synthesis and characterization of soluble polyimides derived from a novel unsymmetrical diamine...

EUROPEAN

European Polymer Journal 43 (2007) 4389–4397

www.elsevier.com/locate/europolj

POLYMERJOURNAL

Synthesis and characterization of soluble polyimidesderived from a novel unsymmetrical diamine monomer:

1,4-(2 0,400-diaminodiphenoxy)benzene

Yu Shao, Yanfeng Li *, Xin Zhao, Tao Ma, Chenliang Gong, Fengchun 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 26 April 2007; received in revised form 6 June 2007; accepted 10 July 2007Available online 20 July 2007

Abstract

A new aromatic unsymmetrical diamine monomer, 1,4-(2 0,400-diaminodiphenoxy)benzene (OAPB), was successfullysynthesized in three steps using hydroquinone as starting material and polymerized with various aromatic tetracarboxylicacid dianhydrides, including 4,4 0-oxydiphthalic anhydride (ODPA), 3,3 0,4,4 0-benzophenone tetracarboxylic dianhydride(BTDA), 2,2 0-bis(3,4-dicarboxyphenyl)-hexafluoropropane dianhydride (6FDA) and pyromellitic dianhydride (PMDA)via the conventional two-step thermal or chemical imidization method to produce a series of the unsymmetrical aromaticpolyimides. The polyimides were characterized by solubility tests, viscosity measurements, IR, 1H NMR, and 13C NMRspectroscopy, X-ray diffraction studies, and thermogravimetric analysis. The polyimides obtained had inherent viscositiesranged of 0.38–0.58 dL/g, and were easily dissolved in common organic solvents. The resulting strong and flexible PI filmsexhibited excellent thermal stability with the decomposition temperature (at 5% weight loss) of above 505 �C and the glasstransition temperature in the range of 230–299 �C. Moreover, the polymer films showed outstanding mechanical propertieswith the tensile strengths of 41.4–108.5 MPa, elongation at breaks of 5–9% and initial moduli of 1.15–1.68 GPa.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Polyimides; Unsymmetrical diamine monomer; Solubility; Polymer films

1. Introduction

Aromatic polyimides are widely used in the aero-space and microelectronic industry in the forms of

0014-3057/$ - see front matter � 2007 Elsevier Ltd. All rights reserved

doi:10.1016/j.eurpolymj.2007.07.002

* Corresponding author. Tel.: +86 931 8912528; fax: +86 9318912113.

E-mail address: [email protected] (Y. Li).

films and moldings, because of their excellent stabil-ities, chemical resistance and electric properties.Other uses for these polymers such as adhesives,gas separation membranes, composite matrices,coating, and foams are rapidly increasing [1–5].Despite their widespread use, most of them havehigh melting temperatures or softening tempera-tures and limited solubility in most solvents becauseof their rigid backbones and strong interaction

.

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4390 Y. Shao et al. / European Polymer Journal 43 (2007) 4389–4397

between chains, which may restrict their applicationin some fields. Because of the interest that they havegenerated, considerable effort has been made tomodify their chemical structure to change theirproperties with regard to a specific application orto a particular property. Recently significant syn-thetic effort has been centered on improving pro-cessability and solubility through the synthesis ofnew diamine or dianhydride monomers. The incor-poration of aryl-ether linkage and asymmetric struc-ture into rigid polymer backbones has led to themost success [6–15].

Ether linkages are the most popular, flexible link-ages introduced into polyimide backbones. It is gen-erally recognized that an aryl-ether linkage impartsproperties such as better solubility and melt-pro-cessing characteristics and improved toughness incomparison with those of polymers without anaryl-ether linkage [16–19]. In general, the introduc-tion of geometrically or molecularly unsymmetricaldiamine components into the polyimide main chainhas led to new polyimides with improved solubilityand melt processability and other desirable proper-ties [10–15,20].

In the preceding paper, we have reported a ser-ies of polyimides based on the new unsymmetricaldiamines PAPB (Fig. 1) [21]. In continuation ofour studies in the search for novel polyimideswith improved processability and solubility, weherein synthesize a new aromatic unsymmetricaldiamine monomer, 1,4-(2 0,400-diaminodiphen-oxy)benzene (OAPB). It has been designed as apotentially convenient condensation monomerfor polyimides, capable of imparting solubilityand good thermal properties at the same time.Meanwhile, a series of all-aromatic, organosolublepolyimides bearing unsymmetrical ether structurewere synthesized from the diamine with fourkinds of commercial dianhydrides via a conven-tional two-stage process. The characterizationsof the diamine and related intermediates, as wellas the resulting polyimides based on this diaminewere carried out by means of FT-IR, 1H NMR,13C NMR, DSC, TGA and elemental analysismethods.

Fig. 1. Structures of P

2. Experimental

2.1. Materials

Commercially available hydroquinone (Cheng-dou Chemical reagents Corp., China), p-chloroni-trobenzene (Shanghai Chemical reagents Corp.,China), o-chloronitrobenzene (Shanghai Chemicalreagents Corp., China), hydrazine monohydrate(Beijing Chemical reagents Corp., China), potas-sium carbonate (Fuchen Chemical reagents Corp.,Tianjin, China), and 5% Pd/C (Acros) were usedwithout further purification. 4,4 0-Oxydiphthalicanhydride (ODPA, Shanghai Nanxiang ChemicalCo., China), 3,3 0,4,4 0-benzophenonetetracarboxylicdianhydride (BTDA, Beijing Chemical ReagentsCorp., China) and 2,2 0-bis(3,4-dicarboxyphe-nyl)hexafluoropropane dianhydride (6FDA,Aldrich) were recrystallized from acetic anhydridebefore use. Pyromellitic dianhydride (PMDA, Bei-jing Chemical Reagents Corp., China) was purifiedby sublimation under vacuum at 200–220 �C. N,N-dimethylacetamide (DMAc) and N-methyl-2-pyr-rolidone (NMP) were purified by distillation underreduce pressure over calcium hydride and storedover 4 A molecular sieves. All other solvents wereobtained from various commercial sources and usedwithout further purification.

2.2. Measurements

The inherent viscosities of the resulting polyi-mides were measured with an Ubbelohde viscometerat 30 �C. FT-IR spectra (KBr) were recorded on aNicolet NEXUS670 fourier transform infrared spec-trometer. 1H NMR and 13C NMR spectra were mea-sured on a JEOL EX-300 and a Bruker-400spectrometer. Elemental analyses were determinedby a Perkin–Elmer model 2400 CHN analyses. Themechanical properties were measured on an Instron1122 Tensile Apparatus with 100 · 5 mm specimensin accordance with GB 1040-79 at a drawing rateof 100 mm/min. Testing of differential scanning cal-orimetry (DSC) were performed on a Perkin–Elmerdifferential scanning calorimeter DSC 7 or Pyris 1

APB and OAPB.

Y. Shao et al. / European Polymer Journal 43 (2007) 4389–4397 4391

DSC at a scanning rate of 20 �C/min in flowingnitrogen (30 cm3/min), and glass transition tempera-tures (Tg) were read at the DSC curves at the sametime. Thermogravimetric analysis (TGA) was con-ducted with a TA Instruments TGA2050, and exper-iments were carried out on approximately 10 mg ofsamples in flowing nitrogen (flowing rate = 100 cm3/min) at a heating rate of 20 �C/min.

2.3. Synthesis of the monomer

2.3.1. 4-(4-nitrophenoxy)phenol (NPP)NPP was synthesized from hydroquinone by pro-

cedures described in the literature with a yield of53% [21].

2.3.2. 1,4-(2 0,400-dinitrodiphenoxy)benzene (ONPB)

A 100 ml three necked round bottom flask con-taining 6.02 g (0.026 mol) of NPP, 4.25 g(0.027 mol) of o-chloronitrobenzene, 3.72 g ofpotassium carbonate and 60 ml of DMF were fittedwith a magnetic stirring bar, condenser, nitrogenpad and a thermometer. This mixture was heatedto 150–155 �C with stirring. After heating this mix-ture for 8 h, it was cooled down to ambient temper-ature. The mixture was poured into an excessamount of water. The precipitate was collected byfiltration, washed with water, and air-dried. Thecrude product was recrystallized from ethanol togive a yellow product. 8.18 g was obtained (89.2%).

The mp is 120.5 �C by DSC (5 �C/min). Electron-impact mass spectrometry provided a value of 352(M+; Calc. For C18H12N2O6: 352.3). IR (KBr) spec-trum indicates absorption peaks at 1519, 1344 cm�1

(–NO2 stretching), 1241, 1184 cm�1 (C–O stretch-ing). 1H NMR (300 MHz, DMSO-d6, ppm) showsthe signals of different protons at d values of 8.24(m, 2H), 8.06 (dd, J = 1.5, 7.8 Hz, 1H), 7.70 (td,J= 7.8, 1.2 Hz, 1H), 7.36 (td, J = 7.8, 1.2 Hz, 1H),7.27–7.12(m, 7H). 13C NMR (75 MHz, DMSO-d6,d, ppm) showed values of 163.71, 153.43, 151.26,150.03, 142.97, 141.76, 135.76, 126.87, 126.36,124.97, 123.10, 121.41, 117.91.

ELEM. ANAL. Calc. For C18H12N2O6: C, 61.37%;H, 3.43%; N, 7.95%. Found: C, 61.26%; H, 3.34%;N, 8.11%.

2.3.3. 1,4-(2 0,400-diaminodiphenoxy)benzene

(OAPB)

The dinitro compound ONPB (7.74 g, 0.022 mol)and 5% Pd/C (0.2 g) were suspended in 150 ml ofethanol in a 250 ml flask. The suspension solution

was heated to reflux, and hydrazine monohydrate80% (15 ml) was added dropwise to the mixture over1 h. After a further 8 h of reflux, the resultant clear,darkened solution was filtered hot to remove Pd/C,and the filtrate was distilled to remove some solvent.The obtained mixture was poured into 200 ml ofstirring water, giving rise to a precipitate that wasisolated by filtration. The crude product was puri-fied by column chromatography over silica gel(petroleum ether/ethyl acetate 5:1). The yield ofOAPB was 5.91 g (92%).

The mp is 105.9 �C by DSC. (5 �C/min).electron-impact mass spectrometry provided a value of 292(M+; Calcd. For C18H16N2O2: 292.3). IR (KBr)spectrum indicates absorption peaks at 3458,3376 cm�1 (N–H stretching), 1208 cm�1 (C–Ostretching). 1H NMR(400 MHz, DMSO-d6, ppm)showed signals of different protons at d values of6.89–6.83 (m, 5H), 6.79 (dd, J = 1.6, 7.6 Hz, 1H),6.75–6.72 (m, 3H), 6.58 (d, J = 8.8 Hz, 2H), 6.54–6.50 (m, 1H) 4.92, (s, 2H), 4.89(s, 2H). 13C NMR(100 MHz, DMSO-d6, d, ppm) showed values of153.68, 151.97, 146.51, 144.98, 142.48, 140.04,124.41, 120.12, 119.32, 118.18, 117.98, 116.27,115.61, 114.80.

ELEM. ANAL. Calc. For C18H16N2O2: C, 73.95%;H, 5.52%; N, 9.52%. Found: C, 73.70%; H, 4.65%;N, 9.35%.

2.4. Polymer synthesis

The polyimides were synthesized from variousdianhydrides and diamine OAPB via a two-stepmethod [21]. The synthesis of polyimide PI-1(ODPA–OAPB) was used as an example to illus-trate the general synthetic route used to producethe polyimides. To a solution of 0.4512 g(1.54 mmol) of diamine OAPB in 5.3 ml of CaH2-dried NMP, 0.4777 g (1.54 mmol) of ODPA wasadded in one portion. The solution was stirred atroom temperature under N2 for 24 h to yield a vis-cous polyamic acid (PAA) solution. PAA was con-verted into polyimide using thermal-imidization orchemical-imidization methods. For the thermal-imi-dization method, the PAA solution was cast onto aclean glass plate and heated (80 �C/3 h, 120 �C/30 min, 150 �C/30 min, 180 �C/30 min, 210 �C/30 min, 250 �C/30 min, 300 �C/1 h) to produce afully imidized polyimide film. Chemical imidizationwas carried out by adding an equimolar mixture ofacetic anhydride and pyridine into the aforemen-tioned PAA solution (with magnetic stirring) at


ambient temperature for 30 min and heating at80 �C for 4 h. The polyimide solution was pouredinto methanol. The precipitate was collected by fil-tration, washed thoroughly with methanol, anddried at 80 �C in a vacuum to give the following.

PI-2 (BTDA–OAPB), PI-3 (6FDA–OAPB) andPI-4 (PMDA–OAPB) were synthesized by the simi-lar method described above.

3. Results and discussion

3.1. Monomer synthesis and characterization

The new aromatic unsymmetrical diamine mono-mer, 1,4-bis(20,400-diaminodiphenoxy)benzene (OAPB),was successfully synthesized by using hydroquinoneas starting material, as shown in Scheme 1. Firstly,4-(4-nitrophenoxy)phenol (NPP) was synthesizedthrough the nucleophilic etherification of 4-chloro-nitrobenzene with potassium phenolate of hydro-quinone in dimethylformamide. Then 1,4-(2 0,400-dinitrodiphenoxy)benzene (ONPB) was synthe-sized by nucleophilic substitution reaction of NPPand o-chloronitrobenzene in the presence of potas-sium carbonate in DMF. Finally, ONPB wasconverted to the corresponding diamine monomerOAPB by hydrazine Pd/C-catalyzed reduction.The structures of OAPB and the intermediates wereconfirmed by elemental analysis, IR spectra, and 1HNMR and 13C NMR spectroscopy. In the IR spec-tra, diamine OAPB exhibited characteristic absorp-tions of amino and ether groups at 3458, 3376 and1208 cm�1, respectively. Elemental analysis is alsoin agreement with the unsymmetrical ether diamineOAPB. The 1H and 13C NMR spectrum also sup-ported the formation of a desired compound havingthe proposed structure. As shown in Fig. 2a and b,each proton and carbon was assigned to the NMRspectra of the diamine monomer. All the spectro-scopic data obtained were in good agreement withthe expected structures. The enhanced processabilityof the fully imidized polyimides is expected due to

Scheme 1. Synthesis of 1,4-(2 0,400-

the improved solubility of the polymers resultingfrom the presence of ether linkages, geometricallyasymmetric diamine components and ortho-linkedphenylene units.

3.2. Polymer synthesis and solubility

New polyimides were prepared from OAPB andcommercially available aromatic dianhydrides, suchas PMDA, BTDA, ODPA, and 6FDA, via a con-ventional two-step procedure as shown in Scheme2. The polymerization was carried out by reactingstoichiometric amounts of diamine monomer OAPBwith aromatic dianhydrides at a concentration of15% solids in N-methylpyrrolidone (NMP). Thering-opening polyaddition at room temperaturefor 24 h yielded poly(amic acid)s, followed bysequential heating to 300 �C or mixture of Ac2O/Py to obtain the corresponding polymers. Transfor-mation from poly(amic acid) to polyimide was pos-sible via the thermal or chemical cyclodehydration;merits of the former were easy to handle and to castinto thin film, and the latter was suited to preparesoluble polyimides.

The chemical structures of polyimides were char-acterized by FT-IR, 1H NMR and element analysis.All of the polyimides showed characteristic imideabsorption bands at 1782–1785 cm�1 attributed tothe asymmetrical carbonyl stretching vibrations,and at 1720–1730 cm�1 attributed to the symmetri-cal carbonyl stretching vibrations. The absorptionat 1370–1380 cm�1 was assigned to C–N stretching,and the C–O multiple stretching absorptions werealso detected in the range of 1300–1100 cm�1. Therewas no existence of the characteristic absorptionbands of the amide group near 3200–3363 (N–Hstretching) and 1650–1674 (C=O stretching) cm�1,indicating polymers had been fully imidized. Inaddition to the IR spectra, the elemental analysisvalues of the polymers generally agreed well withthe calculated values for the proposed structures.Fig. 3 shows the high-resolution 1H NMR spectrum

diaminodiphenoxy)benzene.

Fig. 2. NMR spectra of OAPB: (a) 1H NMR; and (b) 13C NMR.


of PI-1 derived from OAPB and ODPA by chemicalimidization, in which all the protons in the polymerbackbone can be assigned. Table 1 shows theelement analysis data for the unsymmetrical poly-imides, which are in good agreement with the calcu-lated values for the proposed chemical structures.Since all polyimides was soluble in DMAc, intrinsicviscosities were evaluated in DMAc at 30 �C with anUbbelohde viscometer and the results fell within the0.38–0.58 dL/g range. The above results demon-strate that the diamine monomer OAPB holds agood polymerization activity to form polyimides,

meanwhile, 100% chemical imidization could beachieved at lower temperatures, and this should besuitable to soluble polyimide.

The solubilities of the resulting polyimides bychemical imidization were investigated for the sam-ples in different organic solvents. The solubilitybehavior of these polymers in different solvents ispresented in Table 2. These polymers exhibited verygood solubility behavior in polar solvents such asDMSO, DMF, N,N-dimethyl acetamide (DMAc),and N-methyl-2-pyrrolidinone. Upon heating, theycould also be dissolved in common solvents such

Scheme 2. Synthesis of the unsymmetrical polyimides.

Fig. 3. 1H NMR spectra of PI-1 (DMSO-d6).


as chloroform and toluene. The solubility of theunsymmetrical polyimides depended on, to someextent, their chemical structures. The good solubil-ity might be attributed to their more bent chainstructure of polyimides resulted from the nonlinear

and asymmetric structure provided by OAPB. The2-position of the unsymmetrical diamine is occupiedby amino group, furthermore, the imide groupsinhabit interchain interaction and chain packing,thus increasing solubility. It should be noted that

Table 1Inherent viscosity and element analysis of the polyimides

Polyimides code PI ginha (dL/g) Elemental analysis (%) of polyimidesb

Formula of PI (formula weight) C H N

PI-1 0.58 (C34H18N2O7)n Calc. 72.08 3.20 4.94(566.53)n Found 71.24 2.99 4.88


PI-3 0.38 (C37H18N2O6F6)n Calc. 63.44 2.59 4.00(700.55)n Found 63.20 2.38 4.10


a Inherent viscosity (g) determined on 0.5% solutions in a solvent (DMAc) at 30 �C.b Obtained by thermal cyclization from the corresponding poly(amic acid).

Table 2Solubility data of the polyimides

Solventa polymerb

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

NMP ++ ++ ++ ++DMAc ++ ++ ++ ++DMF ++ ++ ++ ++DMSO ++ ++ ++ ++THF + – + –CHCl3 ++ + ++ –Toluene + + ++ +Acetone – – + –

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

PAAs. ++: soluble at room temperature; +: soluble on heating,and –: insoluble even on heating.


good solubility in low boiling point solvents is thecritical to prepare polyimide films or coatings,which is desirable for advanced microelectronicsmanufacturing applications. Comparing with thoseobtained by chemical imidization, the poor solubil-ity of PI obtained by thermal imidization was possi-bly due to the presence of partial inter molecularcrosslinking during the thermal imidization stage.

3.3. Mechanical properties

All of the polyimides could be processed intoclear, flexible, and tough films. These films weresubjected to tensile tests. Table 3 shows the mechan-ical properties of the unsymmetrical polyimides,including the tensile strength, tensile modulus aswell as elongation at breakage. The polyimide filmshad tensile strength of 41.4–108.5 MPa, elongationsat breakage of 5–13%, and tensile moduli of 1.1–1.6 GPa, which indicated strong and toughmaterials.

3.4. Thermal properties

DSC and TGA methods applied to evaluate thethermal properties of the polyimides, TGA curvesof the polyimides are shown in Fig. 4, and thermalanalysis data from the TGA and DSC curves ofthe polyimides are summarized in Table 4. DSCrevealed rapid cooling from 400 �C to room temper-ature produced predominantly amorphous samples,so that the Tg of polymer could be easily read in thesecond-heating trace of DSC. The Tg values of PI-1–PI-4 were in the range of 233–299 �C, and no melt-ing endotherms were observed in the DSC traces.The observations revealed the amorphous natureof the polymers. The crystallinity of the polyimideswas further evaluated by WAXD. All the polymersexhibited amorphous patterns. The amorphous nat-ure of the polyimides could be attributed to theirasymmetric structural units and aryl ether linkage,which decreased the intra- and inter-polymer chaininteraction, resulting in loose polymer chain pack-aging and aggregates.

As we expected, the Tg values of these PIsdepended on the structure of the dianhydride com-ponent, and decreased with increasing flexibility ofthe polyimides backbones based on the appliedstructure of dianhydride. PI-1 obtained from ODPAshowed a lower Tg, because of the presence of a flex-ible ether linkage between the phthalimide units. PI-4 derived from PMDA exhibited the highest Tg dueto the rigid pyromellitimide unit. (ortho-aminophen-oxy)benzene residues force neighbouring phthali-mide units together, restricting chain flexibilitywhich reverses the trend that ortho-catenated unitsreduce Tg.

For the thermal stability of the polyimides, Table4 gives the temperature of the initial decomposition,

Table 3Tensile properties of polyimide films

Polymera Tensile strength (Mpa) Elongation at break (%) Initial modulus (Gpa)

PI-1 102.5 9 1.68PI-2 108.5 13 1.44PI-3 41.4 5 1.15PI-4 101.4 8 1.62

a Polyimide film samples were obtained by thermal imidization method.

Fig. 4. TGA curves of the unsymmetrical polyimides.

Table 4Thermal properties of the polyimides

Polymer Tga (�C) Td

b (�C) Weight loss(�C) Char yieldc (%)

Td5d Td10

d

PI-1 230.2 549.4 528.8 560.2 54.5PI-2 244.2 548.5 505.8 540.2 49.2PI-3 253.1 519.9 525.0 539.8 46.6PI-4 299.0 562.0 515.2 558.8 40.2

a Glass transition temperature determined by DSC.b Onset decomposition temperature.c Residual weight retention at 700 �C.d 5 and 10% weight loss temperature measured by TGA in N2.


5% and 10% gravimetric loss in nitrogen, i.e. Td, T5

and T10. Td were determined in the range of 519–562 �C for all of polyimides tested. T5 and T10 ofthe polyimides reached 505–528 �C and 539–560 �C in nitrogen, which is an important evidencefor thermal stability. Also char yields at 700 �Cfor all polymers were in the range of 40.2–54.5 wt%. Obviously, the data from thermal analy-

sis show that the resulting polyimides have fairlyhigh thermal stability.

4. Conclusions

A novel unsymmetrical diamine was synthesizedand characterized. A series of polyimides wereobtained from the diamine monomer with various


aromatic dianhydrides by two-step thermal orchemical imidization methods. Experimental resultsindicated that the resulting polymers showed verygood solubility in different organic solvents andcould be cast into transparent, tough, and flexiblefilms. These polymers also have good thermal stabil-ity and mechanical properties. Excellent solubility,moderate Tg value may be attributed to the morebent chain structure of polyimides resulted fromthe nonlinear and asymmetric structure providedby OAPB.

Acknowledgement

The authors acknowledge the financial supportof the Research Foundation of the State Key Labo-ratory of Applied Organic Chemistry.

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