A comparison study of high thermal stable and resistant polyimides Open Access

Polyimides (PIs), a class of polymers with imide-containing heterocyclic rings, were first reported in 1908 by Liaw.¹ From then on, a lot of attention has been focused on PIs due to their excellent overall properties, including thermo-oxidative stability, unique electrical properties, and excellent mechanical properties and dimensional stability.^2–4 Benefitting from the excellent comprehensive properties, PI films are always used as heat-resistant insulating materials, flexible printed circuits, dielectric and flexible connecting materials for multichip model systems, tape automated bonding, substrates for flexible devices, and lithium-ion battery separators.^5–8 Especially in the portable energy product field, the utilization of PI films as substrates can effectively improve the flexibility and reduce the weight of devices, such as Cu(In,Ga)Se₂ (CIGS) or Cu₂ZnSnS₄ (CZTS) solar cells. However, common commercial PI films cannot be compatible with the high temperature deposition process of CIGS solar cells, which is usually carried out at above 530 °C.⁹ 

Researchers have focused on PIs with excellent comprehensive properties and hunted for practical synthetic methods. From the designing of molecule structures, it has been found that the introduction of certain functional segments, such as imidazole rings, oxazole rings, pyridine, etc., can enhance the thermal endurance of PIs.^10–14 Therefore, in this study, we employed bisbenzimidazole and bisbenzoxazole based diamines to prepare poly(amic acid) (PAA) solutions with dianhydrides via two-step solution-based polymerization under even stirring. Then two kinds of PI thin films could be obtained after hot-pressing amination and cyclodehydration processes with PAA solutions. The synthetic process of PI thin films is shown in Scheme 1. For comparison, two other kinds of commercial PI thin films were also prepared through the same synthetic process. After measuring the thermal, mechanical, and hygroscopic properties, the PI thin films containing bisbenzimidazole and bisbenzoxazole segments showed excellent thermal stabilities (with a thermal decomposition temperature of higher than 530°C), outstanding mechanical properties (with a tensile strength of around 200 MPa), and relatively low water absorption percentages.

4,4′-([6,6′]bis[benzimidazolyl]-2,2′-diyl)-bis-aniline (BAPBBI) was used as the bisbenzimidazole diamine monomer, with the synthetic process shown in Fig. 1. First, 150 g polyphosphoric acid (PPA, Aladdin) was added into a 500 ml three-necked round bottom flask with mechanical stirring under a N₂ atmosphere at 70°C. Then, 10.713 g (0.05 mol) 3,3′-diaminobenzidine (DAB, Alfa Aesar) and 14.399 g (0.105 mol) p-aminobenzoic acid (PABA, Aladdin) were successively added into the reaction flask with thermal heating to 110°C slowly and rested for 2 h. After that, the flask was thermal heated to 200°C slowly and rested for 6 h. After the reaction, when the solution cooled down to around 100°C, it was mixed with a large amount of deionized (DI) water, which owns about six times the volume of the reaction solution. After filtration, the pH value of the residue on the filter would be adjusted by 0.5 M NaHCO3 (Aladdin) solution to 7–8. Then the residue was washed with DI water at 60°C under stirring. The filtration was repeated until a clear supernatant was found after washing the residue. After final filtration, the residue was dried in vacuum at 80–100°C overnight. Finally, the BAPBBI could be obtained with further sublimation, with a yellow brown color, ∼ 70% yield, and a 327°C melting point.

Figure 2 shows the 1H-NMR of 4,4′-([6,6′]bi[benzimidazolyl]-2,2′-diyl)-bis-aniline. The protons at the 2, 2′, 3, and 3′ positions are separated from each other due to the presence of protons on the carbobenzene ring at adjacent positions, which are double peaks with chemical shifts of 6.71 ppm and 7.95 ppm, respectively; other protons are single peaks. The chemical shifts of the proton peaks on the trisubstituted benzene ring are in the range of 7.46–7.79 ppm. It is to be noted that the chemical shift of the proton on the N–H bond on the benzimidazole ring is at 12.46 ppm in the ultra-low field region, which is due to the easy-to-form intermolecular hydrogen bonds, and the hydrogen bond proton H has a small shielding effect in the low-field and high-frequency region.

The synthetic reaction of polyimides containing bisbenzimidazole units is shown in Fig. 3. First, 4.161 g (0.01 mol) BAPBBI and a small amount of N,N-dimethylacetamide (DMAc, Aladdin) solvent were added into a three-necked round bottom flask equipped with a mechanical stirrer at room temperature and under a N₂ atmosphere. When the BAPBBI powder was dissolved completely, 2.942 g (0.01 mol) 3,3′,4,4′-biphenyltetracarboxylic dianhydride (s-BPDA, Aladdin) and the remaining DMAc solvent were added (total 63.5 g DMAc solvent used) with stirring for 12 h, obtaining a slurry with ∼10% solid content. The PAA solution could be obtained by filtering the slurry with a sand core filter funnel (with 40–80 µm core size) and leaving for defoaming. Then, glass substrates of 2 mm thickness were used for film coating (with a speed of 250 mm/s and thickness of 300 µm) with the PAA solution after washing and drying. The coated substrates were transferred to a horizontal hot plate for thermal treatment at 30°C for 1 h and 60°C for 2 h in sequence. Then they were placed in a vacuum oven, which was cleaned and filled with He gas, and kept at 80, 120, 150, 200, 250, 300, 330, 350, and 370°C for 1, 1, 1, 1, 1, 1, 0.5, 0.5, and 0.5 h, respectively (heating rate: 1°C/min below 200 and 2°C/min above 200). Finally, a piece of bisbenzimidazole PI film of ∼30 µm thickness that peeled off automatically in warm water (40°C) was obtained by drying in an oven, which would be named as the s-BPDA/BAPBBI film.

4,4′-([6,6′]bis[benzoxazolyl]-2,2′-diyl)-bis-aniline (BAPBBOA) was used as the bisbenzoxazole diamine monomer, with the synthetic process shown in Fig. 4. First, 150 g PPA was added into a 500 ml three-necked round bottom flask with mechanical stirring under a N₂ atmosphere at 70°C. Then, 10.813 g (0.05 mol) 3,3′-dihydroxybenzidine (DHB, Alfa Aesar), 14.399 g (0.105 mol) PABA, and a small amount of an amino protective agent (SnO · H₂O, Aladdin) and desiccant (P₂O₅, Aladdin) were successively added into the reaction flask with thermal heating to 110°C slowly and rested for 2 h. After that, the flask was heated to 200°C slowly and rested for 6 h. When the solution cooled down to ∼100°C, it was mixed with a large amount of DI water, which was about six times the volume of the reaction solution. After filtration, the pH value of the residue on the filter was adjusted by 0.5 M NaHCO₃ solution to 7–8. Then the residue was washed with DI water at room temperature under stirring. The filtration was repeated until a clear supernatant found after washing the residue. After final filtration, the residue was dried in vacuum at 80°C overnight. Finally, the BAPBBOA could be obtained with further sublimation, with a luminous yellow color, ∼ 70% yield, and a 350°C melting point. Figure 5 shows the 1H-NMR of 4,4′–([6,6′]bi[benzoxazolyl]-2,2′-diyl)-bis-aniline. Compared to imidazolediamine, the H peak on the aromatic ring is in the range of 7.72–8.06 ppm; especially the chemical shifts of the proton peaks at positions four and five on the trisubstituted benzene ring are the same, superimposed at 7.72 ppm, showing a sharp the single peak.

The synthetic process of bisbenzoxazole containing PIs is shown in Fig. 6. First, 4.181 g (0.01 mol) BAPBBOA and a small amount of DMAc were added into a three-necked round bottom flask equipped with a mechanical stirrer at room temperature and under a N₂ atmosphere. When the BAPBBOA powder was dissolved totally, 2.942 g (0.01 mol) s-BPDA and the remaining DMAc solvent were added (a total of 64.1 g DMAc solvent was used). After 12 h reaction, a slurry was obtained with ∼10% solid content. The PAA solution could be obtained by filtering the slurry with a sand core filter funnel (with 40–80 µm core size) and leaving for defoaming. Then, glass substrates of 2 mm thickness were used for film coating (with a speed of 250 mm/s and thickness of 300 µm) with the PAA solution after washing and drying. The coated substrates were transferred to a horizontal hot plate for thermal treatment at 30°C for 1 h and 60°C for 2 h in sequence. Then they were placed in a vacuum oven, which was cleaned and filled with He gas, and kept at 80, 120, 150, 200, 250, 300, 330, 350, and 370°C for 1, 1, 1, 1, 1, 1, 0.5, 0.5, and 0.5 h, respectively (heating rate: 1°C/min below 200 and 2°C/min above 200). Finally, a piece of bisbenzoxazole PI film of ∼30 µm thickness that peeled off automatically in warm water (40°C) was obtained after drying in an oven, which would be named as the s-BPDA/BAPBBOA film.

The synthetic process of DuPont PI thin film is shown in Fig. 7. First, 38.631 g DMAc and 2.002 g 4,4′-oxydianiline (ODA) were added into a three-necked round bottom flask (100 ml) with stirring for complete dissolution room temperature and under a N₂ atmosphere. Then 2.290 g pyromellitic dianhydride (PMDA) was quickly added into the above-mentioned flask and allowed to polymerize for 12 h to obtain the homogeneous PAA solution (solid content: ∼10% and viscosity: ∼2 Pa · s), which was filtered by serial sand core filter funnels (with 40–80/15–40 µm core size) and left for defoaming. Then, glass substrates of 2 mm thickness were used for film coating (with a speed of 250 mm/s and thickness of 300 µm) with PAA solution after washing and drying. The coated substrates were transferred to a horizontal hot plate for thermal treatment at 30°C for 1 h and 60°C for 2 h in sequence. Then they were placed in a vacuum oven, which was cleaned and filled with He gas, and kept at 80, 120, 150, 200, 250, 300, 330, and 350°C for 1, 1, 1, 1, 1, 1, 0.5, and 0.5 h, respectively (heating rate: 1°C/min below 200°C and 2°C/min above 200). Finally, a piece of DuPont PI film of ∼30 µm thickness that peeled off automatically in warm water (40°C) was obtained by drying in an oven, which would be named as the PDMA/ODA film.

The synthetic process of Ube PI film is shown in Fig. 8. First, 36.207 g DMAc and 1.081 g p-phenylenediamine (PPD) were added into a three-necked round bottomed flask (100 ml) at room temperature and under a N₂ atmosphere with stirring for complete dissolution. Then 2.942 g s-BPDA was quickly added into the above-mentioned flask and allowed to polymerize for 12 h to obtain a homogeneous PAA solution (solid content: 10% and viscosity: ∼2 Pa · s), which was filtered by serial sand core filter funnels with 40–80/15–40 µm core size and left for defoaming. Then, glass substrates of 2 mm thickness were used for film coating (with a speed of 250 mm/s and thickness of 300 µm) with the PAA solution after washing and drying. The coated substrates were transferred to a horizontal hot plate for thermal treatment at 30°C for 1 h and 60°C for 2 h in sequence. Then they were placed in a vacuum oven, which was cleaned and filled with He gas, and kept at 80, 120, 150, 200, 250, 300, 330, and 350°C for 1, 1, 1, 1, 1, 1, 0.5, and 0.5 h, respectively (heating rate: 1°C/min below 200°C and 2°C/min above 200). Finally, a piece of Ube PI film of ∼30 µm thickness that peeled off automatically in warm water (40°C) was obtained by drying in an oven, which would be named as the s-BPDA/PPD film.

The thermogravimetric (TGA) curves were collected by using a TGA2-SF (Mettler Toledo), with thin film samples cut to strips (5–10 mg) and placed in crucibles. The measurement parameters were set to a temperature range of 25–800°C, to a ramp rate of 10°C/min, and under N₂ protection. The dynamic mechanical analysis (DMA) was conducted using a Q800 of the American TA company, using 6.35 × 6.35 mm² size PI films of a thickness of about 30 µm. The measurement parameters were set as choosing the stretching mode, a temperature range of 25–500°C, and a heating rate of 5°C/min under N₂ protection. The mechanical properties were tested using an H5K–S machine (Hounsfield, UK), with 50 × 10 mm² size PI films. The thickness would be measured three times to find the average value. The mechanical testing would be conducted three times with a tensile rate of 5 mm/min to find the average value. High resolution nuclear magnetic resonance (NMR) measurement was conducted using an American Varian 400-MR with a frequency of 400 MHz. Deuterated DMSO-d6 was used as solvent, and tetramethylsiloxane (TMS) was used as a reliable internal chemical shift reference.

The Fourier transform infrared spectroscopy (FT-IR) measurements were conducted for s-BPDA/BAPBBI, s-BPDA/BAPBBOA, PDMA/ODA, and s-BPDA/PPD, with results shown in Fig. 9. According to previous reports, the typical peaks of the vibration absorption bands for aromatic imides are at around 1780, 1720, 1380, and 725 cm⁻¹.¹⁵ From the infrared spectra, all the four samples, s-BPDA/BAPBBI, s-BPDA/BAPBBOA, PDMA/ODA, and s-BPDA/PPD, in sequence show peaks of carbonyl C=O in the imide rings at 1780 cm⁻¹ (1778, 1776, 1776, and 1777 cm⁻¹, respectively), 1720 cm⁻¹ (1731, 1728, 1737, and 1730 cm⁻¹, respectively), and 725 cm⁻¹ (735, 742, 727, and 733 cm⁻¹, respectively), which are caused by the large dipole moment and the stretching and bending vibrations of the carbonyl group.^16–18 Meanwhile, the four samples possess peaks at 1380 cm⁻¹ (1380, 1377, 1382, and 1374 cm⁻¹, respectively), corresponding to the stretching vibration of the C–N group in the imide ring.^19,20 Moreover, they also have peaks at 3480 cm⁻¹ (3380, 3483, 3495, and 3482 cm⁻¹, respectively), according to the stretching vibration of the N–H group.²¹ These characteristic absorption peaks confirm the aromaticity of the prepared PI films by the successful synthesis of imide groups through the dehydration and ring closure of PAA.

The degree of thermal imidization also could be reflected through the infrared spectra, which would demonstrate the effect of the curing temperature. Form the infrared spectrum of PAA, it can be seen that the carbonyl C=O (in carboxy –COOH) has a characteristic absorption peak in the range of 1720–1706 cm⁻¹ and the C–O (in carboxyl –COOH) has a characteristic stretching peak in the range of 1320–1210 cm⁻¹.^22,23 From this perspective, in the four kinds of PI films prepared by us, the typical characteristic peaks of the PAA could not be found from the infrared spectra (Fig. 9), illustrating the high degree of imidization. The result also verifies the competence of the curing temperature designed for the four samples. For more in-depth analysis, compared to the s-BPDA/BAPBBOA film, the s-BPDA/BAPBBI film shows a relatively strong peak at 3610 cm⁻¹ from the infrared spectra due to the hydrogen bond formed between the N–H bonds in the diamine structure. Meanwhile, it could be observed that the PMDA/ODA film exhibits unique strong peaks at 1166 and 1120 cm⁻¹, when compared to the s-BPDA/PPD film. The peaks would be ascribed to the C–O–C bond in the molecular chain skeleton structure of the PMDA/ODA film, which could not be observed for the s-BPDA/PPD film.

As mentioned above, when used as a flexible substrate of CIGS thin film solar cells, PI thin films need to meet high standards of thermal stability. The thermal stability of polymers is usually represented by T_d (thermal decomposition temperature), which could be obtained through the TGA experimentally. Due to the great challenge in finding the precise initial drop point of temperature from TGA curves, T_d5% is often used as the changing point and to evaluate the thermal stability of polymers. From the TGA curves in Fig. 10, it can be clearly observed that s-BPDA/BAPBBI and s-BPDA/BAPBBOA show T_g5% values of 581°C and 584°C, respectively. In comparison, the lab-synthesized DuPont (PMDA/ODA) and Ube (s-BPDA/PPD) films have lower thermal stability, with T_g5% values of 505 and 560°C, respectively. Because of owning the rigid molecular chain skeleton and strong intermolecular forces, the PIs with bisbenzimidazole and bisbenzoxazole groups show relatively higher thermal stability and can basically withstand the temperature during the production of thin film solar cells.

In the process of preparing flexible solar cells, the PI substrate also needs to meet the requirement of physical thermal resistance, which determines the working time of PI during the solar cell fabrication. The thermal resistance of polymers is usually represented by T_g (glass-transition temperature), which could be obtained by finding the peak point of the DMA or thermomechanical analysis (TMA) curves. The DMA measurements were conducted for s-BPDA/BAPBBI and s-BPDA/BAPBBOA, with results shown in Fig. 11. By finding the peaks from the curves, the T_g values could be obtained for s-BPDA/BAPBBI and s-BPDA/BAPBBOA as 421 and 339°C, respectively. At the glass-transition point, the change in energy storage capacity is caused by the rapid movement of the molecular chain. The existence of the biaromatic rings inside the bisbenzimidazole and bisbenzoxazole groups enhances the rigidity of molecular chains. Meanwhile, the N atoms inside the bisbenzimidazole group and the O atoms inside the bisbenzoxazole group can form hydrogen bonds between and within molecular chains to further enhance the intermolecular force. Therefore, both PI films have relatively high energy storage capacity.

Apart from excellent thermal properties, superior mechanical property was also required of PI films for commercial applications, such as tensile strength and elongation at break. The stress–strain curves of the four kinds of PI films were measured, with results shown in Fig. 12. It can be observed that the tensile strengths of s-BPDA/BAPBBI and s-BPDA/BAPBBOA are 218 and 192 MPa, respectively. In comparison, the tensile strengths of PMDA/ODA and s-BPDA/PPD films are only 110 and 137 MPa, respectively. The excellent mechanical properties of s-BPDA/BAPBBI and s-BPDA/BAPBBOA should be ascribed to the rigid molecular chain skeleton and the hydrogen bonds formed between and inside the molecular chains. Besides, the PMDA-ODA film shows relatively good elongation at break (18.8%), which would be due to the flexible -O- segment in the molecular chain skeleton.

The water absorption percentage (W) refers to the film’s absorption ability of moisture in the air. For measuring the water absorption percentage, a piece of PI film (2 × 2 cm²) was placed in a vacuum oven at 100°C for 6 h, with the initial weight measured as G₀. Then the PI film was moved to a constant temperature and humidity chamber (25°C and 50% air humidity) for 24 h, with the weight measured as G₁. The calculation equation of the water absorption percentage is as follows:

The results of the water absorption percentage with the oxygen and nitrogen contents are provided in Table I for the four kinds of PIs. It can be observed that the water absorption percentage is according to the number of hydrophilic groups in the molecular chains of each sample. The water absorption of polymers was related to molecular packing and the polar units with high affinity.²⁴ PMDA/ODA and s-BPDA/PPD show higher water absorption percentages due to more oxygen and nitrogen contents as hydrophilic groups in the molecular chains. The existence of benzoxazole units in the main chain and accordingly the strong intermolecular association also contribute to the lower water absorption of PI in our study.²⁵ The water absorption percentages of s-BPDA/BAPBBI and s-BPDA/BAPBBOA are similar and less than 1.0%, which could be well qualified for practical utilization.

In this work, we synthesized four kinds of PI thin films, with two kinds of commercial products and two kinds of lab-fabricated products containing bisbenzimidazole and bisbenzoxazole groups. s-BPDA/BAPBBI and s-BPDA/BAPBBOA possess excellent thermal properties, with T_d5% of 581 and 584°C and T_g of 421 and 339°C, respectively. The mechanical properties of s-BPDA/BAPBBI and s-BPDA/BAPBBOA are also outstanding, with a tensile strength of 218 and 192 MPa, respectively. In addition, their water absorption percentages are also lower than those of commercial films. The excellent properties of s-BPDA/BAPBBI and s-BPDA/BAPBBOA are attributed to the rigid molecular chain skeleton, strong force, and hydrogen bonds between and inside molecular chains.

This work was financially supported by the Natural Science Foundation of Guangdong Province, Grant No. 2021A1515011409, the Science and Technology Major Project of Guangdong Provincial Education Department, Grant No. 2020ZDZX2081, the Science and Technology Major Project of Zhongshan Polytechnic, Grant No. KYA2002, and Shenzhen Basic Research Grant No. JCYJ20210324115406019.

The authors have no conflicts to disclose.

J. Li., W. Zhao, C. Zhao, and T. Qi contributed equally.

Jiang Li: Data curation (equal); Investigation (equal). Wenhua Zhao: Data curation (equal). Chenchen Zhao: Data curation (equal); Investigation (equal); Methodology (equal). Tongqing Qi: Data curation (equal); Methodology (equal). Pengchang Ma: Data curation (equal); Methodology (equal). De Ning: Data curation (equal); Investigation (equal); Methodology (equal). Chunlei Yang: Funding acquisition (equal). Weimin Li: Funding acquisition (equal).

The data that support the findings of this study are available within the article.