Thermal Rearrangement of Functionalized Polyimides: IR-Spectral

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Thermal rearrangement of functionalized polyimides: IR-spectral, quantum chemical studies and gas permeation parameters of TR-polymers Julia Kostina, Olga Rusakova, Galina N Bondarenko, Alexander Alentiev , Tamara Meleshko, Nina Kukarkina, Alexander Yakimansky, and Yuri Yampolskii Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie3034043 • Publication Date (Web): 28 Mar 2013 Downloaded from http://pubs.acs.org on April 2, 2013

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Industrial & Engineering Chemistry Research

Thermal rearrangement of functionalized polyimides: IR-spectral, quantum chemical studies and gas permeability of TR polymers Julia Kostina1, Olga Rusakova1, Galina Bondarenko1, Alexander Alentiev1, Tamara Meleshko2, Nina Kukarkina2, Alexander Yakimanskii2, YuriYampolskii1* 1

A.V.Topchiev Institute of Petrochemical Synthesis, RAS, Moscow, Russia 2

Institute of Macromolecular Compounds, RAS, S.-Petersburg, Russia

ABSTRACT Thermal rearrangement of polyimides with OH side groups results in insoluble products (TR polymers) that reveal extremely high gas permeability combined with good permselectivity. However, the mechanism of the formation of these polymers remains unclear, as well as there are some doubts about their chemical structure. In order to elucidate this problem, IR spectra of TR polymers were recorded and compared with those of several polyimide precursors. IR spectra in the range 400–4000 cm–1 showed numerous bands that evidenced the formation of aromatic lactam structure in the final products of thermal rearrangement, while only weak bands indicated the presence of benzoxazole structure. It was also shown that thermal rearrangement of functionalized polyimides studied leads to highly permeable TR polymers: permeability coefficients P increased by a factor 5–20 as compared to those of the polyimide precursors.

Introduction Among the most intriguing results of the late decade in gas separation membrane material science is a discovery of thermal rearrangement of OH-containing polyimdes (PI).1,2 A feature of the polyimides that contain OH groups in ortho-position to nitrogen atom of imide cycle is a possibility to intramolecular thermochemical reaction (ITR) that leads, at temperature about 400°C, to formation of novel structures of insoluble polymers with sometimes fascinating physicochemical properties. In particular, it has been shown that these so-called thermally rearranged (TR) polymers reveal strongly increased gas permeability with little or no accompanying loss of permselectivity as compared with the corresponding parameters of the precursors.1-3 . First, such reaction was described for PI based on pyrromelitic dianhydride and 3,3′dioxybenzidine when the sample was heated up to 600 °С.4. Two alternative mechanisms were proposed by Kardash and Pravednikov4: one with formation of benzoxazole and another with formation of ladder ester-amide structure.

*To whom correspondence should be addressed. E-mail: [email protected]

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COOH N

N O

O

O

O HOOC

N

*

HO

N

OH O

O H N

O H N C

O

O

O C

C

O

O

C

Scheme 1 Later it was established using IR-spectroscopy and mass-spectrometric analysis of the gaseous products that carbon dioxide was formed in the process of the reaction and the formation of the polymer with benzoxazole structure was proved. In subsequent studies1,2,5 these ideas obtained a development and the following mechanism was proposed: O HO N

HO

400 ºC

400 ºC

N OH

N

O

I

O

O

O

II

400 ºC -CO2

O N

O

III

IV

Scheme 2 Entirely different mechanism was proposed by Hodgkin and Dao.6 These authors assumed that water is eliminated under elevated temperatures and, hence, the following structure is produced: O HO N O

O 400 °C°C - -H2OO

N O

Scheme 3 According to the literature data, benzoxazole ring in the formed polymers should reveal the following absorption bands: 1630, 1555 and 935 cm–1 ,7 1474 and 1059 cm–1 ,2,8 and 1617 and 1058 cm–1 .5 Thus, PI based on dianhidride of 3,3′,4,4′-benzophenontetracarbonic acid and 3,3′-dihydroxy4,4′-diaminebiphenyl should exhibit new intensive absorption bands at 1617, 1500, 1250, 1058, 850– 900 cm–1 in its IR spectrum that are characteristic for the appearance of benzoxazole rings in the polymer structure and modification of aromatic rings due to intramolecular rearrangement.5 Assignment of the bands in the range 1300–1000 cm–1 is complicated thanks to the presence of C-F bonds and ether groups connecting the aromatic rings. Indeed, in this range a number of intensive absorption bands are located and their position and intensity should be sensitive to chain conformation.9,10


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Earlier Likhatchev et al.8 compared IR spectra of PI based on 2,2-bis(3-amino-4hydroxyphenyl)hexafluoropropane (6F-OH diamine) and 3,3', 4,4'-benzophenonetetracarboxylic dianhydride (BTDA) before and after thermal treatment at 425 °С in nitrogen atmosphere with the spectrum of deliberately prepared polybenzoxazole. Likhatchev et al.8 made a conclusion that the spectra of both polymers are very similar, however, the presence of the bands at ~ 3200 and ~1660 cm– 1

in the spectrum of thermally treated polymer is not commented, meanwhile they are absent in the

spectrum of polybenzoxazole. Both these bands can be ascribed to stretching vibrations of the NH and C=O bands that can be formed in the process of thermal rearrangement of the amide group.10 Thus, a critical analysis of the literature data does not allow one to make unambiguous conclusion on the structure of the polymer that is formed via ITR. In attempting to clarify the situation we investigated the mechanism of ITR and the reaction products of several novel PIs with systematically changed structure of the main chain. All PIs contained OH groups in the diamine fragment. In addition, the gas permeability coefficients of two novel PI precursors and corresponding TR polymers were measured.

Experimental Source and purity of reactants and solvents. N-methyl-pyrrolidone (NMP) (98%, Aldrich) and toluene (analytical grade, Vekton, Russia) were dried above anhydrous calcium hydride (99.9%, Aldrich) and distilled under vacuum. Pyridine (≥99%, Aldrich) was boiled with KOH (reagent grade, Vekton, Russia) and distilled from calcium hydride. 4,4’-(2,2-hexafluoropropylidene)-bisphthalic anhydride (99%, Aldrich) was dried in vacuum at 140 °С. 2,4-diaminophenol dihydrochloride (98%, Lancaster), 4,6-diaminoresorcin dihydrochloride (97%, Aldrich), and 3,3’-dihydroxybenzidine (99%, Aldrich) were dried in vacuum at 100°С. Polycondensation The technique used earlier11 for the synthesis of PI4 and PI5 (Table 1) was extended

here

to

the

preparation

of

polyimides

PI1-PI3

on

the

basis

of

4,4’-(2,2-

hexafluoropropylidene)-bisphthalic anhydride. PI solutions were prepared via the standard two-stage procedure without an isolation of polyamic acid precursors, the latter being obtained as 20 wt% solutions in NMP via the room-temperature polycondensation of the dianhydride and a particular diamine (2,4-diaminophenol, 4,6-diaminoresorcin, and 3,3’-dihydroxybenzidine for the synthesis of PI1, PI2, and PI3, respectively). 3,3’-Dihydroxybenzidine was used as a free base, while 2,4diaminophenol and 4,6-diaminoresorcin were introduced into the reaction mixture in the form of their corresponding dihydrochlorides in order to protect their unstable amino-groups. These diamines reacted with the dianhydride immediately after their deprotection in situ by dehydrochlorination reactions with specially added pyridine.11 Then, the cyclodehydration (imidization) of polyamic acid precursors was performed in NMP solutions at 170–180 °C. The evolved water was removed from the reaction solution as a toluene–water azeotrope. Toluene was ACS Paragon Plus Environment


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added into NMP solution of polyamic acid at the volume ratio toluene:NMP = 1:3. PIs were precipitated into methanol from a native solution diluted to 5 wt % and filtered. The polymers were purified from NMP by boiling with diethyl ether in a Soxhlet apparatus for two days. It should be noted that the imidization temperature (170–180 °C) was sufficiently high to convert polyamic acids into polyimides completely, being, at the same time, too low for side reactions with the participation of OH groups to occur. Intrinsic viscosities of the studied PI (Table 1) were measured in Ubbelohde viscosimeter in NMP solution at 20 °C. The work reports the spectral data for PI1-PI5 (Table1) and the correspondent TR-polymers. The IR spectra of PI4 and PI5 have been also discussed by Rusakova et al.

11

It should be noted that

Smith et al.12 and Sanders et al.13 recently reported results on the polymer similar to PI-3. In those works the authors used the precursor with acetate group in the o-position. Thermal transformations at 250oC resulted in formation of hydroxyl also in o-position. This opens a possibility for comparison of the spectra of PI3 and the corresponding TR polymer, which is discussed later in this work. Since the features of all five TR polymers turned out to be very similar we considered it possible to limit our study of gas permeation of only two TR polymers prepared from PI4, and PI5, which showed better film forming properties. On the other hand, the transport parameters for PI3 and its TR polymer can be taken for comparison from Ref. 13. Table 1 Structure of polyimide precursors

Intrinsic Structure

№

viscosity, dL/g (NMP, 20ºC)

O

F 3C

O

CF3

N

1

0.50

N n

O

O

O

F3C

O

CF3

N

2

OH

0.56

N n

O

O

3

N O

O HO

F3C

CF3

OH

O

2.27

N O HO


OH

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O

4

H3C

O

CH3

O

O

N

N

O

O

O

2.42

O O

5

n OH

N

O

0.55

N

O

O

n OH

Spectral study. ITR was studied using films of PIs formed from the solutions in N,Ndimethylformamide (DMF).(99%, Sigma-Aldrich). 5% mass solutions of PIs were coated over the surface Si optical window (flat discs of crystalline silicon purified by zone melting method, provider “Elektrosteklo”, Russia). This support is transparent in middle IR region. After drying, the films were

transparent and had slightly yellowish color, their thickness was in the range 5–7 µm. The samples were heated in a vacuum oven in the range 25 – 430 °С in nitrogen atmosphere at the heating rate of 5 K/min, then the samples were kept for a period 0.5- 2 h. and cooled to the ambient temperature together with the support. After heating the samples became brittle, insoluble in chloroform and amide solvents, their color became dark brown. FTIR spectra of the initial, the thermally treated and cooled samples were recorded at a spectrometer IFS 66v/s (Bruker) in the range 400–4000 cm–1 in the regime of the absorbance (50 scans, resolution 1 cm-1). The spectra were treated using a software package OPUS (Bruker). No changes in the process of recording od IR spectra were noted. (Figure S1 in Supporting Information). Thermoanalysis. TGA was carried out using an instrument STA Jupiter® 449С (NETZSCH). It was combined with IR analysis of gaseous products of the reaction. The samples were heated in the range 20 − 600 °С at the rate 5 K/min while sweeping by argon (100 ml/min). Different scanning calorimetry (DSC) scans were obtained using a Mettler instrument DSC823e in the range of 25-400oC with a heating rate of 10 K/min.

Quantum chemical calculations. Quantum chemical calculations were conducted by the Density functional theory (DFT) method using the package Gaussian 03.14 Optimization of all the model compounds (reactants, transition states, and products) as well as computation of calculated vibration spectra were performed with accounting electronic correlation by means of DFT; in this procedure hybrid functional B3LYP in the basis 6-31+G(d,p) was used. The addition of the diffusive function into the basis set was caused by the necessity to improve the description of neighbor atoms and ACS Paragon Plus Environment


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account for the affinity to proton that is impossible without a good description of non-bonding electrons. Such approach is applicable in calculation of the molecules that include cycles and in calculations of the theoretical vibration spectra needed for a comparison with the experimental data. Gas permeation. Determination of the permeability coefficients was performed by the barometric method using a MKS Baratron instrument. Measurements were made for the following gases: He, O2, N2, and CO2 at 25±3°С and the pressure drop of 1 bar (upstream pressure 1 bar and downstream pressure less than 0.001 bar). The average error in the determination of P was 10%. The films of PI precursors were cast from 5% solutions in DMF at 60 °С with subsequent removal of the residual solvent at 160 °С in a vacuum oven. The films with uniform thickness of 20-30 µm were used. The thickness was determined with the accuracy of ± 1 µm. ITR was carried out at 430 °С for 2 h in atmosphere of nitrogen. Results and Discussion Physicochemical and spectral studies Some physicochemical properties of the polyimide precursors and obtained TR polymers are shown in Table 2. Table 2. Glass transition temperatures and densities* of the PI precursor and TR polymers ρ, g/cm3

Тg, °C No Precursor

TR polymer

Precursor

TR polymer

1

330

Not found

1.34

1.28

2

219

Not found

1.42

-

3

331**

Not found

1.44

1.40

4

230

Not found

1.33

1.04

5

260

273

1.37

1.29

* Hydrostatic weighing in isopropanol was used in the determination of the density **See Ref. 15 It is seen that than in most cases thermal rearrangement results in disappearance of Tg, that is, the chains become much more rigid and the supposed glass transition occurs above the onset of thermal decomposition, i.e. above 480-500 °C. The density of the TR polymers decreases as compared with the PI precursors. A typical DSC curve is given in Supporting Materials (Figure S2) The study of the process of thermal rearrangement was started by conducting TGA tests with IR spectral analysis of the formed gases. A typical TGA trace is shown on Figure 1. It was proven that the main gaseous product at 400 – 450 °С is carbon dioxide, (See Figure S3) The mass loss at 350-450oC ACS Paragon Plus Environment

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corresponds to 7–8% of the initial mass of PIs with one OH group. It means that one СО2 molecule is formed per the repeat unit of PI. In this temperature range the intensity of the bands that belong to H2O is small. It can

be interpreted as an evidence of a small degree of polymer destruction and not as manifestation of dehydration reaction. So the mechanism proposed by Hodgkin and Dao6 can be discarded. Figure 1 also displays the peak that characterizes the elimination of CO2. It is interesting that the maximum of this peak is observed at the temperatures of bigger mass loss according to the TGA curve.

Figure 1. TGA curve and the changes of optical density at 2330-2360 cm-1 (range of CO2 absorption) for thermal rearrangement of PI4; the right axis: optical density of CO2 in gas (arbitrary units).

Similar results were obtained for other precursors. A comparison of mass loss and the quantity of carbon dioxide formed shows that these two quantities are very similar (Table 3).

Table 3. The results of TGA and spectral control of elimination of carbon dioxide. No

Starting of CO2 Tmax, °C of CO2 Loss of mass, % o

elimination, C

elimination

Expected loss of mass, %

1

365

450

7.0±1.0

8.3

2

360

445

12.0±1.0

16.0

3

365

420

12.5±1.0

14.0

4

340

418

7.0±1.0

7.2

5

340

428

7.7±1.0

9.0

It is worth noting that bigger mass loss is observed for the polyimides 2 and 3 which contain two OH groups in the diamine component. The last column of the table shows the mass loss found on the basis of assumption that the complete elimination of CO2 takes place. There is no quantitative agreement between the values given in the last two columns but the trend is rather similar. ACS Paragon Plus Environment


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IR spectra of the samples before and after thermal rearrangement are shown in Figure 2. The changes of IR spectra of PI4 and PI5 are discussed in detail by Rusakova et al 11. A general pattern of IR spectra of all PIs and the corresponding TR polymers is the same. (Figure S4). Now we shall discuss the most informative details of these spectra. In the spectra of all the films of the precursors one can see the bands that can be assigned to stretching vibrations of С=О in amide groups of residual DMF (1660 cm–1). In the process of thermal treatment these bands steadily disappear (Figures 2а and S1). In the spectra of thermally treated polymers the bands at 1619 and 1060 cm–1 are exhibited. According to the literature,2,5,8 they can be ascribed to absorption of benzoxazole rings. Thus, our spectral observations do not contradict formation of benzoxazole rings due to thermal rearrangement. Additional evidences in favor of this structure of TR polymers were reported in several works.7, 16 However, the intensity of these bands (1619 and 1060 cm–1) is insignificant in our spectra. It is interesting to note that these absorption bands are especially strong for TR polymer of PI1 (Figure 2b) and TR polymer of PI3 (Figure 2b, as well as Figure S1) On the other hand, in the spectra of TR polymers of all PIs one can see the new bands at ~1680 cm–1, which in combination with the band at ~1556 cm–1 can serve as an evidence of the formation of secondary amides. In spectroscopic literature

10,17

the band corresponding to the

vibrations С=О in amide group at 1670–1680 cm–1 is usually named Amide I, while the band at 1560– 1570 cm–1 is named Amide II. These bands cannot belong to residual DMF since (1) only one band at 1670 cm-1 can belong to DMF (i.e. tertiary amide); (2) after thermal treatment above 300°С no bands that belong to residual DMF are registered in the IR spectra (Figures 2а and S1). It should be noted that the presence of the band at 1560 cm–1 (Amide II) and the ratio of the intensities of the bands of Amide I and Amide II in the IR spectra are characteristic only for primary and secondary amides. Beyond the bands at 1680 and 1560 cm-1 (νC=O Amide I and Amide II) of primary and secondary amides the spectra show the bands of stretching vibrations of N-H bonds in the range above 3300 cm-1. Unfortunately, it is difficult to interpret this range in the IR spectra of TR polymers, because after cooling of the samples in contact with atmosphere a broad band in the range 3700-3450 сm–1 appears. It has low intensity and is caused by water adsorbed on the surface of the sample. One, however, can note an increase in intensity at 3400 сm–1 for the films, having the greatest intensity of the absorption of Amide I and Amide II. As we shall show later on in this work, stretching vibrations of NH bond in the theoretical spectrum of a model of aromatic lactam correspond to the band at wave number 3462 сm–1 with low intensity. Significant changes in the IR spectra after thermal treatment indicate a formation of new structures.


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a) 1680 1660 1619 1060 Absorbance

PI 2-ITR

o

PI 2-350 C

PI 2

1800

1600

1400

1200

1000

Wavenumber, cm

800

-1

Figure 2а. IR spectra of the films of the following PI 2 and the products of their ITR

1680

b)

1660 1619 1556

1060

PI 3-ITR

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PI 3

PI 1-ITR

PI 1 1800

1600

1400

1200

Wavenumber, cm

1000

800

-1

Figure 2b. IR spectra of the films of the following PIs 1-3 and the products of their ITR



c) 1680

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1660

1558 1060

PI 5-ITR PI 5 PI 4-ITR PI 4 1800

1600

1400

1200

1000

Wavenumber, cm

800

600

400

-1

Figure 2c. IR spectra of the films of the following PIs 4-5 and the products of their ITR; the film of PI4 formed from chloroform; its IR spectrum does not show bands that could belong to amide groups. The primary amides have 2 intensive bands in the range 1690-1630 см–1 и 1620-1590 см–1, while for secondary amides the bands in the area 1690-1630 и 1550-1510 см–1 are characteristic.10,17 The spectra of the products of ITR show the bands in this range, and this is consistent with the proposed structure of the aromatic amide. The differences in the relative intensities of the bands at 1600, 1400, 1250, and 1100 cm–1 in the polymers before and after thermal treatment can be explained also by intramolecular rearrangement with formation of entirely different structure of condensed aromatic rings. Earlier, we assumed11 that the following aromatic lactam can be formed from PI4 and PI5 (Scheme 4).

O O

I

II

4000° °CC

O

N H

40 400 °C C -C-CO2

NH O

V

VI

Scheme 4. Structures I and II are the same as in Scheme 2. The proposed mechanism can be realized by elimination of CO2 from etheramide4 (Scheme 1). Quantum chemical calculations were carried out for elucidation of theoretical possibility of such route of thermal rearrangement with formation of aromatic lactam. In these calculations we considered


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the routes leading to the benzoxazole structure (route 1, Scheme 5) and to aromatic lactam (route 2, Scheme 5). O

O

O N+ OH

N

N

O O H

OH O

I

O

I-II

H

II-III

O

O

N

N

O O

III-IV

IV

Route 1 O

O

O

N

N

N

O

O O H

OH O

I

O

H

I-II

II-V

O

O NH

NH

O O V-VI

VI

Route 2 Scheme 5

For calculations we selected the optimized model of PIs that includes the functional groups that can participate in the intramolecular rearrangement:

O

O N

N OH O


O NH


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A search for the corresponding transition states was conducted for both reaction routes. The details of the calculations were disclosed earlier.11 The activation energies of these transition states were found and are displayed in Figure 4.

B

А

Figure 4. Cross-sections along the surface of potential energy for different routes of thermal transformations with the formation of benzoxazole structure (route 1) (a) and via the mechanism with formation of aromatic lactam (route 2) (b).

The arrows in Figures 4A and 4B indicate the differences between the initial energy of a model (energy minimum at the surface of the potential energy) and energy of the transition state. The activation barrier is virtually the same for the both routes of the reaction: 403 kJ/mol for the formation of benzoxazole structure and 414 kJ/mol for the formation or aromatic lactam. However, the structure of the aromatic lactam is more favorable because in this case the total energy minimum (Etot) is by 121-33 = 88 kJ/mol lower than in the case of benzoxazole structure. As mentioned earlier, the theoretical vibration spectra were computed for modeling final products according to the two mechanisms, namely: O O N

O N O

Model a and


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O NH O

O N O

Model b and compared with the experimental spectra. The calculated vibration spectrum of the aromatic lactam agrees better with the observed spectrum. (Table 4 and Figure S5). In the theoretical spectrum, the bands at 3490 cm-1 can be assigned to the vibrations νNH, while the bands at 1690 and 1560 cm-1 to the vibrations of the bond С=O in the amide group; this interpretation finds confirmations in the absorption bands in the experimental IR spectrum. Table 4. Comparison of some bands in the observed IR spectrum of the products of thermal rearrangement of PIs and the calculated spectrum of aromatic lactam as well as the assignment of the frequencies. Theoretical IR Observed IR spectrum, spectrum, Assignment wavenumber, cm-1 -1 wavenumber, cm Mixed deformation vibrations of aromatic 1050 (shoulder) 1054 structure 1100 1120 Deformation vibrations of aromatic cycle Deformation vibrations of aromatic 1167 1166 structure νCOC, 1230 1227 Deformation vibrations of aromatic structure νCOC, 1260 1262 Deformation vibrations of aromatic structure ν C-N 1340 Deformation vibrations of aromatic Wide band with the structure maximum at 1360 Deformation vibrations of aromatic 1358 structure 1440 1444 ν arom (C=С) Mixed vibrations 1477 1477 ν arom (C=С) Amide II 1560 1570 ν arom (C=С) Wide band at 1620-1590 1600 ν arom (C=С) 1670 1671 Amide I 1720 1711 Imide νs(C=O) 1780 1760 Imide,νas(C=O) 3070 3081 ν arom (CН) Wide band at 3600-3490 3462 ν N-H ACS Paragon Plus Environment


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Table 5. Experimental and predicted frequences of benzoxazole structure. Observed IR spectrum, wave number, cm-1

Theoretical IR spectrum, wave number, cm-1

No band

1025

Weak shoulder at 1230

1230

No band

1536

No band

1605

Assignment νCOC (benzoxazole ring) Mixed vibrations (benzoxazole ring) νС=N ν arom (C=С) νС=N ν arom (C=С)

In the calculated spectrum for the model with formation of benzoxazole structure (Model а) only the wave numbers that belong to the imide cycle agree well with the experimental spectrum of the produce of ITR. The greatest intensity in the calculated spectrum for this model (1230 cm-1) that belongs to the mixed (stretching and deformation) C-O-C vibrations in the benzoxazole cycle can be compared but to a very weak shoulder in the observed spectrum. This shoulder can be assigned also to the theoretical wave number 1227 cm-1 that corresponds to mixed (stretching and deformation) vibrations of the bonds Ph-O-Ph and mixed deformation vibrations of aromatic rings in the model of cyclic lactam (Table 4). The second wave number of the group С-О-С benzoxazole cycle (1025 cm-1) is absent in the experimental spectrum. The vibration frequencies of the bonds C=N (1605 and 1536 cm-1) are noncharacteristic, mixed in form with skeleton vibrations of aromatic rings. Thus, intramolecular thermal rearrangement of PIs with OH groups in the diamine fragment in оposition to nitrogen atom in the imide cycle can proceed with formation of the products of different chemical structure. The results of the present work emphasize the role of formation of aromatic lactam in addition to the formation of benzoxazole cycle as is claimed by Calle et al.16 It can be assumed that the composition of the final product can be controlled by temperature and the rate of heating.

Gas permeation studies The permeability coefficients P were determined for the following gases: He, O2, N2, CO2 and for the precursors and the products of thermal rearrangement. Table 5 present the obtained results. The polyimide precursors studied can be characterized as low permeable, rather permselective polymers. In some cases (e.g. P(N2) for PI5) the permeability coefficients are close to the limit of the determination using the Baratron instrument. Ideal separation factor α(O2/N2) relatively high (ca. 8.7 for PI5). A transition to TR polymer is accompanied by a dramatic increase in permeability. Thus, for PI4 thermal treatment results in four-five fold increase in permeability coefficients for all the gases, ACS Paragon Plus Environment

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while for PI5 this effect is much greater: permeability coefficients increase by a factor of 10–20. This is the same trend as has been observed for other thermally rearranged polymers.3,13.

Table 6. Gas permeation parameters of PI precursors and TR polymers.

Sample

Gas

P, Barrer PI

TR polymer

PI4

PI5

He

5.8

21

O2

0.28

1.4

N2

0.067

0.33

CO2

1.20

5.7

He

2.5

22

O2

0.076

1.4

N2

0.0087

0.41

CO2

0.22

5.9

Another important characteristics of membrane materials, separation factors αij=Pi/Pj, also changes to some extent due to thermal rearrangement. Its trend can be visualized on Robeson diagrams18 that characterize the variations of permeability and permselectivity of polymers. Such diagrams are shown in Figure 5 (for oxygen/nitrogen pair) and Figure 6 (for carbon dioxide/nitrogen pair). It is seen that in one case (O2/N2 diagram) drastic increases in permeability are accompanied by some reduction of the separation factors, while in another case (CO2/N2 diagram) the selectivity does not change noticeably. For comparison, Figures 5 and 6 also display the data for one of the PI precursors of this work (PI3) and the values for the corresponding TR polymers. These results are based on the recent work13 where similar PI and TR polymers had been studied. Interestingly, the data points obtained in our work and by Sanders et al.13 form a general succession: it is characterized by strong increases in permeability accompanied by only small variation of the permselectivity. The group of the data points slowly moves in the direction of the Upper bound 2008 of Robeson.18



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Figure 5. Robeson diagram for O2/N2 gas pair and PI precursors (filled points) and TR polymers (open points). The line is Upper bound of 200818 The data for PI3 and PI3 TR are based on the results by Sanders et al.13; here PI3 corresponds to OH-containing polyimide (HAB-6FDA according to the terminology of the authors).

Again, it can be noted that such behavior is a common feature of all TR polymers. Thus, Wang and Chung3 observed steady increases in permeability of CO2 accompanied by some decrease in the separation factor P(CO2)/P(CH4), so the data point for the samples that underwent the thermal treatment at the highest temperature (450oC) is located relatively close to the Upper Bound.

Figure 6. Robeson diagram for CO2/N2 gas pair and PI precursors (filled points) and TR polymers (open points). The line is Upper bound of 2008.18 The data for PI3 and PI3 TR are based on the results by Sanders et al.13; here PI3 corresponds to OH-containing polyimide (HAB-6FDA according to the terminology of the authors). ACS Paragon Plus Environment

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Conclusions Investigation of thermal rearrangement of five novel OH-functionalized PIs was performed using a number of physicochemical methods: TGA with spectral analysis of gas eliminated in the process of thermal reaction, IR spectral study of the precursors and TR polymers, quantum chemical calculations and measurement of the gas permeation parameters of the PI precursors and TR polymers. It was shown that the gaseous products of the reaction comprise exclusively carbon dioxide. The analysis of the spectra of TR polymers in the range 400-4000 cm-1 gave a strong indication of formation of aromatic lactam as the prevailing product of the thermal rearrangement. The reaction leads, as has been noted for other PI precursors, to a strong increase in permeability coefficients, while the separation factors changed insignificantly.

ASSOCIATED CONTENT S Supporting Information Additional spectra are given in the Supporting Information. This information is available free of charge via the Internet at http://pubs.acs.org/. AUTHOR INFORMATION Corresponding Author E-mail: [email protected]. Notes The authors declare no competing financial interest

References (1). Park, H. B.; Jung, C. H.; Lee, Y. M.; Hill, A. J.; Pas, S. J.; Mudie, S. T.; Van Wagner, E.; Freeman, B. D.; Cookson, D. J. Polymers with Cavities Tuned for Fast Selective Transport of Small Molecules and Ions Science 2007, 318. 254. (2) Park, H. B.; Han, S. H.; Jung, C. H.; Lee, Y. M. Thermally rearranged (TR) polymer membranes for CO2 separation J. Membr. Sci. 2010. 359 11. (3). Wang, H.; Chung, T.-S. The evolution of physicochemical and gas transport properties of thermally rearranged polyhydroxyamide (PHA), J. Membr. Sci., 2011, 385-386, 86. (4). Kardash, I. E.; Pravednikov, A. N. Aromatic polyimides containing oxy-and methoxy groups, Vysokomol. Soed., B, 1967. 9, 873. (5). Tullos, G. L.; Powers, J. M.; Jeskey, S. J. Thermal Conversion of Hydroxy-Containing Imides to Benzoxazoles: Polymer and Model Compound Study, Macromolecules 1999, 32. 3598. (6) Hodgkin, J. H.; Dao, B. N. Thermal conversion of hydroxy-containing polyimides to polybenzoxazoles. Does this reaction really occur? Eur. Polym. J. 2009, 45, 3081.



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(9) Kostina, J. V.; Bondarenko, G. N.; Alentiev, A. Yu.; Yampol’skii, Yu. P. Effect of Structure and Conformational Composition on the Transport Behavior of Poly(ether imides), Polymer Science, A, 2007, 49, 77. (10) Bellamy L.J. The Infra-red Spectra of Complex Molecules. London: Methuen and Co LTD, 1954 (11) Rusakova, O.; Kostina, J.; Rodionov, A. S.; Bondarenko, G. N.; Alentiev, A. Yu ; Meleshko, T. K. Kukarkina, N. V.; Yakimanskii, A. V. Study of the mechanism of thermochemical reaction of polyimides with hydroxyl groups using the methods of vibration spectroscopy and quantum chemistry Polymer Science A, 2011, 53, 791. (12) Smith, Z. P.; Sanders, D. F.; Ribeiro, C. P.; Guo, R.; Freeman, B. D.; Paul, D. R.; McGrath, J. E.; Swinnea, S. Gas sorption and characterization of thermally rearranged polyimides based on3,3’-dihydroxy-4,4’diamino-biphenyl (HAB) and 2,2’-bis-(3,4-dicarboxyphenyl)hexafluoropropanedianhydride (6FDA), J. Membr. Sci., 2012, 415–416, 558. (13) Sanders, D. F.; Smith, Z. P.; Ribeiro, C. P.; Guo, R.; McGrath, J. E.; Paul, D. R.; Freeman, B. D. Gas permeability, diffusivity, and free volume of thermally rearranged polymers based on 3,3dihydroxy-4,4-diamino-biphenyl (HAB) and 2,2-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA), J. Membr. Sci., 2012, 409–410, 232. (14) Frisch M. J., Trucks G. W., Schlegel H. B., Scuseria G. E., Robb M. A., Cheeseman J. R., Montgomery J. A., Vreven Jr. T., Kudin K. N., Burant J. C., Millam J. M., Iyengar S. S., Tomasi J., Barone V., Mennucci B., Cossi M., Scalmani G., Rega N., Petersson G. A., Nakatsuji H., Hada M., Ehara M., Toyota K., Fukuda R., Hasegawa J., Ishida M., Nakajima T., Honda Y., Kitao O., Nakai H., Klene M., Li X., Knox J. E., Hratchian H. P., Cross J. B., Bakken V., Adamo C., Jaramillo J., Gomperts R., Stratmann R. E., Yazyev O., Austin A. J., Cammi R., Pomelli C., Ochterski J. W., Ayala P. Y., Morokuma K., Voth G. A., Salvador P., Dannenberg J. J., Zakrzewski V. G., Dapprich S., Daniels A. D., Strain M. C., Farkas O., Malick D. K., Rabuck A. D., Raghavachari K., Foresman J. B., Ortiz J. V., Cui Q., Baboul A. G., Clifford S., Cioslowski J., Stefanov B. B., Liu G., Liashenko A., Piskorz P., Komaromi I., Martin R. L., Fox D. J., Keith T., Al-Laham M. A., Peng C. Y., Nanayakkara A., Challacombe M.,. Gill P. M. W, Johnson B., Chen W., Wong M. W., Gonzalez C., Pople J. A., GAUSSIAN 03, Revision C.02, Wallingford: Gaussian Inc., 2004 (15) Calle, M.; Chan, Y.; Jo, H. J.; Lee, Y. M. The relationship between the chemical structure and thermal conversion temperatures of thermally rearranged (TR) polymers, Polymer, 2012, 53 2783. ACS Paragon Plus Environment

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(16) Calle, M., A.E. Lozano, and Y.M. Lee, Formation of thermally rearranged (TR) polybenzoxazoles: Effect of synthesis routes and polymer form. European Polymer Journal, 2012. 48, 1313. (17) Nakanishi, K. Infrared Absorption Spectroscopy Holden-Day Inc. S-Francisco and Nakodo Company Limited, Tokio 1962 P.58 (18) Robeson, L.M. The upper bound revisited, J. Membr. Sci., 2008, 320, 390.



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For Table of Contents only

O N

O HO N

400 oC

O NH O


Thermal Rearrangement of Functionalized Polyimides: IR-Spectral

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