass transition temperature (Tg). The attack by AO struck off some soft organic fraction. The remaining should consist of more rigid domain impeding the mobility of polymer chains in the glass transition temperature zone such that the

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mobility of polymer chains had to occur at a higher temperature. The upshift in Tg was attributed to strong interfacial interaction between the formed SiO2 and PI matrix as well [9]. In the case of Corin XLS, the storage modulus did not decline, which might be because of very little change on the film surface. Yet all the other three types of materials exhibited some decreases in storage moduli after AO irradiation. Similarly, there was a report on 15-30% loss in tensile moduli for AO-exposed carbon reinforced polysulfone composites compared with the unexposed [3]. The results are also in agreement with nanoindentation tests in which the AO-treatment induced significant decrease in hardness and elastic modulus for Kapton polyimide [29]. Plasma glassification did not change these properties clearly since the treatment was on the surface only. Good stability in mechanical properties under AO environment was observed in the case of PM1215. The affected region on the POSS-polyimide nanocomposite should be very limited right on the surface rather than going deeper due to self-healing. Although the Thermalbright samples have very high inorganic loading of TiO2, TiO2 does not have the ability to form a protective layer upon AO irradiation while only 5 wt.% POSS was too dilute to offer protection against AO [1].

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0.4

PI modulus PI AO-exp. modulus PI tan Tan δ PI AO-exp. Tan δ

3000

0.3

2000

0.2

1000

0.1

0

Tan δ

Storage modulus (MPa)

4000

0 0

100

200 300 Temperature (oC)

400

500

Fig. 13. Typical dynamical viscoelastic properties of PI films for Control PI and AO exposed PI.

Table 7 DMA tested dynamical viscoelastic properties of the films. Material PI Thermalbright Thermalbright PM1215 PM1215 Corin XLS Corin XLS

Plasma treatment time (min.)

POSS (wt.%)

0 0 3 0 3 0 3

0 5 5 10 10 30 30

Storage modulus at 45 oC (GPa) AO Control Exposed 2.65 2.34 3.34 3.07 3.58 3.45 3.02 2.46 3.26 2.26 1.67 1.76 2.25 2.15

Tg (oC) Control 425.5 298.2 297.7 399.3 399.2 265.8 267.8

AO Exposed 428.9 305.7 309.6 401.4 402.6 271.6 271.9

4.5 Molecular dynamics simulation The initial and final snapshots of the PI-POSS systems undergoing AO bombardment and the normalized mass loss for the two systems relative to that of the neat PI are given in Fig. 14. In Fig. 14b, the instantaneous mass of the systems is normalized with respect to their initial mass. 27

As seen in this figure, with an increase in the concentration of POSS, the onset of material degradation is shifted to longer AO exposure times. Moreover, while the rate of degradation (slope of the curves) is nearly the same for the two systems, the total mass loss for the PI-POSS30 system is lower than that of PI-POSS-15. The trends observed here match the experimental data for the PI, Thermalbright, PM 1215, and Corin XLS systems, which have, in the same order, higher POSS content in their formulation (Table 4). In Fig. 15, normalized mass density is given as a function of system dimension (z direction perpendicular to the material surface). A damage propagation depth is defined as the distance between the point corresponding to the onset of drop in the normalized mass density to the point corresponding to the material surface when a comparison is made between the initial and final systems. The damage propagation depth for the Neat PI system is about 25 Å, which is higher than those of the PI-POSS-15 (~18 Å) and PIPOSS-30 (~16 Å) systems. This observation again indicates that at higher POSS concentration, better mitigation of the AO attack damage can be achieved.

a)

b)

Fig. 14. a) Initial (t = 0) and final (t = 35 ps) snapshots of the PI-POSS systems and b) normalized mass loss as a function of simulation time.

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b)

a)

c) Fig. 15. Normalized mass density as a function of the system dimension perpendicular to the material surface (z direction) for a) Neat PI, b) PI-POSS-15, and c) PI-POSS-30 systems. A damage propagation depth is defined as the distance from the onset of drop in the mass density at the end of the simulation when compared to the initial mass density.

5. Conclusions Some effect of oxygen plasma glassification to improve the resistance against AO attacks under the selected durations on the tensile properties was observed for polyimide of low POSS

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loading. The synergetic effect of POSS and oxygen plasma leads to higher values of tan delta. The selected materials with higher POSS percentages generally performed better in terms of mass loss of the tested systems due to the rapid and more complete formation of an oxidized protective SiO2 layer that significantly limits further degradation once it formed. POSS-polyimide nanocomposite materials in this study were found to substantially enhance the resistance against AO erosion due to their self-healing under AO irradiation. In a molecular dynamics simulation study, the mitigations in mass loss, damage propagation depth, and erosion yield of the PI-POSS nanocomposites over those of the neat PI were confirmed. The resistance against AO erosion increased with an increase in the POSS concentration, consistent with the experimental observations. The mechanism of self-healing was the formation of a silica (SiO2) thin layer on the AOradiated surface. The mass loss was reduced dramatically when compared with neat PI. The polyimide containing 10 wt.% POSS showed excellent stability in tensile properties. The POSS was incorporated into the PI matrix as simple filler, realizing similar effects from the study in which POSS nanoparticles were covalently grafted into the backbone of PI macromolecules or to the side of the chains [17]. POSS nanoparticles appear promising in such applications because its organic component makes it soluble in organic solvents and compatible with polymer matrices while the inorganic component can be converted to SiO2 which is resistant to AO erosion. Therefore, POSS-polyimide nanocomposite may be a very good space-survivable material.

Acknowledgment Authors acknowledge the financial support received under a subcontract from National Aeronautics and Space Administration (Grant No. NNX13AD24A).

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Graphical abstract

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Highlights of the submitted research paper “Substantially enhanced durability of polyhedral oligomeric silsequioxane-polyimide nanocomposites against atomic oxygen erosion”

It is found in this research that incorporated polyhedral oligomeric silsequioxane (POSS) significantly enhances the durability of polyimide under atomic oxygen (AO) radiation evidenced by substantial reduction in erosion yield and much smoother surface in Atomic Force Microscope (AFM) images. The selected POSS exhibits self-healing on the AO-radiated surface to form an inert layer to block further erosion. Molecular dynamics simulation confirms the experimental results.

Xiaobing Li Research Associate Department of Civil Engineering 106 Carrier Hall University of Mississippi University, MS 38677 Phone: 1-662-915-1975

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