synthesis and properties of fluorinated polyimides from a new unsymmetrical diamine:...

Synthesis and Properties of Fluorinated Polyimidesfrom a New Unsymmetrical Diamine:1,4-(20-Trifluoromethyl-40,400-diaminodiphenoxy)benzene

YU SHAO, YAN-FENG LI, XIN ZHAO, XIAO-LONG WANG, TAO MA, FENG-CHUN YANG

State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering,Institute of Biochemical Engineering and Environmental Technology, Lanzhou University, Lanzhou 730000, China

Received 15 May 2006; accepted 31 August 2006DOI: 10.1002/pola.21777Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: A new aromatic, unsymmetrical ether diamine with a trifluoromethylpendent group, 1,4-(20-trifluoromethyl-40,40 0-diaminodiphenoxy)benzene, was success-fully synthesized in three steps with hydroquinone as a starting material and polymer-ized with various aromatic tetracarboxylic acid dianhydrides, including 4,40-oxydiph-thalic anhydride, 3,30,4,40-benzophenone tetracarboxylic dianhydride, 2,20-bis(3,4-dicar-boxyphenyl)-hexafluoropropane dianhydride, and pyromellitic dianhydride, via aconventional two-step thermal or chemical imidization method to produce a series offluorinated polyimides. The polyimides were characterized with solubility tests, viscos-ity measurements, IR, 1H NMR, and 13C NMR spectroscopy, X-ray diffraction studies,and thermogravimetric analysis. The polyimides had inherent viscosities of 0.56–0.77dL/g and were easily dissolved in both polar, aprotic solvents and common, low-boiling-point solvents. The resulting strong and flexible polyimide films exhibited excellentthermal stability, with decomposition temperatures (at 5% weight loss) above 522 8Cand glass-transition temperatures in the range of 232–272 8C. Moreover, the polymerfilms showed outstanding mechanical properties, with tensile strengths of 74.5–121.7 MPa,elongations at break of 6–13%, and initial moduli of 1.46–1.95 GPa, and good dielectricproperties, with low dielectric constants of 1.82–2.53 at 10 MHz. Wide-angle X-raydiffraction measurements revealed that these polyimides were predominantly amor-phous. These outstanding combined features ensure that the polymers are desirablecandidate materials for advanced microelectronic applications. VVC 2006 Wiley Periodicals,

Inc. J Polym Sci Part A: Polym Chem 44: 6836–6846, 2006

Keywords: films; fluorinated polyimides; high performance polymers; low dielectricconstant; polyimides; solubility; unsymmetrical ether diamine

INTRODUCTION

Aromatic polyimides are some of the most usefuland important polymers because of their excel-lent thermal stability, good chemical resistanceand mechanical strength, and low dielectric con-stants. Consequently, they have been widely used

in many applications such as electronics, coat-ings, composite materials, and membranes.1–5

Despite their widespread use, most of them havehigh melting temperatures or softening tempera-tures and limited solubility in most solventsbecause of their rigid backbones and strong inter-actions between chains, which may restrict theirapplications in some fields. To overcome theselimitations, structural modifications of the poly-mer backbone, such as the introduction of bulkylateral substituents, flexible aryl–ether linkages,

Correspondence to: Y.-F. Li (E-mail: [email protected])

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 44, 6836–6846 (2006)VVC 2006 Wiley Periodicals, Inc.

6836

perfluoroalkoxy groups, and unsymmetrical struc-tures, have been used to modify the polymer prop-erties by the design and preparation of new mono-mers.6–15

Aromatic ether linkages inserted in polymermain chains provide them with significantlylower energy of internal rotation.16–20 In general,such a structural modification leads to a lowerglass-transition temperature (Tg) and crystallinemelting temperature as well as a significantimprovement in the solubility and process char-acteristics without greatly sacrificing thermalstability. The introduction of bulky substituentsinto the polymer backbones is expected todecrease in order along the chain, enhance solu-bility, and reduce crystallinity because of the ran-dom arrangement of substituents. In addition,the introduction of geometrically or molecularlyunsymmetrical diamine components into thepolyimide main chain has led to new polyimideswith improved solubility and melt processabilityand other desirable properties.10–15,21 Polyimidescontaining hexafluoroisopropylidene or pendenttrifluoromethyl groups are of special interest.22–24

The incorporation of these groups serves to in-crease the free volume of the polyimides, therebyimproving various properties, including the solu-bility and electrical insulating properties, withoutforfeiture of thermal stability. These groups alsoreduce water absorption and color, whereas theyincrease flame resistance, gas permeability, andoptical transparency.

In this study, a new aromatic, unsymmetri-cal ether diamine with a trifluoromethyl pendentgroup, 1,4-(20-trifluoromethyl-40,400-diaminodiphe-noxy)benzene (PAPB), was successfully synthe-sized. It was designed as a potentially convenientcondensation monomer for polyimides, capableof imparting good solubility and outstanding elec-trical properties at the same time. Meanwhile,a series of all-aromatic, organosoluble polyimidesbearing a unsymmetrically structured, aromaticring with a trifluoromethyl pendent group weresynthesized from the diamine with four kindsof commercial dianhydrides via a conventionaltwo-stage process. The characterizations of thediamine and related intermediates, as well asthe resulting polyimides based on this diamine,were carried out by means of Fourier trans-form infrared (FTIR), 1H NMR, 13C NMR, dif-ferential scanning calorimetry (DSC), thermo-gravimetric analysis (TGA), wide-angle X-raydiffraction (WAXD), and elemental analysismethods.

EXPERIMENTAL

Materials

Commercially available hydroquinone (ChengdouChemical Reagents Corp., China), p-chloronitroben-zene (Shanghai Chemical Reagents Corp., China),2-chloro-5-nitrotrifluoromethylbenzene (Acros), hy-drazine monohydrate (Beijing Chemical ReagentsCorp., China), potassium carbonate (Fuchen Che-mical Reagents Corp., Tianjin, China), and 5% Pd/C(Acros) were used without further purification. 4,40-oxydianiline (ODA; Aldrich), 4,40-oxydiphthalic an-hydride (ODPA; Shanghai Nanxiang Chemical Co.,China), 3,30,4,40-benzophenone tetracarboxylic di-anhydride (BTDA; Beijing Chemical Reagents), and2,20-bis(3,4-dicarboxyphenyl)hexafluoropropane di-anhydride (6FDA; Aldrich) were recrystallized fromacetic anhydride before use. Pyromellitic dianhy-dride (PMDA; Beijing Chemical Reagents) was puri-fied by sublimation at 200–220 8C. N,N-Dimethyl-acetamide (DMAc) and N-methyl-2-pyrrolidinone(NMP) were purified by distillation under reducedpressure over calcium hydride and stored over 4-Amolecular sieves. All other solvents were obtainedfrom various commercial sources and used withoutfurther purification.

Synthesis of the Monomers

4-(4-Nitrophenoxy)phenol (NPP)

A mixture of 12.0 g (0.109 mol) of hydroquinone,12.045 g (0.0765 mol) of p-chloronitrobenzene,15 g (0.109 mol) of potassium carbonate, and120 mL of N,N-dimethylformamide (DMF) wasstirred at room temperature for 5 h. The mixturewas continuously reacted at 100 8C for 10 h.Then, it was poured into cold, dilute hydrochloricacid. The crude product was obtained by filtra-tion, washed with water, and dried in vacuo. Theobtained product was purified by column chroma-tography over silica gel (10:1 petroleum ether/ethyl acetate). After recrystallization from etha-nol, 9.37 g of NPP was obtained in a yield of 53%.

The melting point was 170–173 8C accordingto DSC at a scanning rate of 20 8C/min. Fastatom bombardment mass spectrometry (m/z) pro-vided a value of 231 (Mþ; calcd. for C12H9NO4:231.2). The IR (KBr) spectrum indicated absorp-tion peaks at 3436 (O��H stretching), 1241(��O�� stretching), and 1343 cm�1 (NO2).

1HNMR (300 MHz, acetone-d, ppm) showed signalsof different protons at d values of 8.22 (d, J ¼ 9.6Hz, 2H), 7.05 (d, J ¼ 9.3 Hz, 2H), 7.02 (d, J ¼ 9.0

NEW UNSYMMETRICAL DIAMINE 6837

Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola

Hz, 2H), and 6.95 (d, J ¼ 9.0 Hz, 2H). 13C NMR(300 MHz, acetone-d, d, ppm) showed values of164.24, 155.13, 146.28, 141.91, 126.25, 122.07,116.84, 116.51.

ELEM. ANAL. Calcd. for C12H9NO4: C, 62.34%;H, 3.92%; N, 6.06%. Found: C, 62.30%; H, 3.71%;N, 6.32%.

1,4-(20-Trifluoromethyl-40,400-dinitrodiphenoxy)benzene (PNPB)

A 250-mL, three-necked, round-bottom flaskcontaining 13.87 g (0.06 mol) of NPP, 13.51 g(0.06 mol) of 2-chloro-5-nitrotrifluoromethylben-zene, 8.30 g of potassium carbonate, and 150 mLof DMF was fitted with a magnetic stirring bar,condenser, nitrogen pad, and thermometer. Thismixture was heated to 100–110 8C with stirring.After this mixture was heated for 6 h, it wascooled to the ambient temperature. The mixturewas poured into an excess amount of ice water.The precipitate was collected by filtration, washedwith water, and air-dried. The crude product wasrecrystallized from ethanol to give a yellow pro-duct; 23.5 g was obtained (93.2%).

The melting point was 188–189 8C accordingto DSC (5 8C/min). Electron-impact mass spec-trometry provided a value of 420 (Mþ; calcd. forC19H11O6N2F3: 420.3). The IR (KBr) spectrumindicated absorption peaks at 1523 and 1340 cm�1

(��NO2 stretch) and at 1285, 1243, 1179, and1138 cm�1 (C��F and C��O stretching). 1H NMR[300 MHz, dimethyl sulfoxide (DMSO), ppm]showed signals of different protons at d values of8.50 (s, 1H), 8.47 (d, J ¼ 10.2 Hz, 1H), 8.25 (d, J¼ 9.3 Hz, 2H), 7.35 (t, 4H), 7.21 (d, J ¼ 9.8 Hz,1H), and 7.20 (d, J ¼ 9.1 Hz, 2H). 13C NMR(300 MHz, DMSO, d, ppm) showed values of163.46, 160.88, 152.61, 151.49, 143.07, 142.42,130.79, 129.47, 126.87, 126.88, 123.85, 123.44,123.31, 118.75, 118.13, and 93.79.

ELEM. ANAL. Calcd. for C19H11F3N2O6: C,54.30%; H, 2.64%; N, 6.67%. Found: C, 54.25%;H, 2.62%; N, 6.68%.

PAPB

The dinitro compound PNPB (4.75 g, 0.011 mol)and 5% Pd/C (0.2 g) were suspended in 100 mL ofethanol in a 250-mL flask. The suspension solu-tion was heated to refluxing, and 80% hydrazinemonohydrate (10 mL) was added dropwise to themixture over 0.5 h. After a further 4 h of reflux-ing, the resultant clear, darkened solution wasfiltered hot to remove Pd/C, and the filtrate wasdistilled to remove some solvent. The obtainedmixture was poured into 200 mL of stirring water,giving rise to a pale yellow product that was iso-lated by extraction (diethyl ether). The crude prod-uct was purified by column chromatography oversilica gel (5:1 petroleum ether/ethyl acetate). Theyield of colloid PAPB was 3.52 g (86.4%);

Electron-impact mass spectrometry provideda value of 360 (Mþ; calcd. for C19H15N2O2F3:360.3). The IR (KBr) spectrum indicated absorp-tion peaks at 3443 and 3369 cm�1 (N��H stretch)and at 1220, 1189, 1161, and 1125 cm�1 (C��Fand C��O stretching). 1H NMR (300 MHz,CDCl3, ppm) showed signals of different protonsat d values of 6.91 (d, J ¼ 2.4 Hz, 1H), 6.87 (s,4H), 6.83 (dd, J1 ¼ 2.1 Hz, J2 ¼ 6.9 Hz, 2H), 6.81(d, J ¼ 8.7 Hz, 1H), 6.71 (dd, J1 ¼ 2.4 Hz, J2

¼ 8.7 Hz, 1H), and 6.63 (dd, J1 ¼ 2.1 Hz, J2 ¼ 6.9Hz, 2H). 13C NMR (300 MHz, CDCl3, d, ppm)showed values of 154.11, 152.84, 149.10, 146.79,142.45, 142.13, 128.47, 125.05, 122.96, 122.72,122.48, 122.06, 121.63, 120.46, 119.27, 119.04,118.52, 118.21, 116.17, and 112.81.

ELEM. ANAL. Calcd. for C19H15F3N2O2: C,63.33%; H, 4.19%; N, 7.77%. Found: C, 63.25%;H, 4.23%; N, 7.69%.

Scheme 1. Synthesis of PAPB.

6838 SHAO ET AL.


Polymer Synthesis

The polyimides were synthesized from variousdianhydrides and diamine PAPB via a two-stepmethod.25,26 The synthesis of polyimide PI-1(ODPA–PAPB) is used as an example to illustratethe general synthetic route used to produce thepolyimides. To a solution of 0.3600 g (1.0 mmol)of diamine PAPB in 5.3 mL of CaH2-dried NMP,0.3099 g (1.0 mmol) of ODPA was added in oneportion. The solution was stirred at room temper-ature under N2 for 24 h to yield a viscous poly(amic acid) (PAA) solution. PAA was converted intoa polyimide with thermal or chemical imidizationmethods. For the thermal imidization method,the PAA solution was cast onto a clean glass plateand heated (80 8C/3 h, 120 8C/30 min, 150 8C/30min, 180 8C/30 min, 210 8C/30 min, 250 8C/30min, 300 8C/1 h) to produce a fully imidized polyi-mide film. Chemical imidization was carried outby the addition of an equimolar mixture of acetic

anhydride and pyridine to the aforementionedPAA solution (with mechanical stirring) at theambient temperature for 30 min and via heatingat 80 8C for 4 h. The polyimide solution waspoured into methanol. The precipitate was col-lected by filtration, washed thoroughly withmethanol, and dried at 80 8C in vacuo to give thefollowing.

PI-2 (BTDA–PAPB), PI-3 (6FDA–PAPB), and PI-4(PMDA–PAPB) were synthesized by a similarmethod.

Measurements

The inherent viscosities of the resulting polyimideswere measured with an Ubbelohde viscometer at30 8C. FTIR spectra (KBr) were recorded on aNicolet Nexus 670 FTIR spectrometer. 1H NMRand 13C NMR spectra were measured on a JEOLEX-300 spectrometer with tetramethylsilane asthe internal reference. Elemental analyses were

Figure 1. (a) 1H and (b) 13C NMR spectra of PAPB.



determined with a PerkinElmer model 2400 CHNanalyzer. The mechanical properties were mea-sured on an Instron 1122 tensile apparatus with100 � 5 mm2 specimens in accordance with GB1040-79 at a drawing rate of 100 mm/min. Thedielectric constant was determined on an Agilent4291B instrument with 25-lm-thick specimens ata frequency of 10 MHz at 25 8C. DSC testing wasperformed on a PerkinElmer DSC 7 or Pyris 1 dif-ferential scanning calorimeter at a scanning rateof 20 8C/min in flowing nitrogen (30 cm3/min), andthe Tg values were read at the DSC curves at thesame time. TGA was conducted with a TA Instru-ments TGA 2050, and the experiments were car-ried out with approximately 10-mg samples inflowing air (flow rate ¼ 100 cm3/min) at a heatingrate of 20 8C/min. WAXD measurements were per-formed at room temperature (ca. 25 8C) on a Sie-mens Kristalloflex D5000 X-ray diffractometerwith nickel-filtered Cu Ka radiation (wavelength¼ 1.5418 A) at 40 kVand 30 mA.

RESULTS AND DISCUSSION

Monomer Synthesis and Characterization

The new aromatic, unsymmetrical ether diaminewith a trifluoromethyl pendent group, PAPB, was

successfully synthesized with hydroquinone as astarting material, as shown in Scheme 1. First,NPP was synthesized through the nucleophilicetherification of 4-chloronitrobenzene with potas-sium phenolate of hydroquinone in dimethylfor-mamide. Then, PNPB was synthesized by anucleophilic substitution reaction of NPP and 2-chloro-4-nitrotrifluoromethylbenzene in the pres-ence of potassium carbonate in DMF. Finally,PNPB was converted to the corresponding di-amine monomer, PAPB, by hydrazine Pd/C-cata-lyzed reduction. The structures of PAPB and theintermediates were confirmed by elemental anal-ysis, mass spectrometry, IR spectra, and 1H NMRand 13C NMR spectroscopy. In the IR spectra,diamine PAPB exhibited characteristic absorp-tions of amino and ether groups at 3443, 3369,and 1220 cm�1. The absorption at 1161 cm�1,assigned to the C��F stretching vibration, wasalso observed. The elemental analysis was alsoin agreement with the unsymmetrical ether di-amine PAPB. The 1H and 13C NMR spectra alsosupported the formation of the desired compoundwith the proposed structure. As shown in Figure1, the absorption peaks at 6.6–7.0 ppm in 1HNMR were assigned to the aromatic protons. Thearomatic proton H6 of diamine PAPB showed adoublet of doublet absorption because of the split-

Scheme 2. Synthesis of the fluorinated polyimides.

6840 SHAO ET AL.


ting of H5 and H7. The signal of proton H5 wasalso a doublet peak as a result of the splitting ofproton H6. In the 13C NMR spectra of diaminePAPB, C10 and C15 exhibited clear quartetabsorptions at 116–130 and 122 ppm, respec-tively, caused by 2JC��F and 1JC��F coupling of thecarbons with fluorines. All the spectroscopic dataobtained were in good agreement with theexpected structures. The structure of the diaminewas designed to impart several desirable proper-ties to polyimides. The polymers that wereformed were expected to have low dielectric con-stants because of the presence of geometricallyunsymmetrical diamine components and elec-tron-withdrawing CF3 substituents. In addition,enhanced processability of the fully imidized poly-imides was expected because of the improved solu-bility of the polymers resulting from the presenceof ether linkages, geometrically unsymmetricaldiamine components, and CF3 substituents.

Polymer Synthesis and Solubility

New polyimides were prepared from PAPB andcommercially available aromatic dianhydrides,such as PMDA, BTDA, OPDA, and 6FDA, via aconventional two-step procedure, as shown inScheme 2. The polymerization was carried outthrough the reaction of stoichiometric amounts ofdiamine monomer PAPB with aromatic dianhy-drides at a concentration of 15% solids in NMP.The ring-opening polyaddition at room tempera-ture for 24 h yielded PAAs, and this was followedby sequential heating to 300 8C or a mixtureof Ac2O/Py to obtain the corresponding polymers.The transformation from PAA to a polyimidewas possible via thermal or chemical cyclodehy-dration; the merits of the former include easyhandling and casting into thin films, and thelatter is suited to the preparation of solublepolyimides.

Figure 2. FTIR spectra of the fluorinated polyimides.



The chemical structures of the polyimideswere characterized with FTIR, 1H NMR, and ele-mental analysis. All the polyimides showed char-acteristic imide absorption bands at 1776–1780 cm�1,which were attributed to the asymmetrical car-bonyl stretching vibrations, and at 1722–1728 cm�1,which were attributed to the symmetrical car-bonyl stretching vibrations (Fig. 2). The absorp-tion at 1374–1378 cm�1 was assigned to C��Nstretching, and the characteristic bands around1234 and 1136 cm�1 were attributable to theether and trifluoromethyl groups, respectively.There were no characteristic absorption bands ofthe amide group near 3200–3363 (N��H stretch-

ing) or 1650–1674 cm�1 (C¼¼O stretching), andthis indicated that the polymers had been fullyimidized. In addition to the IR spectra, the ele-mental analysis values of the polymers generallyagreed well with the calculated values for theproposed structures. Figure 3 shows the high-resolution 1H NMR spectrum of PI-1 derivedfrom PAPB and ODPA by chemical imidization;the absorption peaks at 7.2–8.4 ppm wereassigned to the aromatic protons in the polymerbackbone. No absorption was detected in a rangegreater than 8.4 ppm, and this indicated that theconcentration of the amide group [��C(O)NH��]in the polymer chains was lower than the detec-

Figure 3. 1H NMR spectrum of PI-1 (DMSO-d6).

Table 1. Inherent Viscosities and Elemental Analyses of the Polyimides

PolyimideInherent

Viscosity (dL/g)a

Elemental Analysis (%)b

Formula (Formula Weight) C H N

PI-1 0.77 (C35H17F3N2O7)n [(634.52)n] Calcd. 66.25 2.71 4.41Found 65.84 2.78 4.32




a Determined with 0.5% solutions in a solvent (DMAc) at 30 8C.b Obtained by thermal cyclization from the corresponding PAA.

6842 SHAO ET AL.


tion limit of 1H NMR (<5%). In other words, thechemically imidized polyimide possessed an imid-ization degree greater than 95%. Table 1 showsthe elemental analysis data for the fluorinatedpolyimides, which are in good agreement withthe calculated values for the proposed chemicalstructures. Because all the polyimides were solu-ble in DMAc, the intrinsic viscosities wereevaluated in DMAc at 30 8C with an Ubbelohdeviscometer, and the results fell within the 0.56–0.77 dL/g range, in which PI-1 showed the high-est value (0.77 dL/g). These results demonstratethat the diamine monomer PAPB has good poly-merization activity for forming polyimides de-spite the presence of electron-withdrawing CF3

substituents.The solubilities of the resulting polyimides by

chemical imidization were investigated in differ-ent organic solvents. The solubility behavior ofthese polymers in different solvents is presentedin Table 2. These polymers exhibited very good

solubility behavior in polar solvents such asDMSO, DMF, N,N-dimethyl acetamide (DMAc),and NMP. Upon heating, they could also be dis-solved in common solvents such as chloroform,tetrahydrofuran (THF), and toluene. In compari-son, the polyimides derived from 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene with BTDAand PMDA were partially soluble in DMSO,DMF, and DMAc and insoluble in CHCl3 andTHF.27 The solubility of the fluorinated polyi-mides depended on, to some extent, their chemi-cal structures. The good solubility might beattributed to the more bent chain structure of thepolyimides, which resulted from the bulky CF3

groups and the unsymmetrical structure pro-vided by PAPB, which increased the disorder inthe chains and hindered dense chain stacking,thus reducing the chain–chain interactions toenhance solubility. It should be noted that goodsolubility in low-boiling-point solvents is criticalfor preparing polyimide films or coatings at a rel-atively low processing temperature, which is de-sirable for advanced microelectronics manufac-turing applications. The polyimides prepared bythermal curing had much poorer solubility in or-ganic solvents than those prepared by chemicalimidization. PI-2 and PI-4 did not dissolve in anyorganic solvents, whereas PI-1 and PI-3 weresoluble in polar, aprotic solvents at an elevatedtemperature. The poor solubility of the polyimideobtained by thermal imidization was possibly dueto the presence of partial intermolecular cross-linking during the thermal imidization stage.

X-Ray Diffraction of the Polyimides

The crystallinity of the polyimides was examinedby WAXD analysis with graphite-monochromat-ized Cu Ka radiation, with 2h ranging from 0 to

Table 2. Solubility Data of the Polyimidesa

Solvent

Polymerb

PI-1 PI-2 PI-3 PI-4

NMP þþ þþ þþ þþDMAc þþ þþ þþ þþDMF þþ þþ þþ þþDMSO þþ þþ þþ þþTHF þþ þ þþ þCHCl3 þþ þ þþ þToluene þþ þ þþ þAcetone þ � þ �

a The qualitative solubility was determined at 3.0% (w/v).b Measured by chemical cyclization from the correspond-

ing PAAs. þþ ¼ soluble at room temperature; þ ¼ solubleon heating; � ¼ insoluble even on heating.

Table 3. Dielectric Properties of the Polyimide Films

Polymera

FilmThickness

(lm)

Dielectric Constant (Dry)b

10 MHz 20 MHz 200 MHz 1 GHz

PI-1 36 2.53 2.53 2.51 2.55PI-2 30 1.96 1.96 1.95 1.99PI-3 32 1.82 1.82 1.82 1.85PI-4 31 2.05 2.05 2.03 2.06PMDA–ODA 36 3.44 3.42 3.32 3.25

a The polyimides were obtained by thermal imidization.b Measured by Agilent 4291B at room temperature (dry dielectric constant).



608; polyimide powders obtained by chemicalimidization were used as samples. The WAXDpatterns of these polyimides showed an amor-phous phase. The amorphous nature of the poly-imides could be attributed to their unsymmetri-cal structural units and the bulky CF3 groups,which reduced the intra- and interpolymer chaininteractions, resulting in loose polymer chainpackaging and aggregates.

Mechanical and Electrical Properties

All the polyimides could be processed into clear,flexible, and tough films. These films were sub-jected to tensile tests. Table 4 later shows the me-chanical properties of the fluorinated polyimides,including the tensile strength, tensile modulus,and elongation at break. The polyimide films hadtensile strengths of 74.5–121.7 MPa, elongationsat break of 6–13%, and tensile moduli of 1.46–1.95 GPa, which indicated strong and toughmaterials.

The dielectric constants of the fluorinatedpolyimides are presented in Table 3. Polyimidesbased on PAPB have lower dielectric constants(1.82–2.53 at 10 MHz) than conventional poly-imides such as PMDA–4,40-oxydianiline (ODA)polyimide films (3.44 at 10 MHz). The decreaseddielectric constants could be attributed to thepresence of the bulky CF3 groups and aromatic,unsymmetrical structure, which resulted in lessefficient chain packing and increased free vol-ume. In addition, the strong electronegativity ofthe fluorine atom resulted in very low polarizabil-ity of the C��F bonds, thereby reducing thedielectric constant. Polyimide PI-3 exhibited thelowest dielectric constant among this series of

polyimides because of the highest fluorine con-tent in the repeat unit. It presents potential util-ity for the microelectronics industry, in which alow dielectric constant is desired to prevent cross-talk between conducting paths.

Thermal Properties

DSC and TGA methods were applied to evaluatethe thermal properties of the polyimides; theTGA curves of the polyimides are shown inFigures 4, and thermal analysis data from theTGA and DSC curves of the polyimides are sum-marized in Table 4. DSC revealed that rapid cool-ing from 400 8C to room temperature producedpredominantly amorphous samples, so that Tg ofthe polymer could be easily read in the secondheating trace of DSC. The Tg values of PI-1 toPI-4 were in the range of 232–273 8C. As weexpected, the Tg values of these polyimidesdepended on the structure of the dianhydridecomponent and decreased with increasing flexi-bility of the polyimide backbones according to theapplied structure of the dianhydride. PI-1,obtained from ODPA, showed a lower Tg becauseof the presence of a flexible ether linkage betweenthe phthalimide units. PI-4, derived from PMDA,exhibited the highest Tg because of the rigidpyromellitimide unit. PI-2 showed a lower Tg

(237.1 8C) than PI-3 (260.8 8C), and this was prob-ably due to the different packing densities of thepolymer aggregation and the interactions of thepolymer chains. Clearly, the difference in Tg’scould be attributed to the rigidity and packing ofthe polymer chains.

For the thermal stability of the polyimides,Table 4 gives the temperatures of the initial

Table 4. Thermal and Tensile Properties of the Polyimides

PolymerTg

(8C)aTd

(8C)b

Weight Loss(8C) Char

Yield(%)c

TensileStrength(MPa)

Elongation(%)

Modulus(GPa)Td5

d Td10e

PI-1 232.3 546.2 545.0 570.4 17 121.7 13 1.49PI-2 237.1 540.2 538.2 560.8 31 74.5 6 1.95PI-3 260.8 525.2 522.8 535.2 5 104.6 11 1.51PI-4 272.2 582.2 540.2 562.8 14 79.9 10 1.46

a Determined by DSC.b Onset decomposition temperature.c Residual weight retention at 800 8C.

d Five percent weight loss temperature.e Ten percent weight loss temperature.

6844 SHAO ET AL.


decomposition and 5 and 10% gravimetric lossesin nitrogen. The temperatures of the initial de-composition were determined to be in the rangeof 525–582 8C for all polyimides tested. The tem-peratures of 5 and 10% gravimetric losses of thepolyimides reached 522–545 and 535–570 8C innitrogen, and this is important evidence for ther-mal stability. Also, the char yields at 800 8C for allthe polymers were in the range of 5–31 wt %.Obviously, the data from thermal analysis showthat the resulting polyimides had fairly high ther-mal stability.

CONCLUSIONS

A novel unsymmetrical ether diamine with a tri-fluoromethyl pendent group was synthesized andcharacterized. A series of polyimides were ob-tained from the diamine monomer with variousaromatic dianhydrides by two-step thermal orchemical imidization methods. The introductionof trifluoromethyl groups into the polyimidesresulted in dramatic changes in their properties;in particular, the solubility and dielectric proper-ties were improved. The experimental resultsindicated that the resulting polymers had verygood solubility in different organic solvents andcould be cast into transparent, tough, and flexiblefilms. These polymers also had good thermal sta-bility and mechanical properties with low dielec-tric constants. These features are desirable for

polyimides as potential candidates for packagingapplications for microelectronics.

The authors acknowledge the financial support of theResearch Foundation of the State Key Laboratory ofApplied Organic Chemistry.

REFERENCES AND NOTES

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2. Wilson, D.; Stenzenberger, H. D.; Hergenrother,P. M. Polyimides; Chapman & Hall: New York,1990; p 227.

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Figure 4. TGA curves of the fluorinated polyimides.



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6846 SHAO ET AL.


synthesis and properties of fluorinated polyimides from a new unsymmetrical diamine:...

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