Synthesis of High Molecular Weight

Research Article Received: 25 April 2017

Revised: 16 June 2017

Accepted article published: 30 June 2017

Published online in Wiley Online Library: 22 September 2017

(wileyonlinelibrary.com) DOI 10.1002/pi.5426

Synthesis of high molecular weight polybenzimidazole using a highly pure monomer under mild conditions Eun-Ki Kim,a,b So Young Lee,a Sang Yong Nam,c Sung Jong Yoo,a Jin Young Kim,a Jong Hyun Jang,a Dirk Henkensmeier,a Hyoung-Juhn Kima* and Jong-Chan Leeb* Abstract An isophthalaldehyde bisulfite adduct (IBA) was synthesized using a reaction between sodium sulfite and isophthalaldehyde. Isophthalaldehyde monosulfite adduct (IMA) was inevitably synthesized during the reaction. Because IBA and IMA have similar solubility in water, it is difficult to separate them through a recrystallization process. In order to obtain pure IBA, an excess of sodium sulfite was used. Highly pure IBA was subsequently obtained without the need for recrystallization. The IBA was then used as the starting monomer for the synthesis of poly[2,2′ -(m-phenylene)-5,5′ -bisbenzimidazole] (PBI). Previous isolation methods for IBA hindered the synthesis of high molecular weight PBI. This new synthetic procedure produces high-purity IBA, which can be used to synthesize high molecular weight PBI. © 2017 Society of Chemical Industry Keywords: engineering plastics; isophthalaldehyde bisulfite adduct; polybenzimidazole; solvent polymerization

INTRODUCTION

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Engineering plastics (EPs) are plastics that are used for engineering purposes. They typically have better mechanical and thermal properties than commonly used plastics, such as polyethylene and polypropylene. Examples of EPs include polyamide, polyimide, polyethylene terephthalate and polycarbonate. These materials are expected to be used instead of some metals due to their significant physical properties and low weight. Poly[2,2′ -(m-phenylene)-5,5′ -bibenzimidazole] (PBI) is known as one of the most thermally stable EPs. It has a high decomposition temperature (over 600 ∘ C) and is fire resistant.1 These traits have led to it being extensively used in the manufacture of fire protective clothing and aerospace suits. Recently, there has been a lot of progress to increase the application of PBI. These applications include fiber,2–4 membranes,5,6 nano-filtration7 and gas separation.8 Currently, commercial PBI is available in global markets as Celazole® and SCM 7000. Although PBI has some drawbacks such as the difficulty in dyeing and its low solubility in organic solvents, a large number of researchers are focusing on improvement of these properties by various methods. Improvement can be achieved by modification of the PBI backbone,9 nanocomposites,10 PBI copolymers11 and PBI blending.12 Although researchers have investigated many modified PBI structure derivatives,13–15 the basic meta-PBI (m-PBI) is the most useful form of the material. In industry, m-PBI is mainly produced using the solvent polymerization method with polyphosphoric acid (PPA). PPA is very hygroscopic and as such it is effective for polycondensation reactions, like PBI synthesis. However, PPA is too viscous to be used effectively in mass production processes. Also, Polym Int 2017; 66: 1812–1818

the PBI exists as the H3 PO4 salted structure after polymerization in PPA. H3 PO4 salted PBI has to be neutralized by base treatment to make the PBI soluble in common organic solvents such as 1-methyl-2-pyrrolidinone (NMP) or N,N-dimethylacetamide (DMAc). This work-up stage is very complex and should be simplified for large volume production. If a milder solvent can be found, then PBI can be produced more efficiently. Unfortunately, few papers have been published on this topic, and those that have contain some problems. For example, one of the papers describes producing PBI using DMAc with a bisulfite adduct, but the group had difficulty in increasing the purity of the adduct monomer.16 This research group also used sulfolane as the solvent in the synthesis process.17 However, a high molecular weight of m-PBI could not be achieved due to the low solubility of m-PBI in sulfolane. More recently, it has been reported that a high molecular weight PBI was obtained by polymerizing the monomer isophthalaldehyde bisulfite adduct (IBA) with 3,3′ -diamino-

∗

Correspondence to: H-J Kim, Fuel Cell Research Center, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea or J-C Lee, School of Chemical and Biological Engineering, Seoul National University, Seoul 08826, Republic of Korea. E-mail: [email protected] (Kim); [email protected] (Lee)

a Fuel Cell Research Center, Korea Institute of Science and Technology, Seoul, Republic of Korea b School of Chemical and Biological Engineering, Seoul National University, Republic of Korea c Department of Materials Engineering and Convergence Technology, Engineering Research Institute, Gyeongsang National University, Jinju, Korea

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© 2017 Society of Chemical Industry

Synthesis of polybenzimidazole

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benzidine (DAB) in DMAc.18 According to that paper, the purity of the isophthalaldehyde bisulfite was greater than 97%, and less than 3% isophthalaldehyde monosulfite was produced as an impurity. In the present report, we develop a method for improving the purity of this bisulfite adduct monomer. Our method produces a 100% pure isophthalaldehyde bisulfite, which can be used to make a high molecular weight PBI. The results can be used as reference for the design of PBI mass production processes.

EXPERIMENTAL Materials and characterization Sodium sulfite, methanol, dimethyl sulfoxide (DMSO), DAB, DMAc, lithium bromide (LiBr), NMP, 2,2,4-trimethylpentane, N,N-dimethylformamide (DMF), chloroform (CF), isophthalic acid, PPA and tetrahydrofuran (THF) from (Sigma-Aldrich, St. Louis, MO, USA) were used without any purification. Isophthalaldehyde from (Tokyo Chemical Industry, Tokyo, Japan), DMSO-d6 from (Cambridge Isotope Laboratories, Tewksbury, MA, USA) and isopropanol from (Daejung Chemistry, Cheong-ju, Korea) were also used as received. Commercial PBI was purchased from (PBI Advanced Materials Korea, Bucheon, Korea). NMR data were obtained using a 400 MHz Bruker NMR spectrometer with a DMSO-d6 solvent. Infrared (IR) spectroscopy data were obtained using a Lambda FTIR-7600. The molecular weight and polydispersity (PDI) were obtained through gel permeation chromatography (GPC) with a Waters Empower 3 Personal gel permeation chromatograph. The GPC samples were made using a 0.05 mol L−1 LiBr/NMP solution, and the results were based on a polystyrene reference material. TGA experiments were conducted using a TA Instrument Q50 at 10 ∘ C min−1 under a nitrogen atmosphere. A Sartorius MSA124S-DU with 2,2,4-trimethylpentane was used to measure density. A solubility test was conducted by checking whether polymer samples (2–15 wt%) were soluble in an organic solvent, such as NMP, DMAc or THF. All of the dissolution tests were conducted at temperatures above 60 ∘ C in order to accelerate the procedure. The mechanical properties, including tensile strength, were recorded using a Tinius Olsen H5KT at a speed of 10 mm min−1 . The glass transition temperature (T g ) was determined using a DSC curve that was recorded with a TA Instrument Q10 DSC. This DSC test was done using a secondary heating procedure that was conducted at 10 ∘ C min−1 . Synthesis of PBI by solvent polymerization using PPA PBI was synthesized through a condensation reaction between DAB and isophthalic acid in PPA, as described in a previously published paper.19 Synthesis of the isophthalaldehyde bisulfite adduct (IBA) Sodium sulfite (22.5 g, 0.216 mol) was dissolved in 75 mL distilled water, and isophthalaldehyde (14.5 g, 0.108 mol) was dissolved in 500 mL MeOH. The two were mixed in a 1000 mL round flask and stirred for 24 h. A white precipitate subsequently formed, and it was filtered and dried in a vacuum oven for 24 h. Different amounts of sodium sulfite were used for the synthesis of IBA, while the amount of isophthalaldehyde used was fixed.

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PBI synthesis through solvent polymerization using DMAc The DMAc and IBA were mixed in a 100 mL four-necked round flask with a condenser, argon inlet/outlet and a mechanical stirrer. After 1 h of drying at 100 ∘ C under an inert argon gas environment, 35 mL of DMAc was added to the flask. The synthesis was conducted for 18 h at 180 ∘ C while the solution was slowly stirred. A brown-colored polymer solution was then precipitated in distilled water before being washed repeatedly using water and methanol. The final polymer product was filtered and dried in a vacuum oven for 48 h.

RESULTS AND DISCUSSION Imperfect reaction between sodium sulfite and isophthalaldehyde Instead of using isophthalic acid, IBA was obtained for the synthesis of PBI (Scheme 1). In the first synthesis trial, a chemically equivalent ratio (molar ratio of sodium sulfite and isophthalaldehyde 2:1) of the starting monomers was used for the IBA synthesis; this resulted in an imperfect reaction occurring between the two. In the 1 H NMR spectrum obtained for the IBA (Fig. 1(b)), peaks appeared that were not related to the IBA structure. These peaks were attributable to the totally unreacted isophthalaldehyde or the half-reacted adduct material (isophthalaldehyde monosulfite adduct (IMA)). This postulation is reasonable, as most of the structure-unrelated peaks had similar chemical shifts to the 1 H NMR spectrum peaks of isophthalaldehyde (Fig. 1(a)). If the purity of the IBA is low, the final polymer product will have a low molecular weight according to Carother’s equation: degree of polymerization =

1 1−p

The degree of polymerization refers to the number of repeating units that constitute a polymer chain, and p refers to the monomer conversion of a reaction. As such, a low purity monomer results in a low degree of polymerization. For this reason, we needed to find a way to separate the imperfect by-products from the reaction mixture. Although purification methods like recrystallization or column chromatography are typically used for this purpose, they were not effective in this instance. With regard to recrystallization, both IBA and IMA were too soluble in water for pure IBA to be recrystallized, and their solubility in distilled water was still high even at very low temperatures. However, the IBA was not soluble in alcohol, even near the alcohol’s boiling point; as such, recrystallization using alcohol was impossible. With regard to column chromatography, aside from this process being very time-consuming it would not be suitable for the sulfite structure due to problems related to solubility.20 As a result, we needed to find another method for separating the by-products from the IBA. New synthetic strategy utilizing an excess of sodium sulfite To improve the yield of IBA, we amended the reaction ratio by adding an excess of the reactant, sodium sulfite. As the molar ratio of sodium sulfite in the reaction increased, the intensity of the peaks that were not related to the structure of IBA decreased (Fig. 2). More specifically, the peak around 10.0 ppm, which is not related to IBA, decreased dramatically.


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IBA purification process After synthesizing the IBA using an excess of sodium sulfite, 5 g of the reaction mixture was dissolved in 50 mL DMSO. The insoluble precipitate was filtered using a Whatman 45 𝜇m polytetrafluoroethylene syringe filter. Following this, the solution was poured

into acetone, and pure white IBA was again precipitated. After the filtration, the precipitate was dried in a vacuum oven at 60 ∘ C.

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E-K Kim et al.

Scheme 1. Synthesis of isophthalaldehyde bisulfite adduct (IBA).

Figure 1. 1 H NMR spectrum of (a) isophthalaldehyde and (b) the reaction mixture from a 2:1 equivalent ratio of the starting materials.

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However, this may result in a sodium sulfite residue remaining in the final IBA product. In order to separate the two, the mixture was added to DMSO. We did this because, while sodium sulfite is not soluble in DMSO, IBA is. The dissolution time should be no more than 30 min, however, because IBA will degrade in DMSO if left too long (Fig. 3), which may cause a reverse reaction (i.e. from IBA back to isophthalaldehyde and sodium sulfite); the reaction mechanism behind this should be investigated in the near future. When a similar stability test was conducted in air and with another organic solvent, DMAc, the reverse reaction did not occur. Based


on the NMR results, it is reasonable to conclude that intact IBA structures can be obtained when dissolution is conducted for less than 30 min in DMSO. After filtration, the IBA solution was poured into acetone to once again precipitate pure IBA. The IBA was precipitated as a white powder, and it had a very high purity of nearly 100% (Fig. 3(a)). Unfortunately, this purification process was rather complex, and as a result it is not ideal for the mass production of PBI. We therefore need to find other methods that will make the work-up stage simpler; for example, a potential solution could be that, during the IBA


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Figure 2. 1 H NMR spectrum of IBA made using (a) 0.216 mol (2:1 ratio), (b) 0.324 mol, (c) 0.389 mol and (d) 0.432 mol of sodium sulfite.

reaction, the isophthalaldehyde solution is gradually added dropwise over a long period of time. This could make the reaction conditions better because the isophthalaldehyde reacts with excess sodium sulfite.

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Synthesis and characterization of polybenzimidazole (PBI) By using the pure IBA, synthesized above, and DAB as monomers, we carried out the solvent polymerization of PBI in DMAc (Scheme 2). The tetra-amine monomer acted as a nucleophile, and one of its amine groups attacked the carbon bonded to the –OH and –SO3 − Na+ in IBA. This attack occurred twice, as the same carbon was attacked by the amine group adjacent to the amine group that had just reacted. Because this reaction was a condensation, small molecules, such as H2 O and NaHSO3 , were produced during the synthesis procedure. This scheme has a couple of advantages over the scheme that uses PPA as a solvent. First, a DMAc solvent is used as the synthesis solvent instead of PPA. PPA is a good polycondensation solvent but it is too viscous to be used for mass production purposes. Additionally, a lot of complex work-up steps are needed in order to compensate for the strong acidity of PPA. Second, the reaction temperature is lower; using DMAc, the reaction temperature can be around 180 ∘ C, which is 40 ∘ C lower than that used in the PPA scheme. The successful polymerization of PBI was confirmed using the data from the NMR, IR and GPC measurements (Fig. 4 and Table 1). In the NMR spectrum, all the peaks were confirmed to be related to the PBI structure. Similarly, the PBI structure was checked using

the IR spectrum; the spectrum was almost the same as those of well-known references.21 In terms of molecular weight, the PBI produced using DMAc had a number-average molecular weight of 78 kDa, which is a little higher than that produced using the PPA scheme. Based on these data, we can conclude that the synthesis of PBI using a milder solvent (namely DMAc) was successful. In order to evaluate the thermal stability of the PBI produced using DMAc as a solvent, we characterized the PBI with TGA and DSC (Figs 5 and 6, respectively). On the TGA curve, the weight loss that occurred from room temperature to around 250 ∘ C was due to evaporation of the residual water and DMAc. The thermal degradation temperature was found to be about 600 ∘ C, which is similar to that found in previous research.1 The glass transition temperature (T g ) was found using the inflection point of the DSC curve, and it was 427 ∘ C; this figure is also similar to that in previous studies.22 In order to obtain information about the crystallinity of the PBI using DMAc as a solvent, we measured its density (Table 2). Normally, the greater the crystalline fraction in a structure, the higher the density of the polymer; by considering that PBI has a compact structure, this average figure could be reasonable. Furthermore, solubility is a very important factor when evaluating the processability of a polymer. Various organic solvents were used to dissolve the PBI using DMAc as a solvent (Table 3). In highly polar solvents, such as NMP, DMAc, DMF and DMSO, the PBI was very soluble at moderately high temperatures, such as 60 ∘ C. However, in relatively non-polar solvents, like CF and THF, it was not soluble. This result can be explained by the ‘like dissolves like’

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E-K Kim et al.

Figure 3. 1 H NMR spectrum of IBA according to the dissolution time in DMSO during purification.

Scheme 2. Solvent (DMAc) polymerization of PBI.

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Figure 4. (a) 1 H NMR and (b) IR spectra of PBI synthesized using DMAc.



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Table 1. GPC results for the PBI using DMAc and PPA as a solvent Mw

Mn PBI (DMAc) PBI (PPA)

78 kDa 74 kDa

Table 4. Mechanical properties of PBI and SCM 7000 Tensile strength (MPa)

PDI

144 kDa 112 kDa

1.84 1.50

PBI (DMAc) SCM 7000

Young’s modulus (MPa)

100.5 (0.7) 103.5 (0.7)

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Elongation at break (%) 8.18 (0.1) 9.47 (0.1)

theory; that is, as PBI has a highly polar imidazole structure, the interactions between PBI and polar solvents can be strong enough for it to be more soluble in such solvents. These results confirm that PBI has good solubility. Moreover, the solvent polymerization method described in this paper can be used to synthesize PBI derivatives that have a modified structure. Lastly, the mechanical properties of the PBI were compared to those of commercial PBI, SCM 7000 (Table 4). By taking into consideration that the PBI was produced on a laboratory scale, we find that the small differences between the two are negligible. As such, the PBI has similar mechanical properties to those of commercial PBI.

CONCLUSION PBI was synthesized using a bisulfite monomer and a DMAc solvent. Particular effort was put into studying the purity of the synthesized monomer. By using our proposed procedure a high purity bisulfite monomer was obtained, and as a result a high molecular weight PBI could be synthesized (78 kDa). The mechanical properties of this PBI were investigated, and they were found to be mostly equivalent to those of the commercial PBI, SCM 7000. Additionally, the density, thermal properties and solubility of the PBI that we synthesized were also similar to the properties of PBI that was produced by other means. Although the extra purification steps required by our process limit the simplicity of this scheme, we believe that our purification and synthesis methods will enable PBI to be applied in factory production processes in the future.

Figure 5. TGA curve of PBI using DMAc as solvent.

ACKNOWLEDGEMENT This work was partially supported by the projects KIST Institutional Program from the Korea Institute of Science and Technology. Moreover, it was partially supported by the National Research Foundation of Korea (NRF) Grant funded by the Ministry of Science, ICT and Future Planning (NRF-2016M1A2A2937136).

Figure 6. DSC curve of PBI using DMAc as a solvent.

Table 2. Density measurement of PBI using DMAc as a solvent 1 Density (g cm−3 )

1.229

2

3

1.320

4

1.297

Average

1.253

1.274 (0.04)

Table 3. Solubility tests for PBI using DMAc as a solvent

3 wt% 6 wt% 8 wt% 10 wt% 12 wt% 15 wt%

NMP

DMAc

DMF

DMSO

CF

THF

++ ++ ++ ++ + +

++ ++ ++ ++ + +

++ ++ ++ ++ + +

++ ++ ++ ++ + +

− − − − − −

− − − − − −

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++, very soluble at 60 ∘ C; +, soluble at 100 ∘ C; −, not soluble.

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