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Phenylphosphine oxide-containing aromatic polyamide films with high atomic oxygen erosion resistance
Polymer Degradation and Stability 97 (2012) 981e986
Contents lists available at SciVerse ScienceDirect
Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Phenylphosphine oxide-containing aromatic polyamide films with high atomic oxygen erosion resistance Limin Su a, b, Liming Tao a, Tingmei Wang a, *, Qihua Wang a a b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China Graduate University of Chinese Academy of Sciences, Beijing 100049, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 December 2011 Received in revised form 1 March 2012 Accepted 9 March 2012 Available online 19 March 2012
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 from eroding by atomic oxygen (AO), as compared with a kind of common aromatic polyamide (PA) without PPO groups. The chemical composition, surface morphology, mass loss, optical properties and tensile strength of the two samples, before and after AO irradiation, were compared in detail. XPS results indicated that, during the AO exposure, a passive phosphate rich layer, which protected the following under-layer from attacking by AO, was formed on the PA surface with PPO segments. SEM micrographs showed the surface morphology of both films changed intuitively. However, the PA with PPO segments didn’t take out so much surface change during the irradiation. The surface of PA was changed from smooth before irradiation, to a carpet-like character after 2 h irradiation and to a tree-root feature after 6 h irradiation. PA film with PPO segments also turned out lower mass change, less tensile strength reduction, and higher transmission after the AO irradiation as compared with the common PA film. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Aromatic polyamide Phenylphosphine oxide group Atomic oxygen Morphology
1. Introduction Aromatic polyamides (PAs), as a kind of high performance polymers, are attractive and desirable spacecraft materials due to their excellent thermal stability and mechanical properties, as well as outstanding chemical resistance [1]. High tensile strength fibers of aromatic polyamide, such as Kevlar and Nomex, were widely used as fabrics and fillers in the aerospace industry [2]. However, the polymeric materials including aromatic polyamides are usually damaged both chemically and physically by many space environmental threats like high vacuum, high-energy ultraviolet 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 is substantially increased by the high speeds of the spacecraft in the LEO [7]. Low molecular polymer and volatile oxides, such as CO, CO2, were formed on the material when exposed to AO. Typically they become matte in appearance because the microscopic surface fibrils or cone-like structures formed as a result of the oxidation of the directed AO flux. Such erosion can affect the performance of the
* Corresponding author. E-mail address: [email protected] (T. Wang). 0141-3910/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2012.03.013
spacecraft significantly and may result in structural failure problems [8e10]. There have been several approaches to protect polymeric materials from attacking by AO. Inorganic oxide coatings [11e13] like silicone dioxide, aluminum oxide and indium tin oxide sputter deposited on polymers have been widely used to provide AO protection. However, coatings thicker than 100 nm cracked or spalled easily under the thermal cycling in LEO, due to either their intrinsic stress or inability to conform with compression or expansion flexure of polymer substrates [14]. Of late, atomic oxygen-resistant polymers have been developed. The incorporation of silica into organic material is a recognized means of improving the AO resistance [15,16]. When irradiated by AO the formation of sufficient silicon dioxide surface populations will protect the underlying polymer from further erosion. Similarly, researchers found the phosphorous containing polymers, which will form a rich polyphosphonate layer under the AO environment, also have AO resistance [17]. Unfortunately, there are not so many studies on them, especially on aromatic polyamides. Therefore, developing a kind of aromatic PA containing PPO groups seems to be a reasonable approach. In the present study, PA with PPO segment was synthesized from bis(4-carboxyl phenyl) phenyl phosphine oxide (BCPPO). A common PA without PPO group was also prepared for comparison. The chemical composition,
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L. Su et al. / Polymer Degradation and Stability 97 (2012) 981e986
Fig. 1. Synthesis of aromatic polyamides.
surface morphology, mass loss, optical properties and tensile strength of the two samples, before and after AO irradiation, were investigated systematically. Atomic oxygen resistance of the phenylphosphine oxide-containing PA films were confirmed from the study. 2. Experimental 2.1. Material Bis(4-carboxyl phenyl) phenyl phosphine oxide (BCPPO) was synthesized according to a reported method [18]. Isophthalic acid (IPA) and Oxydianiline (ODA) and pyridine (Py) were supplied by Sinopharm Chemical Reagent Co., Ltd. Calcium chloride (CaCl2) and triphenyl phosphite (TPP) were produced by Tianjin Chemical Reagent No. 1 Plant. They were all used as received. N-methyl-2pyrrolidone (NMP, Shantou Xilong Chemical Co., Ltd) was simply disposed by calcium hydroxide (CaH2) to remove water and stored with molecular sieve. 2.2. Preparation of aromatic polyamide films 2.2.1. Preparation of aromatic polyamide BCPPO 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 round bottom flask equipped with a mechanical stirrer, condensing tube and nitrogen inlet. The reaction mixture was refluxed 8 h at 125 C and then cooled to room temperature. After that, it was poured into anhydrous ethanol and washed with hot water and anhydrous
ethanol for four times, respectively. Finally, it was dried in vacuum oven for 24 h at 110 C. The synthesis step and the polymer code were shown in Fig. 1. 2.2.2. Film preparation 1.5 g aromatic PA was dissolved in 8.5 g NMP to form a 15% solid content viscous solution. It was directly casted onto a clean glass plate 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 remove the solvent. The obtained PA films were immerged in warm water and peeled off from the glass plates. 2.3. Analysis techniques AO irradiation was carried out in a coaxial ground-based atomic oxygen simulation facility in Lanzhou Institute of Physics, Chinese Academy of Sciences (Lanzhou, China). The average energy of the AO produced by this system was 5 eV and the flux of AO was determined to be 5.0 1015 AO/cm2 s. In the present study, each kind of the sample was 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 tensile experiments machine (Shimadzu AG-X, Japan). Five rectangular specimens (3.5 cm 0.6 cm 0.006 cm) were tested for each film. Surface element analysis and light transmission were determined by 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 emission scanning electron microscopy (FE-SEM, JSM-6701F, Japan). The surfaces of the polymer samples were coated with a thin gold film before SEM observation in order to prevent charge accumulation. 3. Results and discussion The FTIR spectra in Fig. 2 are used to characterize the functional groups of the PA films. The broad peak at 3300 cm 1 and the one at 1529 cm 1, which we labeled on the spectra, are the peaks of the NeH stretching and bending vibration, respectively. The peak at 1670 cm 1 is the characteristic peak of C]O stretching vibration of
Table 1 Atomics change in concentration. Polymer
Photopeak
Atomic concentration (%) 0h
2h
4h
6h
Theory
PA-1
C1s O1s N1s C1s O1s N1s P2p
75.8 18.34 5.86 74.69 18.82 4.79 1.71
55.21 38.43 6.36 38.12 38.3 9.88 13.7
52.87 40.52 6.61 32.8 41.43 11.34 14.43
49.8 41.65 8.55 31.71 41.38 14.1 12.81
76.92 15.38 7.70 76.34 12.72 4.77 6.16
PA-2
Fig. 2. FTIR spectra of the PA films.
L. Su et al. / Polymer Degradation and Stability 97 (2012) 981e986
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Fig. 3. XPS analysis of four kind element spectrums of PA-2 before and after AO irradiation.
the amide group. Meanwhile, the absorptions at 1247 cm l and at 1437 cm l, assigned to the P]O and P-Ph stretching vibration, respectively, were also observed [19]. The FTIR data confirm the formation of the aromatic polyamides.
elements except carbon in PA-2 exhibited broader photopeaks and the binding energies shifted to higher values, which can be seen clearly in Fig. 3. Energy shift and photopeaks broadening indicated that the oxidation reaction occurred when the AO was chemical
3.1. XPS analysis of the PI films before and after AO exposure To determine the change in oxidation states of the surface atoms, thin films of the samples were analyzed by XPS, before and after 2 h, 4 h and 6 h exposure. Obvious change of atomic concentration can be seen from the Table 1. The relative concentration of carbon, both in PA-1 and PA-2, was reduced greatly at 2 h of AO irritation, while oxygen and nitrogen were increased significantly at the same time. However, with the irritation time increasing, the relative concentrations change of the atomics were becoming more gently. Reason of the phenomenon was the surface of the materials were oxidized under the AO irradiation, then CO and CO2 were produced and evacuated from the surface which caused the percentage reduce of carbon and increase of oxygen. Two hours later, the oxidation of the surface becoming at a relatively dynamic balance that all the relative concentrations of the atomics were not changed so significantly as the initial stage. The bonding energy changes of four elements of PA-2 were showed in Fig. 3. All photopeaks were referenced to that of carbon having a maximum taken at 285.0 eV. After exposure to AO, all
Fig. 4. Weight change curves of the PA-1 and PA-2 during AO exposure.
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L. Su et al. / Polymer Degradation and Stability 97 (2012) 981e986
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).
adsorbed on the film surface [20]. After 6 h irradiation of AO, the one oxygen photopeak broken into two peaks, because the double bonds between the oxygen and carbon in amide groups or the oxygen and phosphorus in PPO segments were destroyed by AO treatment and the single bond of oxygen, with bonding energy of 532.8 eV, was formed as a result. The bonding energy of
phosphorus changed from 132.2 eV, attributed to the original triphenyl phosphine oxide moiety in the PA-2, to 134.3 eV due to the highly oxidized form of phosphorus in polyphosphate [21]. PeO bond in polyphosphate consistent with the inference of the single bond of oxygen what we have got before. The concentration of phosphorus in PA-2 changed from 1.71% to 12.81%, which was
L. Su et al. / Polymer Degradation and Stability 97 (2012) 981e986
higher than the theory percentage of 6.16%. It means the formation of phosphorus rich layer on the surface after AO irradiation. The nitrogen photopeak changed from 400.1 eV to two different peaks at 398.3 eV and 401.6 eV, respectively, which was implied the amide bond of PA-2 was partially damaged and formed a new oxidation state of nitrogen [22]. High speed of AO brought high load and temperature, which may produce residual stress in the material. These residual stresses generate a local increase in the polymer free volume, which facilitates oxygen diffusion into the polymer, thus initiating the process of local high-rate degradation [23]. The formation of polyphosphate layer can be assigned as a passivation layer on the surface, which could reduce the influence of residual stress on the basic material.
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As mentioned above, phenylphosphine oxide-containing polymers have good resistance to the irradiation of AO. In order to further prove it, tensile strength of PA-1 and PA-2 films was studied in Fig. 7. As we presupposed, tensile strength of both materials was reduced after the irradiation of AO. Tensile strength of PA-1 reduced dramatically from 102 MPa to 75 MPa. Although tensile strength of PA-2 was (86.5 MPa) lower than PA-1 which may be attributed to the structure difference, after 6 h irradiation it was only reduced
3.2. Mass loss and morphology change of both PAs The mass change per unit area of the two PA films as a function of AO irridiation time was presented in Fig. 4. Considerable weight loss was observed on exposure to atomic oxygen for PA-1. The mass of the sample almost linearly decreased, and the total mass loss in 6 h was 0.49 mg/cm2, indicated that the PA-1 was significantly attacked during exposure to AO beam. PA-2 exhibited obviously lower weight-loss rate compared to PA-1. The total mass loss after 6 h AO exposure was 0.15 mg/cm2, which was only 30.6% of the value for PA-1. Two regions of different weight loss rates were exhibited for PA-2. At the initial exposure stage, the mass loss of the film at a relative high rate, which may be attributed to the formation of volatile products such as CO and CO2 during AO irradiation [24]. Then the mass loss rate reduced gradually, for the inorganic phosphate-layer formed at the initial stage protected the underlayer material and thereby reduced the weight loss rate. In order to see the surface change of both materials directly, SEM surface morphologies of PA-1 and PA-2 films under different exposure time of AO beam were illustrated in Fig. 5. Before irradiation, the surfaces of both specimens were very smooth (Fig. 5a and b). However, all films exhibited different topographical under different AO exposure times. Reasons for the surface morphologies change can be attributed to the oxidative nature of AO. For volatile products, CO and CO2 were produced and removed from the surface of 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 more severely. After 6 h AO irradiation it was eroded on the whole and tree-root feature was formed (Fig. 5h). Compared with PA-1, PA-2 had a relatively smooth surface after irradiation, and morphology of the surface was changed unconspicuous with increasing irradiation time. The sufficient polyphosphonate surface formed on the surface protected the under-layer materials effectively from AO erosion [25]. This phenomenon was in well accordance to the mass change what we mentioned above. Obviously, rough surface of the PA-1 and PA-2 also degrades the optical properties which are presented in Fig. 6. Optical transmittance of the films with different AO irridiation time was measured. The transmittance at wavelength of 450 nm was checked as an example. Clearly we can see, the transmittance of PA-1 decreased directly with the irridiation time increasing. After 6 h irridiation of AO, the relative transmittance of PA-1 reduced to 10%, which was attributed to the deterioration of the sample surface morphology after the AO irridiation. This result is consistant with the results what we have got in Fig. 5. In fact a foggy layer was formed on the surface of PA-1 after irridiation which could be seen intutively. In comparison, PA-2 maintained high transmittance after the AO irradiation, indicating the effective protect effect of polyphosphonate surface formed on the surface.
Fig. 6. Transmittance of PA-1 and PA-2 films under different AO irridiation time.
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L. Su et al. / Polymer Degradation and Stability 97 (2012) 981e986
Fig. 7. Tensile strength change of PA-1 and PA-2 films exposure to AO at different time.
9.5 MPa. Obviously, under the AO environment, PA-2 was more suitable for a long period use. 4. Conclusions Phenylphosphine oxide-containing PA exhibited a non-linear mass loss behavior, for an inorganic polyphosphate layer was formed on the surface during the irradiation of AO, which can protect the base material from being damaged. Less erosion of PA-2 with PPO segments helped it keeping higher transmittance and higher tensile strength after the irradiation, as compared with the PA-1 without PPO groups under the same AO exposure conditions. Hence, PA-2 with PPO groups is promising candidates for protecting materials for aerospace industry. Acknowledgment The authors would like to acknowledge the financial supports from the National Science Foundation of China (Grant No.51103168), the National Science Foundation for Distinguished Young Scholars of China (Grant No.51025517) and the National Defense Basic Scientific Research Project (A1320110011). References [1] Cassidy PE, editor. Thermally stable polymers. New York: Marcel Dekker; 1980.
[2] García JM, García FC, Serna F, de la Peña JL. High-performance aromatic polyamides. Prog Polym Sci 2010;35:623e86. [3] Grossman E, Gouzman I. Space environment effects on polymers in low earth orbit. Nucl Instrum Methods Phys Res Sect B 2003;208:48e57. [4] Zhao X, Shen Z, Xing Y, Ma S. An experimental study of low earth orbit atomic oxygen and ultraviolet radiation effects on a spacecraft material e polytetrafluoroethylene. Polym Degrad Stab 2005;88:275e85. [5] Awaja F, Moon JB, Gilbert M, Zhang S, Kim CG, Pigram PJ. Surface molecular degradation of selected high performance polymer composites under low earth orbit environmental conditions. Polym Degrad Stab 2011;96:1301e9. [6] Shimamura H, Nakamura T. Investigation of degradation mechanisms in mechanical properties of polyimide films exposed to a low earth orbit environment. Polym Degrad Stab 2010;95:21e33. [7] Bruce A, Banks SKM, de Groh Kim K. Low earth orbital atomic oxygen interactions with materials. NASA/TMd2004-213223; 2004. p. 4649. [8] Tagawa M, Yokota K. Atomic oxygen-induced polymer degradation Phenomena in Simulated LEO space environments: How do polymers React in a Complicated space environment? Acta Astronaut;62:203e211. [9] Shimamura H, Nakamura T. Mechanical properties degradation of polyimide films irradiated by atomic oxygen. Polym Degrad Stab 2009;94:1389e96. [10] Reddy MR. Effect of low earth orbit atomic oxygen on spacecraft materials. J Mater Sci 1995;30:281e307. [11] Reddy MR, Srinivasamurthy N, Agrawal BL. Atomic oxygen protective coatings for kapton film: a review. Surf Coat Technol 1993;58:1e17. [12] Banks BA, Mirtich MJ, Rutledge SK, Swec DM. Sputtered coatings for protection of spacecraft polymers. Thin Solid Films 1985;127:107e14. [13] Cooper R, Upadhyaya H, Minton T, Berman M, Du X, George S. Protection of polymer from atomic-oxygen erosion using Al2O3 atomic layer deposition coatings. Thin Solid Films 2008;516:4036e9. [14] Yokota K, Abe S, Tagawa M, Iwata M, Miyazaki E, Ishizawa JI, et al. Degradation property of commercially available Si-containing polyimide in simulated atomic oxygen environments for low earth orbit. High Perform Polym 2010; 22:237e51. [15] Verker R, Grossman E, Eliaz N. Erosion of POSS-polyimide films under hypervelocity impact and atomic oxygen: the role of mechanical properties at elevated temperatures. Acta Mater 2009;57:1112e9. [16] Brunsvold AL, Minton TK, Gouzman I, Grossman E, Gonzalez R. An investigation of the resistance of polyhedral oligomeric silsesquioxane polyimide to atomic-oxygen attack. High Perform Polym 2004;16:303e18. [17] Watson KA, Palmieri FL, Connell JW. Space environmentally stable polyimides and copolyimides derived from [2,4-bis(3-aminophenoxy)phenyl]diphenylphosphine oxide. Macromolecules 2002;35:4968e74. [18] Morgan PW, Herr BC. Some dicarboxylic acids and esters containing the phosphine oxide group. J Am Chem Soc 1952;74:4526e9. [19] Wang L-S, Wang X-L, Yan G-L. Synthesis, characterisation and flame retardance behaviour of poly(ethylene terephthalate) copolymer containing triaryl phosphine oxide. Polym Degrad Stab 2000;69:127e30. [20] Connell JW, Smith Jr JG, Hergenrother PM. Oxygen plasma-resistant phenylphosphine oxide-containing polyimides and poly(arylene ether heterocycle)s: 1. Polymer 1995;36:5e11. [21] Smith CD, Grubbs H, Webster HF, Gungör A, Wightman JP, McGrath JE. Unique characteristics derived from poly(arylene ether phosphine oxide)s. High Perform Polym 1991;3:211e29. [22] Packirisamy S, Schwam D, Litt MH. Atomic oxygen resistant coatings for low earth orbit space structures. J Mater Sci 1995;30:308e20. [23] Verker R, Grossman E, Gouzman I, Eliaz N. Residual stress effect on degradation of polyimide under simulated hypervelocity space debris and atomic oxygen. Polymer 2007;48:19e24. [24] Pei X, Li Y, Wang Q, Sun X. Effects of atomic oxygen irradiation on the surface properties of phenolphthalein poly(ether sulfone). Appl Surf Sci 2009;255: 5932e4. [25] Smith Jr JG, Connell JW, Hergenrother PM. Oxygen plasma resistant phenylphosphine oxide-containing poly(arylene ether)s. Polymer 1994;35:2834e9.
Contents lists available at SciVerse ScienceDirect
Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Phenylphosphine oxide-containing aromatic polyamide films with high atomic oxygen erosion resistance Limin Su a, b, Liming Tao a, Tingmei Wang a, *, Qihua Wang a a b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China Graduate University of Chinese Academy of Sciences, Beijing 100049, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 December 2011 Received in revised form 1 March 2012 Accepted 9 March 2012 Available online 19 March 2012
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 from eroding by atomic oxygen (AO), as compared with a kind of common aromatic polyamide (PA) without PPO groups. The chemical composition, surface morphology, mass loss, optical properties and tensile strength of the two samples, before and after AO irradiation, were compared in detail. XPS results indicated that, during the AO exposure, a passive phosphate rich layer, which protected the following under-layer from attacking by AO, was formed on the PA surface with PPO segments. SEM micrographs showed the surface morphology of both films changed intuitively. However, the PA with PPO segments didn’t take out so much surface change during the irradiation. The surface of PA was changed from smooth before irradiation, to a carpet-like character after 2 h irradiation and to a tree-root feature after 6 h irradiation. PA film with PPO segments also turned out lower mass change, less tensile strength reduction, and higher transmission after the AO irradiation as compared with the common PA film. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Aromatic polyamide Phenylphosphine oxide group Atomic oxygen Morphology
1. Introduction Aromatic polyamides (PAs), as a kind of high performance polymers, are attractive and desirable spacecraft materials due to their excellent thermal stability and mechanical properties, as well as outstanding chemical resistance [1]. High tensile strength fibers of aromatic polyamide, such as Kevlar and Nomex, were widely used as fabrics and fillers in the aerospace industry [2]. However, the polymeric materials including aromatic polyamides are usually damaged both chemically and physically by many space environmental threats like high vacuum, high-energy ultraviolet 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 is substantially increased by the high speeds of the spacecraft in the LEO [7]. Low molecular polymer and volatile oxides, such as CO, CO2, were formed on the material when exposed to AO. Typically they become matte in appearance because the microscopic surface fibrils or cone-like structures formed as a result of the oxidation of the directed AO flux. Such erosion can affect the performance of the
* Corresponding author. E-mail address: [email protected] (T. Wang). 0141-3910/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2012.03.013
spacecraft significantly and may result in structural failure problems [8e10]. There have been several approaches to protect polymeric materials from attacking by AO. Inorganic oxide coatings [11e13] like silicone dioxide, aluminum oxide and indium tin oxide sputter deposited on polymers have been widely used to provide AO protection. However, coatings thicker than 100 nm cracked or spalled easily under the thermal cycling in LEO, due to either their intrinsic stress or inability to conform with compression or expansion flexure of polymer substrates [14]. Of late, atomic oxygen-resistant polymers have been developed. The incorporation of silica into organic material is a recognized means of improving the AO resistance [15,16]. When irradiated by AO the formation of sufficient silicon dioxide surface populations will protect the underlying polymer from further erosion. Similarly, researchers found the phosphorous containing polymers, which will form a rich polyphosphonate layer under the AO environment, also have AO resistance [17]. Unfortunately, there are not so many studies on them, especially on aromatic polyamides. Therefore, developing a kind of aromatic PA containing PPO groups seems to be a reasonable approach. In the present study, PA with PPO segment was synthesized from bis(4-carboxyl phenyl) phenyl phosphine oxide (BCPPO). A common PA without PPO group was also prepared for comparison. The chemical composition,
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L. Su et al. / Polymer Degradation and Stability 97 (2012) 981e986
Fig. 1. Synthesis of aromatic polyamides.
surface morphology, mass loss, optical properties and tensile strength of the two samples, before and after AO irradiation, were investigated systematically. Atomic oxygen resistance of the phenylphosphine oxide-containing PA films were confirmed from the study. 2. Experimental 2.1. Material Bis(4-carboxyl phenyl) phenyl phosphine oxide (BCPPO) was synthesized according to a reported method [18]. Isophthalic acid (IPA) and Oxydianiline (ODA) and pyridine (Py) were supplied by Sinopharm Chemical Reagent Co., Ltd. Calcium chloride (CaCl2) and triphenyl phosphite (TPP) were produced by Tianjin Chemical Reagent No. 1 Plant. They were all used as received. N-methyl-2pyrrolidone (NMP, Shantou Xilong Chemical Co., Ltd) was simply disposed by calcium hydroxide (CaH2) to remove water and stored with molecular sieve. 2.2. Preparation of aromatic polyamide films 2.2.1. Preparation of aromatic polyamide BCPPO 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 round bottom flask equipped with a mechanical stirrer, condensing tube and nitrogen inlet. The reaction mixture was refluxed 8 h at 125 C and then cooled to room temperature. After that, it was poured into anhydrous ethanol and washed with hot water and anhydrous
ethanol for four times, respectively. Finally, it was dried in vacuum oven for 24 h at 110 C. The synthesis step and the polymer code were shown in Fig. 1. 2.2.2. Film preparation 1.5 g aromatic PA was dissolved in 8.5 g NMP to form a 15% solid content viscous solution. It was directly casted onto a clean glass plate 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 remove the solvent. The obtained PA films were immerged in warm water and peeled off from the glass plates. 2.3. Analysis techniques AO irradiation was carried out in a coaxial ground-based atomic oxygen simulation facility in Lanzhou Institute of Physics, Chinese Academy of Sciences (Lanzhou, China). The average energy of the AO produced by this system was 5 eV and the flux of AO was determined to be 5.0 1015 AO/cm2 s. In the present study, each kind of the sample was 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 tensile experiments machine (Shimadzu AG-X, Japan). Five rectangular specimens (3.5 cm 0.6 cm 0.006 cm) were tested for each film. Surface element analysis and light transmission were determined by 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 emission scanning electron microscopy (FE-SEM, JSM-6701F, Japan). The surfaces of the polymer samples were coated with a thin gold film before SEM observation in order to prevent charge accumulation. 3. Results and discussion The FTIR spectra in Fig. 2 are used to characterize the functional groups of the PA films. The broad peak at 3300 cm 1 and the one at 1529 cm 1, which we labeled on the spectra, are the peaks of the NeH stretching and bending vibration, respectively. The peak at 1670 cm 1 is the characteristic peak of C]O stretching vibration of
Table 1 Atomics change in concentration. Polymer
Photopeak
Atomic concentration (%) 0h
2h
4h
6h
Theory
PA-1
C1s O1s N1s C1s O1s N1s P2p
75.8 18.34 5.86 74.69 18.82 4.79 1.71
55.21 38.43 6.36 38.12 38.3 9.88 13.7
52.87 40.52 6.61 32.8 41.43 11.34 14.43
49.8 41.65 8.55 31.71 41.38 14.1 12.81
76.92 15.38 7.70 76.34 12.72 4.77 6.16
PA-2
Fig. 2. FTIR spectra of the PA films.
L. Su et al. / Polymer Degradation and Stability 97 (2012) 981e986
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Fig. 3. XPS analysis of four kind element spectrums of PA-2 before and after AO irradiation.
the amide group. Meanwhile, the absorptions at 1247 cm l and at 1437 cm l, assigned to the P]O and P-Ph stretching vibration, respectively, were also observed [19]. The FTIR data confirm the formation of the aromatic polyamides.
elements except carbon in PA-2 exhibited broader photopeaks and the binding energies shifted to higher values, which can be seen clearly in Fig. 3. Energy shift and photopeaks broadening indicated that the oxidation reaction occurred when the AO was chemical
3.1. XPS analysis of the PI films before and after AO exposure To determine the change in oxidation states of the surface atoms, thin films of the samples were analyzed by XPS, before and after 2 h, 4 h and 6 h exposure. Obvious change of atomic concentration can be seen from the Table 1. The relative concentration of carbon, both in PA-1 and PA-2, was reduced greatly at 2 h of AO irritation, while oxygen and nitrogen were increased significantly at the same time. However, with the irritation time increasing, the relative concentrations change of the atomics were becoming more gently. Reason of the phenomenon was the surface of the materials were oxidized under the AO irradiation, then CO and CO2 were produced and evacuated from the surface which caused the percentage reduce of carbon and increase of oxygen. Two hours later, the oxidation of the surface becoming at a relatively dynamic balance that all the relative concentrations of the atomics were not changed so significantly as the initial stage. The bonding energy changes of four elements of PA-2 were showed in Fig. 3. All photopeaks were referenced to that of carbon having a maximum taken at 285.0 eV. After exposure to AO, all
Fig. 4. Weight change curves of the PA-1 and PA-2 during AO exposure.
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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).
adsorbed on the film surface [20]. After 6 h irradiation of AO, the one oxygen photopeak broken into two peaks, because the double bonds between the oxygen and carbon in amide groups or the oxygen and phosphorus in PPO segments were destroyed by AO treatment and the single bond of oxygen, with bonding energy of 532.8 eV, was formed as a result. The bonding energy of
phosphorus changed from 132.2 eV, attributed to the original triphenyl phosphine oxide moiety in the PA-2, to 134.3 eV due to the highly oxidized form of phosphorus in polyphosphate [21]. PeO bond in polyphosphate consistent with the inference of the single bond of oxygen what we have got before. The concentration of phosphorus in PA-2 changed from 1.71% to 12.81%, which was
L. Su et al. / Polymer Degradation and Stability 97 (2012) 981e986
higher than the theory percentage of 6.16%. It means the formation of phosphorus rich layer on the surface after AO irradiation. The nitrogen photopeak changed from 400.1 eV to two different peaks at 398.3 eV and 401.6 eV, respectively, which was implied the amide bond of PA-2 was partially damaged and formed a new oxidation state of nitrogen [22]. High speed of AO brought high load and temperature, which may produce residual stress in the material. These residual stresses generate a local increase in the polymer free volume, which facilitates oxygen diffusion into the polymer, thus initiating the process of local high-rate degradation [23]. The formation of polyphosphate layer can be assigned as a passivation layer on the surface, which could reduce the influence of residual stress on the basic material.
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As mentioned above, phenylphosphine oxide-containing polymers have good resistance to the irradiation of AO. In order to further prove it, tensile strength of PA-1 and PA-2 films was studied in Fig. 7. As we presupposed, tensile strength of both materials was reduced after the irradiation of AO. Tensile strength of PA-1 reduced dramatically from 102 MPa to 75 MPa. Although tensile strength of PA-2 was (86.5 MPa) lower than PA-1 which may be attributed to the structure difference, after 6 h irradiation it was only reduced
3.2. Mass loss and morphology change of both PAs The mass change per unit area of the two PA films as a function of AO irridiation time was presented in Fig. 4. Considerable weight loss was observed on exposure to atomic oxygen for PA-1. The mass of the sample almost linearly decreased, and the total mass loss in 6 h was 0.49 mg/cm2, indicated that the PA-1 was significantly attacked during exposure to AO beam. PA-2 exhibited obviously lower weight-loss rate compared to PA-1. The total mass loss after 6 h AO exposure was 0.15 mg/cm2, which was only 30.6% of the value for PA-1. Two regions of different weight loss rates were exhibited for PA-2. At the initial exposure stage, the mass loss of the film at a relative high rate, which may be attributed to the formation of volatile products such as CO and CO2 during AO irradiation [24]. Then the mass loss rate reduced gradually, for the inorganic phosphate-layer formed at the initial stage protected the underlayer material and thereby reduced the weight loss rate. In order to see the surface change of both materials directly, SEM surface morphologies of PA-1 and PA-2 films under different exposure time of AO beam were illustrated in Fig. 5. Before irradiation, the surfaces of both specimens were very smooth (Fig. 5a and b). However, all films exhibited different topographical under different AO exposure times. Reasons for the surface morphologies change can be attributed to the oxidative nature of AO. For volatile products, CO and CO2 were produced and removed from the surface of 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 more severely. After 6 h AO irradiation it was eroded on the whole and tree-root feature was formed (Fig. 5h). Compared with PA-1, PA-2 had a relatively smooth surface after irradiation, and morphology of the surface was changed unconspicuous with increasing irradiation time. The sufficient polyphosphonate surface formed on the surface protected the under-layer materials effectively from AO erosion [25]. This phenomenon was in well accordance to the mass change what we mentioned above. Obviously, rough surface of the PA-1 and PA-2 also degrades the optical properties which are presented in Fig. 6. Optical transmittance of the films with different AO irridiation time was measured. The transmittance at wavelength of 450 nm was checked as an example. Clearly we can see, the transmittance of PA-1 decreased directly with the irridiation time increasing. After 6 h irridiation of AO, the relative transmittance of PA-1 reduced to 10%, which was attributed to the deterioration of the sample surface morphology after the AO irridiation. This result is consistant with the results what we have got in Fig. 5. In fact a foggy layer was formed on the surface of PA-1 after irridiation which could be seen intutively. In comparison, PA-2 maintained high transmittance after the AO irradiation, indicating the effective protect effect of polyphosphonate surface formed on the surface.
Fig. 6. Transmittance of PA-1 and PA-2 films under different AO irridiation time.
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Fig. 7. Tensile strength change of PA-1 and PA-2 films exposure to AO at different time.
9.5 MPa. Obviously, under the AO environment, PA-2 was more suitable for a long period use. 4. Conclusions Phenylphosphine oxide-containing PA exhibited a non-linear mass loss behavior, for an inorganic polyphosphate layer was formed on the surface during the irradiation of AO, which can protect the base material from being damaged. Less erosion of PA-2 with PPO segments helped it keeping higher transmittance and higher tensile strength after the irradiation, as compared with the PA-1 without PPO groups under the same AO exposure conditions. Hence, PA-2 with PPO groups is promising candidates for protecting materials for aerospace industry. Acknowledgment The authors would like to acknowledge the financial supports from the National Science Foundation of China (Grant No.51103168), the National Science Foundation for Distinguished Young Scholars of China (Grant No.51025517) and the National Defense Basic Scientific Research Project (A1320110011). References [1] Cassidy PE, editor. Thermally stable polymers. New York: Marcel Dekker; 1980.
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