European Polymer Journal 45 (2009) 1139–1148

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European Polymer Journal

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Organo-soluble and transparent polyimides containing phenylphosphineoxide and trifluoromethyl moiety: Synthesis and characterization

Zhuo Li, Jingang Liu *, Zhiqi Gao, Zhihua Yin, Lin Fan, Shiyong Yang *

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

a r t i c l e i n f o

Article history:Received 22 August 2008Received in revised form 26 December 2008Accepted 9 January 2009Available online 22 January 2009

Keywords:PolyimidePhenylphosphine oxideTrifluoromethylSolubilityTransparencyThermal stability

0014-3057/$ - see front matter � 2009 Elsevier Ltddoi:10.1016/j.eurpolymj.2009.01.017

* Corresponding authors. Tel.: +86 10 62564819; f(J. Liu).

E-mail addresses: liujg@iccas.ac.cn (J. Liu),(S. Yang).

a b s t r a c t

A series of aromatic polyimides (PI-IIa–d) containing lateral phenylphosphine oxide (PPO)and trifluoromethyl (–CF3) moiety were prepared from an aromatic diamine, 2,5-bis[(4-amino-2-trifluoromethylphenoxy)phenyl]diphenyl-phosphine oxide (BATFDPO) and vari-ous aromatic dianhydrides via a two-step chemical imidization procedure. In parallel, forcomparison, another series of polyimides (PI-Ia–d) without trifluoromethyl were synthe-sized from a diamine, 2,5-bis[(4-aminophenoxy)-phenyl]diphenylphosphine oxide (BAD-PO) and the same dianhydrides. It was found that both of the two series of polyimides(PIs) were soluble in polar aprotic solvents, such as N-methyl-2-pyrrolidinone (NMP)and the solubility of PI-IIa–d was highly enhanced by the introduction of the bulky –CF3

group. Flexible and tough PI films with tensile strengths higher than 70 MPa were cast fromthe PI solution. The introduction of –CF3 moiety slightly sacrificed the thermal stability andmechanical properties of the PI films. For example, PI-IIa–d showed 5% weight loss at 472–476 �C, which was about 50 �C lower than those of their PI-Ia–d analogues. However, –CF3

group apparently improved the optical transparency and decreased the refractive indicesof the PI films. PI-IId derived from BATFDPO and 4,4’-hexafluoroisopropylidenediphthalicanhydride (6FDA) exhibited the highest optical transparency with the transmittance of90% at 400 nm and the refractive index as low as 1.5511 at 1310 nm.

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1. Introduction family of polymers, including epoxy, [7] poly(arylene

During the last decades, research on the conventionalthermally stable polymers has centered on endowing themwith special functionality while maintaining their intrinsiccharacteristics so as to expand their applications tomicroelectronics, optoelectronics, and other high-techfields [1–3]. Recent research of phosphorus chemistry hasbeen focused on developing high-temperature-resistantphosphorous-containing polymers for advanced applica-tions [4]. Phosphorous-containing polymers have beenwell known for their good flame retardancy, atomic oxygenresistance in low earth orbit (LEO), and in some cases, hightransparency and low dielectric constants [5,6]. Thus, this

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shiyang@iccas.ac.cn

ether) [8], and perfluorocyclobutyl polymers [9] have beenwidely investigated as functional coatings or structuralmaterials for industrial applications.

Recently, phosphorous substituents have been incorpo-rating into the molecular structures of polyimides (PIs).The pioneering and systematic work on phosphorous-con-taining PIs includes the polymers with phenylphosphineoxide (PPO) moiety either in the main chains or in the sidechains developed by National Aeronautics and SpaceAdministration (NASA) for space applications [10–12],and the PIs reported by Yoon and co-workers for develop-ing PIs with improved adhesion in order to enhance thereliability of electronic devices [13–16]. In our continuouseffort to develop PI optical coatings, a series of fluoro-con-taining PIs have been designed and synthesized [17–20].Based on the well-established knowledge on PPO-contain-ing PIs, we can conclude that it should be an effective wayto develop novel PI optical coatings by introducing both of

1140 Z. Li et al. / European Polymer Journal 45 (2009) 1139–1148

the PPO moiety and fluoro substituents into the molecularskeletons of PIs. However, to our knowledge, few effortshave been reported up to now on fluoro- and PPO-contain-ing PIs for optical applications.

Thus, in the present work, a series of novel PIs containingboth –CF3 and PPO moiety were synthesized, aiming atincreasing the solubility and optical transparency of the PIswithout scarifying their thermal stability. For this purpose,an aromatic diamine 2,5-bis[(4-amino-2-trifluoromethyl-phenoxy)phenyl]diphenylphosphine oxide (BATFDPO) wassynthesized first. The PIs based on BATFDPO are expectedto possess high solubility, high transparency, low refractiveindices, and low dielectric constants due to the bulky andelectron-withdrawing nature of the –CF3 and PPO moiety.For comparison, the present study was performed in parallelwith another system of PIs without –CF3 in their structures,that is, the PIs derived from 2,5-bis[(4-aminophen-oxy)phenyl]diphenylphosphine oxide (BADPO) and the samedianhydrides. The synergic effects of –CF3 and PPO moiety onthe properties of the PIs were investigated in detail.

2. Experimental

2.1. Materials

2,5-Dihydroxydiphenylphosphine oxide (DHDPO) [21] and2-bromo-5-nitrobenzo-trifluoride [22] were synthesizedin our laboratory according to the reported procedure. 4-Fluoronitrobenzene, cesium fluoride (CsF), palladium onactive carbon (Pd/C, 5%), and hydrazine hydrate (80%) werepurchased from Aldrich and used as received. 3,3’,4,4’-Biphenyltetracarboxylic dianhydride (BPDA, TCI, Japan),3,3’,4,4’-benzophenonetetracarboxylic dianhydride (BTDA,Aldrich, USA), 4,4’-oxydiphthalic anhydride (ODPA, TCI, Ja-pan), and 4,4’-hexafluoroisopropylidenediphthalic anhy-dride (6FDA, Hoechst Celanese Corp., USA) wererecrystallized from acetic anhydride and dried in vacuumat 160 �C overnight prior to use. Commercially availableN-methyl-2-pyrrolidinone (NMP), m-cresol, N,N-dimethyl-formamide (DMF), N,N-dimethylacetamide (DMAc), cyclo-pentanone (CPA), and other reagents were purified bydistillation prior to use.

2.2. Synthesis of 2,5-bis[(4-nitro-2-trifluoromethylphenoxy)-phenyl]diphenylphosphine oxide (BNTFDPO)

To a three-necked 500-mL flask equipped with amechanical stirrer, a nitrogen inlet, and a condenser wasadded a mixture of 2,5-dihydroxydiphenylphosphine oxide(24.82 g, 0.08 mol), 2-bromo-5-nitrobenzotrifluoride (64.80 g,0.24 mol), anhydrous CsF (30.38 g, 0.20 mol), and freshly-distilled N,N-dimethylacetamide (240 mL). After 30min ofstirring at room temperature, the mixture was heated toreflux in nitrogen for 24 h. The obtained mixture was fil-tered hot. The filtrate was poured into an excess amountof water. The precipitate was collected by filtration,washed with water, and dried in vacuo at 60 �C for 24 h.The obtained brown powder was recrystallized from etha-nol to afford pale yellow crystals (BNTFDPO). Yield: 24.0 g(43.6%); mp: 191.6 �C (DSC peak).

FT-IR (KBr, cm�1): 1597.2, 1531.7, 1465.8, 1353.0,1048.4, 908.9, 843.5, and 732.0. 1H NMR (400MHz,DMSO-d6/TMS, ppm): 6.75–6.77 (d, 1H), 7.34–7.67 (m,14H), 8.18–8.21 (t, 1H), 8.33–8.34 (d, 1H), 8.49–8.52 (t,1H), 8.53–8.54 (d,1H). C32H19F6N2O7P: Calcd. C,55.83%, H, 2.78%, N, 4.07%; Found: C, 55.70%, H, 2.87%, N,4.06%.

2.3. Synthesis of 2,5-bis[(4-amino-2-trifluoromethylphenoxy)phenyl]diphenylphosphine oxide (BATFDPO)

A 500-mL three-necked flask fitted with a mechanicalstirrer, a thermometer and a dropping funnel was chargedwith a mixture of BNTFDPO (20.65 g, 0.03 mol), absoluteethanol (200 mL), and a catalytic amount (0.5 g) of 5% pal-ladium on activated carbon. The reaction mixture washeated to reflux and then hydrazine hydrate (22 mL)was added dropwise over a period of 1.5 h. After the addi-tion, the reaction system was refluxed for 24 h. Then, themixture was filtered hot to remove the catalyst. Distilledwater was added to the filter until white precipitate ap-peared. The white granular crystals were filtered, washedwith cold ethanol and dried in vacuo at 80 �C overnight toafford BATFDPO. Yield: 15.40 g (81.9%); m.p. 199.6 �C (DSCpeak).

FT-IR (KBr, cm�1): 3325.9, 3217.6, 1467.0, 1341.0,1261.3, 1221.4, 1121.7, 1046.4, and 853.1. 1H NMR(400MHz, DMSO-d6, ppm): 5.38 (s, 1H), 5.48 (s, 1H),6.06–6.08 (d, 1H), 6.55–6.62 (h, 2H), 6.76–6.82 (t, 2H),6.90–6.95 (t, 2H), 7.10–7.13 (t, 1H), 7.29–7.33 (t, 1H), and7.46–7.62 (t, 10H). 13C NMR (400MHz, DMSO-d6, ppm):111.5, 112.0, 119.8, 120.0, 122.3, 122.5, 122.6, 122.8,122.9, 123.1, 123.3, 123.5, 124.1, 124.4, 125.1, 125.8,126.2, 129.6, 129.7, 132.6, 132.7, 133.1, 143.1, 143.5,147.2, 147.3, 154.8, 154.9, 155.5. MS (EI, m/e, percentageof relative intensity): 628 (M+, 7), 452 (M+-176, 100).C32H23F6N2O3P: Calcd. C, 61.15%, H, 3.69%, N, 4.46%; Found:C, 60.69%, H, 3.76%, N, 4.47%.

2.4. Synthesis of 2,5-bis[(4-aminophenoxy)phenyl]-diphenylphosphine oxide (BADPO)

2,5-Bis[(4-aminophenoxy)phenyl]diphenylphosphine oxide(BADPO) was synthesized by a similar procedurementioned above. First, 2,5-bis[(4-nitrophenoxy)-phenyl]diphenylphosphine oxide (BNDPO) was synthe-sized from DHDPO and 4-fluoronitrobenzene, followed bycatalytic reduction to afford BADPO with a total yield of63%. Melting point: 208.7 �C (DSC peak). FT-IR (KBr,cm�1): 3345.5, 3224.3, 1463.4, 1399.5, 1252.7, 1204.7,1171.0, 1119.4, and 841.8. 1H NMR (400MHz, DMSO-d6,ppm): 4.96–4.98 (d, 4H), 6.32–6.45 (t, 4H), 6.56–6.63 (h,3H), 6.75–6.77 (d, 2H), 7.04–7.07 (t, 1H), 7.24–7.28 (t,1H), 7.48–7.69 (m, 10H). 13C NMR (400 MHz, DMSO-d6,ppm): 116.0, 116.3, 117.8, 117.9, 121.9, 122.0, 122.6,123.9, 129.7, 129.8, 132.6, 132.7 133.0, 133.1, 133.6,134.7, 145.6, 146.8, 147.2, 147.3, 154.5, 154.6, 155.9,156.0. MS (EI, m/e, percentage of relative intensity):492(M+, 27), 384(M+-108, 100). C30H25N2O3P: Calcd. C,73.16%, H, 5.12%, N, 5.69%; Found: C, 73.10%, H, 5.18%, N,5.73%.

Z. Li et al. / European Polymer Journal 45 (2009) 1139–1148 1141

2.5. Polyimide synthesis and film preparation

The two series of polyimides PI-Ia–d and PI-IIa–d weresynthesized via a two-step chemical imidization procedurewith NMP as the solvent, acetic anhydride as the dehydrat-ing agent, and pyridine as the catalyst. For the synthesis ofPI-IId, BATFDPO (31.4255 g, 0.05 mol) was added to a 500-mL three-necked flask equipped with a mechanical stirrer,a nitrogen inlet and a cold water bath. NMP (200 mL) wasadded and a gentle stream of nitrogen was passed throughthe solution. After stirring for 10 min, a clear diamine solu-tion was obtained. 6FDA (22.2120 g, 0.05 mol) was thenadded in one batch and an additional volume of NMP(15 mL) was added to wash the residual dianhydride, andat the same time to adjust the solid content of the reactionsystem to be 20 wt%. The cold water bath was removedafter 4 h. The mixture was stirred at room temperaturefor 20 h to yield a viscous pale-yellow solution. Aceticanhydride (47 mL) and pyridine (32 mL) were added tothe solution, and the reaction mixture was stirred at roomtemperature for another 24 h to afford a viscous pale-yel-low solution. The PI solution was carefully poured into eth-anol (1000 mL) to yield silky resin. The precipitate wascollected and dried at 80 �C in vacuum overnight to affordPI-IId resin. Yield: 50.80 g (98%).

The well-dried silky PI-IId resin was first dissolved inNMP at room temperature with a solid content of 15wt%. The obtained PI solution was filtered through a0.45-lm Teflon syringe filter to remove any contaminatesthat might affect the quality of the film. Then, the solutionwas cast onto a clean glass and PI-IId film was obtained bythermally baking the solution with the following heatingprocedure: 80 �C/2 h, 150 �C/1 h, 200 �C/1 h, 250 �C/1 hand 280 �C/1 h.

PI-IIa, PI-IIb and PI-IIc were prepared according to thesimilar procedure as PI-IId, except that 6FDA was replacedby BPDA for PI-IIa, BTDA for PI-IIb and ODPA for PI-IIc. PI-Ia–d

resin and films were also prepared according to the proce-dure mentioned above, except that BATFDPO was replacedby BADPO.

2.6. Measurements

Inherent viscosity was measured using an Ubbelohdeviscometer with a 0.5 g dL�1 NMP solution at 30 �C. Abso-lute viscosity was measured using a Brookfield DV-II+ Proviscometer at 25 �C. FT-IR spectra were obtained with aTensor 27 Fourier transform spectrometer. UV–vis spectrawere recorded on a Hitachi U-3210 spectrophotometer atroom temperature. Prior to test, PI samples were dried at100 �C for 1 h to remove the absorbed moisture. 1H NMRand 13C NMR were performed on a AV 400 spectrometeroperating at 400 MHz in DMSO-d6. 1H–13C HeteronuclearSingle Quantum Coherence (HSQC) spectra were per-formed on a AV600 spectrometer operating at 600 MHzin DMSO-d6. DSC and TGA were recorded on a TA-Q seriesthermal analysis system at a heating rate of 10 �C min�1

and 20 �C min�1 in nitrogen, respectively. The tensile prop-erties were performed on an Instron 3365 Tensile Appara-tus with 80 � 10 � 0.05 mm3 specimens in accordancewith GB1447-83 at a drawing rate of 2.0 mm min�1.

Solubility was determined as follows: 1.0 g of the testedPI resin was mixed with 9.0 g of the solvent at room tem-perature (10 wt% solid content), which was then mechani-cally stirred in nitrogen for 24 h. The solubility wasdetermined visually as three grades: completely soluble( ), partially soluble (s), and insoluble (d). The completesolubility is defined as a homogenous and clean solution isobtained, in which no phase separation, precipitation or gelformation is detected.

Refractive index of the PI film formed on a 3-in siliconwafer was measured at the wavelength of 1310 nm atroom temperature with a SaironTech Model SPA-4000prism coupler. The in-plane (nTE) and out-of-plane (nTM)refractive index were determined using linearly polarizedlaser light parallel (transverse electric, TE) and perpendic-ular (transverse magnetic, TM) polarizations to the filmplane, respectively. The average refractive index (nav) wascalculated according to Eq. (1):

nav ¼ ð2nTE þ nTMÞ=3 ð1Þ

3. Results and discussion

3.1. Monomer synthesis

As shown in Scheme 1, two PPO-containing diaminesBADPO and BATFDPO were synthesized via a two-step pro-cess. First, the dinitro intermediates were prepared fromDHDPO and 4-fluoronitrobenznene for BNDPO and 2-bro-mo-5-nitrobenzene-trifluoride for BNTFDPO, respectively,under the catalysis of CsF, followed by catalytic reductions.BADPO has been previously reported by Hergenrother andcoworkers by the reaction of 4-chloronitrobenzene andDHDPO using potassium carbonate (K2CO3) as the base[23]. However, it was found that in the present work, usingK2CO3 as the catalyst was not successful for preparingBATFDPO. In the literature, it has been reported that thenucleophilic aromatic substitution reaction of phospho-rous-containing dihydroxy compound, 2-(6-oxido-6H-dibenz<c,e><1,2>-oxaphosphorin-6-yl)-1,4-benzenediol(DHDOPO) and 4-fluoronitrobenznene under the catalysisof K2CO3 would cause the alkaline hydrolysis reaction ofP-Ar linkage, resulting in the finial product without phos-phorous side chain [24]. Very recently, the reaction condi-tion of DHDOPO and 4-fluoronitrobenznene was modifiedby Lin and coworkers by using CsF as the catalyst insteadof K2CO3 and the target dinitro compound was obtainedwith good yield and identified structure [25]. In the pres-ent work, low reaction yields and complex side reactionswere also observed for the preparation of BNTFDPO whenusing K2CO3. Thus, CsF was utilized for the synthesis ofthe diamine and satisfying results were obtained.

Fig. 1 compares the FT-IR spectra of BATFDPO and its di-nitro intermediate BNTFDPO. It can be seen that theabsorptions at 1597 and 1353 cm�1 assigned to the asym-metric and symmetric stretching of the –NO2 group inBNTFDPO disappeared in BATFDPO. Instead, the character-istic absorptions of amino groups at 3325 and 3217 cm�1

appeared, indicating the successful transition from –NO2

to –NH2. As for BATFDPO, the characteristic absorptions

Scheme 1. Synthesis of BADPO and BATFDPO.

Fig. 1. FT-IR spectra of BATFDPO and BNTFDPO.

1142 Z. Li et al. / European Polymer Journal 45 (2009) 1139–1148

of C-F stretching vibration at �1121 cm�1 and P@O at�1221 cm�1 were also observed.

The 1H NMR spectrum of BATFDPO, shown in Fig. 2a,clearly indicates that the two amino protons show differ-ent absorptions due to the asymmetric substitution ofdiphenylphosphine oxide group. Similarly, the protonpairs, including H2 and H02, H5 and H05 and H6 and H06 alsoexhibit separate signals. The asymmetric chemical struc-ture of BATFDPO is also reflected in the 13C NMR spectrumin Fig. 2b. The absorptions of the C atoms located in theamino-substituted benzenes all exhibit different, but veryclose signals. The carbons in –CF3 (C17) or ortho-to –CF3

(C3) show the typical quartet absorptions at 120–130ppm because of the 2JC–F and 3JC–F coupling of the C with

F atoms in the diamine. Considering the complex molecu-lar structure of BATFDPO, the 2D-NMR technique was uti-lized to confirm the C-H correlations. As can be seen inFig. 2c, all the H and C signals correlate well, which is veryhelpful to confirm the individual H and C absorption.

The elemental analysis data are in good agreement withthe expected values. Thus, all of the characterizations con-firm the expected chemical structure of BATFDPO.

3.2. Polyimide synthesis and film properties

Two series of PIs were synthesized according to thetwo-step procedure outlined in Scheme 2 involving theformation of poly(amic acid)s and the subsequent chemicalimidization. Chemical imidization is an effective procedureto synthesize organo-soluble PIs, by which the PI resinswith high imidization degree could be obtained [26]. How-ever, the chemical imidization procedure is dependent onthe molecular structures of the PIs. In the present work,the lateral bulky PPO substituents endow the PIs with goodsolubility in the reaction medium and the reaction pro-ceeded smoothly and homogeneously during the wholechemical imidization process. In all cases, the polymeryields were essentially quantitative. The inherent viscosi-ties (Table 1) of the PIs are in the range of 0.66–1.22dL.g-1, indicating moderate to high molecular weights ofthe polymers. PI-IIa–d have lower inherent viscosity valuesthan those of their PI-Ia–d analogues probably due to theslight decrease of the reactivity of amino in BATFDPO bythe electron-withdrawing –CF3 group. Nevertheless, allthe PI/NMP solution with a solid content of 15 wt% affor-ded flexible and tough films. The tensile properties of thePI films are presented in Table 1. The creasable films have

Fig. 2. (a) 1H NMR, (b) 13C NMR, and (c) 1H–13C HSQC spectra of BATFDPO (a and b: 400 MHz, DMSO-d6; c: 600 MHz, DMSO-d6).

Z. Li et al. / European Polymer Journal 45 (2009) 1139–1148 1143

tensile strength higher than 70 MPa, elongation at breakshigher than 3.7%, and tensile modulus higher than 2.7GPa. The introduction of –CF3 groups slightly decreasedthe mechanical properties of the films.

The structure of the PIs was confirmed by FT-IR and 1HNMR measurements. The FT-IR spectra of PI-IIa–d areshown in Fig. 3. The characteristic absorption bands dueto the vibration of the carbonyl groups in the imide linkageare clearly observed in the region around 1780 cm�1 (tas)and 1720 cm�1 (ts). In addition, the stretching vibrationof C-N bond located at 1375 cm�1 and the imide ring defor-mation at 721 cm�1 further confirm the formation of polyi-mides. The strong C-F absorptions between 1000 and1200 cm�1 and the characteristic absorption of P=O at1130–1180 cm�1 are also found in all of the polymers.

Fig. 4 presents the representative 1H NMR spectra of PI-IId and PI-Id. In order to investigate the effects of –CF3 onthe structure of the polymers, the serial numbers of theprotons in both of the spectra were uniformed. Thus, inFig. 4b, the labels lack the number ‘‘3” and ‘‘30” due tothe symmetric structure. In Fig. 4a, total 25 protons are

identified for PI-IId based on the integral values of theabsorption peaks, whereas 2 more protons are found forPI-Id in Fig. 4b. This is in good agreement with the antici-pated structure of the PIs. Generally, the protons in thedianhydride moiety (H10, H11, H12) appear at the farthestdownfield in the spectra due to the synergic effects of theelectron-withdrawing imide ring and –CF3 groups. How-ever, in the case of PI-IId containing –CF3 in both of the dia-mine and dianhydride unit, the protons ortho-to the –CF3

group (H01and H1) show absorptions mixed with H10 andH12. In both of the spectra, the protons in the ether-linkedbenzene (H4, H5, H6) all show absorptions at the farthestupfield area.

3.3. Solubility

Good solubility in common organic solvents is oftenimportant for polymer optical coatings. As we know, thecommon PIs usually have poor solubility, thus can onlybe utilized with their soluble poly(amic acid) or poly(amicester) precursors, which are then chemically converted

Fig. 2. (continued)

Table 1Polymerization and thermal and mechanical properties of PI films.

PI [g]inhadL g�1 Tg

b(�C) T5%b(�C) T10%

b(�C) Rw700b(%) Ts

c(MPa) Ebc(%) TM

c(GPa)

Ia 1.22 265 534 557 63 100.3 5.9 3.6Ib 1.08 253 525 540 59 92.8 5.8 3.1Ic 1.13 242 531 549 60 86.3 5.6 3.0Id 0.86 265 522 539 62 90.0 6.3 2.9IIa 0.74 266 476 506 59 87.0 4.7 3.0IIb 0.68 257 473 495 59 70.7 4.1 2.7IIc 0.69 244 477 495 58 99.0 6.5 2.9IId 0.66 265 472 493 53 79.0 3.7 3.1

a Measured with PI resin at a concentration of 0.5 g dL�1 in NMP at 30 �C.b Tg, glass transition temperature; T5%, T10%, temperatures at 5% and 10% weight loss, respectively; Rw700, residual weight ratio at 700 �C in nitrogen.c Ts, tensile strength; Eb, elongation at break; TM, tensile modulus.

1144 Z. Li et al. / European Polymer Journal 45 (2009) 1139–1148

into PIs at temperatures as high as 300–350 �C. The highcuring temperatures often cause damage to the optical de-vices and at the same time, drawbacks (such as pinhole orthickness ununiformity) often occur in the PI films duringthe thermally dehydration process. A highly disordered PIresin is expected to possess better solubility in commonorganic solvents, thus better processability, than its or-dered analogues. This is confirmed in the present workby evaluating and comparing the solubility of PI-Ia–d andPI-IIa–d in 8 common solvents. As shown in Table 2, allPIs exhibit good solubility in NMP due to the existence of

the bulky diphenylphosphine oxide groups. The solubil-ity of PI-IIa–d is further enhanced by the introductionof –CF3 group. For example, the BATFDPO-PIs exceptPI-IIa show good solubility not only in polar aprotic sol-vents, but in many common solvents, such as CPA, THF,and CHCl3. Surprisingly, PI-IId (6FDA/BATFDPO) is evensoluble in acetone. The good solubility of the presentPIs in solvents with low boiling points makes it possibleto fabricate the PI coatings at relatively low tempera-tures, which is critical for temperature-sensitive opticalapplications [27].

Fig. 3. FT-IR spectra of the PI-IIa–d films.

Z. Li et al. / European Polymer Journal 45 (2009) 1139–1148 1145

Although the PIs are all soluble in NMP, the solubility ofPI-Ia–d and PI-IIa–d are different. For example, plots of theabsolute viscosity vs. solid content of PI-Ia and PI-IIa areshown in Fig. 5. As expected, the viscosity of both PI-Ia

and PI-IIa increases with the increasing of the solid con-tents. However, at the same solid content, such as 15wt%, the viscosity of PI-Ia (24440 mPa s) is much higherthan that of PI-IIa (125 mPa s), indicating the relativelylower solubility of the former. We can mainly attributethe superior solubility of PI-IIa–d to their more loose molec-ular packing caused by the bulky –CF3 and PPOsubstituents.

Scheme 2. Synth

3.4. Thermal properties

The thermal properties of the PIs were evaluated usingTGA and DSC measurements. Fig. 6 illustrates the TGAtraces of the PI films in nitrogen. All the PIs exhibit goodthermal stability without apparent weight loss up to400 �C. We can clearly observe that the incorporation of –CF3 slightly decrease the initial thermal decompositiontemperatures of the PIs. For instance, PI-Ia–d show 5%weight loss temperatures at 522–534 �C, which are about50 �C higher than those of PI-IIa–d. When the temperatureis higher than 500 �C, the PIs decompose rapidly and thethermal decomposition behavior of PIs is more compli-cated. PI-IIa–d exhibits a second decomposition in the rangeof 550–600 �C, after which the decomposing rate tends tobe smooth. Residual weights higher than 53% are observedat 700 �C. This phenomenon indicates that the diphenyl-phosphine oxide side chains might decompose first at ele-vated temperatures, leaving the more thermal stable mainchain structures. Then, the PI skeleton would decomposesecondly at much higher temperature. At the same time,it should be noticed that the phosphorous-rich residuemight prevent further decomposition of the PIs, resultingin high char yield at 700 �C. DSC scans of the PIs were re-corded up to 400 �C as shown in Fig. 7. Clear endothermictransition are observed around 240–265 �C. Within a givendiamine family, the Tg values of the PIs decrease withdecreasing rigidity of the dianhydrides in the order ofBPDA > BTDA > ODPA (Table 1). Interestingly, the PIs de-rived from 6FDA (PI-Id and PI-IId) show very close Tg values

esis of PIs.

Fig. 4. 1H NMR spectra of PIs, (a) PI-IId and (b) PI-Id (400MHz, DMSO-d6).

1146 Z. Li et al. / European Polymer Journal 45 (2009) 1139–1148

to those derived from BPDA, although the formers havemore flexible structures. This might be contributed tothe high molecular weight and of the repeating unit ofPI-IId (1036.72 g/mol), thus resulting in the hindrance ofmotion of the molecular chain segments at the testedtemperatures.

3.5. Optical properties

Fig. 8 shows the representative UV–vis spectra of the PIfilms and the optical values are listed in Table 3. PI-IIa–d

films show higher optical transmittance in the visible lightregion than their unfluorinated analogues. For example, PI-IId showed a cutoff wavelength at 312 nm, which was14 nm lower than that of PI-Id. The transmittances of thePI-IIa–d films at 400 nm increased in the order of PI-IIa <PI-IIb � PI-IIc < PI-IId depending on the dianhydride used.It has been well-established that the main source of opticalloss in PI film is due to absorption of visible light by thecharge-transfer-complex (CTC) formed by the overlap ofthe highest occupied molecular orbital of the diamine withthe lowest unoccupied molecular orbital of the five-mem-

Fig. 5. Correlations of the viscosities and solid contents of PI solution.

Fig. 6. TGA curves of the PI films (in nitrogen, 20 �C min�1).

Fig. 7. DSC curves of the PI films (in nitrogen, 10 �C min�1).

Fig. 8. UV–vis spectra of the typical PI films.

Table 3Optical properties of the PI films.

PI Fca(%) kcutoff

b(nm) T400c(%) dd(lm) nav

e ef

Ia 0 369 37 3.08 1.6461 2.98Ib 0 327 59 2.02 1.6362 2.94Ic 0 346 66 3.64 1.6355 2.94Id 12.66 326 82 2.41 1.5894 2.78IIa 12.86 368 49 5.18 1.5994 2.81IIb 12.46 325 70 2.48 1.5908 2.78IIc 12.63 343 72 2.69 1.5827 2.79IId 21.99 312 90 3.56 1.5511 2.65

a Fluoro content.b Cutoff wavelength.c Transmittance at 400 nm.d Film thickness for refractive index measurements.e Average refractive index.f Dielectric constant estimated from Maxwell’s equation as e ¼ 1:10n2

av.

Table 2Solubility of the PIs.

PI Solventsa

NMP DMAc DMF DMSO CPA THF CHCl3 acetone

Ia s s d d d s d

Ib s d d

Ic d d

Id d

IIa s s s d s d

IIb s s

IIc s

IId

a : wholly soluble; s: partially soluble; d: not soluble; NMP: N-methyl-2-pyrrolidinone; DMAc: N,N-dimethylacetamide; DMF: N,N-dimethylformamide; DMSO: dimethylsulfoxide; CPA: cyclopentanone.

Z. Li et al. / European Polymer Journal 45 (2009) 1139–1148 1147

bered imide rings [28]. In addition, the steric hindrance ofthe molecular chains often lowers the formation of CTC. Inthe present study, the bulky lateral PPO substituents andthe electron-withdrawing –CF3 group significantly de-crease the CTC formation through the steric hindranceand inductive effect, respectively. The effect of fluorinecontents on the optical properties of PI films could be obvi-ously observed from the data listed in Table 3. PI-IId with –

CF3 both in the diamine and dianhydride moiety, thus hav-ing the highest fluoro content, shows the highest transmit-tance of 90% at 400 nm. Thus, it proves to be effective toincrease the optical transmittance of the PI films by intro-ducing both PPO and –CF3 group.

The refractive indices of the PI films were investigatedand the results are tabulated in Table 3. It has been well

1148 Z. Li et al. / European Polymer Journal 45 (2009) 1139–1148

known that phosphorous-containing groups, such as phos-phonates [29], phosphazenes [30], and phenylphosphineoxide can often increase the refractive indices of the poly-mers. However, in the present study, the PPO group wasintroduced as side chains, whose bulky molecular volumewould decrease the refractive index to some extent accord-ing to the Lorentz–Lorenz equation [31]. PI-IIa–d containingboth bulky PPO and –CF3 group exhibit average refractiveindices (nav) of 1.5511–1.5994, which is about 0.03–0.05lower than those of their PI-Ia–d analogues. The decreaseof the nav values, on one hand is attributed to the decreaseof molar refraction of the polymers caused by the introduc-tion of low-molar-refraction –CF3 group; on the otherhand, to the increase of molecular volumes resulted fromthe synergic effects of bulky –CF3 and PPO groups. PI-IId hasa nav value of 1.5511, which is quite lower than those of com-mon aromatic PIs [32]. The dielectric constants of the PI filmscan be estimated from the refractive indices using the modi-fied Maxwell equation: e ¼ 1:10n2

av [33]. Thus, the estimatede values of the PIs are in the range of 2.65–2.98.

4. Conclusion

The major purpose of the present work is the improvingof the optical transparency and solubility of the aromaticPIs so as to meet the requirements of advanced opticalapplications. This was successfully achieved by introduc-tion of –CF3 and PPO group into the molecular structuresof the PIs. PIs derived from BATFDPO and aromatic dianhy-drides possessed many desired properties as optical coat-ings, including good thermal stability up to 470 �C, glasstransition temperatures as high as 265 �C, tensile strengthshigher than 70 MPa, refractive index as low as 1.5511, andgood solubility in common organic solvents. The good com-bined properties make them good candidates as compo-nents for advanced optical fabrications. The application ofthe present PIs in the fabrication of optical waveguides isnow under investigation and will be reported in the future.

Acknowledgements

Fundings from the National Natural Science Foundationof China (NSFC) and from Beijing Municipal Science &

Technology Commission (No. Z08080302110801) aregratefully acknowledged.

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