synthesis and evaluation of properties of novel poly ... en pdf/2008/jpspc-2008-46... · synthesis...

Synthesis and Evaluation of Properties of NovelPoly(benzimidazole-amide)s

JUAN J. FERREIRO,1 JOSE G. DE LA CAMPA,1 ANGEL E. LOZANO,1 J. DE ABAJO,1 JACK PRESTON2

1Instituto de Ciencia y Tecnologıa de Polımeros, Consejo Superior de Investigaciones Cientıficas.Juan de la Cierva 3, Madrid 28006, Spain

2College of Textiles, Department of Textile Engineering, Chemistry and Science,North Carolina State University, Raleigh, North Carolina 27695-8301

Received 11 July 2008; accepted 27 August 2008DOI: 10.1002/pola.23049Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Novel aromatic polyamides have been prepared by a combination of diac-ids containing preformed benzimidazole rings and aromatic diamines. By the phos-phorylation method of polycondensation, polymers of high molecular weight (inherentviscosities between 0.81 and 2.13 dL/g) were obtained, which showed good solubilityin polar aprotic solvents. The combination of aromatic amide linkages and benzimid-azole rings along the polymer chain endowed the polymers with high thermal resist-ance and excellent mechanical properties. Glass transition temperatures fell in therange of 290–330 �C as measured by differential scanning calorimetry, and initialdecomposition temperatures under nitrogen were over 480 �C as measured by ther-mogravimetric analysis. Some polymer films showed outstanding tensile strength(over 150 MPa) and moduli (up to 5 GPa). The presence of benzimidazole rings in thecurrent polyamides greatly enhanced their hydrophilicity in comparison with clas-sical wholly aromatic polyamides; thus, although aromatic polyamide films normallyshow water sorption values of only 4–8%, some of the current poly(benzimidazoleamide)s show water sorption up to 19% in a 65% relative humidity atmosphere. VVC 2008

Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7566–7577, 2008

Keywords: benzimidazole; heteroatom-containing polymers; high temperaturematerials; hydrophilic polymers; mechanical properties; monomer synthesis;monomers; polyamides; properties; water sorption

INTRODUCTION

Aromatic polyamides are a class of polymers thatshow a combination of outstanding properties.They have excellent thermal resistance, withglass transition temperatures over 250 �C and ini-tial decomposition temperatures over 450 �C asmeasured by dynamic thermogravimetric analysis(TGA). In film and fiber form, they exhibit very

high tensile strengths and moduli. Furthermore,some of them, for example polyisophthalamides,show relatively good processability from solution.

Aromatic polyheterocycles, such as polyimides,polyquinoxalines, and polybenzimidazoles (PBI),have even higher thermal resistance than poly-amides, showing in some cases initial decomposi-tion temperatures around 600 �C by TGA.1–3

However, most of them present poor solubilityand tractability (they are rather insoluble and donot melt without decomposition), which limitstheir application in many fields.

Consequently, attempts have been made tocombine heterocycles with other stable linkages

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 46, 7566–7577 (2008)VVC 2008 Wiley Periodicals, Inc.

Correspondence to: J. de Abajo (E-mail: [email protected])

7566

along the polymer backbone to obtain more tracta-ble materials. In this way, many polyarylenes con-taining heterocyclic units, poly(amide-imide)s,4,5

poly(ether-benzimidazole)s,6 poly(amide-benzoxa-zoles)s,7,8 and poly(ester-benzoxazoles)s9 havebeen reported previously.

Poly(benzimidazole-amide)s have also beenreported. They were first investigated as high-strength, high heat-resistant fibers, for whichthey were prepared from linear monomers to yieldquasi rod-like, high-molecular weight species.10

These polymers actually showed excellent fiber-forming abilities and high mechanical and ther-mal resistance. Poly(benzimidazole-imide)s werealso reported as early as 1967,11 and some otherpapers on this class of polymers have appearedduring the last two decades.12,13

PBIs were discovered by Marvel and coworkersmore than 40 years ago, and they received specialattention in the 1960s and 1970s just because oftheir outstanding thermal, chemical, and mechan-ical properties. However, their high cost and theirability to absorb water gradually reduced theirtechnical interest in the following decades. None-theless, PBIs have been the object of many scien-tific and application studies since then.14,15

Furthermore, PBIs are at present being evaluatedas proton exchange membranes, an applicationwhere high water uptake and retention is a man-datory condition.16,17 Polyamides with benzimida-zole pendent groups have also been reported, andan improved hydrophilicity has been observed forthese polyamides with respect to unmodified poly-amides.18,19 The investigation of novel polymermaterials of improved hydrophilicity is well justi-fied since there is a growing interest in specialapplications where water is present, such aswater purification by means of membrane tech-nologies, drying of air or other vapors and gases,or high-temperature polymer membranes for pro-ton exchange fuel cells.

In previous papers, the preparation and char-acterization of several poly(ether-amide)s contain-ing benzimidazole rings have been reported.19,20

It was observed that the presence of benzimida-zoles greatly enhanced the inherent ability of poly-amides to take up water. It was also stated thatoxyethylene short sequences incorporated in thepolyamide chains brought about a decrease ofwater sorption, because the ether linkages com-pete with water molecules for hydrogen bondingwith amide groups of the polymers. Thus, itappeared interesting to investigate the generalproperties of amide-benzimidazole polymers and,

particularly, their behavior as regards waterabsorption.

In this article, the syntheses of diacids contain-ing benzimidazole groups are reported along withthe preparation of polyamides from them andsuitable commercial diamines. These diacids weredesigned with the aim of providing good physicalproperties to the polymers, in particular good me-chanical and thermal properties and a markedhydrophilic character at the same time. Conven-ient monomer combinations were outlined toobtain polymers that could be dissolved in polarorganic media, as solution casting is the onlysuitable way to process these rather stiff chainpolymers.

EXPERIMENTAL

Materials and Intermediates

Solvents and starting materials, for example, 3,4-diaminobenzoic acid, 4-cyanobenzoic acid methylester, terephthalaldehyde, and isophthalaldehyde(Aldrich) were used without purification. Tri-phenylphosphite (TPP; Aldrich) was distilled underreduced pressure over calcium hydride just beforeuse. Reagent grade lithium chloride was driedat 300 �C before use. N-Methyl-2-pyrrolidinone(NMP) was purified by distillation twice underreduced pressure over calcium hydride andstored over molecular sieves (4 A). Pyridine wasrefluxed over potassium hydroxide overnight,distilled twice on KOH at atmospheric pressureand stored over molecular sieves (4 A).

The diamines, m-phenylendiamine, MPD(Aldrich), and 4,40-diaminobenzanilide, DAB(Aldrich) were purified by sublimation just beforeuse. 3,30-Diaminobenzanilide, 3DAB, was synthe-sized and purified in our laboratory according tothe method previously reported.21

Benzimidazole Monomer Synthesis

4-[5(6)-Carboxy-1H-benzimidazole-2-yl]-benzoicAcid (4BI)

The preparation of this monomer involved severalsteps of synthesis. First, 3,4-diamino-methylben-zoate was prepared in high yield from 3,4-diami-nobenzoic acid and methanol in the usual manner,by treatment with excess methanol at reflux inthe presence of a catalytic amount of sulfuric acid.The other main reactant was the chlorhydrate of4-(ethoxyimino) benzoic acid methyl ester, which

SYNTHESIS AND PROPERTIES OF POLY(BENZIMIDAZOLE-AMIDE)S 7567

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

was prepared from 4-cyano methylbenzoate andethanol in a solution of toluene by continuouspassing of HCl into the solution for 24 h. Fromthe two intermediates, a dimethyl ester precursor,2-(4-methoxycarbonylphenyl) benzimidazole-5-carboxy methyl ester, was prepared in the follow-ing manner: A 250-mL flask was charged with 4-[ethoxy(imino)methyl] methylbenzoate hydro-chloride (10.2 g, 0.042 mol), 3,4-diamino-methyl-benzoate (7.0 g, 0.042 mol), and 140 mL methanol.Then, the temperature was raised to reflux andthe reaction allowed to proceed for 1 h. After cool-ing to room temperature, the solid which hadformed was filtered off and dried in a vacuumoven for 24 h at 90 �C. The yield of crude materialwas nearly quantitative, but was decreased to85% after crystallization from methanol/water (9/1). mp 238 �C (unc.).

Analysis (C17H14N2 O4, 310.31): Calc.: C, 65.80;H, 4.55; N, 9.03. Found: C, 65.71, H, 4.30, N, 8.90.

To a stirred solution of the dimethyl ester pre-cursor (10.0 g, 0.032 mmol) in 135 mL ethanol, 90mL was added of a 25% NaOH solution, and thetemperature was raised to reflux. After 3 h, meth-anol and water were removed in a rotavaporator,and the dry residue was dissolved in 30 mL ofwater. Acetic acid was added dropwise to the solu-tion until it was acid, forming a white precipitatethat was collected by filtration. The latter wasthoroughly washed with water and dried in a vac-uum oven for 24 h at 100 �C. Yield was 96%.Upon recrystallization from water/DMF, a crystal-line, white powder of mp: 370 �C (DSC) wasobtained.

Analysis (C15H10N2O4, 282.26): Calc.: C, 63.83;H, 3.57; N, 9.92. Found: C, 63.58; H, 3.40; N, 9.89.

1,4-Bis(5-carboxy-1H-benzimidazole-2-yl)Benzene (p-DBI) and 1,3-Bis(5-carboxy-1H-benzimidazole-2-yl) Benzene (m-DBI)

These two bisbenzimidazole monomers were pre-pared by the same general method from 3,4-dia-minobenzoic acid and terephthalaldehyde or iso-phthalaldehyde. The general synthetic route wasas follows.

In a 500-mL flask, a solution of the dialdehyde(5.0 g, 0.037 mol) in 90 mL of 40% aqueous so-dium hydrogen sulfite was prepared by magneticstirring for 2 h. Then, a suspension of 3,4-diami-nobenzoic acid in 200 mL ethanol was added, andthe temperature was raised to reflux and main-tained for 3 h. After cooling, the solid was filtered

off, washed with water three times and driedunder vacuum for 24 h at 100 �C.

Average yield of several batches was 90% form-DBI and 76% for p-DBI; the first was recrys-tallized from DMF (mp. 354 �C, DSC) and the sec-ond from a mixture water/DMF (1/9), mp. 378 �C(DSC).

Analysis for m-DBI (C22H14N4O4, 398.38):Calc.: C, 66.33; H, 3.54; N, 14.06. Found: C, 66.12,H, 3.80, N, 13.95.

Analysis for p-DBI (C22H14N4O4, 398.38):Calc.: C, 66.33; H, 3.54; N, 14.06. Found: C, 66.22;H, 3.70; N, 14.00.

Polymer Synthesis

All of the polyamides reported here were preparedby the phosphorylation method of polycondensa-tion. An illustrative example is described indetail.

A 100-mL three-necked glass flask fitted withmechanical stirring was purged for 30 min with aflow of dry nitrogen. The flask was charged withdiacid (10.0 mmol), 20 mL NMP, 6 mL pyridine,and 1.4 g LiCl. Once the mixture became totallydissolved, diamine (10.0 mmol) and TPP (10.0mmol) were added and the temperature wasraised to 110 �C and held for 4 h under a gentleflow of dry nitrogen. Upon cooling to room temper-ature, the viscous polymer solution was droppedinto a large volume of ethanol giving rise to apearl-like polymer precipitate. In some cases, sol-vent had to be added to dilute the solutions beforeprecipitation because of their very high viscosity.The poly(benzimidazole-amide)s were washed sev-eral times with ethanol, extracted with ethanol ina Soxhlet for 18–24 h, and finally dried in a vac-uum oven for 24 h at 100 �C. Yields over 95%were obtained in all cases.

Measurements

Elemental analyses were performed with a CarloErba EA1108 elemental analyzer.

Melting points of intermediates and monomerswere measured in a Buchi apparatus, and occa-sionally by differential scanning calorimetry(DSC) on a Perkin–Elmer DSC-7 device, undernitrogen at a heating rate of 5 �C/min.

FTIR spectra were recorded on a Perkin–ElmerRX-1 device, on KBr pellets for monomers andintermediates, and on films of a few micronswidth for polymers. An attenuated total reflection

7568 FERREIRO ET AL.


accessory (ATR) was occasionally used for thelatter as well.

1H and 13C NMR spectra were recorded on aVarian-Innova 300 spectrometer tuned at 299.95and 75.43 MHz, respectively.

Viscosities were measured on 0.5 g/dLNMP solutions at 25 � 0.1 �C in an Ubbelohdeviscometer.

Qualitative solubility was determined using 10mg of polymer in 1 mL of solvent at room temper-ature. Samples that did not dissolve after stirringat room temperature for 24 h were heated to sol-vent boiling temperature.

DSC and TGA were made with Perkin–Elmeranalyzers DSC-7 and TGA-7, respectively. All ofthe samples were tested under nitrogen at a heat-ing rate of 20 �C/min for DSC and 10 �C/min forTGA. Curves were registered at the second run,after a first one up to 250 �C to eliminate absorbedwater.

Measurements of mechanical properties wereconducted on a MTS Synergie 200 Universal Test-ing dynamometer of vertical extension on 30 mmlength, 5 mm width film strips, using mechanicalclamps, with an initial separation of 10 mm and10 mm/min extension rate.

Powdered polymer samples of 200–300 mg, pre-viously dried at 120 �C over phosphorus pentoxidefor 24 h in a vacuum oven, were placed in a boxcontaining a supersaturated solution of sodiumnitrite at 25 �C, which provided a relative humid-ity of 65%. The samples were periodically weighedthrough 48 h, at which time they had equilibratedwith their surrounding, as indicated by no weightchange.

Molecular Simulation

Molecular simulations were performed by meansof the Materials Studio suite of Accelrys Inc.22

The cells were constructed with the amorphouscell module, starting with a density of 0.1 g/mLand applying constant pressure molecular dynam-ics to compress the cell until the correspondingvolume. The Discover module was used to performthe molecular dynamics simulations, using peri-odic boundary conditions with a cut off of 12.5 Afor the van der Waals and Coulomb nonbond pa-rameters and a spline of 3 A. The polymer consist-ent force field (pcff)23 was used to minimize thestructures and in the molecular dynamics simula-tions.

RESULTS AND DISCUSSION

Monomers and Polymers

For the preparation of polyamides containingbenzimidazole rings, suitable diacid monomerswere first synthesized, using available methodswhich in some cases had to be optimized, particu-larly as regards to purification methods, becauseof the severe requirements of purity for condensa-tion monomers.

The preparation of 2-phenyl-benzimidazolescan be carried out by the traditional route fromortho-phenylenediamines and aromatic acids inan acid medium at high temperature (Phillipsreaction). This method, which is useful for benzoa-zoles in general, involves the use of very strongcyclodehydrating reactants, such as polyphos-phoric acid24,25 or mixtures P2O5/CH3SO3H.24,26,27

In fact, synthesis of the monomer 4BI, 4-[5(6)-car-boxy-1H-benzimidazole-2-yl]-benzoic acid, wasattempted by this method, through the prepara-tion of the intermediate 4(5)-methyl-1H-2-(4-methyl-phenyl) benzimidazole. However, the labo-rious purification of the intermediate and the dif-ficulties encountered to achieve a reasonable yieldof the diacid monomer by oxidation of the di-methyl intermediate made it advisable to look foranother more convenient synthetic route. Thus,monomer 4BI was attained by an elegant, high-yield synthetic route, which first led to a dimethylester precursor starting from 3,4-diaminobenzoicacid methyl ester and the chlorhydrate of 4-(ethoxyimino) benzoic acid methyl ester. The syn-thetic route is depicted in Scheme 1. The route tothis monomer can be recommended not onlybecause of the high yield but also because it is amuch cleaner route than the traditional methodsused in the preparation of benzimidazoles by theaction of acid cyclodehydrating agents at hightemperature. In fact, on reacting ortho-phenylene-diamine and benzoic acid at high temperature,the diamine often competes for the protons of theacid catalyst and this can inhibit attack at thecarbonyl carbon, so that it is convenient in manyinstances to replace the carbonyl oxygen with themore basic imino group.

The bisbenzimidazole diacids m-DBI and p-DBI could be synthesized by a facile, high-yieldmethod, from 3,4-diaminobenzoic acid and iso-phthalaldehyde and terephthalaldehyde, respec-tively. Scheme 2 shows the general route to mono-mers m-DBI and p-DBI. This aldehyde route tobenzimidazoles is particularly suited to the



synthesis of compounds that show a relativelyhigh concentration of aromatics, as is the case ofmonomers m-DBI and p-DBI.28

All three of the monomers could be isolated inhigh purity. They were identified by FT infraredspectroscopy, nuclear magnetic resonance, andelemental analysis. As an example, the 1H and13C NMR spectra of the diacid monomer 4BI havebeen reproduced in Figures 1 and 2.

All of the polyamides were prepared by the syn-thetic route outlined in Scheme 3. The phospho-

rylation method first described by Yamazaki andHigashi29,30 was used as a general approach forall of the polymers, using the system TPP-pyri-dine as a condensing promoter and LiCl as solu-bility enhancer. The polycondensation reactionsproceeded in homogeneous solutions and highyields and relatively high molecular weights wereachieved.

Table 1 shows the inherent viscosities (ginh)measured in NMP. They all were well above 0.8dL/g, which indicates high molecular weight in

Scheme 2. Synthesis of monomers m-DBI and p-DBI.

Scheme 1. Synthesis of monomer 4BI.



every case. In fact, the high molecular weight wasalso confirmed because tough films could beobtained from the polymers that were soluble bycasting from NMP solutions. Polymers were char-acterized by their spectroscopic features, usingFTIR and NMR spectroscopies. Figure 3 reprodu-ces the 1H spectrum of polymer 4BI-MPD. As wasalso the case for the rest of polymers, chemicalshifts could be readily ascribed to the signal

observed in the spectra, which did not showstrange signal attributable to solvent, impurities,or end groups.

Polymer Properties

Most of the polyamides were soluble at roomtemperature in organic aprotic solvents, such asNMP, N,N-dimethylacetamide (DMA), or dimethyl

Figure 1. 1H NMR spectrum of monomer 4BI.

Figure 2. 13C spectrum of monomer 4BI.



sufoxide, at concentrations over 10% w/v (Table1). However, all of them were resistant to com-mon solvents such as tetrahydrofuran, chloro-form, or alcohols. They all were insoluble even inm-cresol, which was an unexpected resultbecause the acidic nature of m-cresol should havehelped to solvate both amide groups and imidaz-ole groups at the same time. Only ordered polya-

mides were prepared from DAB and the symmet-ric monomersm-DBI and p-DBI resulting in poly-mers that were insoluble in aprotic polarsolvents. This result agrees fairly well with thechemical composition of the polymers. Monomer4BI is an asymmetric monomer that introducesinto a polymer chain an element of irregularityand a less effective molecular packing through

Scheme 3. Synthesis of poly(benzimidazole amide)s.

Table 1. Properties of Poly(benzimidazole amide)s

Polymer ginh (dL/g) Tg (�C) Td (�C)

Solubility

NMP DMA DMF m-Cresol

4BI-MPD 1.3 325 500 þþ þþ þþ �4BI-DAB 2.3 295 495 þþ þþ þ �4BI-3DAB 1.1 292 495 þþ þþ þþ �p-DBI-MPD 1.3 294 490 þ þ þþ �p-DBI-DAB � 300 495 � � � �p-DBI-3DAB 0.8 312 480 þþ þþ þþ �m-DBI-MPD 1.0 304 480 þþ þþ þþ �m-DBI-DAB � 322 490 � � � �m-DBI-3DAB 0.9 331 480 þþ þþ þþ �

þþ, Soluble at room temperature; þ, soluble on heating; �, insoluble.



the formation of head to head, head to tail, andtail to tail sequences. Therefore, all polymersfrom 4BI showed good solubility, as was also thecase for polymers from 3DAB. As a rule, poly-mers from 3DAB are more soluble than thosederived from DAB, which is consistent with thehigher relative linearity and rigidity of diamineDAB. Furthermore, the diamine 3DAB showsfour possible conformations, which also favor mo-lecular mobility and solubility.31

A combination of meta- and para-orientedphenylene rings are present in the current poly-mers, which does not help for the formation of lyo-tropic dopes. Thus, no liquid crystalline polymer/solvent system was observed in the manipulationof solutions, even for polymer p-DBI-DAB, whichhas all-para-oriented phenylene rings.

Glass transition temperatures (Tg), listed in Ta-ble 1, were in the range 290–330 �C. Thus, it wasdemonstrated again that the combination of aro-matic amide and benzimidazole units within apolymer chain led to high Tg polymers. It shouldbe noted that slightly higher Tgs were found forpolymers derived from monomer m-DBI, thanthose found for polymers from p-DBI. This un-usual behavior may be justified by consideringrotational energy barriers as m-phenylenes cancertainly hinder the rotation of the backbone in agreater extension than the p-phenylenes. To an-swer the question as to whether m-phenylenelinkages can actually account for less mobility

and higher Tg than p-linkages, a computer-assisted calculation has been carried out of rota-tional barriers and of the statistical probability ofthe isomeric systems m-DBI-MPD and p-DBI-MPD.

A study of the rotational barriers of the isolatedrepeat units centered in m- and p-substitutedrings did not show any difference in both cases,because the rotation around one bond is not influ-enced by the position, meta or para, of the otherone. The differences between both cases have tobe explained by the different conformational spaceoccupied by the repeat units when the rotationsare made around meta or para phenylene rings.As it can be seen in Figure 4, the rotation aroundthe benzimidazole-phenyl bond occupies a greatamount of conformational space in the m-isomer,whereas the space is much smaller in the case ofthe p-isomer. Therefore, in the bulk, a greatamount of cooperation in the movement of thechain is needed to allow the rotation in the metaisomer. To visualize the differences between bothisomers, a molecular dynamics study was per-formed on both polymers. To do this, two cells con-taining the m- and p-polymers (20 repeat units),respectively, were created with a density of 1.30 g/mL. Both cells were submitted to a NVT (constantnumber of moles, volume, and temperature ¼ 500K) molecular dynamics for 300 ps, with a timestep of 1 fs (300,000 time steps). The dihedralangles formed by N¼¼CAC¼¼C (1-2-3-4 in Fig. 4)

Figure 3. 1H NMR spectrum of polymer 4BI-MPD.



between the imidazole and the phenyl rings weremeasured for all the repeat units in the initialconformation and in the final one. Figures 5 and 6show the distribution of initial and final dihedralangles for m- and p-polymers. As it can be seen,this distribution is almost the same in the m-poly-mer, what indicates that its mobility is very lowand the chain does not suffer any significantchange during 300 ps at 500 K. However, in the p-polymer, the mobility is clearly enhanced and asignificant number of flips take place. This can beseen in the figure because several repeat unitsadopt conformations with the dihedral anglehigher than 90�.

Having p-oriented and m-oriented isomericamorphous polymers, the intermolecular forces,mostly due to hydrogen bonds, can be presumedto be rather equivalent; thereby rigidity (rota-tional freedom) should be the dominant influenceon the glass transition temperature. These resultsare consistent with previously reported diffusionmeasurements, where it was demonstrated thatgases diffuse faster through p-linked amorphous,aromatic polymers than through m-linked iso-mers. This means that p-oriented polymers offer ahigher mobility and a higher effective free volumeas they have lower chain stiffness than the corre-sponding m-oriented isomers.32,33 It should be

stressed that these results apply well only fordiheterocyclic symmetrical monomers; asymmet-rical monomers, such as 4BI, do fit the generalrule and para phenylenes provide higher Tg poly-mers than meta-oriented phenylenes.

As to thermal resistance, as measured by TGA,the list of decomposition temperatures shown inTable 1 indicates that all of the polymers begin todecompose in the same temperature range,around 480–490 �C, regardless of the monomer

Figure 4. Rotation around the benzimidazole-phenyl bond.

Figure 5. Distribution of initial dihedral angles form- and p-polymers.

Figure 6. Distribution of final dihedral angles form- and p-polymers.



combinations. PBI are among the most heat-re-sistant polyheterocycles, and initial decompositiontemperatures have been reported around 600 �Cfor them.13,14 Therefore, polyamides from m-DBIand p-DBI should have shown higher Tds thanthose from 4BI, as the former have two benzimid-azole groups per repeating unit and the latterhave only one benzimidazole per repeating unit;however, they did not. At present, no reliable ex-planation has been formulated for this unex-pected result. Only a slight difference could beobserved in the char yield at 800 �C, which was75–78% for polymers prepared from the symmet-ric bisbenzimidazole monomers and less than 75%for polymers from 4BI (Figure 7).

The tensile strength and the Young’s modulusof the polyamides (Table 2), ranging between 115and 170 MPa and 2.8–5.1 GPa, respectively, canbe considered as outstanding for unoriented poly-amide films made by casting at a laboratory scalewithout any post-treatment. This means that theincorporation of benzimidazole groups greatlyimproved the mechanical properties of aromaticpolyamides, which generally do not exhibit tensilestrength over 90–100 MPa, with moduli nothigher than 2.0–2.2 GPa.25,26,34 Exceptionallyhigh are the resistance and the modulus of poly-mer 4BI-DBA, which has a great density of p-ori-ented moieties and a very high molecular weight(ginh 2.3 dL/g). This is an indication of great poten-tial for high-strength, high-modulus fibers andcomposites. Comparatively high values of elonga-tion at break may be caused by water absorbed.

As can be seen in Table 2, the water uptake isquite high, much higher than that of classical aro-matic polyamides. That means that the incorpora-tion of benzimidazole units greatly increased theaffinity of polyamides for water. This characteris-

tic is very attractive for some specific applications,such as water filtration membranes, thermallystable man-made fibers, or moisture sensors. Fur-thermore, benzimidazoles seem to be unique inthis respect since homologous polyamides contain-ing benzoxazole instead of benzimidazole moietiesshow a much lower affinity for water.26 If the ratiobenzimidazole/amide is considered, one canobserve that, as a rule, the greater the density ofbenzimidazole groups, the greater is the ability toabsorb water. This observation confirms previousresults where the beneficial presence of benzimi-dazoles on the water uptake of sequenced poly-amides had been observed.18,19 Moreover, meas-ured values of water uptake over 15% for many ofthe current polymers qualify them as actuallyvery hydrophilic. Especially high is the watersorption of polymers p-DBI-MPD (19.7%) and

Table 2. Water Uptake and Mechanical Properties of Poly(benzimidazole amide)s

PolymerWater

Uptake (%)Tensile

Strength (MPa)Modulus(GPa) Elongation (%)

4BI-MPD 15.3 152 2.9 304BI-DAB 8.1 171 5.1 6.74BI-3DAB 13.2 144 2.9 11p-DBI-MPD 19.7 143 3.1 19p-DBI-DAB 13.5 – – –p-DBI-3DAB 17.4 125 3.3 14m-DBI-MPD 18.0 116 2.8 9m-DBI-DAB 14.9 – – –m-DBI-3DAB 15.0 129 3.0 9

Figure 7. TGA curves of poly(benzimidazole amide)sin N2 at 10 �/min. [Color figure can be viewed in theonline issue, which is available at www.interscience.wiley.com.]



m-DBI/-MPD (18%), both of which have the great-est ratio of benzimidazole/amide. As a matter offact, the values are in the same range of wateruptake reported for pure PBI. Polymers from 4BI,which have the lowest ratio benzimidazole/amideshowed the lowest ability to absorb water, butthey also showed comparatively high values ofwater uptake (8.1–13.2%) with respect to conven-tional wholly aromatic polyamides. The lowerwater uptake exhibited by polymers preparedfrom 4BI seems to indicate a more efficient molec-ular packing, which would provide higher me-chanical resistance and less elongation at yieldthan for p-DBI and m-DBI polymers, whichactually does happen.

CONCLUSIONS

Convenient synthetic routes have been optimizedto prepare three new dicarboxylic acids containingbenzimidazole groups, in high yield and highpurity. They have been used as condensationmonomers, in combination with three aromaticdiamines, to yield novel polyamides containingbenzimidazole heterocyclic rings. The direct poly-condensation of aromatic dicarboxylic acids anddiamines has proved to be a very convenientmethod for the preparation of polyamides in highyield and high molecular weight. The combinationof aromatic amide and benzimidazole groupsalong the polymer backbone endow the currentpolymers with some special characteristics, suchas good solubility in organic aprotic solvents, highglass transition temperatures (up to 330 �C), goodthermal resistance, and excellent mechanicalproperties. The nature of the monomers and theirspecial chemical composition can be related withthe individual behavior of each polymer; thus,meta-substitution in symmetric bisbenzimidazolemonomers provides a higher Tg than para-substi-tution, while para-substitution provides a lowersolubility than meta-substitution. Poly(benzimida-zole-amide)s from monomer 4BI show a greatpotential for high-modulus materials, muchhigher than for polymers from the symmetricmonomers m-DBI and p-DBI. The high concentra-tion of amide and benzimidazole groups per poly-mer repeat unit accounts for the high affinity forwater, and thus, water uptake values mostly over13% and up to 19.7% have been measured at 65%relative humidity. Polymers which have the high-est ratio of benzimidazole/amide units show thehighest value of water uptake.

This research has been supported by the SpanishMinisterio de Ciencia e Innovacion (MAT2007-62392).

REFERENCES AND NOTES

1. Ateri, G. R.; Hillman, M. E. D. In High Perform-ance Polymers: Structure, Properties, Composites,Fibers; Baer, E.; Moer, A., Eds.; Hanser Verlag:Munich, 1990.

2. High Performance Polymers and Polymer MatrixComposites.; Eby, R. K.; Evers, R. C.; Meador, M.A., Eds.; MRS: Pittsburg, 1992.

3. Mansen, G. In Polymeric Materials Engineering;Salamone, J. C., Ed.; CRC Press: Boca Raton, 1996.

4. Kakimoto, M.; Akiyama, R.; Negi, Y. S.; Imai, Y.J Polym Sci Part A: Polym Chem 1988, 26, 99–105.

5. Yang, C.-P.; Chen, R.-S.; Wei, C.-S. Eur Polym J2002, 38, 1721–1729.

6. Hergenrother, P. M.; Connell, J. W.; Labadie, J.W.; Hedrick, J. L. Adv Polym Sci 1994, 117, 67–110.

7. Caruso, E.; Centore, R.; Roviello, A.; Sirigu, A.Macromolecules 1992, 25, 2290–2293.

8. Mercer, F. W.; McKenzie, M. T.; Bruma, M.;Schulz, B. High Perform Polym 1996, 8, 395–406.

9. Kricheldorf, H. R.; Thomsen, S. A. MacromolChem Rapid Commun 1993, 14, 395–400.

10. (a) Preston, J.; DeWinter, W.; Black, W. B.; Hoffer-bert, W. L., Jr. J Polym Sci Part A-1: Polym Chem1969, 7, 3027–3037; (b) Preston, J. In Encyclope-dia of Polymer Science and Engineering; Mark,H. F.; Bikales, N. M.; Overberger, C. G.; Menges,G., Eds.; Wiley: New York, 1988; Vol. 11, p 381.

11. Preston, J.; Black, J. J Polym Sci Part A-1: PolymChem 1967, 5, 2429–2439.

12. Wang, H.-H.;Wu, S.-P. J Appl Polym Sci 2004, 91, 378–386.

13. Berrada, M.; Carriere, F.; Abboud, Y.; Abourriche,A.; Benamara, A.; Lajrhed, N.; Kabbaj, M.; Ber-rada, M. J Mater Chem 2002, 12, 3551–3559.

14. Neuse, E. W. Adv Polym Sci 1982, 47, 1–42.15. Cheng, T.-S. J Macromol Sci Macromol Chem

Phys C 1997, 37, 277–301.16. Jones, D. J.; Roziere, J. J. Membrane Sci 2001, 185,

41–58.17. (a) Li, Q.; He, R.; Jensen, J. O.; Bjerrum, M. J.

Fuel Cells 2004, 4, 147–159; (b) Ronghuan, H.;Qingfeng, L.; Jens, O. J.; Niels, J. B. J Polym SciPart A: Polym Chem 2007, 45, 2989–2997.

18. Mikroyannidis, J. A. Polymer 1996, 37, 2715–2721.

19. Ayala, V.; Maya, E. M.; Garcıa, J. M.; de laCampa, J. G.; Lozano, A. E.; de Abajo, J. J PolymSci Part A: Polym Chem 2005, 43, 112–121.

20. Ayala, V.; Munoz, D. M.; Lozano, A. E.; de laCampa, J. G.; de Abajo, J. J Polym Sci Part A:Polym Chem 2006, 44, 1414–1423.



21. Lozano, A. E.; de la Campa, J. G.; de Abajo, J. JPolym Sci Part A: Polym Chem 1999, 37, 4646–4655.

22. Accelrys Software Inc. Materials Studio v. 4.3;Accelrys Software Inc.: San Diego, CA, 2008.

23. Sun, H.; Mumby, S. J.; Mapple, J. R.; Hagler, A.T. J Am Chem Soc 1994, 116, 2978–2987.

24. Backes, J.; Heinz, B.; Ried, W. G. In Methodender Organische Chemie, 4th ed.; Schaumann, E.S., Ed.; Georg Thieme Verlag: Stuttgart, 1994;Hetarene III, Vol. E, p 216.

25. Lozano, A. E.; de la Campa, J. G.; de Abajo, J.;Preston, J. Polymer 1994, 35, 872–877.

26. Lozano, A. E.; de Abajo, J.; de la Campa, J. G.;Preston, J. Polymer 1994, 35, 1317–1321.

27. Marcos-Fernandez, A.; Lozano, A. E.; de Abajo, J.;de la Campa, J. G. Polymer 2001, 42, 793–800.

28. Kitazume, T.; Ishikawa, N. Bull Chem Soc Jpn1974, 47, 785–786.

29. Yamazaki, N.; Higashi, F.; Jawabata, J. J. PolymSci Part A: Polym Chem 1974, 12, 2149–2154.

30. Yamazaki, N.; Higashi, F. Adv Polym Sci 1981,38, 1–25.

31. Ferreiro, J. J.; de la Campa, J. G.; Lozano, A. E.;de Abajo, J.; Preston, J. J Polym Sci Part A:Polym Chem 2007, 45, 4671–4683.

32. Wang, X.-Y.; in’t Veld, P. J.; Freeman, B. D.; San-chez, I. C. Polymer 2005, 46, 9155–9161.

33. Sannigrahi, A.; Arumbabu, D.; Sankar, R. M.;Jana, T. J Phys Chem B 2007, 111, 12124–12132.

34. Lozano, A. E.; de Abajo, J.; de la Campa, J. G.J Polym Sci Part A: Polym Chem 1993, 31,1203–1210.



synthesis and evaluation of properties of novel poly ... en pdf/2008/jpspc-2008-46... · synthesis...

Documents