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Polymer Degradation and Stability 97 (2012) 981e986

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Polymer Degradation and Stability

journal homepage: www.elsevier .com/locate /polydegstab

Phenylphosphine oxide-containing aromatic polyamide films with high atomicoxygen erosion resistance

Limin Su a,b, Liming Tao a, Tingmei Wang a,*, Qihua Wang a

a State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR ChinabGraduate University of Chinese Academy of Sciences, Beijing 100049, PR China

a r t i c l e i n f o

Article history:Received 18 December 2011Received in revised form1 March 2012Accepted 9 March 2012Available online 19 March 2012

Keywords:Aromatic polyamidePhenylphosphine oxide groupAtomic oxygenMorphology

* Corresponding author.E-mail address: wangtin3088@sina.com (T. Wang)

0141-3910/$ e see front matter � 2012 Elsevier Ltd.doi:10.1016/j.polymdegradstab.2012.03.013

a b s t r a c t

A kind of aromatic polyamide (PA) film containing phenylphosphine oxide group (PPO) was prepared.The experiment results indicated that the incorporation of the PPO group protects the PA film fromeroding by atomic oxygen (AO), as compared with a kind of common aromatic polyamide (PA) withoutPPO groups. The chemical composition, surface morphology, mass loss, optical properties and tensilestrength of the two samples, before and after AO irradiation, were compared in detail. XPS resultsindicated that, during the AO exposure, a passive phosphate rich layer, which protected the followingunder-layer from attacking by AO, was formed on the PA surface with PPO segments. SEM micrographsshowed the surface morphology of both films changed intuitively. However, the PA with PPO segmentsdidn’t take out so much surface change during the irradiation. The surface of PA was changed fromsmooth before irradiation, to a carpet-like character after 2 h irradiation and to a tree-root feature after6 h irradiation. PA film with PPO segments also turned out lower mass change, less tensile strengthreduction, and higher transmission after the AO irradiation as compared with the common PA film.

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1. Introduction

Aromatic polyamides (PAs), as a kind of high performancepolymers, are attractive and desirable spacecraft materials due totheir excellent thermal stability and mechanical properties, as wellas outstanding chemical resistance [1]. High tensile strength fibersof aromatic polyamide, such as Kevlar and Nomex, were widelyused as fabrics and fillers in the aerospace industry [2].

However, the polymeric materials including aromatic poly-amides are usually damaged both chemically and physically bymany space environmental threats like high vacuum, high-energyultraviolet and various radiations [3e6]. In fact, atomic oxygen(AO), the dominant chemical constituent of the Low Earth Orbit(LEO), is the most serious threat and its erosive potential issubstantially increased by the high speeds of the spacecraft in theLEO [7]. Low molecular polymer and volatile oxides, such as CO,CO2, were formed on the material when exposed to AO. Typicallythey become matte in appearance because the microscopic surfacefibrils or cone-like structures formed as a result of the oxidation ofthe directed AO flux. Such erosion can affect the performance of the

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spacecraft significantly and may result in structural failure prob-lems [8e10].

There have been several approaches to protect polymericmaterials from attacking by AO. Inorganic oxide coatings [11e13]like silicone dioxide, aluminum oxide and indium tin oxidesputter deposited on polymers have been widely used to provideAO protection. However, coatings thicker than 100 nm cracked orspalled easily under the thermal cycling in LEO, due to either theirintrinsic stress or inability to conform with compression orexpansion flexure of polymer substrates [14]. Of late, atomicoxygen-resistant polymers have been developed. The incorporationof silica into organic material is a recognized means of improvingthe AO resistance [15,16]. When irradiated by AO the formation ofsufficient silicon dioxide surface populations will protect theunderlying polymer from further erosion. Similarly, researchersfound the phosphorous containing polymers, whichwill form a richpolyphosphonate layer under the AO environment, also have AOresistance [17]. Unfortunately, there are not so many studies onthem, especially on aromatic polyamides.

Therefore, developing a kind of aromatic PA containing PPOgroups seems to be a reasonable approach. In the present study, PAwith PPO segment was synthesized from bis(4-carboxyl phenyl)phenyl phosphine oxide (BCPPO). A common PAwithout PPO groupwas also prepared for comparison. The chemical composition,

Fig. 1. Synthesis of aromatic polyamides.

L. Su et al. / Polymer Degradation and Stability 97 (2012) 981e986982

surface morphology, mass loss, optical properties and tensilestrength of the two samples, before and after AO irradiation, wereinvestigated systematically. Atomic oxygen resistance of the phe-nylphosphine oxide-containing PA films were confirmed from thestudy.

2. Experimental

2.1. Material

Bis(4-carboxyl phenyl) phenyl phosphine oxide (BCPPO) wassynthesized according to a reported method [18]. Isophthalic acid(IPA) and Oxydianiline (ODA) and pyridine (Py) were supplied bySinopharm Chemical Reagent Co., Ltd. Calcium chloride (CaCl2) andtriphenyl phosphite (TPP) were produced by Tianjin ChemicalReagent No. 1 Plant. They were all used as received. N-methyl-2-pyrrolidone (NMP, Shantou Xilong Chemical Co., Ltd) was simplydisposed by calcium hydroxide (CaH2) to remove water and storedwith molecular sieve.

2.2. Preparation of aromatic polyamide films

2.2.1. Preparation of aromatic polyamideBCPPO or IPA 10 mmol and ODA 10 mmol, CaCl2 10 mmol, NMP

20 mL, Py 5 mL and TPP 5.3 mL were put into a three-neck roundbottom flask equipped with a mechanical stirrer, condensing tubeand nitrogen inlet. The reaction mixture was refluxed 8 h at 125 �Cand then cooled to room temperature. After that, it was poured intoanhydrous ethanol and washed with hot water and anhydrous

Fig. 2. FTIR spectra of the PA films.

ethanol for four times, respectively. Finally, it was dried in vacuumoven for 24 h at 110 �C. The synthesis step and the polymer codewere shown in Fig. 1.

2.2.2. Film preparation1.5 g aromatic PAwas dissolved in 8.5 g NMP to form a 15% solid

content viscous solution. It was directly casted onto a clean glassplate and the film was thermally baked at 80 �C for 4 h and 100 �C,150 �C, 200 �C and 250 �C for 1 h each in air, respectively, to removethe solvent. The obtained PA films were immerged in warm waterand peeled off from the glass plates.

2.3. Analysis techniques

AO irradiation was carried out in a coaxial ground-based atomicoxygen simulation facility in Lanzhou Institute of Physics, ChineseAcademy of Sciences (Lanzhou, China). The average energy of theAO produced by this system was 5 eV and the flux of AO wasdetermined to be 5.0 � 1015 AO/cm2 s. In the present study, eachkind of the samplewas irradiated for a period of 0 h, 2 h, 4 h and 6 h,respectively.

Tensile strength of the films was got from a universal tensileexperiments machine (Shimadzu AG-X, Japan). Five rectangularspecimens (3.5 cm � 0.6 cm � 0.006 cm) were tested for each film.Surface element analysis and light transmission were determinedby an X-ray photoelectron spectroscopy ESCALAB 210 (England)and a Hitachi U-3010 spectrophotometer (Japan), respectively.Morphologies of the surface were observed via a field emissionscanning electron microscopy (FE-SEM, JSM-6701F, Japan). Thesurfaces of the polymer samples were coated with a thin gold filmbefore SEM observation in order to prevent charge accumulation.

3. Results and discussion

The FTIR spectra in Fig. 2 are used to characterize the functionalgroups of the PA films. The broad peak at 3300 cm�1 and the one at1529 cm�1, which we labeled on the spectra, are the peaks of theNeH stretching and bending vibration, respectively. The peak at1670 cm�1 is the characteristic peak of C]O stretching vibration of

Table 1Atomics change in concentration.

Polymer Photopeak Atomic concentration (%)

0 h 2 h 4 h 6 h Theory

PA-1 C1s 75.8 55.21 52.87 49.8 76.92O1s 18.34 38.43 40.52 41.65 15.38N1s 5.86 6.36 6.61 8.55 7.70

PA-2 C1s 74.69 38.12 32.8 31.71 76.34O1s 18.82 38.3 41.43 41.38 12.72N1s 4.79 9.88 11.34 14.1 4.77P2p 1.71 13.7 14.43 12.81 6.16

Fig. 3. XPS analysis of four kind element spectrums of PA-2 before and after AO irradiation.

Fig. 4. Weight change curves of the PA-1 and PA-2 during AO exposure.

L. Su et al. / Polymer Degradation and Stability 97 (2012) 981e986 983

the amide group. Meanwhile, the absorptions at 1247 cm�l and at1437 cm�l, assigned to the P]O and P-Ph stretching vibration,respectively, were also observed [19]. The FTIR data confirm theformation of the aromatic polyamides.

3.1. XPS analysis of the PI films before and after AO exposure

To determine the change in oxidation states of the surfaceatoms, thin films of the samples were analyzed by XPS, before andafter 2 h, 4 h and 6 h exposure. Obvious change of atomicconcentration can be seen from the Table 1. The relative concen-tration of carbon, both in PA-1 and PA-2, was reduced greatly at 2 hof AO irritation, while oxygen and nitrogen were increased signif-icantly at the same time. However, with the irritation timeincreasing, the relative concentrations change of the atomics werebecoming more gently. Reason of the phenomenonwas the surfaceof the materials were oxidized under the AO irradiation, then COand CO2 were produced and evacuated from the surface whichcaused the percentage reduce of carbon and increase of oxygen.Two hours later, the oxidation of the surface becoming at a rela-tively dynamic balance that all the relative concentrations of theatomics were not changed so significantly as the initial stage.

The bonding energy changes of four elements of PA-2 wereshowed in Fig. 3. All photopeaks were referenced to that of carbonhaving a maximum taken at 285.0 eV. After exposure to AO, all

elements except carbon in PA-2 exhibited broader photopeaks andthe binding energies shifted to higher values, which can be seenclearly in Fig. 3. Energy shift and photopeaks broadening indicatedthat the oxidation reaction occurred when the AO was chemical

Fig. 5. SEM photographs of the pristine PA-2 and PA-1 films (a, b) and AO-irradiated samples of PA-2 (c: 2 h, e: 4 h, g: 6 h) and PA-1 (d: 2 h, f: 4 h, h: 6 h).

L. Su et al. / Polymer Degradation and Stability 97 (2012) 981e986984

adsorbed on the film surface [20]. After 6 h irradiation of AO, theone oxygen photopeak broken into two peaks, because the doublebonds between the oxygen and carbon in amide groups or theoxygen and phosphorus in PPO segments were destroyed by AOtreatment and the single bond of oxygen, with bonding energy of532.8 eV, was formed as a result. The bonding energy of

phosphorus changed from 132.2 eV, attributed to the original tri-phenyl phosphine oxide moiety in the PA-2, to 134.3 eV due to thehighly oxidized form of phosphorus in polyphosphate [21]. PeObond in polyphosphate consistent with the inference of the singlebond of oxygen what we have got before. The concentration ofphosphorus in PA-2 changed from 1.71% to 12.81%, which was

Fig. 6. Transmittance of PA-1 and PA-2 films under different AO irridiation time.

L. Su et al. / Polymer Degradation and Stability 97 (2012) 981e986 985

higher than the theory percentage of 6.16%. It means the formationof phosphorus rich layer on the surface after AO irradiation. Thenitrogen photopeak changed from 400.1 eV to two different peaksat 398.3 eV and 401.6 eV, respectively, which was implied theamide bond of PA-2 was partially damaged and formed a newoxidation state of nitrogen [22].

High speed of AO brought high load and temperature, whichmay produce residual stress in the material. These residualstresses generate a local increase in the polymer free volume,which facilitates oxygen diffusion into the polymer, thus initiatingthe process of local high-rate degradation [23]. The formation ofpolyphosphate layer can be assigned as a passivation layer on thesurface, which could reduce the influence of residual stress on thebasic material.

3.2. Mass loss and morphology change of both PAs

The mass change per unit area of the two PA films as a functionof AO irridiation time was presented in Fig. 4. Considerable weightloss was observed on exposure to atomic oxygen for PA-1. The massof the sample almost linearly decreased, and the total mass loss in6 h was 0.49 mg/cm2, indicated that the PA-1 was significantlyattacked during exposure to AO beam. PA-2 exhibited obviouslylower weight-loss rate compared to PA-1. The total mass loss after6 h AO exposure was 0.15 mg/cm2, which was only 30.6% of thevalue for PA-1. Two regions of different weight loss rates wereexhibited for PA-2. At the initial exposure stage, the mass loss of thefilm at a relative high rate, which may be attributed to the forma-tion of volatile products such as CO and CO2 during AO irradiation[24]. Then the mass loss rate reduced gradually, for the inorganicphosphate-layer formed at the initial stage protected the under-layer material and thereby reduced the weight loss rate.

In order to see the surface change of bothmaterials directly, SEMsurface morphologies of PA-1 and PA-2 films under differentexposure time of AO beam were illustrated in Fig. 5. Before irradi-ation, the surfaces of both specimens were very smooth (Fig. 5a andb). However, all films exhibited different topographical underdifferent AO exposure times. Reasons for the surface morphologieschange can be attributed to the oxidative nature of AO. For volatileproducts, CO and CO2were produced and removed from the surfaceof the films during the irradiation of AO. After 2 h AO irradiation,PA-1 formed a carpet-like morphology on the surface (Fig. 5d).With increasing the irradiation time, the surface was eroded moreseverely. After 6 h AO irradiation it was eroded on the whole andtree-root feature was formed (Fig. 5h). Compared with PA-1, PA-2had a relatively smooth surface after irradiation, andmorphology ofthe surface was changed unconspicuous with increasing irradiationtime. The sufficient polyphosphonate surface formed on the surfaceprotected the under-layer materials effectively from AO erosion[25]. This phenomenon was in well accordance to the mass changewhat we mentioned above.

Obviously, rough surface of the PA-1 and PA-2 also degrades theoptical properties which are presented in Fig. 6. Optical trans-mittance of the films with different AO irridiation time wasmeasured. The transmittance at wavelength of 450 nmwas checkedas an example. Clearly we can see, the transmittance of PA-1decreased directly with the irridiation time increasing. After 6 hirridiation of AO, the relative transmittance of PA-1 reduced to 10%,which was attributed to the deterioration of the sample surfacemorphology after the AO irridiation. This result is consistant withthe results what we have got in Fig. 5. In fact a foggy layer wasformed on the surface of PA-1 after irridiation which could be seenintutively. In comparison, PA-2maintained high transmittance afterthe AO irradiation, indicating the effective protect effect of poly-phosphonate surface formed on the surface.

As mentioned above, phenylphosphine oxide-containing poly-mers have good resistance to the irradiation of AO. In order tofurther prove it, tensile strength of PA-1 and PA-2 films was studiedin Fig. 7. As we presupposed, tensile strength of both materials wasreduced after the irradiation of AO. Tensile strength of PA-1 reduceddramatically from 102 MPa to 75 MPa. Although tensile strength ofPA-2 was (86.5 MPa) lower than PA-1 which may be attributed tothe structure difference, after 6 h irradiation it was only reduced

Fig. 7. Tensile strength change of PA-1 and PA-2 films exposure to AO at different time.

L. Su et al. / Polymer Degradation and Stability 97 (2012) 981e986986

9.5 MPa. Obviously, under the AO environment, PA-2 was moresuitable for a long period use.

4. Conclusions

Phenylphosphine oxide-containing PA exhibited a non-linearmass loss behavior, for an inorganic polyphosphate layer wasformed on the surface during the irradiation of AO, which canprotect the base material from being damaged. Less erosion of PA-2with PPO segments helped it keeping higher transmittance andhigher tensile strength after the irradiation, as compared with thePA-1 without PPO groups under the same AO exposure conditions.Hence, PA-2 with PPO groups is promising candidates for protect-ing materials for aerospace industry.

Acknowledgment

The authors would like to acknowledge the financial supportsfrom the National Science Foundation of China (GrantNo.51103168), the National Science Foundation for DistinguishedYoung Scholars of China (Grant No.51025517) and the NationalDefense Basic Scientific Research Project (A1320110011).

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