2,6-diamino-4-phenylphenol (dapp) copolymerized polyimides: synthesis and characterization

Polymer International Polym Int 55:93–100 (2006)DOI: 10.1002/pi.1924

2,6-Diamino-4-phenylphenol (DAPP)copolymerized polyimides: synthesis andcharacterizationBor-Kuan Chen,1∗ Yu-Jie Tsai2 and Sun-Yuan Tsay1

1Department of Polymer Materials, Kun Shan University of Technology, Tainan, 710 Taiwan2Chi-Mei Optoelectronics Co., Tainan, 741 Taiwan

Abstract: Novel soluble copolyimides containing phenyl and hydroxyl pendant groups were synthesizedfrom pyromellitic dianhydride (PMDA) and two diamines, 2,6-diamino-4-phenylphenol (DAPP) and4,4′-oxydianiline (ODA), in various ratios via thermal imidization. The structures and physical propertiesof the copolyimides were characterized by FTIR, elemental analysis, DSC, dynamic mechanical analysis(DMA), TGA, a universal testing machine for stress–strain behaviour, and a dielectric analyzer to studythe effect of DAPP on the physical properties of the modified polymers. Copolyimides containing morethan 40 mol% DAPP were soluble in hot N-methyl-2-pyrrolidone (NMP), dimethylacetamide (DMAc)and dimethylformamide (DMF), and possessed a high glass transition temperature (358 ◦C) and a highmodulus (3.9 GPa). Introduction of the diamine DAPP could also reduce the dielectric constant. Asegment of imide linkages could convert to benzoxazole linkages by decarboxylation at temperatureshigher than 420 ◦C under vacuum. Although the heat-treated polybenzoxazoles (PBOs) exhibited manygood properties, they were found to be too rigid and brittle to be processable for microelectronicapplications. 2005 Society of Chemical Industry

Keywords: polyimide; 2,6-diamino-4-phenylphenol (DAPP); diamines; polybenzoxazoles; copolymerization;Kapton; solubility

INTRODUCTIONPolyimides (PIs) are utilized in a wide rangeof advanced technologies in the microelectronic,aerospace and automotive industries because oftheir excellent mechanical properties, high thermalstability, good electrical properties and good resistanceto organic solvents.1,2 A polyimide synthesized bypolymerizing an aromatic dianhydride, pyromelliticdianhydride (PMDA), and an aromatic diamine,4,4′-oxydianiline (ODA), commercialized by DuPontunder the name Kapton, is one of the mostcommonly used PIs in the electronics industry.3

However, many PI applications are limited owingto process difficulties, as their insolubility in organicsolvents and their extremely high glass transition ormelt temperatures (Tg) preclude melt processing.Therefore, much research has been carried out toimprove the processability of PIs without sacrificingtheir high performance properties.

The modification of PI structure is a major researchinterest. Some general approaches have an emphasison reducing molecular regularity, rigidity and cohesiveenergy density. One approach to improve solubility

and processability of PI is by the introductionof flexible linkage groups such as ether, sulfone,ketone and methylene groups.4,5 Some commercialproducts, such as Ultem 1000 polyetherimide (PEI)resin, have outstanding processability on conventionalmoulding equipment. Incorporating bulky pendantgroups along the polymer backbone to modify PIproperties, particularly by improving the solubilityof aromatic PIs, is a general approach that hasoften proven to be effective because it enhancesfree volume and reduces the packing force.6,7 Otherapproaches to improve organic solubility are alsocommonly used, such as replacing aromatic groupswith alicyclic groups,8 using meta-substituted phenylmonomers,9 and introducing fluorinated groups.10

Successful results can be achieved by using one ora combination of these approaches. In further studies,hydroxyl-containing PIs were synthesized and lookedvery promising for membranes, photoresistors11 andnonlinear optical applications.12 Additionally, giventheir bulky and polar hydroxyl substituents andtheir rigid backbone structure, some of the PIscontaining OH groups resulted in high Tg while

∗ Correspondence to: Bor-Kuan Chen, Department of Polymer Materials, Kun Shan University of Technology, Tainan, 710 TaiwanE-mail: [email protected]/grant sponsor: National Science Council of Taiwan; contract/grant number: NSC93-2216-E168-002(Received 20 December 2004; revised version received 7 June 2005; accepted 8 June 2005)Published online 9 November 2005

2005 Society of Chemical Industry. Polym Int 0959–8103/2005/$30.00 93

BK Chen, YJ Tsai, SY Tsay

retaining solubility in organic solvents.13 AlthoughPI synthesized from PMDA and ODA is well known,few papers deals with the chemical modification ofthis famous PI.14 Most deal with PI/inorganic hybridmaterials.14–17

In this study, we synthesized a diamine, 2,6-diamino-4-phenylphenol (DAPP), containing bulkypendant phenyl and hydroxyl groups to copolymerizewith ODA and PMDA in an attempt to modify Kaptonand obtain organic-soluble polyimides which havehigh Tg, high functionality and excellent mechanicalproperties. The effects of various molar ratios of DAPPon the soluble, thermal, mechanical and dielectricproperties of PI copolymers were also studied.

EXPERIMENTALMaterialsPyromellitic dianhydride (Chriskev, 99%) was recrys-tallized from acetic anhydride and vacuum driedbefore use. ODA (Acros, 98%), 4-phenylphenol (Lan-caster, 98%), hydrazine monohydrate (Katayama,98%), nitric acid (Showa) and acetic acid (Tedia)were used without further purification. N-methyl-2-pyrrolidone (NMP, Tedia) was purified by distillationunder reduced pressure and stored over 4 A molecularsieves. The other reagents were used as received.

Synthesis of DAPP2,6-Dinitro-4-phenylphenol (DNPP) was synthesizedby nitration of 4-phenylphenol in the presence ofacid.18 DNPP (10.0 g, 0.0384 mol) and 0.01 g of10 % Pd/C were added to 120 mL of ethanol ina 500 mL Pyrex reactor with stirring. The mixturewas then heated to 78 ◦C and 25 mL of hydrazinemonohydrate was added dropwise over the courseof 1 h. As the hydrazine monohydrate was added,the mixture initially became a red slurry and thenturned black-brown after 30 min. After completionof the addition, the mixture was stirred at 78 ◦C foranother 2 h. The reaction mixture was then filtered toremove Pd/C, resulting in a purple-black solution. Itwas cooled to room temperature and held overnightto yield white short-needle crystals of DAPP. Theproduct was filtered and dried at 60 ◦C for 4 hunder vacuum. Yield: 85 %. Melting point: 208 ◦C.IR (KBr): 3344 and 3427 cm−1 (–NH2 stretching).1H-NMR (DMSO-d6): δ 7.42–7.45 (s, PhH, 2H),7.32–7.37 (s, PhH, 2H), 7.18–7.23 (m, PhH, H),6.28 (s, PhH, 2H). Elemental analysis: C12H12N2O,Calculated (%) C 72.00; H 6.00; N 14.00, Found (%)C 71.92; H 6.11; N 14.02.

Synthesis of copolyimidesThe copolyimides were synthesized by reacting DAPPand ODA with PMDA as shown in Scheme 1. Thereactor was purged with dry nitrogen for 30 min.Various molar ratios of ODA and DAPP were chargedinto the reactor through an addition funnel (theamounts used are listed in the Table 1). The vessel

Table 1. Monomer compositions used for copolyimide synthesis

Polymercode

DAPP(mol%)

ODA(mol%)

PMDA(mol%)

Polymercodea

PI 0.0 1.0 1.0 —CPI1 0.1 0.9 1.0 PBO1

CPI2 0.2 0.8 1.0 PBO2CPI3 0.3 0.7 1.0 PBO3

CPI4 0.4 0.6 1.0 PBO4

CPI5 0.5 0.5 1.0 PBO5CPI10 1.0 0.0 1.0 PBO10

a Polymers after heating isothermally for 1 h at 480 ◦C under vacuum.

was then charged with NMP (9.75 g, solid content30 %). PMDA (2.18 g, 10 mmol) was added to thereactor in two portions, 80 % and 20 %, respectively,through another addition funnel over a period of1 h until complete dissolution of the diamines wasachieved. After the PMDA was charged, residual NMP(13.94 mL) was added into the reactor to dilute thereaction mixture (solid content 15 %) and to washthe PMDA that adhered to the addition funnel. Thereaction mixture was stirred at room temperature in anitrogen atmosphere resulting in a viscous copolyamicacid solution after 2 h. The copolyamic acid solutionwas then spread on a glass plate using a spin-coaterto control the film thickness at 10–15 µm for IRand dynamic mechanical analysis (DMA), and at70–80 µm for the stress– strain testing. The films werethermally dried at 50 ◦C in a forced air oven for 2 h toremove most of the solvent and they were convertedto the copolyimide with a heating program of 110 ◦Cfor 2 h followed by 1 h each at 200 and 320 ◦C. Thefilms were then cooled to room temperature, soakedin water and stripped from the plates. After drying thefilms at 110 ◦C for 24 h in vacuum, the light-yellowPI and copolyimide films denoted CPI1–10 in Table 1were obtained.

Conversion of imide rings to benzoxazole ringsSome of the imide linkages thermally converted tobenzoxazole linkages by decarboxylation at temper-atures higher than 420 ◦C under vacuum, and thereaction mechanism is illustrated in Scheme 2. Thecopolyimide films CPI1–10 were fixed on the glassplate and heated under vacuum (<133 Pa) for 1 h at480 ◦C. After being cooled to room temperature, thepolybenzoxazole (PBO) films were obtained.

CharacterizationPhysical and chemical properties1H-NMR spectra were obtained on a Bruker AMX-400 spectrometer with deuterated dimethylsulfoxide(DMSO-d6) as the solvent. Fourier-transfer infrared(FTIR) spectra were recorded on a Bio-Rad FTS-40A spectrometer. The inherent viscosities weredetermined with an Ubbelohde viscometer (SchottGerate AVS310) at 30 ◦C. An Instron universal testermodel 4467 was used to study the stress–strainbehaviour of the samples. The load cell used

94 Polym Int 55:93–100 (2006)

Synthesis and characterization of DAPP copolymerized polyimides

Scheme 1. Reaction scheme for synthesis of the copolyimides.

was 5 kg and the crosshead rate was 5 mm min−1.Measurements were performed with film specimensthat were 1.35 cm wide, 6 cm long and 70–80 µmthick. The gauge length was 2 cm. The solubility ofthe copolymers was determined for the film samples invarious solvents. The amount of moisture absorptionwas determined by using 100 mg rectangular strips ofpolymer films that had been dried in a vacuum oven at110 ◦C overnight, immersing them in 30 ◦C water for48 h, and measuring the weight difference before andafter immersion.

Thermal analysisThermogravimetric analysis (TGA) was performedwith a Perkin-Elmer TGA-7 thermal analyzer at aheating rate of 20 ◦C min−1 in nitrogen (10 mL min−1)over the temperature range 30–800 ◦C. Differentialscanning calorimetry (DSC) data were obtained witha Perkin-Elmer DSC-7. Samples of ∼5 mg weresealed in hermetic aluminium pans and scanned inthe calorimeter at a heating rate of 5 ◦C min−1 overthe range 30–400 ◦C under a nitrogen atmosphere(10 mL min−1). The Tg values were taken as themidpoint of the change of the specific heat in the heat

flow curves. Dynamic mechanical analysis (DMA)was performed on a Perkin-Elmer DMA-7 thermalanalyzer system in the tensile mode at a frequencyof 1 Hz over the temperature range 50–410 ◦C at aheating rate of 5 ◦C min−1. A sample 15 mm in length,5 mm in width and approximately 1.5 mm in thicknesswas used.

Dielectric constantThe dielectric properties of the polymer films weretested with an IM6 electrochemical workstation(ZAHNER-elektrik GmbH & Co. KG) at a frequencyof 1 kHz with an AC amplitude of 5 mV rms. Thesample holder of the IM6 had two stainless steelcircular plates and its diameter was 1.0 cm. Polymerfilms were pressed between the two circular plateswhen measuring their dielectric constant.

RESULTS AND DISCUSSIONMonomer and polymer synthesisDAPP was obtained in high yield (85 %) by thecatalytic reduction of DNPP with hydrazine mono-hydrate and Pd/C catalyst in refluxing ethanol in

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Scheme 2. Reaction mechanism for conversion of imide to benzoxazole linkages.

Figure 1. FTIR spectra of (a) DNPP and (b) DAPP.

comparison with reduction by sodium hydrosulfite.18

The structures of DNPP and DAPP monomer wereconfirmed by 1H-NMR, FTIR and elemental analysis.Figure 1 shows the FTIR spectra of the dinitro com-pound DNPP and the diamine DAPP. The nitro group

of DNPP gave two characteristic bands, at 1533 and1325 cm−1 (NO2 asymmetric and symmetric stretch-ing). After reduction, the characteristic absorptionsof the nitro group disappeared, and the amino groupshowed a pair of typical N–H stretching bands in theregion of 3300–3500 cm−1. Figure 2 shows the 1H-NMR spectra of DNPP and DAPP. The absorptionsignals of aromatic protons of DNPP appeared in therange 7.4–8.5 ppm, and those of DAPP appeared inthe range 6.3–7.5 ppm. The Ha protons of DNPPresonated at the furthest downfield region because ofthe inductive effect of the electron-withdrawing –NO2

groups. The 1H-NMR spectrum of DNPP confirmedthat the nitro groups were all ortho substitutions. Afterreduction, all protons of DAPP were shifted upfield bythe electron-donating property of the amino groups;the Ha protons shifted the most because they werenear the amino groups whose electron-donating effectswere largest. The chemical shift between 4–5 ppm (notshown in expanded spectrum) was caused by the pro-tons of the amino groups. The characteristic peakscorrelated well with the proposed structure.

The inherent viscosities of the intermediatepolyamic acids of PI and CPI1–5 ranged from 0.80 to

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Figure 2. Expanded 1H-NMR spectra of (a) DNPP and (b) DAPP in DMSO-d6. The accompanying hydroxyl group resonance of DNPP (–OH at11.04 ppm) and DAPP (–OH at 10.82 ppm), and the amine group of DAPP (–NH2 at 4.51 ppm) are not shown.

Figure 3. FTIR spectra of copolyimides.

1.18 dL g−1, indicating the formation of high molec-ular weights. The FTIR spectra of the copolyimidesare shown in Fig. 3, where the characteristic imidegroup absorptions at 1783 and 1728 cm−1 (imide car-bonyl asymmetrical and symmetrical stretching), at1376 cm−1 (C–N stretching), and at 728 cm−1 (C–Nbending) are evident.

The hydroxyl-containing polyimide can undergorearrangement to an aromatic PBO via in situ ther-mal conversion.19,20 Figure 4 shows the FTIR spectraof CPI4 at various elevated temperatures under vac-uum. It can be seen that the characteristic absorbance

Figure 4. FTIR spectra of CPI4 after heating for 1 h in vacuum at theindicated temperatures.

peak of benzoxazole stretching (1617 cm−1) increasedwith temperature while the characteristic peak of –OHstretching (3400–3600 cm−1) disappeared gradually.This demonstrated that DAPP-containing PI con-verted to PBO at higher temperatures. The color ofthe PI film also changed from yellow to purple-brownwhen converted to PBO.

Mechanical properties of PIsTable 2 summarizes the tensile properties of polyimidefilms of PI and CPI1–5 obtained from the stress–straincurves. The introduction of phenyl and hydroxyl

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Table 2. Mechanical properties of copolyimides and inherent

viscosity of polyamic acids

Tensile properties

Polymercode

Tensilestrength(MPa)

Elongationat break

(%)

Initialmodulus

(GPa)Film

propertiesηinh

(dL g−1)a

PI 80.5 14.9 2.85 Flexible 0.90CPI1 88.7 9.5 3.31 Flexible 1.10CPI2 83.3 7.6 3.40 Flexible 0.95CPI3 81.2 6.3 3.56 Flexible 0.80CPI4 80.2 5.9 3.81 Flexible 1.00CPI5 71.7 3.5 3.93 Flexible 1.18CPI10

b — — — Brittle 0.56

a Measured at a polyamic acid concentration of 1 g dL−1 in NMP at30 ◦C.b The polymer film was too brittle for measurement of the mechanicalproperties.

pendant groups caused the tensile strength to initiallyincrease (from 80.5 to 88.7 MPa) and then to decrease(to 71.7 MPa). When the DAPP content was lower,the polar characteristics of the hydroxyl group weremore significant than the phenyl group, which madeCPI1 a polymer material with great strength andflexibility. However, when the DAPP ratio rose tomore than 40 %, the asymmetric and free volumes ofthe phenyl pendant groups increased. This resultedin greater separation between chains, reducing thestrength of the interchain interactions,7,13,21 anddecreasing the elongation at break (from 14.9 % to3.5 %). The polymer became more rigid because thestrain was reduced considerably more than the tensilestrength, while maintaining a higher initial modulus(from 2.85 to 3.93 GPa). The polymer materials thuschanged from strong and flexible to strong and rigidas the content of DAPP increased. Nonetheless, thevalues of modulus and tensile strength measuredon these phenyl polyimides corresponded to fairlygood mechanical properties. The PBO films were,however, too rigid and brittle for measurement oftheir mechanical properties.

Thermal propertiesThe thermal properties of polymers were investigatedby TGA and DSC, and these are listed in Table 3.The DSC scans of the polymers exhibited only oneTg, indicating that the various repeating units wererandomly distributed along the polymer chain, andthat a random copolymer structure had been formed.22

The Tg of PI and CPI1–5 were found to be in the range351–391 ◦C by DSC and in the range 363–398 ◦C byDMA. Their thermal decomposition temperatures at5 % weight loss (Td,5 %) in a nitrogen atmosphere werein the range 488–603 ◦C. Both the Tg and Td,5 % valuesdecreased gradually with increasing DAPP fraction.Compared with the Tg values estimated by the Foxequation23 for copolymers,

1Tg

= W1

Tg1+ W2

Tg2(1)

Table 3. Thermal properties of copolyimides

Tg (◦C) TGA

by Td,5 % (◦C) CharPolymer by by Fox yieldcode DSC DMA eqn In air In N2 In N2

b (%)a

PI 391 398 — 597 603 — 60.7CPI1 386 393 386 595 598 598 47.0CPI2 384 391 382 571 573 603 57.5CPI3 372 377 378 496 547 605 57.7CPI4 361 371 374 491 513 609 58.8CPI5 358 363 370 481 488 612 60.3CPI10 351 —c — 461 478 614 61.9

a CPI residual weight at 800 ◦C in N2.b TGA of polymers (PBO1–10) after heating isothermally for 1 h at 480 ◦Cunder vacuum.c The film was too brittle to test.

Figure 5. Relationship between Tg and molar ratio of DAPP/ODA.

where W1 and W2 are the weight fractions ofcomponents 1 and 2, respectively, the actual Tg valueswere lower than those predicted when the DAPPcontent was higher, as shown in Fig. 5. The relativelylower Tg of the polymers can be explained by theasymmetric bulky pendant phenyl groups in DAPP,which significantly increased the free volume of thepolymers and reduced their Tg.24

There are two sets of Td,5 % values under nitrogen,as listed in Table 3. The first Td,5 % of all the sampleswas due to thermal decomposition of molecular chainsor partial conversion to PBO with the release ofCO2. If the polymers were preheated isothermallyfor 1 h at 480 ◦C under vacuum, then the thermaldecomposition temperatures (the second set of Td,5 %

values) would be much higher than the first set. Thiswas attributed to the thermal conversion of PI to athermal stable PBO that possessed a higher Td.19,20

Also, as illustrated in Fig. 6, the weight reduction ofCPI10 between 320 and 550 ◦C was due to thermalcyclodecarboxylation (CO2) and conversion to PBO.The polyimide thermal conversion was accompaniedby quantitative loss of carbon dioxide, indicating thatan intramolecular reaction was occurring. The weight

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losses of copolyimides observed are consistent withthe calculated CO2 weight loss values as listed inTable 4. These results demonstrated that at highertemperatures, the CPI conversion to PBO was due tocyclodecarboxylation. With increasing DAPP content,the heat-treated polymers exhibited higher Td withTd,5 % reaching as high as 614 ◦C for PBO10. Thechar yield increased gradually with increasing DAPPcontent. This suggested that the copolymer with ahigher DAPP content was transformed to higher charyield structures of PBO with heat treatment.

SolubilityIntroducing bulky pendant groups has proved to bean effective approach for modifying the solubility

Figure 6. TGA curves in nitrogen for CPI10 (solid line) and PBO10

(dashed line).

Table 4. Weight loss after decarboxylation of copolyimides

CO2 weight loss (%)

Polymer code Calculated Found (320–550 ◦C)

CPI1 1.15 1.24CPI2 2.30 2.41CPI3 3.45 3.40CPI4 4.60 4.65CPI5 5.75 5.66CPI10 11.52 11.43

of aromatic polyimides.7,25 The qualitative solubilityof the polymers in various solvents is shown inTable 5. CPI10 was soluble in organic solvents, andCPI4 and CPI5 were soluble in hot, polar aproticsolvents such as NMP, dimethylacetamide (DMAc)and DMSO. The introduction of monomer DAPP canthus effectively improve the solubility of copolymersbecause of its bulky pendant group and polar hydroxylsubstituents, which cause loss of chain packing.All the copolyimides could dissolve in aqueous1 mol L−1 sodium hydroxide, presumably owing tophenoxide ion formation. However, PBO1∼10 wereinsoluble in organic solvents, concentrated sulfuricacid and aqueous 1 mol L−1 sodium hydroxide,suggesting that some degree of cross-linking hadoccurred.

Dielectric constantThe results of the dielectric constant measurementsare listed in Table 6. Copolyimides CPI1–5 hadlower dielectric constants (3.41–3.79) than polyimidePI (3.84), whose structure is the same as Kaptonpolyimide. The reduction might be attributed to thepresence of bulky phenyl groups, which resultedin less efficient chain packing and increased freevolume. PBO1–5 had lower dielectric constants thanCPI1–5 because the former possessed fewer polarcarbonyl groups in its backbone when the conversionof the imide to the benzoxazole rearrangement tookplace.

Table 6. Moisture absorption and dielectric constant of polymers

Polymer code Dielectric constant Moisture absorption (%)

PI 3.84 2.03CPI1 3.79 2.26CPI2 3.68 2.34CPI3 3.58 2.65CPI4 3.50 2.78CPI5 3.41 3.06PBO1 3.72 1.70PBO2 3.60 1.56PBO3 3.47 1.34PBO4 3.39 1.12PBO5 3.25 0.93

Table 5. Solubility behaviour of copolyimides

Organic solvent

Polymer code NMP DMAc DMF m-cresol CHCl3 THF NaOH (1 mol L−1) H2SO4 (97 %)

PI −− −− −− −− −− −− + ++CPI1 −− −− −− −− −− −− −− −−CPI2 −− −− −− −− −− −− −− −−CPI3 −− −− −− −− −− −− −− −−CPI4 + + + −− −− −− ++ ++CPI5 + + + −− −− −− ++ ++CPI10 ++ ++ ++ ++ ++ ++ ++ ++Symbols: ++, soluble at 30 ◦C; +, soluble when heating to solvent’s boiling point; −−, insoluble.PBO1–10 were insoluble in all solvents tested.

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Moisture absorptionThe moisture absorption of the polymers is listedin Table 6. As DAPP increased, the moistureabsorption of polyimides increased from 2.03 % to3.06 % because of the hydroxyl substituents. Afterdecarboxylation, the moisture absorption of PBO1–5

decreased to 0.93–1.70 %. This was caused by thereduction of polar carbonyl and hydroxyl groups.

CONCLUSIONSIncorporation of DAPP containing bulky pendanthydroxyl and phenyl groups into the polyimidebackbone can improve the solubility of polymers.Copolyimides are able to dissolve in hot NMP,DMAc and DMF when the DAPP content ismore than 40 mol%. Copolyimides exhibit betterinitial modulus but less elongation at break thanpolyimide without DAPP. Introduction of DAPPreduced the dielectric constant of the copolyimides.Copolyimides containing DAPP were found toundergo decarboxylation and rearrange to PBOsupon heating above 420 ◦C under vacuum. Althoughthe resultant PBOs had lower dielectric constants,reduced moisture absorptions, and better thermalstabilities than the copolyimides, the PBO films wereinsoluble and too rigid and brittle to be processablefor microelectronic applications.

ACKNOWLEDGEMENTSWe are grateful to the National Science Council ofTaiwan for the support of this research work (grantNSC93-2216-E168-002).

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2,6-diamino-4-phenylphenol (dapp) copolymerized polyimides: synthesis and characterization

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