European Polymer Journal 45 (2009) 1716–1727

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European Polymer Journal

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Synthesis and gas transport properties of ODPA–TAP–ODA hyperbranchedpolyimides with various comonomer ratios

J. Peter *, A. Khalyavina, J. Kríz, M. BlehaInstitute of Macromolecular Chemistry, Heyrovsky Sq. 2, 16206 Prague 6, Czech Republic

a r t i c l e i n f o a b s t r a c t

Article history:Received 27 November 2008Received in revised form 1 March 2009Accepted 4 March 2009Available online 13 March 2009

Keywords:HyperbranchedPolyimideCopolyimideGas separationMembranePermeability

0014-3057/$ - see front matter � 2009 Elsevier Ltddoi:10.1016/j.eurpolymj.2009.03.003

* Corresponding author. Tel.: +420 296 809 246; faE-mail address: peter@imc.cas.cz (J. Peter).Abbreviations: 6FDA, 4,40-(hexafluoroisoprop

anhydride; ANH-HBPI, anhydride-terminated hyperHBPI, hyperbranched polyimide; NH2-HBPI, aminbranched polyimide; NMP, N-methylpyrrolidone; ODODPA, 4,40-oxydiphthalic anhydride; PAA, poly(amtriaminopyrimidine.

A series of hyperbranched copolyimides (HBPI)s based on commercially available mono-mers 4,40-oxydiphthalic anhydride (ODPA), 2,4,6-triaminopyrimidine (TAP) and 4,40-oxydi-aniline (ODA) were prepared. The synthesis involved the formation of hyperbranchedpolyamic acid (PAA) precursors in the first step and the thermal imidization of cast thinPAA films in the second step. Two basic types of HBPIs were prepared by controlling themolar ratio of ODPA and an amine mixture of TAP and ODA. When the molar ratio was1:1, the amine-terminated HBPIs were obtained; with the molar ratio of 2:1 anhydride-ter-minated HBPIs were prepared. Degree of branching was estimated by 1H and 13C NMR anal-ysis. It was found that approximately 48% of TAP units presented in ODPA:TAP:ODA =1:0.75:0.25 HBPI macromolecules create the branching unit. Amine-terminated HBPIsshowed moderate weight-average molecular weights and these values rather higher thanfor the anhydride-terminated HBPIs. With increasing ODA comonomer content in amine-terminated HBPIs increased their molecular weight and thermal and mechanical stability,whereas in anhydride-terminated HBPIs these trends were opposite. Amine-terminatedHBPIs generally exhibited higher thermal stability than the anhydride-terminated ones.Gas permeability coefficients of both HBPIs types increased with increasing content ofODA comonomer. Prepared membranes exhibited high separation performance and havea potential to be utilized in industrial gas separation applications.

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1. Introduction

Dendritic polymers have received increasing attentionin recent years because they are expected to have uniqueproperties (i.e., low solution viscosity, high solubility)when compared with their linear analogues [1–9]. Thehighly branched structure and a large number of terminalfunctional groups are two important structural featuresof dendritic polymers, which clearly distinguish them from

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ylidene) diphthalicbranched polyimide;e-terminated hyper-A, 4,40-oxydianiline;ic acid); TAP, 2,4,6-

linear (nonbranching) polymers. Dendrimers are perfectlybranched and monodisperse molecules. The dendrimersynthesis involves multistep procedures (protection, cou-pling, and deprotection cycles), leading to high cost andproblems in large-scale preparation.

Hyperbranched polymers do not have the well-definedarchitectures as dendrimers. Their architecture is moreirregular and generally consisted of three kinds of struc-tural units: linear (L), dendritic (D), and terminal (T) units.The degree of branching as defined by Frey [10] as a sum ofdendritic and terminal units divided by sum of all units isthe most widely accepted term used to describe hyper-branched polymers. Nevertheless, hyperbranched poly-mers are assumed to have similar physical properties todendrimers and can be used to replace dendrimers formost cases [9]. Since hyperbranched polymers can besimply prepared by direct ‘‘one-step” polymerization of

J. Peter et al. / European Polymer Journal 45 (2009) 1716–1727 1717

multifunctional monomers, they are of more significance thandendrimers from the viewpoint of industrial applications.

Many hyperbranched polymers have been synthesizedsuch as polyphenylene, poly(ether ketone), poly [4-(chlo-romethyl)styrene], m-polyaniline, polycarbonate, polyes-ters, aromatic polyamides, etc. [1–9]. Recently, there wasobserved a remarkable increase in the number of publica-tions dealing with synthesis of hyperbranched aromaticpolyimides (HBPIs) [11–22]. It is well known that aromaticpolyimides represent an important class of high-perfor-mance polymeric materials because of their many out-standing properties such as high mechanical strength,high modulus, unusual thermooxidative stability, excellentelectrical properties, irradiation and chemical resistance.Because of these merits, polyimides have found wide appli-cations as high-temperature protection films and adhe-sives in aircraft and space industry, isolation layers andphotoresists in microelectronics, etc. [23].

Polyimides have also been identified as the high-per-formance membrane materials for gas separations [24].The permeability of polyimides is generally attributedto the fractional free volume which is closely related totheir highly rigid structure, while the high selectivity isdue to the high diffusion selectivity and/or the high sol-ubility selectivity. It is reported that there are manyopen and accessible cavities (typically several angstromsin size) in a rigid branched structure [9,25]. These cavi-ties may function as the pathways for the transport ofgas molecules.

Hyperbranched polyimides (HBPI) have been preparedfrom either ABx monomers or A2 + B3 monomers, where Arepresent anhydride functional group and B represent ami-no functional group [11–19]. Both methods are impracticalfor preparation of HBPIs on a large scale because of therequirement of special ABx monomers for the former meth-ods, or the stringent polymerization conditions for thelatter to avoid gel formation (e.g. low monomer concentra-tions, strictly controlled slow addition rates, and molarratios of monomers) [20].

Facile syntheses of HBPIs were realized by additionpolymerization of A2 + BB02 monomers: 4,40-(hexafluoro-isopropylidene) diphthalic anhydride) (6FDA) and 2,4,6-triaminopyrimidine (TAP). 6FDA corresponds to the A2

monomer and TAP to the BB02 monomer [21]. TAP showsdifferent reactivities of the amino groups, which is not inaccord with basic assumption of the same reactivity ofthe monomer functional groups in Flory’s theory of gel for-mation [26,27]. Based on this fact, no gel formation wasobserved during the synthesis although the monomer con-versions exceeded those at the theoretical gel points. An-other advantage of this approach is that commerciallyavailable monomers were used, which can be crucial forlarge-scale production.

It is known that many hyperbranched polymers havepoor mechanical properties probably due to a considerablylower number of physical entanglements between themacromolecules. Because it is very difficult to prepareHBPIs in the form of a self-standing membrane, there arealmost no data on their gas transport properties. Gas trans-port characterization of self-standing HBPI membraneswas reported in work [11]. Sufficient mechanical proper-

ties of these HBPI membranes were achieved bycrosslinking.

In our work, we similarly prepared hyperbranchedpolyimides based on 4,40-oxydiphthalic anhydride (ODPA)as A2 monomer and 2,4,6-triaminopyrimidine (TAP) asBB02 monomer. Our goal was to prepare HBPIs in theform of membranes with improved mechanical proper-ties and thus suitable for gas separation applications. Incontrast to HBPIs, their linear analogues exhibit excellentmechanical properties. In accord with this fact, we triedto improve mechanical properties of HBPIs by incorpora-tion of difunctional B2 comonomer into the polymerstructure. Higher number of B2 comonomer could in-crease distances among branches and thus afford moreflexibility in relatively rigid branched structure and final-ly they could increase a number of chain physical entan-glements. As such B2 comonomer we used difunctional4,40-oxydianiline (ODA). This approach could be describedas A2 + BB02 + B002 or more simply A2 + B3 + B2 copolymeri-zation (if we suppose the same B functional group reac-tivity). The prepared membranes were tested forthermal and chemical stability and their gas transportproperties were investigated. Additionally, moleculardynamics simulations were performed to demonstrateand visualize probable conformations of the preparedHBPIs macromolecules.

2. Experimental

2.1. Materials

4,40-oxydiphthalic anhydride (ODPA), m.p. 225–229 �Cwas purified by recrystallization from acetic anhydrideand dried for 6 h at 180 �C under vacuum, 2,4,6-triamino-pyrimidine (TAP), m.p. 249–251 �C was purified by subli-mation and 4,40-oxydianiline (ODA), m.p. 190 �C wasrecrystallized from tetrahydrofuran and then dried at140 �C under vacuum. N-Methylpyrrolidone (NMP) wasdistilled in vacuum after drying over P2O5. All chemicalswere purchased from Sigma–Aldrich.

2.2. Synthesis

2.2.1. Amine-terminated ODPA–TAP hyperbranchedpolyimide

The monomers were reacted in the molar ratio ODPA:-TAP = 1:1, the NMP was chosen to produce 10 wt.% poly-mer solution. 1 g of ODPA was dissolved in 8 ml of NMPin a dried 50-ml three-neck flask at 35 �C under N2 flow.To this solution was consequently added 0.403 g of solidTAP through a funnel and another 4.6 ml NMP was usedfor washing out of the monomer traces on the glassware.The obtained poly(amic acid) precursor was further stir-red for 24 h. The solution was then cast on dry and cleanglass plate to create a thin film. Solvent was evaporatedin oven with air stream at 80 �C for 12 h. Film was sub-sequently imidized (125 �C/2 h, 150 �C/2 h, 175 �C/2 hand 200 �C/1 h). The resulting polyimide film was delami-nated in distilled water and dried in vacuum oven at150 �C for 12 h.

1718 J. Peter et al. / European Polymer Journal 45 (2009) 1716–1727

2.2.2. Anhydride-terminated ODPA–TAP hyperbranchedpolyimide

The monomers were used in the molar ratio ODPA:-TAP = 2:1. Similarly to the previous procedure, 1 g of ODPAwas dissolved in 7 ml of NMP in a dried 50-ml three-neckflask at 35 �C under N2 flow. To this solution was added0.202 g of solid TAP and 3.8 ml of NMP. The reaction mix-ture was then stirred for 24 h. The cast film was dried andimidized under the same conditions as in the previousprocedure.

2.2.3. Amine-terminated ODPA–TAP–ODA hyperbranchedpolyimides

A series of hyperbranched polyimides were preparedwith different molar ratios between TAP and ODA. The mo-lar ratio between ODPA and the total amount of both TAPand ODA amines was kept 1:1. For example, a hyper-branched polyimide with molar ratio of comonomers OD-PA:TAP:ODA = 1:0.75:0.25 was prepared by dissolving 1 gof ODPA in 9 ml of NMP in a 50-ml dried three-neck flaskat 35 �C under N2 flow. To this solution 0.303 g of solidTAP and 0.161 g of solid ODA were alternately added (fivebatches of each comonomer). Each addition was performedafter previous batch completely dissolved. 4.2 ml of NMPwas added to wash-out monomer traces on glassware.The solution mixture was then stirred for 24 h. The castfilm was dried and imidized under the same conditionsas in the previous procedure.

2.2.4. Anhydride-terminated ODPA–TAP–ODA hyperbranchedpolyimides

The above procedure was followed except that the mo-lar ratio ODPA/(TAP + ODA) was kept 2:1.

2.3. Molecular modeling and molecular dynamics simulationof HBPIs

Geometric characterization of unmodified NH2-HBPI(molecular weight 37,250) and ANH-HBPI (molecularweight 36,550) single macromolecules was performedusing molecular modeling software Hyperchem� 7.5. TheCHARMM molecular mechanics force field [28] (Charmm27parameter set) was used for molecular dynamics simula-tion at a constant temperature of 600 K with the time step0.1 fs, the total runtime of molecular dynamics simulationwas 20 ps.

2.4. Measurements

Infrared (IR) spectra were recorded on a Perkin-ElmerParagon 1000 PC spectrometer with resolution of 2 cm�1.One hundred and twenty eight spectra were collectedand signal-averaged for each sample. All the films used inthis study were sufficiently thin to absorb in the rangewhere the Lambert–Beer law is obeyed. 1H and 13C NMRspectra were measured at 300.13 and 75.46 MHz, respec-tively, with an upgraded Bruker Avance 300 DPX NMRspectrometer. The measurements were done at 330 K in a10% w/v DMSO-d6 solution using HMDSO internal stan-dard. In the case of 1D spectra, 32 kpoints and 32 scanswere taken for 1H NMR (repetition time 10 s) and 64

kpoints and 2000 scans (repetition time 4 s) for 1H–13CDEPT45 polarization transfer spectra using a direct-detec-tion broadband probe. For 2D DQF-COSY, HSQC and HMBCspectra, an inverse-detection z-gradient probe was usedtaking 1 kpoint and 80 scans in F2 domain and 256 incre-ments filled up to 512 points in F1 domain. Sine-bellweighting functions were used in both dimensions beforeFourier transform.

Dynamical mechanical analysis was performed using aRheometric mechanical spectrometer (RSA II) at a fre-quency of 1.0 Hz at temperatures ranging from �150 to500 �C under nitrogen. Wide-angle X-ray scattering pat-terns were obtained using a powder diffractometer HZG/4A (Freibeger Praezisionsmechanik, Freiberg GmbH, Ger-many). Measurements were performed in the reflectionmode. Cu Ka radiation was used. Thermogravimetric anal-ysis (TGA) was performed using a Perkin-Elmer thermalanalysis controller TAC 7/DX from 30 to 700 �C at a heatingrate of 10 �C/min in air atmosphere.

Permeability, diffusivity a solubility coefficients P, D, Swere determined using a time-lag apparatus based onthe barometric method. The measurement error of theapparatus was 0.002 � 10�10 cm3 (STP) cm/(cm2 s cm Hg),i.e., 0.002 Barrer. Measurements were performed usingthe gas pressure of 1.5 bar at constant temperature of35 �C. Thickness of all tested HBPI membranes was inrange of 25–50 lm. The ideal selectivity is calculated fromthe ratio of permeability coefficients (Eq. (1))

aA=B ¼PA

PBð1Þ

where PA and PB refer to the permeability coefficients ofpure gases A and B, respectively. The diffusion coefficientD is calculated from Eq. (2):

D ¼ L2

6hð2Þ

where L is the thickness of the membrane and h is thetime-lag. The apparent solubility coefficient S, is expressedby P/D.

3. Results and discussion

3.1. Polymerization

Molar ratios of ODPA, TAP and ODA monomers used inthe polyimide synthesis are shown in Table 1. Two basictypes of hyperbranched polyimides (HBPI) were preparedby controlling the molar ratio of ODPA and a TAP–ODAmixture. When the molar ratio ODPA/(TAP + ODA) was1:1, the hyperbranched polyimide with amino end-groups(NH2-HBPI) was obtained, whereas with the molar ratio of2:1 we prepared HBPI with anhydride end-groups (ANH-HBPI). By changing the molar ratio of the TAP and ODAwe can easily control the number of linear units and thedegree of branching.

The polymerization was performed via soluble HBPIprecursors – poly(amic acids). The precursors were pre-pared by alternate addition of small amounts of TAP andODA into an ODPA solution in NMP (Fig. 1). Preliminary

Table 1Molecular weight averages and polydispersity of synthesized poly(amicacid)s; thermal stability represented by 10 wt.% weight loss temperature(T10) and glass transition temperature (Tg) of resulting HBPIs.

ODPA/TAP/ODAa

Resultingpolymer

PAA characterization PIcharacterization

Mw Mn Mw/Mn T10

(�C)Tg

(�C)

1/1/0 NH2-HBPI 15,100 9900 1.5 350 2221/0.95/0.05 NH2-HBPI 16,600 12,000 1.4 355 2271/0.75/0.25 NH2-HBPI 25,500 13,900 1.8 360 2381/0.5/0.5 NH2-HBPI 32,900 16,400 2.0 380 2501/0.25/0.75 NH2-HBPI 36,300 19,600 1.9 420 2551/0/1 Linear PI 47,100 24,400 1.9 550 2662/1/0 ANH-HBPI 31,300 16,200 1.9 395 2312/0.95/0.05 ANH-HBPI 20,600 13,000 1.6 360 2182/0.75/0.25 ANH-HBPI 10,700 6000 1.8 345 2082/0.5/0.5 ANH-HBPI 9200 6300 1.5 340 1992/0.25/0.75 ANH-HBPI 8700 6300 1.4 335 195

a Molar ratios of comonomers.

J. Peter et al. / European Polymer Journal 45 (2009) 1716–1727 1719

experiments showed that the order of adding monomersdoes not significantly influence polymer properties. Allsynthesized poly(amic acid)s were soluble in NMP giving

O

OO

O

O

O

O

N

N

NH2

N

N

6

5

4

1

2

3

O

OC

COOHHOOC

CO8

R 9

14

1312

1110 OCO

HOOC

N21

2223

N

24 NHHN

NH

oxydiphthalic anhydride (ODPA)

2,4,6-triaminop (TAP)

NMP, 35 °C

IMIDIZATION

- H2OResulting polyimide structure

poly(amic acid) -

+

R = or R

A B

Fig. 1. A Scheme of synthesis of hyperbranched polyimide precursor – poly(am

transparent amber-colored solutions. Because ODPA haslimited solubility in NMP, and also the final membranethickness depends on the concentration of casting solution,we kept the poly(amic acid) content in NMP at 10 wt.%. Nogel formation was observed during polymerizations, prob-ably due to reduced reactivity of the TAP amino group inposition 2 [21]. Poly(amic acid) solutions were cast onglass plates cleaned with a mixture of H2SO4 and K2Cr2O7

to achieve perfect wettability. The imidization was carriedout by thermal treatment after evaporation of the solventfrom cast films.

3.2. Structure characterization

Chemical structures of HBPIs prepared by imidization ofcorresponding (PAA)s are presented in Fig. 2. A part ofchemical structure of ANH-HBPI is given by scheme (I),and that of NH2-HBPI in scheme (II). Structures of NH2-HBPI copolymers containing ODA comonomer are inscheme (III). Structures of ANH-HBPI copolymers are anal-ogous to the structure (III) but with anhydride end-groups.

More detailed information about inner structure of syn-thesized (PAA)s was investigated by NMR. Corresponding

H2

H2

ONH2NH2

O

2019

1817

15

16HOOC

COOH

CO

CORCO

COOH

2528

2726

OHN NH

+

yrimidineoxydianiline (ODA)

s presented in Figure 2

(PAA)

=

C

ic acid) (PAA) by copolymerization of commercially available monomers.

1720 J. Peter et al. / European Polymer Journal 45 (2009) 1716–1727

HBPIs cannot be characterized directly due to their insolu-bility in organic solvents. The analysis of both 1H and 13CNMR spectra of the branched (PAA)s of ODPA, TAP andODA is extremely difficult due to the large variety of struc-tural units in them illustrated in Fig. 1 (where the numbersdesignate carbon atoms as well as possible protons at-tached to them). In contrast to the corresponding polyi-mides, we have at least three possible types of boundODPA, A and B with axial symmetry and C with an ‘‘anti-symmetric” structure. Moreover, the chemical shifts ofprotons and carbons in these units somewhat depend on

)I(

N

O

O

O

N

N

NN

N

N

O

O

O

O

O

N

NN

N N

OO

O

O

OO

O

OO

N

NN

N N

N N

N

N

N

O

O

OO

O

O

O

OO

O OO

N OO

ODA

Fig. 2. Chemical structures of synthesized polymers: (I) anhydride-terminated‘‘linear” ODA units; in the structure (II) the linear (L), dendritic (D) and termina

the nature of R and, additionally, on the conformations en-forced by spatial strain of the copolymer. Therefore, thesignal assignments given below are not quite completeand subject to possible revisions in spite of the fact thatthey rely on a combination of DQF-COSY, HSQC, and HMBCspectra.

1H NMR spectra of a linear ODPA–ODA PAA and twobranched ODPA–TAP–ODA PAA, one with prevailing linearbranches (1:0.25:0.75) and the other one with a highbranching degree (1:0.75:0.25) are shown in Fig. 3. The sig-nal assignment corresponds to scheme in Fig. 1. As one can

)III(

)II(

NH2

NH2N

N N

O

O

O

N

NN

N N

OO

O

O

O

OO

N OO

O

O

O

“linear” unit

NH2

NH2N

N N

O

O

O

N

NN

N N

N

NN

N N

NH2 N

N

N

N

O

O

OO

OO

O

OO

O

O

O

OO

N OO

Linear (L)

Dendritic (D)

Terminal (T)

HBPI, (II) amine-terminated HBPI, (III) amine-terminated HBPI containingl (T) units are presented.

J. Peter et al. / European Polymer Journal 45 (2009) 1716–1727 1721

see, the TAP proton 22 has a clear signal with stable chem-ical shift and additive integral intensity so that it can serveas a check of the content of branching units. In addition,free NH2 groups of TAP have a group of separate signalsat 6.2–6.3 ppm. There are no such signals in Fig. 3b, sothe content of TAP gives the degree of branching. InFig. 3c, the intensity ratio of these signals to 22 is 1.04,i.e., there are 0.52 free NH2 units for one TAP. As there isone NH2 unit in a linear TAP-containing structure and nonein a branched one, it can be concluded that 0.48 of thepresent TAP units (i.e., about 0.25 per one ODPA molecule)form a branching center.

It would be nice to check this result by comparing itwith the intensity of CONH signals at 10.3–10.5 ppm but,as one can see in Fig. 3, these signals are not reliable beingsubject to broadening by chemical exchange in particularin highly branched samples.

From the rest of the spectra, signals of ODA (26, 27) areclearly prominent if ODA is present in longer linear se-quences but become strangely obscured in a branchedpolymer. Conversely, the variously bonded variants ofODPA give a rather complex signal pattern in mostly linearpolymers (showing that all possibilities of reacting areprobably used) whereas one group of signals (17, 19, 20)and thus one bonding form gets prominence under higherbranching (Fig. 3c).

Fig. 3. 1H NMR spectra (300.13 MHz) of (a) ODPA–ODA PAA and ODPA–TAP–1:0.75:0.25 (c); DMSO, 330 K.

A very analogous pattern is offered by 13C NMRDEPT45 spectra, containing only signals of carbons withattached protons (Fig. 4). Again, signals 26, 27 are prom-inent in spectra of polymers with prevailing linear ODAbranches but these signals are surprisingly split in ahighly branched polymer where, as in proton spectra,one bonded form of OPDA also prevails (signals 17, 19,20). As a whole, the DEPT spectra are much better re-solved than their proton counterparts. In contrast to con-ventional 13C NMR spectra, they can be obtained in areasonable measurement time. As the intensity of thesignals depends on the value of the JCH coupling constant,the additivity of the integral signal intensity is not quiteassured. However, assuming that their value is approxi-mately the same, we can obtain the content of individualcomonomers from the integral intensities of the corre-sponding signals. Thus, in the ODPA–ODA polymer, weget 49.2 mol% of ODPA and 50.8 mol% of ODA, which cor-responds, within the error limits, to the original ratio ofthe reactants. Similarly, the ODPA:TAP:ODA molarratio 1:0.25:0.75 in the batch (Fig. 4b) gives the apparentratio 1:0.23:0.78 in the spectrum whereas that with theratio 1:0.75:0.25 (Fig. 4c) gives the apparent ratio1:0.76:0.23. These deviations of the found ratios fromthe original ones probably reflect the integration errorsrather than real changes of the comonomers’ contents.

ODA PAA with the components in the molar ratio 1:0.25:0.75 (b) and

Fig. 4. 13C NMR (75.45 MHz) DEPT45 spectra of (a) ODPA–ODA PAA and ODPA–TAP–ODA PAAs with the components in the molar ratio 1:0.25:0.75 (b) and1:0.75:0.25 (c); DMSO, 330 K.

1722 J. Peter et al. / European Polymer Journal 45 (2009) 1716–1727

As seen in Fig. 5 showing the Cq sections of correspond-ing HMBC spectra, the amide and carboxylic carbonsresonate at surprisingly similar frequencies. This is proba-bly due to strong internal hydrogen bonds of the corre-sponding groups in ortho-position. The rest of the signalscorroborate the above signal assignment.

There is generally lack of information about three-dimensional structures of hyperbranched polymers and al-most no information about HBPIs. One review on molecularmodeling of dendritic polymers could be found in ref. [29].To demonstrate hypothetical 3D structures of ODPA–TAPHBPI we performed simple molecular dynamics simula-tions. The simulations were based on minimization of totalenergy of single macromolecules, located in vacuum at theconstant temperature. From the screenshots of the molecu-lar dynamics simulation of NH2-HBPI macromolecule,shown in Fig. 6, it is evident that the polymer chain is rela-tively flexible. Approximately half of end-groups are lo-cated in the inside of the polymer coil whereas the rest islocated on its surface. This fact could be useful to take in ac-count for possible polymer modifications (e.g., crosslink-ing). On the other hand, our simple simulation of singleHBPI macromolecule conformations cannot clearly confirmor reject the existence of cavities accessible to small gasmolecules.

3.3. Polymer characterization

No information is available on suitable polymer stan-dards for gel permeation chromatography (GPC) whichcould describe molecular weight distribution of hyper-branched polyimides with sufficient accuracy. Polystyrenestandards, commonly used for many linear polymers, couldgive just apparent values because of large differences inpolymer chemical structure and smaller size of dendriticmacromolecules compared with linear polymers of thesame molecular weight. However, GPC analysis of hyper-branched polyimides using PS standards was reported inliterature [16,20] and was used also in our work. GPC anal-ysis was performed on the samples of polyimide precur-sors and results are shown in Table 2. We supposenegligible changes of polymer molecular weight distribu-tion during imidization process. Weight-average molecularweights (Mw) and number-average molecular weights (Mn)of amine-terminated precursors were in the range of15,100–36,300 and 9900–19,600 respectively. These val-ues were slightly higher than the respective anhydride pre-cursors. Molecular weights of synthesized polyimideprecursors were significantly dependent on the ODA con-tent. In case of NH2-HBPIs precursors were Mw and Mn

increasing with increasing content of ODA comonomer.

Fig. 5. Parts of 1H–13C HMBC spectra of (a) ODPA–ODA and (b) ODPA–TAP–ODA (1:0.75:0.25 mol) polyamides (DMSO, 330 K).

Fig. 6. Molecular dynamics simulation of amine-terminated HBPI macromolecule in vacuum starting from the initial 2D idealized structure to 3Dmacromolecule.

J. Peter et al. / European Polymer Journal 45 (2009) 1716–1727 1723

4000 3000 2000 1000

1498

1785

1384

1716

1601

1231

138415

9317

1317

80

1/0

0.75/0.25

0.5/0.5

0.25/0.75

cm-1

Fig. 7. FTIR spectra of the amine-terminated hyperbranched polyimidewith different TAP/ODA ratios.

1724 J. Peter et al. / European Polymer Journal 45 (2009) 1716–1727

On the other hand, Mw and Mn of ANH-HBPI precursorswere in the range 8700–31,300 and were decreasing withincreasing ODA comonomer content. The number of anhy-dride functional groups of prepared HBPIs is not generallyequal to numbers of amino groups. In such cases we prin-cipally cannot obtain high molecular polymers, neverthe-less the molecular weights are high enough to producegood quality membranes. The NH2-HBPIs anhydride/aminegroups ratios are more closer to unity with higher ODAcontents and hence these polymers exhibited highermolecular weights then ANH-HBPIs with reverse trend.Higher ODA contents in case of ANH-HBPIs led to molecu-lar weight decrease. With a higher content of anhydridegroups in reaction system there is also higher probabilityof their hydrolysis with moisture resulting in a Mw de-crease. Linear ODPA-ODA comonomer with the molar ratioequal to unity exhibited the highest values of molecularweight.

Polydispersity indices (PDI) of all synthesized polymerswere in the range 1.4–2.0, close to the values typical of lin-ear polycondensation. PDI values of HBPIs reported in [21]were in the range 1.4–1.63 whereas in [20] the values aremuch higher. ANH-HBPI generally exhibited a lower appar-ent molecular weight than HBPI with amino end-groups.

The prepared polymers were characterized by FTIR anal-ysis. Absence of the characteristic peaks of poly(amic acid)around 1680 cm-1 and the appearance of the characteristicpeaks of the imide ring at 1780 (C@O asymmetrical stretch-ing), 1716 (C@O symmetrical stretching), 1384 (C–Nstretching), and 742 (C@O bending) in FTIR spectra suggesta complete conversion of all poly(amic acid)s into the polyi-mides. FTIR spectra of NH2-HBPIs with different ratios TAP/ODA are shown in Fig. 7. It can be seen that the aromaticamine absorption of ODA at 1498 cm�1 decreases withdecreasing content of the ODA comonomer and disappearswhen no ODA was incorporated in the HBPI structure.

All prepared films were brown–orange in color andwere completely transparent. The morphology of preparedHBPIs was investigated using wide-angle XRD patterns. Allfilms showed a broad peak reflecting completely amor-phous structure. Results were in an agreement with DSCanalysis which showed no peaks related to crystallizationenthalpy. SEM micrographs showed no inhomogeneitiesin polymer films.

Table 2Gas permeability coefficients and ideal selectivities of hyperbranched polyimides

HBPIs PI type Permeability [10�10 cm3 (STP) cm/(cm2 s cm

ODPA/TAP/ODAa H2 CO2 O2 N2

1/1/0 NH2-HBPI 0.887 0.096 0.024 0.0021/0.95/0.05 NH2-HBPI 1.187 0.156 0.043 0.0041/0.75/0.25 NH2-HBPI 1.843 0.246 0.056 0.0061/0.5/0.5 NH2-HBPI 2.514 0.352 0.101 0.0131/0.25/0.75 NH2-HBPI 2.032 0.451 0.087 0.0101/0/1 Linear PI 2.389 0.595 0.132 0.0172/1/0 ANH-HBPI 0.803 0.0792/0.95/0.05 ANH-HBPI 0.923 0.140 0.024 0.0032/0.75/0.25 ANH-HBPI 1.631 0.135 0.054 0.0052/0.5/0.5 ANH-HBPI 1.923 0.257 0.065 0.0072/0.25/0.75 ANH-HBPI 2.131 0.285 0.071 0.008

a Molar ratios of comonomers.

Glass transition temperatures (Tg) as determined byDSC analysis, showed higher values for amine-terminatedHBPIs than for HBPIs terminated with anhydride groups(Table 1). With increasing ODA comonomer content, theTg temperature was increasing for NH2-HBPIs, but de-creased for ANH-HBPIs. It seems that lower molecularweights of ANH-HBPIs caused higher segment mobilityand hence lower values of Tg. Thermal stability of the pre-pared HBPIs was determined by TGA analysis. As can beseen from Table 1, NH2-HBPI with no content of ODA have10 wt.% loss temperature T10 around 350 �C whereas ANH-HBPI exhibits a lower T10, around 335 �C. Higher thermalstability of NH2-HBPI could be explained by higher molec-ular weights and by hydrogen bonds between free aminogroups of TAP resulting in stronger interactions betweenmacromolecules. With increasing content of ODA, inNH2-HBPI structures, the thermal stability increases up tovery high T10 (550 �C) which is contributed to the linearODPA–ODA polyimide. With increasing content of ODA

at 1.5 bar and 30 �C.

Hg)] Ideal selectivity

CH4 a(H2/CH4) a(O2/N2) a(CO2/CH4) a(CO2/CH4)

0.002 444 12.0 48 480.003 396 10.8 52 390.004 461 9.3 62 410.007 356 7.5 50 260.006 339 8.7 75 450.010 239 7.8 60 35

0.003 308 8.6 47 500.004 431 10.8 36 270.004 493 9.4 66 370.006 380 8.8 51 36

J. Peter et al. / European Polymer Journal 45 (2009) 1716–1727 1725

units the number of branches decreases. The reduction ofsteric hindrances, caused by high chain branching, couldlead into more efficient chain–chain packing and strongerchain–chain interactions (van der Waals interaction, polarinteraction, charge transfer interactions between the aciddianhydride moieties and the diamine moieties) [30].

HBPI membranes with no or very low contents of ODAunits in their molecular structure exhibited poor mechan-ical properties (high brittleness). This could be attributedto lower molecular weights of unmodified HBPI and tothe presence of a low number of physical enlargements.Measurement of their transport properties was then diffi-cult and for some cases unrealizable because of crackingin the membrane cell. With increasing content of theODA comonomer, acceptable mechanical properties ofmembranes (especially based on NH2-HBPIs) were ob-tained. On the other hand HBPIs with lowest molecularweights were on the border of measurability because ofhigh brittleness. This led to easier membrane handling.

3.4. Transport properties

Transport properties of prepared membranes weremeasured in the time-lag apparatus, where the pressureof permeating gas was measured in the constant volume.Each membrane was evacuated at least for 24 h beforemeasurement. The permeability coefficients (P) of H2, N2,

Table 3Gas diffusivity and solubility coefficients of HBPIs at 1.5 atm and 30 �C.

HBPIs Diffusivity and solubility coefficientsb

ODPA/TAP/ODAa DH2 SH2 DCO2 SCO2 DO2 SO2 DN2 SN2

1/1/0 50 0.60 0.04 811/0.95/0.05 60 0.66 0.10 52 0.28 5.18 0.12 1.121/0.75/0.25 67 0.93 0.13 62 0.45 3.96 0.21 0.981/0.5/0.5 88 0.96 0.13 105 0.38 6.87 0.20 2.001/0.25/0.75 77 0.89 0.16 95 0.33 4.81 0.18 1.921/0/1 85 0.94 0.24 84 0.52 8.59 0.24 2.382/1/0 35 0.77 0.03 882/0.95/0.05 0.08 59 0.22 3.66 0.10 0.712/0.75/0.25 51 1.07 0.08 57 0.28 6.47 0.14 1.242/0.5/0.5 63 1.02 0.10 86 0.28 7.79 0.13 1.862/0.25/0.75 71 1.01 0.12 80 0.33 7.22 0.16 1.69

a Molar ratios of comonomers.b D and S are in 10–12 m2 s�1and 10�5 mol m3 Pa�1, respectively.

Table 4Gas diffusivity and solubility selectivities of prepared hyperbranched polyimides

HBPIs Selectivities

ODPA/TAP/ODAa aD (H2/CO2) aS (CO2/H2) aD (CO2/N2) aS (

1/1/0 1240 1341/0.95/0.05 604 79 0.83 471/0.75/0.25 501 67 0.65 631/0.5/0.5 717 110 0.64 531/0.25/0.75 481 107 0.91 491/0/1 358 89 0.99 352/1/0 1167 1152/0.95/0.05 513 78 0.84 832/0.75/0.25 0.59 462/0.5/0.5 630 84 0.80 462/0.25/0.75 592 79 0.75 47

a Molar ratios of comonomers.

O2, CH4 and CO2 respectively, and ideal selectivities (a) ofvarious gas pairs through HBPI membranes are shown inTable 2. Diffusion coefficients (D) and solubility coeffi-cients (S), are presented in Table 3, diffusivity selectivities(aD) and solubility selectivities (aS) for H2/CO2, CO2/N2 andO2/N2, are shown in Table 4. In some cases, the time-lagvalues of H2 were to short to determine diffusion and con-sequently solubility coefficients. Similarly, we did notdetermine these coefficients when the time-lags were tolong (especially for CH4 and in some cases for N2). In addi-tion the values of permeability coefficients of CH4 and N2

were at the lower limit of measurability of our apparatus.It is well known that the fractional free volume (includ-

ing cavities in the interior of globular hyperbranched mac-romolecules) is mainly responsible for the gas transportproperties in polymers [11,31]. Also inter-macromolecularpacking of the polymer chains has considerable effect onthe polymer gas permeation properties [11]. From the pre-sented data it is evident that almost all NH2-HBPIs andANH-HBPIs have considerably lower permeability coeffi-cients for all gasses than a linear ODPA–ODA polyimide.On the other hand, the ideal selectivities of those HBPIsare significantly higher compared to linear ODPA–ODApolyimide. The fact that prepared hyperbranched polyi-mides have lower permeability coefficient then the linearpolyimide would seem surprising, but it is necessary totake in account that the chemical structure of difunctionalODA monomer is considerably different from that of 2,4-diaminopryimidine or 4,6-diaminopyrimidine (their chem-ical structure is closest to the structure of TAP). Thesemonomers combined with ODPA would demonstrate theeffect of cavities much better but due to the low molecularweights of the polymer we failed to prepare self-standingmembrane from them.

HBPIs with high contents of ODA comonomer are lessbranched and thus it can be expected that they will bemore tightly packed with smaller cavities. However, exper-imental results showed that the permeability coefficientsincreased with ODA comonomer increase for all gasses.Similar tendency was found for the diffusion and solubilitycoefficients. This indicates that increasing content of ODAincreases the fractional free volume of polyimide [32]and in addition it increases the interactions of gas mole-cules with the polymer, represented by the solubility coef-

at 1.5 atm and 30 �C.

CO2/N2) aD (O2/N2) aS (O2/N2) aD (H2/N2) aS (H2/N2)

2.3 4.6 503 0.592.2 4.0 326 0.942.0 3.4 462 0.471.9 2.5 440 0.462.1 3.6 354 0.40

2.3 5.22.1 5.2 378 0.872.2 4.2 504 0.552.1 4.3 444 0.60

1726 J. Peter et al. / European Polymer Journal 45 (2009) 1716–1727

ficient. Exceptional data were found for H2, where thehighest permeability, diffusivity and solubility coefficientswere found for ODPA/TAP/ODA = 1/0.5/0.5 HBPI. We do nothave a supported explanation for this behavior, but itmight be due to an existence of smaller cavities whichare accessible only for the smallest molecules like the H2.With higher contents of ODA units, the behavior ofmacromolecules seems to be similar to linear polyimidesand permeabilities slightly drop.

The ideal selectivities of both types HBPIs were in therange 308–493 for H2/CH4, 7.5–12 for O2/N2, 47–75 forCO2/CH4 and 27–50. These values were decreasing withincreasing content of ODA units. The highest ideal selectiv-ity for H2/CH4 were found with the value of 461 for NH2-HBPI with the ODA content 25% (related to TAP) and 490for ANH-HBPI with content of ODA 50%. In case of O2/N2

selectivity, the highest value of 12 was found for neatODPA-TAP NH2-HBPI.

Gas diffusivities given in Table 3 were decreasing in theorder D(H2) >> D(O2) > D(N2) � D(CO2). This trend is consis-tent with the kinetic diameters of gas molecules, except forCO2 whose diffusion could be suppressed by polar interac-tion with the polymer. Solubility coefficients decreased inthe order S(CO2) >> D(O2) > D(N2) > D(H2). These valuesshowed that the overall separation performance of preparedHBPI membranes is based on the differences in diffusivities.On the other hand, separation of gas pairs containing CO2 isstrongly influenced by solubility coefficient differences (e.g.solubility selectivity CO2/N2 is around 50, diffusivity selec-tivity of this gas pair is just 0.7–0.9).

Gas permeation properties of membranes depend alsoon the type of chemical functional groups in the polymer[31,32]. In this study, the NH2-HBPI membranes with largenumber of amine end-groups consistently displayed higherpermeability and solubility coefficients than those pre-pared from ANH-HBPI with anhydride end-groups. Amineend-groups show evidently stronger polar interactionswith gasses than the anhydride ones. This holds, in partic-ular, for the solubility coefficients for carbon dioxide.

Compared to other polyimides the synthesized HBPIsexhibited slightly lower permeabilities but very high selec-tivities which makes this materials very promising for usein many gas separation applications like O2/N2, H2/CH4,CO2/CH4, CO2/N2, etc.

4. Conclusions

A series of self-standing HBPI membranes were success-fully prepared by copolymerization of comonomers ODPA,TAP and ODA at various molar ratios of comonomers. Nogelation occurred during polymerizations probably be-cause of different reactivities of the amino groups at the2- and 4-/6-positions in TAP.

Structure characterization of the products was foundto be only partly possible by 1H NMR due to strong signaloverlap and relative shifts of the analogous signals in dif-ferent structure types. Using 13C NMR, the content ofODPA, TAP and ODA in the products was found to beapproximately equal to that in the charge. Using 1HNMR signals of the TAP ring and NH2 protons, we found

that all of TAP was utilized as branching center in the1:0.25:0.75 (ODPA:TAP:ODA) product, whereas only48 mol% of it formed branches in the 1:0.75:0.25 product.Molecular dynamics simulation of idealized ODPA–TAPmacromolecule isolated in vacuum gave a hypotheticalidea about its three-dimensional structure, showed highflexibility. Results also show that around half of endinggroups are located in the inner space of macromoleculecoil so they could be stericly hindered for possiblemodifications.

NH2-HBPIs showed moderate molecular weights andtheir values were slightly higher than those of ANH-HBPIs.Mw and Mn of NH2-HBPIs precursors were increasing withincreasing content of ODA comonomer. On the other hand,molecular weights decreased with increasing ODA contentin case of ANH-HBPI precursors. Polydispersities were inthe range 1.4–2.0.

All prepared membranes were transparent (HBPIs arecompletely amorphous). They have very good thermal sta-bility, which increased with higher content of ODA como-nomer for NH2-HBPIS and decreased for ANH-HBPIs.NH2-HBPIs generally exhibited higher thermal stabilitythan ANH-HBPIs.

Most of HBPIs had considerably lower permeabilitycoefficients for all gasses than a linear ODPA–ODA polyim-ide. Gas permeability coefficients of NH2-HBPIs were gen-erally higher than those of ANH-HBPIs. Permeabilitieswere increasing with increasing content of ODA comono-mer for both HBPI types. Prepared HBPIs exhibited excel-lent separation factors (in range of 300–490). Separationperformance of the prepared membranes could be veryinteresting in potential industrial applications.

Acknowledgements

This work was supported by the Ministry of Industryand Trade of the Czech Republic (grant 2A-1TP1/116) andby the Ministry of Education, Youth and Sports of the CzechRepublic (grant 1P05ME797) as a part of the ‘‘Nanocom-posite Polymeric Layered Charged Membranes” projectwhich is in contract with National Science Foundation ofUSA.

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