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Progress in Organic Coatings 75 (2012) 49– 58

Contents lists available at SciVerse ScienceDirect

Progress in Organic Coatings

jou rn al h om epage: www.elsev ier .com/ locate /porgcoat

tomic oxygen-resistant and transparent polyimide coatings from3,5-bis(3-aminophenoxy)phenyl]diphenylphosphine oxide and aromaticianhydrides: Preparation and characterization

huo Li, Haiwang Song, Minhui He, Jingang Liu ∗, Shiyong Yang ∗

aboratory of Advanced Polymer Materials, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China

r t i c l e i n f o

rticle history:eceived 16 October 2011eceived in revised form 19 February 2012ccepted 10 March 2012vailable online 1 April 2012

eywords:olyimidestomic oxygenransparency

a b s t r a c t

A novel meta-substituted aromatic diamine, [3,5-bis(3-aminophenoxy)phenyl]diphenylphosphine oxide(m-BADPO) was synthesized by the Williamson reaction of 3,5-difluorophenyldiphenylphosphine oxide(DFPPO) and meta-aminophenol. The diamine was then polymerized with several commercially availablearomatic dianhydrides to afford a series of aromatic polyimides (PI-1–PI-4). The meta-substituted molec-ular skeleton and the pendant bulky phenylphosphine oxide (PPO) group endowed the PIs many desiredproperties for their potential applications in space environments. For instance, the solubility of the PIswas enhanced due to the synergic effects of meta structure and the bulky PPO groups, making it possi-ble to fabricate the PI films via solution procedure. The films exhibited flexible and tough natures withlight color and high transparency in the visible light region. The transmittance up to 87% at 400 nm was

hermal propertiesiphenylphosphine oxide

achieved in the films. The atomic oxygen (AO) degradation behavior of the PI films in the ground-basedsimulation facility was investigated. The preliminary X-ray photoelectron spectroscopy (XPS), scanningelectron microscopy (SEM) measurement results indicated that the PI films exhibited good durability inAO-rich environments. Inert phosphorous-containing protecting layer might form at the surface of PI filmduring AO exposure. In addition, the intrinsic merits of typical high-performance PIs were maintained inthe present polymers.

. Introduction

Recent years have witnessed a rapid progress demonstrating theide applications of aromatic polyimides (PIs) as thermal control

oatings or optical components in low earth orbit (LEO) spacecrafts1–3]. The beneficial properties of PIs used for such optical appli-ations include their superior thermal and chemical resistance,igh dimensional stability, and good dielectric properties to otherrganic polymers. However, many disadvantageous properties ofonventional PIs, including the high solar absorptivities originatedy their highly colored appearance, poor processability in commonrganic solvents, and particularly their inferior atomic oxygen (AO)esistance have to be modified in practical applications [4]. In prac-ice, various efforts have been systematically performed to improvehe anti-erosion ability of conventional aromatic PIs in bothround-based and real LEO environments for many decades [5].

any theoretical and practical experiences on improving the AO

esistances of aromatic PIs have been proposed. For instance, pro-edures by combining the AO-resistant species, such as aluminum

∗ Corresponding authors. Tel.: +86 10 62564819; fax: +86 10 62569562.E-mail addresses: liujg@iccas.ac.cn (J. Liu), shiyang@iccas.ac.cn (S. Yang).

300-9440/$ – see front matter © 2012 Elsevier B.V. All rights reserved.oi:10.1016/j.porgcoat.2012.03.007

© 2012 Elsevier B.V. All rights reserved.

oxide (Al2O3), zirconium oxide (ZrO2) or silicon dioxide (SiO2) withPIs have been widely utilized up to now [6]. Lately, a new concept bydeveloping PI films with “self-healing” ability in AO environmentswas proposed [7]. Based on this concept, a series of inherentlyAO-resistant PIs has been reported, typically the ones contain-ing phenylphosphine oxide (PPO) moiety developed by NASA, USA[8–10], the ones containing polyhedral oligomeric silsesquioxane(POSS) linkage reported by Air Force Research Laboratory (ARFL),USA [11,12], and the silicon-containing PIs demonstrated by JapanAerospace Exploration Agency (JAXA), Japan [13,14]. Phosphorousand silicon elements are mostly often selected due to their poten-tial ability to form inert protecting layer (polyphosphate or silicate)at the surface of PI film upon exposure to atomic oxygen. The inertcovering layer could efficiently prevent the further erosion of theunderlayers; thus, endows the PIs “self-healing” ability.

As for the transparency consideration, from the viewpointof structural characteristics, by minimizing or eliminating thecharge-transfer complexes (CTC) formation in PIs, for exam-ple, introducing alicyclic moiety, electron-withdrawing groups,

bulky substituents, or meta-substituted backbones, low color PIfilms with improved optical transparency can often be obtained[15,16]. Combining these structural features with AO-resistantcomponents, one can develop PI optical materials meeting the

5 anic C

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ho[Bsotsr

2

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ppiB2P(aHamcwa

2

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0 Z. Li et al. / Progress in Org

ritical demands of the severe LEO environments. For instance,elozier et al. reported low color, flexible, and space durableI nanocomposite films based on a PI matrix derived from,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and

meta-substituted diamine, 1,3-bis(3-aminophenoxy)benzene133APB) [17]. Watson et al. reported space environmentallytable PIs derived from a PPO-containing diamine, [2,4-bis(3-minophenoxy)phenyl]diphenylphosphine oxide [18]. In thissymmetrical diamine, a meta-substituted backbone structure wasesigned to inhibit the CTC formation although the PPO groupas substituted ortho to the ether linkage. Very recently, they

eported the AO erosion results of the PI sample derived from theiamine and 4,4′-oxydiphthalic anhydride (ODPA) after exposure

n LEO for 4 years [19]. The PI survived intact whereas Kaptonlm [poly(pyromellitic dianhydride-co-4,4′-oxydianiline)] and theetallized Kapton films were completely eroded.In the present work, as one of our continuous efforts to develop

igh-performance phosphorous-containing polymers [20], a seriesf novel PIs were synthesized from a meta-substituted diamine,3,5-bis(3-aminophenoxy)phenyl]diphenylphosphine oxide (m-ADPO) and aromatic dianhydrides. The symmetrical and meta-ubstituted structure in the diamine is aiming at increasing theptical transparency of the derived PIs while maintaining theirhermal stability. The synergic effects of PPO moiety and the meta-ubstituted main chain structure on the thermal, optical, and AOesistance properties of the PIs were investigated in detail.

. Experiments

.1. Materials

1-Bromo-3,5-difluorobenzene and meta-aminophenol wasurchased from Aldrich and used as received. Diphenylphos-hinyl chloride was purchased from Acros and distilled

n vacuo (b.p.: 232 ◦C at 400 Pa) prior to use. 3,3′,4,4′-iphenyltetracarboxylic dianhydride (sBPDA, TCI, Japan),,3,3′,4′-biphenyltetracarboxylic dianhydride (aBPDA, BeijingOME Sci. Technol. Co. Ltd., China), 4,4′-oxydiphthalic anhydrideODPA, Shanghai Research Institute of Synthetic Resin, China),nd 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA,oechst Celanese Corp., USA) were recrystallized from aceticnhydride and dried in vacuum at 160 ◦C overnight prior to use. N-ethyl-2-pyrrolidinone (NMP), N,N-dimethylacetamide (DMAc),

yclopentanone (CPA), tetrahydrofuran (THF) and other solventsere purified by distillation prior to use. The other commercially

vailable reagents were used without further purification.

.2. Synthesis and sample preparation

.2.1. Synthesis of 3,5-difluorophenyldiphenylphosphine oxideDFDPO)

A 500-mL three-necked round-bottom flask equipped with aechanical stirrer, a nitrogen inlet, a pressure equalizing addi-

ion funnel and a condenser was charged with magnesium turnings4.0 g, 0.167 mol) and anhydrous tetrahydrofuran (THF, 27 mL). The

ixture was cooled to 5 ◦C using an ice-water bath. A solutionf 1-bromo-3,5-difluorobenzene (28.9 g, 0.150 mol) in THF (68 mL)as added dropwise over 3 h. After the addition, the mixture was

lowly warmed to room temperature and stirred for another 4 h.hen the mixture was again cooled to 5 ◦C using an external ice-ater bath. A solution of the freshly distilled diphenylphosphinyl

hloride (29.7 g, 0.125 mol) in THF (40 mL) was added dropwisever 2 h. Then, the mixture was allowed to warm to room tem-erature slowly and stirred for another 15 h. A solution of 10 wt%ydrochloric acid (150 mL) was then slowly added. The organic

oatings 75 (2012) 49– 58

phase was isolated and washed thoroughly with a 5% aqueoussodium bicarbonate followed by fresh water. Then, the organicphase was dried over magnesium sulfate overnight. A pale-yellowsolid was obtained after evaporating the solvent. The crude prod-uct was twice recrystallized from hexane to afford white crystals(yield: 25.9 g, 66.0%); mp: 127.9 ◦C (DSC peak).

FT-IR (KBr, cm−1): 3056, 1590, 1423, 1294, 1188, 1121, 987, and699. 1H NMR (400 MHz, CDCl3, ppm): 6.95–7.00 (m, 1H), 7.17–7.22(m, 2H), 7.48–7.52 (m, 4H), 7.55–7.61 (m, 2H), and 7.64–7.74 (m,4H). MS (EI, m/e, percentage of relative intensity): 313 (M+−1, 100).C18H13F2OP: Cald. C, 68.79%, H, 4.17%; Found: C, 68.64%, H, 4.41%.

2.2.2. Synthesis of3,5-bis[(3-aminophenoxy)phenyl]diphenylphosphine oxide(m-BADPO)

A 500-mL three-necked round-bottom flask equipped with amechanical stirrer, a nitrogen inlet, a thermometer, and a Dean-Stark trap was charged with a mixture of DFDPO (6.29 g, 0.02 mol),meta-aminophenol (4.37 g, 0.04 mol), anhydrous potassium car-bonate (6.91 g, 0.05 mol), NMP (35 mL), and toluene (18 mL). Afterstirring at room temperature for 0.5 h, the mixture was heated to135 ◦C in nitrogen to facilitate dehydration. After complete removalof the water (about 16 h), the toluene was removed from the reac-tion, and the reaction mixture was further stirred at 180 ◦C foranother 4 h. The mixture was then cooled to room temperatureand poured into an excess of ice-water mixture (200 mL) with vig-orous stirring. The precipitate was collected by filtration, washedwith water, and directly recrystallized from ethanol to afford pale-brown crystals (yield: 5.42 g, 55.0%); mp: 195.3 ◦C (DSC peak).

FT-IR (KBr, cm−1): 3341, 3205, 1630, 1608, 1573, 1490, 1416,1371, 1281, and 1177. 1H NMR (400 MHz, DMSO-d6, ppm): 5.26(s, 4H), 6.11–6.13 (d, 2H), 6.19 (s, 2H), 6.32–6.34 (d, 2H), 6.73 (s,1H), 6.78–6.81 (d, 2H), 6.95–6.99 (t, 2H), and 7.51–7.63 (m, 10H).13C NMR (100 MHz, DMSO-d6, ppm): 104.8, 106.7, 110.7, 111.3,115.0, 115.1, 129.3, 129.4, 130.8, 131.9, 132.0, 132.8, 133.1, 135.8,136.8, 151.2, 156.7, 159.2, and 159.4. MS (EI, m/e, percentage of rel-ative intensity): 491 (M+−1, 100). C30H25N2O3P: Cald. C, 73.16%, H,5.12%, N, 5.69%; Found: C, 73.43%, H, 5.27%, N, 5.63%.

2.2.3. Polyimide synthesis and film preparationThe general procedure for the preparation of the PIs could be

illustrated by the synthesis of PI-4. m-BADPO (2.9550 g, 6 mmol)was added to a 100 mL three-necked flask equipped with a mechan-ical stirrer and a nitrogen inlet. NMP (10.9 g) was added and a gentlestream of nitrogen was passed through the solution. After stirringfor 10 min, a clear diamine solution was obtained. 6FDA (2.6654 g,6 mmol) was then added in one batch and an additional volumeof NMP (10 g) was added to wash the residual dianhydride, andat the same time to adjust the solid content of the reaction sys-tem to be 20 wt%. The mixture was stirred at room temperature for24 h to yield a viscous pale-yellow poly(amic acid) (PAA) solution.The PAA solution was then chemically imidized by the addition ofacetic anhydride (3.06 g, 30 mmol) and pyridine (2.37 g, 30 mmol)followed by stirring at room temperature for another 24 h. Thesolution was poured slowly into an excess of methanol (200 mL)to afford a white silky resin. The resin was dried at 80 ◦C in vacuumovernight to afford PI-4. Yield: 5.34 g (95%).

The well-dried PI-4 resin was dissolved in NMP at room tem-perature with a solid content of 20 wt%. After filtering througha 0.45 �m Teflon syringe filter to remove any contaminates, thePI solution was spin-coated on a silicon wafer, and the thicknesswas controlled by the spinning rate. The thickness of a specimen

for FT-IR and UV–vis measurements was controlled to be about10 �m, while the specimen for thermal and mechanical evaluationwas adjusted to be about 50 �m. Then, PI-4 film was obtained bythermally baking the solution in a nitrogen-purged oven with the

anic Coatings 75 (2012) 49– 58 51

f2

ts

2

titrptPmxofioNhwswrtTaS

ws2pctf

wwalppl

n

iUtfitw8ee

E

wl

Z. Li et al. / Progress in Org

ollowing heating procedure: 80 ◦C/2 h, 150 ◦C/1 h, 200 ◦C/1 h, and50 ◦C/1 h.

PI-1, PI-2 and PI-3 resin and films were prepared according tohe similar procedure as PI-4, except that 6FDA was replaced byBPDA for PI-1, aBPDA for PI-2 and ODPA for PI-3.

.3. Measurements

Inherent viscosity was measured using an Ubbelohde viscome-er with a 0.5 g dL−1 NMP solution at 30 ◦C. Fourier transformnfrared (FT-IR) spectra were obtained with a Tensor 27 Fourierransform spectrometer. Ultraviolet–visible (UV–vis) spectra wereecorded on a Hitachi U-3210 spectrophotometer at room tem-erature. The cutoff wavelength was defined as the point wherehe transmittance drops below 1% in the spectrum. Prior to test,I samples were dried at 100 ◦C for 1 h to remove the absorbedoisture. Yellow index (YI) of the PI films was measured using an

-rite color i7 spectrophotometer with PI samples at a thicknessf 30–40 �m. The b* value was used to evaluate the color of thelm. Nuclear magnetic resonance (NMR) spectra were performedn a Bruker Avance 400 spectrometer operating at 400 MHz for 1HMR and 100 MHz for 13C NMR in DMSO-d6 or CDCl3. The 1H–13Ceteronuclear single-quantum coherence (HSQC) NMR spectrumas recorded on a Bruker Avance 600 spectrometer. Differential

canning calorimetry (DSC) and thermogravimetric analysis (TGA)ere recorded on a TA-Q series thermal analysis system at a heating

ate of 10 ◦C min−1 and 20 ◦C min−1 in nitrogen or in air, respec-ively. The tensile properties were performed on an Instron 3365ensile Apparatus with 80 mm × 10 mm × 0.05 mm specimens inccordance with GB1447-83 at a drawing rate of 2.0 mm min−1.even samples of each PI film were tested.

Solubility was determined as follows: 1.0 g of the tested PI resinas mixed with 9.0 g of the solvent at room temperature (10 wt%

olid content), which was then mechanically stirred in nitrogen for4 h. The solubility was determined visually as three grades: com-letely soluble (++), partially soluble (+), and insoluble (−). Theomplete solubility is defined as a homogenous and clean solu-ion is obtained, in which no phase separation, precipitation or gelormation is detected.

Refractive index of the PI film formed on a 3-in. silicon waferas measured at the wavelength of 1310 nm at room temperatureith a Metricon MODEL 2010/M prism coupler. The in-plane (nTE)

nd out-of-plane (nTM) refractive index were determined usinginearly polarized laser light parallel (transverse electric, TE) anderpendicular (transverse magnetic, TM) polarizations to the filmlane, respectively. The average refractive index (nav) was calcu-

ated according to Eq. (1):

av = 2nTE + nTM

3(1)

The atomic oxygen (AO) exposure experiments were testedn a ground-based AO effects simulation facility at Beihangniversity [21]. This apparatus is a filament discharge plasma-

ype facility. The AO exposure was performed on three PIlm samples, PI-1, PI-3 and PI-4. A square film sample withhe size of 20 mm (length) × 20 mm (width) × 0.05 mm (thickness)as used. The films were exposed to AO at a fluence of

.13 × 1020 atoms cm−2, and the mass loss was determined. Therosion yield of the sample, Es, is calculated through the followingquation (2) [22]:

�Ms

s =

As�sF(2)

here Es = erosion yield of the sample (cm3 atom−1); �Ms = massoss of the sample (g); As = surface area of the sample exposed to

Fig. 1. FT-IR spectra of m-BADPO and PIs.

atomic oxygen attack (cm2); �s = density of the sample (g cm−3);F = atomic oxygen fluence (atoms cm−2).

As Kapton film has a well characterized erosion yield; that is3.0 × 10−24 cm3 atom−1 [22], and all the present PI samples aresupposed to possess similar densities and exposed area with Kap-ton in the AO attacking experiments, thus, the Es of the PIs can becalculated using a simplified equation (3):

Es = �Ms

�MKaptonEKapton (3)

where EKapton stands for the erosion yield of Kapton standard, thatis 3.0 × 10−24 cm3 atom−1; �MKapton stands for the mass loss ofKapton standard.

X-ray photoelectron spectroscopy (XPS) data were obtainedwith an ESCALab220i-XL electron spectrometer from VG Sci-entific using a 300 W MgK� radiation. The base pressure was3 × 10−9 mbar. The binding energies were referenced to the C1sline at 284.8 eV from the adventitious carbon. PI surface morphog-raphy was detected by scanning electron microscopy (SEM, Hitachi,S-4300).

3. Results and discussion

3.1. Monomer synthesis

The novel PPO-containing diamine m-BADPO was synthe-sized via a two-step process with a total yield of about36% (Scheme 1). The key factor affecting the yield of thefirst step; that is the Grignard reaction between 1-bromo-3,5-difluorobenzene and diphenylphosphinyl chloride, is the watercontent of the diphenylphosphinyl chloride. It readily hydrolyzesto diphenylphosphinic acid during storage; thus should be distilledin vacuo prior to use. After recrystallizing from hexane, DFDPOcould be obtained as a fine crystal with a moderate yield. In thesecond step, DFDPO was directly reacted with meta-aminophenolto give the diamine m-BADPO via a well-established procedure. Itis necessary to purify the diamine by two recrystallizations fromethanol with charcoal powder treatment to remove the coloredimpurities. The structure of m-BADPO was confirmed by variousspectroscopy measurements.

Fig. 1 compares the FT-IR spectra of DFDPO and m-BADPO.The characteristic absorptions at 3342 and 3205 cm−1 due to theN H stretching of a primary amino group was clearly observed in

the spectrum of m-BADPO, whereas the absorption of C F bondat 1026 cm−1 in the spectrum of DFDPO disappeared. In addi-tion, the characterization bands of P O bond at about 1180 cm−1

appeared in both of the spectra. The 1H NMR and 13C NMR

52 Z. Li et al. / Progress in Organic Coatings 75 (2012) 49– 58

thesi

sspHsOHa(st2

1dusH

ma

3

sdpv0rtaa

The good solubility of the PIs is mainly ascribed to their amor-phous natures, as evidenced by their wide-angle X-ray diffractionpatterns. The pendant bulky PPO groups and the meta structureseffectively prevent the PI molecular chains from regular packing

Scheme 1. Syn

pectra of m-BADPO with assignments of all peaks are pre-ented in Figs. 2 and 3, respectively. As shown in Fig. 2, therotons located at the diphenylphosphine oxide moiety (H7,8, and H9) showed the absorptions at the lowest field in the

pectrum due to the adjacent electron-withdrawing P O bond.n the contrary, those ortho to the ether linkages (H1 and2) resonated at the highest field in the spectrum except thebsorption of protons in amino groups. The 13C NMR spectrumFig. 3a) reveals 14 signals, which is consistent with the structuralymmetry of the diamine. Cj and Ci showed clear double absorp-ions at 135.8–136.8 and 115.0–115.1 ppm, respectively due to theJC-P (coupling constant: 100 MHz) and 3JC-P (coupling constant:0 MHz) coupling of the carbon with phosphorous atoms in theiamine. The two-dimension 1H–13C HSQC spectrum (Fig. 3b) wastilized to confirm the C H correlations, in which all the H and Cignals correlated well. This is very helpful to confirm the individual

and C absorption.The structure of m-BADPO was further confirmed by the ele-

ental analysis, in which the measured C, H and N compositionsgreed well with the theoretical ones.

.2. Polyimide synthesis and film properties

As shown in Scheme 2, the PIs with pendant PPO groups wereynthesized from the polycondensation of m-BADPO and aromaticianhydrides in NMP via a two-step procedure. All the reactionsroceeded homogeneously at room temperature. The inherentiscosity of the obtained PI resins in NMP ranged from 0.61 to.78 dL g−1 (Table 1). These moderate values reflect the moderate

eactivity and high purity of the synthesized m-BADPO. Fig. 1 showshe FT-IR spectra of the PIs. The characteristic absorption bands atbout 1778 cm−1 (�C O,asy), 1720 cm−1 (�C O,sy), 1378 cm−1 (ıC N),nd 720 cm−1 (imide ring deformation) indicate the successful

Fig. 2. 1H NMR spectrum of m-BADPO.

s of m-BADPO.

preparation of the PIs. The typical 1H NMR spectrum of PI-4 isshown in Fig. 4. All the protons are in good agreement with theproposed structure of the polymer. The protons in the dianhydrideresidue resonated farthest downfield of the spectrum due to theelectron-withdrawing imide carbonyl groups.

The solubility of the PI resins is summarized in Table 1. Asexpected, all the PIs are soluble in polar aprotic solvents, NMP andDMAc. They are also soluble in CPA, chloroform and THF at variousdegrees. PI-3 and 4 with flexible ether or hexafluoroisopropyli-dene (6F) linkages exhibit the best solubility in the tested solvents.

Fig. 3. 13C NMR (a) and 1H–13C HSQC (b) spectra of m-BADPO.

Z. Li et al. / Progress in Organic Coatings 75 (2012) 49– 58 53

Scheme 2. Synth

auprg

Nel

TI

Fig. 4. 1H NMR spectrum of PI-4.

nd aggregating; thus prohibit their crystallizability. The good sol-bility of the present PIs in solvents with moderate to low boilingoints (such as CPA) makes it possible to fabricate the PI films atelatively low temperatures. This is beneficial for obtaining homo-eneous PI film with low defects.

Flexible and tough PI films were cast from the PI solutions inMP except PI-2. PI-2 was obtained as a free-standing film; how-ver, it was not creasable. This might be due to the relativelyower reactivity of aBPDA at room temperature. Increasing the

able 1nherent viscosity and solubility of the PIs.

PI [�]inha (dL g−1) Solventsb

NMP DMAc

PI-1 0.73 ++ ++

PI-2 0.61 ++ ++

PI-3 0.78 ++ ++

PI-4 0.68 ++ ++

a Measured with PI solutions at a concentration of 0.5 g dL−1 in NMP at 30 ◦C.b ++, wholly soluble; +, partially soluble; –, not soluble; NMP: N-methyl-2-pyrrolidinon

esis of PIs.

polymerization temperature, that is synthesizing PI-2 by a high-temperature one-step procedure [23] has been proven to be aneffective way to obtain a tough film in our experiment. As tab-ulated in Table 2, PI-1, PI-3 and PI-4 exhibited tensile strengthsfrom 70.7 to 87.2 MPa, elongation at breaks of 3.3–3.9%, and ten-sile modulus of 2.9–3.2 GPa. Apparently, the mechanical propertiesof the present PI films are inferior to their counterparts derivedfrom 2,5-bis[(4-aminophenoxy)phenyl]diphenylphosphine oxide(p-BADPO) reported in our previous work [20]. Relatively infe-rior reactivity of m-BADPO to p-BADPO might be the main reason.Copolymerization of m-BADPO and p-BADPO might be a com-promise developing PIs with good mechanical properties withoutdecreasing the phosphorous contents of the polymers.

3.3. Thermal properties

The thermal properties of the PIs were evaluated by TGA overa range of 50–750 ◦C both in nitrogen and in air. As shown inTable 2 and Fig. 5, all the PIs possess excellent thermal stabil-ity with no significant weight loss up to approximately 500 ◦C.

The 5% weight loss temperatures (T5%) of the PIs are all higherthan 520 ◦C either in nitrogen or in air. The thermal behaviorsof the PIs under an air stream showed that the T5% and T10%values were similar to those under a nitrogen stream, while

CPA CHCl3 THF Acetone

+ – – –++ + – –++ ++ + –++ ++ ++ +

e; DMAc: N,N-dimethylacetamide; CPA: cyclopentanone.

54 Z. Li et al. / Progress in Organic Coatings 75 (2012) 49– 58

Table 2Thermal and mechanical properties of PI films.

PI Tga (◦C) T5%

a (◦C) T10%a (◦C) Rw700

a (%) Tsb (MPa) Eb

b (%) TMb (GPa)

PI-1 209 555 (549) 562 (558) 65 (59) 87.2 3.3 3.2PI-2 208 544 (543) 553 (553) 67 (57) –c –c –c

PI-3 192 541 (539) 552 (549) 64 (49) 76.6 3.9 2.9PI-4 210 523 (524) 540 (542) 65 (59) 70.7 3.5 3.0

a Tg, glass transition temperature; T5%, T10%, temperatures at 5% and 10% weight loss in nitrogen, respectively; Rw700, residual weight ratio at 700 ◦C. The values in thep

t6Bomotoi[Foa4m

fdbe

PI from p-BADPO and 6FDA (PI-4′) at the same thickness [20]. Thetransmittance of the PI-4 film at 400 nm (85%) was also superiorto its para-linked analog (PI-4′: 82%). Yellow index (YI) is usually

arentheses were measured in air.b Ts, tensile strength; Eb, elongation at break; TM, tensile modulus.c Not measured.

he residue weight ratio at 700 ◦C (Rw700) showed a decline of–15%. It is noteworthy that the T5% value of PI-4 (6FDA/m-ADPO) in air is 542 ◦C, which is nearly 100 ◦C higher than thatf PI (6FDA/133APB) [444 ◦C, Ref. 24]. The increase of the ther-al stability in air for PI-4 might be attributed to the interaction

f PPO group with oxygen in the air. This reaction results inhe inert phosphorous-rich residue, preventing the decompositionf the PI. It should also be noticed that the present PIs exhib-ted higher thermal stability compared with the PIs derived from2,4-bis(3-aminophenoxy)phenyl]diphenylphosphine oxide [18].or example, the current PI-4 (6FDA/m-BADPO) has a T5% valuef 524 ◦C in air, whereas the reported value for PI from 6FDAnd [2,4-bis(3-aminophenoxy)phenyl]diphenylphosphine oxide is53 ◦C. The symmetrical molecular structure of the present PIsight induce their better thermal stability.Glass transition temperatures (Tgs) of the PIs were recorded

rom 192 to 210 ◦C in DSC measurements (Fig. 6 and Table 2). PI-3,erived from ODPA and m-BADPO, contains flexible ether linkagesoth in the diamine and dianhydride moiety; thus exhibited a low-st Tg of 192 ◦C in the family. Nevertheless, this value is much higher

Fig. 5. TGA curves of PIs in nitrogen (a) and in air (b) (20 ◦C min−1).

than that of PI from ODPA and 133APB (168 ◦C) [25]. The increaseof the Tg is due to the steric hindrance of the molecular segmentmobility caused by the lateral PPO group at elevated temperatures.For PI-4 (6FDA/m-BADPO), its Tg value (210 ◦C) is also apparentlyhigher than its analog from 6FDA and 133APB (Tg: 199 ◦C) [26].

3.4. Optical properties

The optical properties of the PI films are summarized in Table 3and the UV–vis spectra of the PI films are illustrated in Fig. 7. Thetransmittances of the PI films at 400 nm are in the range of 29–87%and increased in the order of PI-1 < PI-2 < PI-4 ≈ PI-3. PI-4 showeda cutoff wavelength at 319 nm, which was 7 nm lower than that of

Fig. 6. DSC curves of PIs (in nitrogen, 10 ◦C min−1).

Fig. 7. UV–vis spectra of PI films.

Z. Li et al. / Progress in Organic Coatings 75 (2012) 49– 58 55

Table 3Optical properties of the PI films.

PI �a (nm) T400b (%) YIc (b* value) Refractive indices at 1310 nm εf

nTEe nTM

e nave �ne

PI-1 375 29 13.4 1.6432 1.6349 1.6404 0.0083 2.96PI-2 362 75 10.6 1.6329 1.6253 1.6307 0.0076 2.93PI-3 348 87 7.2 1.6208 1.6169 1.6195 0.0039 2.89PI-4 319 85 6.4 1.5568 1.5526 1.5554 0.0042 2.66PI-3′d 346 66 9.7 1.6366 1.6334 1.6355 0.0032 2.94PI-4′d 326 82 8.0 1.5903 1.5876 1.5894 0.0027 2.78

a Cutoff wavelength.b Transmittance at 400 nm.c Yellow indices of the PI films.d PIs derived from p-BADPO and ODPA for PI-3′ and 6FDA for PI-4′ (see Ref. [20]).e See Section 2.3.f Dielectric constant estimated from Maxwell’s equation as ε = 1.10n2

av.

Table 4AO effects and erosion yields for PI films.

Sample Pa (%) W1b (mg) W2

b (mg) �Wc (mg) Esd (10−25 cm3 atom−1)

PI-1 4.13 6.110 5.775 0.335 7.36PI-3 4.04 3.425 3.125 0.300 6.59PI-4 3.44 5.940 5.440 0.500 11.0Kapton 0 2.400 1.035 1.365 30.0

a Phosphorous content in the PI sample.b W1: weight of the sample before irradiation; W2: weight of the sample after irradiation.c

aToc(fvtthbflh

TX

Wight loss of the sample during irradiation, �W = W1 − W2.d Erosion yield, see Section 2.3.

dopted as a criterion evaluating the color of a polymer film [27].his value describes the color changes of a film sample from clearr white toward yellow. Lower YI value usually indicates a weakoloration for a polymer film. As shown in Table 3, the YI valuesb* values) of the m-BADPO-PI films (thickness: ∼40 �m) rangedrom 6.4 to 13.4 dependent on the dianhydrides used. The YIalues of 7.2 for PI-3 and 6.4 for PI-4 were apparently lower thanhose of their p-BADPO analogs (PI-3′: 9.7 and PI-4′: 8.0). The goodransparency and low coloration of PI-3 and PI-4 films, on oneand, could be attributed to its loose molecular packing induced

y the bulky pendant PPO group in the diamine moiety and theexible ether or 6F linkage in the dianhydride part. On the otherand, the irregular structures caused by their meta-substituted

able 5PS results for the pristine and AO-exposed PI films.

Photopeak PI-1 PI-3

PSc ESc PSc ESc

C1sA.Conc.a (%) 75.75 34.34 76.8 41.96B.E.b (eV) 284.8 284.8 284.8 284.8

O1sA.Conc. (%) 17.27 49.36 16.97 43.27B.E. (eV) 531.8 532.6 532.3 532.7

P2pA.Conc. (%) 1.90 12.81 1.40 11.37B.E. (eV) 132.3 134.3 132.3 134.4

N1sA.Conc. (%) 3.93 2.21 3.29 2.78B.E. (eV) 400.1 401.0 400.3 401.0

F1sA.Conc. (%) –d –d –d –d

B.E. (eV) –d –d –d –d

a Atomic concentration.b Binding energy.c PS: pristine sample; ES: exposured sample.d Not detected.

molecular backbone were also advantageous to reducing theformation of charge transfer complexes (CTC).

Influence of the steric hindrance and inductive effects of the PPOgroups and the meta structures on the optical properties of the PIfilms could also be reflected from the refractive index (n) valuesof the films. Generally, polymers containing substituents with lowmolar refractions or bulky molecular volumes often exhibit low nvalues according to the Lorentz–Lorenz equation [28]. For instance,the Kapton-type PI (PMDA-ODA) has a n value of 1.6478 at 1320 nm,whereas PI (6FDA-ODA) with a much looser molecular packing

exhibits a lower n value of 1.5565 [29]. In the present study, the pen-dant bulky PPO substituents and the meta-substituted structure areexpected to decrease the refractive indices of the PIs. As shown in

PI-4 Kapton

PSc ESc PSc ESc

69.12 37.77 63.58 62.83284.8 284.8 284.8 284.8

13.70 39.69 23.75 28.02531.9 532.7 531.8 532.0

1.52 10.55 –d –d

132.3 134.3 –d –d

3.08 2.64 6.36 5.12400.3 400.7 400.1 400.2

10.53 9.35 –d –d

688.5 688.4 –d –d

56 Z. Li et al. / Progress in Organic C

To1oisbir(t

Fig. 8. Mass loss versus AO fluence for the PIs.

able 3, the in-plane (nTE) and out-of-plane (nTM) refractive indicesf the PI films at 1310 nm are in the range of 1.5568–1.6432 and.5526–1.6349, respectively. These values are all lower than thatf the standard PI(PMDA-ODA). PI-4, with the 6F linkage exhib-ted the lowest average refractive index (nav) value, whereas PI-1howed the highest one. The higher nav value for PI-1 could mainlye ascribed to the more dense packing of its molecular chains

nduced by the planar biphenyl structure in sBPDA moiety. Withespect to its isomeric PI-2 derived from the asymmetrical BPDAaBPDA) and the same diamine, the nav value decreased by 0.01 dueo the expanded molecular packing. PI-4 (6FDA/m-BADPO) has a nav

Fig. 9. XPS results for the unexposed (a) and exposed (b) PI-1 film.

oatings 75 (2012) 49– 58

value of 1.5554, which is not only lower than that of PI-4′ (6FDA/p-BADPO) (n: 1.5894, Ref. 20) but lower than PI (6FDA/133APB) (n:1.6073, Ref. 30). Obviously, this is the synergic effects of the pendantPPO group and the meta molecular structure in the polymer.

The flexible ether linkages in m-BAPDO moiety endow the PIslow birefringence (�n) in the increasing order of PI-3 (0.0039) < PI-4 (0.0042) < PI-2 (0.0076) < PI-1 (0.0083). The larger �n value of PI-1is attributable to the rigid and linear chains resulting from the BPDAmoiety, which easily causes in plane orientation of the molecularchains.

The dielectric constants (ε) of the PIs (Table 3) can be roughlyestimated from the refractive indices according to the modifiedMaxwell’s equation [31]. The estimated ε values are all lowerthan that of Kapton (ε = 3.5 at 1 kHz) [32]. Thus, the presentPIs might find applications in advanced microelectronic fabrica-tions.

3.5. Atomic oxygen erosion

Four PI film samples, PI-1, PI-3, PI-4 together with the Kap-ton standard (20 mm × 20 mm × 0.05 mm) were exposed to atomicoxygen at the ground-based simulation facility. The erosion rateof the Kapton was set to be 3.0 × 10−24 cm3 atom−1 as a reference.The samples were exposed to atomic oxygen with a total fluenceof 8.13 × 1020 atoms cm−2. Fig. 8 presents the mass loss versus AOfluence for the PIs and Kapton reference. It can be observed that

the PI samples containing PPO group exhibited a weight loss of5.48–8.76 wt% while that of Kapton was 56.76 wt%. In addition, thePPO-containing PIs exhibited a nonlinear weight loss rate whileKapton exhibited a linear one. The mass loss and calculated AO

Fig. 10. XPS analysis of the element spectra P2p (a) and O1s (b) for PI-1.

Z. Li et al. / Progress in Organic C

F(

eictePiPs

ieifft1bAciPaaifuetw

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ig. 11. SEM images of Kapton (a) and PI-1 (b) exposed to AO8.13 × 1020 atoms cm−2).

rosion yield (Es) according to Eq. (3) for the PIs are summarizedn Table 4. As shown in Table 4, the erosion yields of the PPO-ontaining PIs were much lower than that of Kapton. PI-3 showshe lowest erosion yield of 6.59 × 10−25 cm3 atom−1 while PI-4xhibits the highest one. Obviously, the PPO groups in the presentIs decrease their erosion yields in AO environments. In order tonvestigate the effects of PPO groups on the AO durability of theIs, the surface chemistry of the PI samples after AO exposure wastudied.

The surface composition of the AO-exposed PI films was exam-ned by XPS. The atomic concentration in percent and the bindingnergy are listed in Table 5, while the typical XPS spectra of PI-1 arellustrated in Figs. 9 and 10. As shown in Table 5, the measured sur-ace element composition for pristine PI-1 is 75.75% for C, 17.27%or O, 3.93% for N, and 1.90% for P element. This result is similar withhe theoretical composition of PI-1 [(C46H27N2O7P)n, C: 73.60%; O:4.92%; N: 3.73%; and P: 4.13%]. The difference in P contents mighte attributed to the molecular chain morphography of the PI. AfterO exposure, the amount of C decreased greatly, while O and Pontents increased, as did their respective binding energies. Fig. 11llustrates the individual P2p (a) and O1s (b) spectra for PI-1. The

2p maximum peak shifts from a binding energy of 132.3 eV to much higher value of 134.3 eV. At the same time, the O1s peaklso shows a significant shift from 531.8 eV to 532.6 eV. This resultndicates the formation of a layer of phosphorus oxide at the sur-ace of PI-1. This inert layer might inhibit the further erosion of thenderlying PI film. SEM images of Kapton and PI-1 samples after AO

xposure are shown in Fig. 11. Both of the surfaces of PI-1 and Kap-on films exhibited a carpet-like appearance, which is in agreementith the result reported in the literature [33].

[

[

oatings 75 (2012) 49– 58 57

4. Conclusions

A series of functional PIs containing meta-substituted PPO moi-eties were synthesized. Good combined properties were achievedin the PIs. First, they exhibited an enhanced solubility in com-mon solvents (such as in CPA), making it possible to fabricate afilm via a solution processing technology. The obtained low-defectfilms exhibited an amorphous nature. The PIs exhibited T5% valueshigher than 500 ◦C both in air and nitrogen and Tg values around200 ◦C. Thus, they might withstand the typical thermal cycle in LEOenvironments (±100 ◦C, Ref. 18). PI-3 and PI-4 film also showedgood optical transparency and low yellow indices in the visiblelight region and low refractive indices. Preliminary atomic oxygenexposure measurement results indicate that the PPO-containingPIs have good durability in AO environments. Most of their masswas maintained after a high fluence of atomic oxygen exposure.They showed a nonlinear mass loss during the exposure, exhibit-ing a lowest erosion yield of 6.59 × 10−25 cm3 atom−1. This value isnearly one fifth to the Kapton film. These fundamental experimen-tal results might be useful for their potential applications, such asthermal control coatings for LEO spacecraft.

Acknowledgments

Financial support from the National Natural Science Foundationof China (NSFC, 51173188) and Center for Molecular Science, ICCAS(CMS-LX201003) are gratefully acknowledged.

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