Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Synthesis of Superheat-Resistant Polyimides with High Tg and Low Coefficient of Thermal Expansion by Introduction of Strong Intermolecular Interaction Meng Lian, Xuemin Lu, and Qinghua Lu* School of Chemistry and Chemical Engineering, Shanghai Key Laboratory of Electrical Insulation and Thermal Aging, Shanghai Jiao Tong University, Shanghai 200240, China
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S Supporting Information *
ABSTRACT: The development of polyimides (PIs) with a superheat resistance and a high thermal dimensional stability is required urgently for application in the rapidly growing area of flexible-display substrates. Based on an enhanced intermolecular interaction, 2,2′-p-phenylenebis(5-aminobenzimidazole) (DP) that contains bis-benzimidazole was synthesized, and two series of its copolyimides (PI-a and PI-b) were prepared by copolycondensation with 5-amino-2-(4-aminobenzene)benzimidazole (PABZ) and 5-amino-2-(3-aminobenzene)benzimidazole (i-PABZ), respectively. The high density and packing coefficient of the resulting PIs caused by the strong intermolecular interaction from the hydrogen bonds and the chargetransfer complex provided the PI films with a very high glass-transition temperature (Tg > 450 °C) and an extremely low coefficient of thermal expansion (CTE) below 10 ppm/K for PI-a. Such good thermal properties expand their application as high thermostable materials. Furthermore, the PI-b had a higher Tg than PI-a, whereas the latter had lower CTE values because of the configuration difference of their polymer chains. These data indicate that the resultant thermostable copolyimides have potential application as a flexible-display substrate and provide a feasible method to improve the thermal properties by incorporating bis-benzimidazole moieties.
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properties.10−13 For example, PI films, such as Kapton H (Dupont), Upilex R (Ube Industries), and Novax (Mitsubishi), are commercially available high-performance polymer materials with the highest heat-resistance grade and an excellent comprehensive performance. Their Tg can reach 385 °C for Kapton H, 285 °C for Upilex R, and 350 °C for Novax.12 However, the thermal properties of these PI materials, especially the glass-transition temperature and thermal expansion, limit the maximum process temperature as substrates. Therefore, the development of PIs with a higher Tg and lower CTE is required urgently in science and engineering. Current research has proven that a structural modification and an incorporation of a high-strength hydrogen bond in the PI chains are two effective approaches to improve the thermal stability of intrinsic PI materials.14−18 Specifically, an increase in the linearity/stiffness and intermolecular interactions of the PI chains increases the Tg, modulus, and density and decreases the CTE.19−21 For example, incorporating heterocyclic units that bear functional groups can increase the PI backbone rigidity and lead to strong intermolecular interactions; the method provides an effective way to improve the thermal
INTRODUCTION Flexible active-matrix organic light-emitting-diode (AMOLED) devices based on low-temperature polysilicon thin-film transistors (LTPS TFTs) have attracted intensive attention because of their great potential as optimal candidates in next-generation displays.1−3 The flexible substrate is a key component for future flexible displays. Materials used to manufacture flexible substrates must possess a high heat resistance, a high dimensional stability, a smooth surface, a good water-vapor transmission rate, a ruggedness, and a chemical resistance.4−6 Unlike the water-vapor transmission rate and chemical resistance, which can be improved by the manufacture of a permeation barrier layer,7,8 the thermal properties, including a high glass-transition temperature (Tg), a high thermal-decomposition temperature (Td), and a low coefficient of thermal expansion (CTE), must be solved by performance improvement of the material itself. According to the conventional LTPS process, the supporting substrate of the LTPS is supposed to withstand temperatures up to at least ∼450 °C.9 Organic materials are the most popular candidates for flexible substrates because of their advantage of a high flexibility.10,11 However, the high thermal stability and dimensional stability are the main hindrances for most polymer materials. Among them, aromatic polyimides (PIs) have attracted growing interest because of their high thermal stability, good chemical resistance, and excellent mechanical © XXXX American Chemical Society
Received: October 23, 2018 Revised: November 30, 2018
A
DOI: 10.1021/acs.macromol.8b02282 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Anal. Calcd for C20H16N6: C, 70.57%; H, 4.74%; N, 24.69%. Found: C, 70.93%; H, 4.57%; N, 24.50%. TOF-MS: m/z = 341.15. Synthesis of Poly(amic acid)s. Precursors of the polyimides, poly(amic acid)s (PAAs), were prepared by mixing equimolar amounts of dianhydride BPDA and diamine PABZ or i-PABZ and DP in a mixed solution of DMAc and DMSO (1:1, v/v) under a dried nitrogen atmosphere. For polymerization, the molar ratios of PABZ or i-PABZ/DP were 100:0, 80:20, 60:40, 50:50, 40:60, 20:80, and 0:100. For example, for a molar ratio of PABZ:DP = 50:50, PABZ (1.9103 g, 8.52 mmol) and DP (2.8994 g, 8.52 mmol) were dissolved in a 40 mL mixed solution. The mixture was stirred in a 250 mL three-necked bottle at room temperature until all diamines had dissolved. BPDA (5.0122 g, 17.04 mmol) and 30 mL of mixed solution (DMAc/ DMSO,1:1,v/v) were dispersed uniformly. A viscous solution of PAA with a solid content of 13.8 wt % was obtained after stirring for 20 h. The polymerization procedure for the other ratios of PABZ or iPABZ/DP was similar to the above procedure. Preparation of Polyimide Films. PAA solution was cast onto a clean, dry glass plate at room temperature with a 500 μm depth blade. The solvent was removed by drying the PAA films by successive heating at 70, 90, and 110 °C for 2 h at each temperature. The PAA films were cured in a muffle with a curing program of 120 °C for 2 h, 200 °C for 2 h, 250 °C for 2 h, 300 °C for 2 h, 350 °C for 1 h, and 400 °C for 1 h. The film thickness after curing was approximately 20− 27 μm. Scheme 2 shows the synthetic process and chemical structure of the polyimides. All polyimides were divided into two series: PI-a and PI-b. Characterization. The chemical characterization of diamine DP was performed as follows. 1H NMR and 13C NMR spectra were measured in DMSO-d6 at 400 and 100 MHz on an AVANCE III HD 400 spectrometer (Bruker BioSpin., Germany) with chemical shifts quoted relative to tetramethylsiliane (TMS). Fourier transform infrared (FTIR) spectra were obtained on a Nicolet 6700 FTIR spectrometer (PerkinElmer, Inc., USA) for 4000−650 cm−1 in 32 scans. Elemental analysis was obtained on a Vario EL Cube (Elmentar, Germany), and TOF-MS was recorded on an ACQUITYTM UPLC&Q-TOF MS Pramier (Waters, USA). The chemical characterization of the PI films was monitored in attenuated total reflectance (ATR) mode on a Nicolet 6700 FTIR spectrometer (PerkinElmer, Inc., USA) for 4000−650 cm−1 in 32 scans. Gel permeation chromatography (GPC) was performed using a GPC LC-20AD (Shimadzu, Japan) equipped with an Asahipak GF-7 M HQ column and a laser-refractive index detector using DMF that contained 0.03 mol/L LiBr and 0.03 mol/L H3PO4. The numberaverage (Mn) and weight-average (Mw) molecular weights were estimated by using a polystyrene standard calibration curve. Dynamic mechanical analyses (DMA) of the PI films were performed on a Q800 DMA (TA Instruments, USA) at a heating rate of 5 °C min−1 with a load frequency of 1 Hz under nitrogen. The decomposition behavior of the PI films was investigated by using a Discovery 550 TGA (TA Instruments, USA) at a heating rate of 10 °C min−1 under nitrogen. The CTEs of the films were collected using a Q400 TMA (TA Instruments, USA) at a temperature increase of 5 °C min−1 in a nitrogen flow of 50 mL min−1 under a static load set at 0.05 N. The average CTE values were obtained from the second heating run after the first run, which was performed to eliminate the residual stress. The mechanical properties were collected with a universal electromechanical tester Instron 4465 (Instron Corp., USA). Film densities were obtained from a multifunctional densimeter (FK-120S, Furbs Corp., China) using the solid model and 95% alcohol as a medium. The packing coefficient (K) was estimated from eq 1:23
properties of PI materials. Recently, research has indicated that PIs with rigid benzimidazole in the main chain present an excellent thermal stability.14,15,18 The benzimidazole moieties with a proton donor (N−H group) offer favorable conditions to form hydrogen bonds with electron-rich groups, such as CO. 5-Amino-2-(4-aminobenzene)benzimidazole (PABZ) and 5-amino-2-(3-aminobenzene)benzimidazole (i-PABZ) are representative diamines that contain benzimidazole, and the corresponding PIs possess a Tg greater than 400 °C.14,15,22 PABZ or i-PABZ also can be used as an excellent “additive” to improve the T g of PIs from ordinary diamines and dianhydrides. A novel diamine monomer, 2,2′-p-phenylenebis(5-aminobenzimidazole) (DP), that contained bis-benzimidazole was synthesized. Bis-benzimidazole provides more proton donors and chances for hydrogen-bond formation, which may provide PIs with enhanced thermal properties. Two series of random PIs with PABZ and i-PABZ were synthesized, and their thermal and mechanical properties were investigated. The relationship between the properties and the chemical structure is discussed, which provides a feasible method to develop PIs with a high thermal stability.
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EXPERIMENTAL SECTION
Materials. 1,2,4-Triaminobenzene dihydrochloride, p-phthalic acid, and SnCl2 dihydrate were from TCI Co., Ltd. Poly(phosphoric acid) (PPA), N,N-dimethylacetamide (DMAc), and dimethyl sulfoxide (DMSO) with a trace of water ≤50 ppm were provided by J&K Scientific. 3,3′,4,4′-Biphenyltetracarboxylic dianhydride (BPDA) was obtained from ChinaTech (Tianjin) Chemical Co., Ltd., China. 5-Amino-2-(4-aminobenzene)benzimidazole (PABZ) and 5-amino-2-(3-aminobenzene)benzimidazole (i-PABZ) were from Changzhou Sunlight Medical Raw Material Co., Ltd., China. All materials were used as supplied. Synthesis of 2,2′-p-Phenylenebis(5-aminobenzimidazole) (DP). The diamine DP that contained bis-benzimidazole rings was prepared by a one-step process, as shown in Scheme 1 and Figures
Scheme 1. Synthetic Route of DP
S1−S3. SnCl2 dihydrate (0.23 g), PPA (85 wt %, 110 g), and 1,2,4triaminobenzene dihydrochloride (10 g) were added to a 500 mL three-neck round-bottom flask equipped with a mechanical stirrer, dropping funnel, and nitrogen inlet. The mixture was heated to 80 °C with stirring for 3 h, and 3.85 g of p-phthalic acid was added into the flask and left to stir for 12 h at 200 °C. The mixture was cooled to 80 °C and poured slowly into ice-cold water with stirring. The sediments were collected by filtration and deacidified with a 10% sodium carbonate solution. The mixture was neutralized to pH 8 and filtered to obtain a brown crude product. The crude product was purified by neutral alumina chromatography eluted with ethyl acetate/ethanol (3:1, v/v) to obtain a bright-yellow solid product (6.54 g, 83%). FTIR (KBr, cm−1): 3381 (NH of benzimidazole); 3332, 3237 (NH2); 1632, 1474 (νCN of imidazole); 1304 (imidazole ring breathing). 1H NMR (DMSO-d6, 400 MHz) δ (ppm): 12.38 (s, 2H), 8.19 (d, J = 6.6 Hz, 4H), 7.31 (d, J = 8.3 Hz, 2H), 6.71 (s, 2H), 6.59 (dd, J = 14.4, 8.4 Hz, 2H), 4.90 (s, 4H). 13C NMR (400 MHz, DMSO-d6) δ (ppm): 148.81, 145.64, 138.83, 134.87, 131.25, 126.58, 117.94, 112.64, 96.39.
K=
Vint N ∑ ΔVi = A Vtrue M/ρ
(1)
where K is the ratio of the intrinsic volume (Vint) of the atomic groups that make up the repeating unit to the calculated molar volume (Vtrue) from the experimental measurement of the density (ρ) of the polyimide film. ΔVi represents the volume increments of the atoms, M B
DOI: 10.1021/acs.macromol.8b02282 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Scheme 2. Synthesis of Polyimides Containing Bis-benzimidazole
is the molecular mass of the repeating unit and NA is Avogadro’s number. For the water absorption test, polyimide film specimens with dimensions of 40 mm × 60 mm were weighed before being placed in a distilled water bath at 25 °C for 24 h and then wiped clean with dried tissue paper and weighed. The water absorption ratio was calculated from eq 2.
A (%) =
m2 − m1 × 100% m1
Table 1. Molecular Weights and PDIs of PAAs (a) molecular weights and PDIs of PAA-a m:na
80:20
100:0
3.05 2.59 2.81 3.00 2.63 2.53 7.93 8.51 7.59 9.90 8.94 7.08 2.6 3.3 2.7 3.3 3.4 2.8 (b) molecular weights and PDIs of PAA-b
3.13 9.70 3.1
m:nb
0:100
20:80
40:60
50:50
60:40
80:20
100:0
Mn(×104) Mw(×104) PDI
2.74 7.12 2.6
2.19 5.91 2.7
2.47 6.58 2.7
2.68 7.50 2.8
2.23 7.52 3.4
2.95 9.44 3.2
3.13 9.70 3.1
Mn(×104) Mw(×104) PDI
(2)
where A represents the water absorption ratio; m1 and m2 are the sample mass before and after treatment, respectively.
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0:100
20:80
40:60
50:50
60:40
a
n refers to the diamine PABZ content; m refers to the diamine DP content. bn refers to the diamine i-PABZ content; m refers to the diamine DP content.
RESULTS AND DISCUSSION Polyimide Synthesis. Two series of randomly copolymerized PIs were synthesized according to Scheme 2. The low molecular weight of PAAs may have a significant influence on the performance of the obtained PI films. The molecular weights and the polydispersity indexes (PDIs) of the resultant PAAs are summarized in Table 1. The molecular weights of all PAAs are sufficiently high to guarantee the formation of a flexible and tough polyimide film. The polymerization of PAA is a nucleophilic reaction between diamine and dianhydride, which involves a nucleophilic attack of the amino group on the carbonyl carbon of the anhydride group. Diamines with diverse structures have a different reactivity,24,25 so the difference in nucleophilicity of the diamines PABZ and i-PABZ leads to a higher molecular weight of the PAA-a than the PAA-b. When DP was copolymerized with PABZ or i-PABZ, the molecular weight of the PAAs showed a slight decrease with DP content increase because of the lower reactivity of the amino group in the DP. However, the molecular weight of the PAA (m:n = 100:0), in which only DP was used, is higher than other PAAs. This can be ascribed to the higher molecular weight (M = 340.39 g mol−1) of monomer bis-benzimidazole diamine DP compared with that of PABZ or i-PABZ (M = 224.27 g mol−1).
The polymerization degrees of PAA-a and PAA-b when m:n = 0:100 were approximately 28 and 26 (according to 1H NMR spectra as shown in Figure S4), respectively, whereas the polymerization degree of the PAA when m:n = 100:0 was 23. As a result, the molecular weight of PAA (m:n = 100:0) exhibited a small increase when polymerized with less active DP. The successful preparation of PIs was confirmed by ATR as shown in Figure S5. The stretching band of N−H group can be observed in the range of 3700−3000 cm−1; the broad peak suggests that N−H groups in the benzimidazole ring participate in the formation of hydrogen bonds between the N−H and CO or CN− groups. The characteristic absorption bands at 1773 cm−1 (asymmetric stretching of imide CO), 1704 cm−1 (symmetric stretching of imide C O), and 1357 cm−1 (C−N−C stretching of imide ring) suggest the formation of an imide ring structure.26,27 Peaks at ∼1660 cm−1 (amide-I of PAAs) and 1550 cm−1 (amide-II of PAAs) were hardly observed, which indicates the high imidization C
DOI: 10.1021/acs.macromol.8b02282 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules degree of the obtained PI films.28 The PI films showed the characteristic stretching vibration of CN of benzimidazole at ∼1487 cm−1 29,30 and the imidazole ring-breathing vibration at 1305 cm−1,15,31 which suggests that the benzimidazole moiety had been incorporated into the main polyimide chains. Figures 1a and 1b show the ATR spectra of the CO groups in the PI-a and PI-b series, respectively. With an increase in DP content, the peak position of the asymmetrical stretching vibration band of CO shifted continuously from 1772.4 cm−1 for PI-a and 1773.2 cm−1 for PI-b to 1769.8 cm−1,
and the symmetrical band also shifted to lower frequencies (1704.8 cm−1 for PI-a and 1704.9 cm−1 for PI-b to 1702.9 cm−1). Studies by Dang et al.14 and Liu et al.32 have proven that a red shift would occur upon the formation of a hydrogen bond, and the band positions of CO shifted to lower wavenumbers. For the PI-a series and PI-b series, the extent of shift was ∼3.6 cm−1 for asymmetrical stretching and ∼2.0 cm−1 for symmetrical stretching with an increase in DP content. This suggests a hydrogen-bond formation between CO groups in the imide rings and N−H groups in the benzimidazole moieties. Liu et al.29 indicated that when the annealing temperature reached 400 °C, the position shift of the CN− in the benzimidazole became obvious and the hydrogen bond (CN···N−H) strengthened. All PI films in this study were annealed at 400 °C, and obvious changes appeared at a wavenumber of 1480−1490 cm−1, which was ascribed to the CN− of benzimidazole (Figure S6). Figure 1c shows the CN− band position as a function of DP content. The absorption peak shifted continuously to lower frequencies with an increase in DP content. The results confirm that the hydrogen bond (CN···N−H) formed and was enhanced after the incorporation of DP into the PI main chain. In summary, the intermolecular interactions strengthened as a result of the combined influence of hydrogen bonds CO··· N−H and CN···N−H. Film Density and Water Absorption. The film density was investigated to evaluate the molecular packing in the PI films. As listed in Table 2, the density values range from 1.380 Table 2. Density and Water Absorption of Polyimides m:na
ρ (g m−3)
K
A (%)
m:nb
ρ (g m−3)
K
A (%)
0:100 20:80 40:60 50:50 60:40 80:20 100:0
1.423 1.431 1.443 1.452 1.468 1.450 1.443
0.711 0.716 0.723 0.728 0.737 0.728 0.726
3.71 3.86 4.92 4.87 5.12 5.13 4.95
0:100 20:80 40:60 50:50 60:40 80:20 100:0
1.380 1.393 1.391 1.404 1.417 1.427 1.443
0.689 0.697 0.697 0.704 0.711 0.717 0.726
4.12 5.89 6.08 5.88 6.30 5.76 4.95
a
n refers to the diamine PABZ content; m refers to the diamine DP content. bn refers to the diamine i-PABZ content; m refers to the diamine DP content.
to 1.468 g cm−3, which is comparatively high for other polymers (including polyimides).3323 The packing coefficient of the obtained PIs ranged from 0.689 to 0.737, which is higher than most reported polymers (the average packing coefficient was found to be 0.681).23 The density and packing coefficient of a polymer are related mainly to the packing volume of the atoms that make up the molecule, so a higher PI density and packing coefficient indicate that their molecular packing was much closer. We also noticed that the density and packing coefficient of the PI-a were higher than those of the PI-b. This may be attributed to the difference in the main chain of the two PIs series. The steric structures of the diamines in the PIs were simulated to investigate the molecular configuration influence on density. Figure 2 depicts the optimized molecular structures of these diamines and the angles of the diamine’s backbone. Geometry optimization was performed by using Chem3D Pro 14.0 under a MM2 force-field module (Figure S7). The backbone angles of diamines of PABZ, i-PABZ, and DP were 180.0°, 139.7°, and 166.9°, respectively. The linearity
Figure 1. ATR spectra of CO band for (a) PI-a and (b) PI-b films. (c) Band position of CN stretching as a function of DP content. D
DOI: 10.1021/acs.macromol.8b02282 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Table 3. Thermal Properties of Polyimide Films (a) thermal properties of PI-a m:na
Tg (°C)
0:100 20:80 40:60 50:50 60:40 80:20 100:0
417 452 453 455 457 463 475
m:nb
Tg (°C)
0:100 20:80 40:60 50:50 60:40 80:20 100:0
436 482 472 469 467 463 475
T5% d (°C)
Rwc (%)
579 562 567 557 570 558 564 (b) thermal properties
CTE (ppm/K)
71.88 71.22 72.84 72.65 73.09 71.88 72.81 of PI-b
2.68 6.59 7.00 9.96 8.87 7.89 8.67
T5% d (°C)
Rwc (%)
CTE (ppm/K)
577 567 567 566 557 553 564
71.16 72.57 71.71 71.69 72.22 72.12 72.81
23.90 20.13 16.81 14.83 13.81 11.34 8.67
a
n refers to the diamine PABZ content; m refers to the diamine DP content. bn refers to the diamine i-PABZ content; m refers to the diamine DP content. cResidual weight retention when heated to 800 °C in nitrogen.
rupture. Because of the lower thermal stability of the N−H groups in the benzimidazole moieties, the thermal stability of the PI-a and PI-b decreased to some degree after the introduction of DP. However, the thermal properties of the PIs in terms of weight loss are sufficient for further use. The glass-transition temperatures (Tg) of the PI films examined by DMA (Figure S9) are listed in Table 3. The Tg values of all PIs with DP exceeded 450 °C and were higher than the minimum LTPS process temperature. Figure 3 summarizes the Tg values of the PIs with the same dianhydrides BPDA.15,24,39−43 The polyimides with DP (up to the dotted line) had a higher Tg than other counterparts. Copolymerization with 20 mol % DP led to an increase in Tg of ∼35 °C for PI (PABZ/BPDA) and ∼46 °C for PI (i-PABZ/
Figure 2. Optimized diamine structure.
of the diamines is in the order of i-PABZ < DP < PABZ. Therefore, PI-a that contains PABZ had a higher level of linearity and coplanarity than PI-b that contains i-PABZ. Compared with PI-a, the bent and distorted conformation in PI-b would retard the intermolecular interaction. As a result, the density and packing coefficient of PI-a were higher than those for PI-b. The water absorption behavior was examined to evaluate the molecular packing in the PIs. The water absorption behavior is influenced not only by the molecular packing but also by the concentration of polar groups and their affinity for water molecules.24 The water absorption values (A) of PI-a and PI-b range from 3.71 to 6.30 and are higher than most previously reported PIs.24,34 The primary cause was the higher concentration of the hydrophilic imidazole N−H group from benzimidazole and bis-benzimidazole. N−H groups form a hydrogen bond with water molecules and absorb more water. The water absorption by the PI-b was higher than that by PI-a, which implies that the packing coefficient and density of PI-b are lower than those of PI-a. This conclusion agrees with the density results. Thermal Properties. TGA, DMA, and TMA were used to investigate the thermal properties of the PI-a and PI-b. The thermal analysis data for the PI-a and PI-b are presented in Table 3. The TGA curves of these PIs are illustrated in Figure S8. All PIs showed a similar decomposition behavior in nitrogen that was characterized by a 5% weight loss from 553 to 579 °C, which showed a more attractive thermal stability than previous reported PIs.35−37 Polyimides with a rigid chain possess a higher thermal stability compared with PIs that have flexible, hinged bonds in their chains.38 The PI-a and PI-b contained unbroken conjugated rigid chains, i.e., no hinged heteroatoms in the main chains. When these PIs experience a high temperature, the generalized π-electron system facilitates their rapid distribution to other bonds, which prevents their
Figure 3. T g of polyimides with the same BPDA dianhydrides.15,24,39−43 E
DOI: 10.1021/acs.macromol.8b02282 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
bond angles and intermolecular distances.47,48 Bent and distorted i-PABZ in the PI-b series may lead to an increased difficulty in hydrogen-bond formation. As a integrated consequence, the Tg of the PI-b increased initially and decreased afterward with an increase in DP content. The CTE is believed to be more susceptible to the variation in chain orientation. The CTE decreases with an increase of inplane chain orientation. In previous studies, the in-plane orientation of PI chains was investigated with the help of the physical and optical properties of PI films, such as the CTE, elastic modulus, and refractive index.19,21,49,50 In this work, the in-plane CTEs were measured by TMA and averaged from 50 to 350 °C. The results are shown in Table 3 and Figure S10. Jou19,51 and Hasegawa20,21 pointed out that the in-plane orientation was correlated to a chain stiffness/linearity, interchain interaction, and molecular mobility. The CTE values of the PI-a range from 2.68 to 9.96 ppm/K, whereas the value for PI-b extends from 11.34 to 23.9 ppm/K. The PI-a and PI-b series had relatively low CTE values; the PI-a series especially had a lower CTE than 10 ppm/K. This may be ascribed to a rigid chain structure of two series of PIs, which allows for spontaneous in-plane orientation during thermal imidization.21,52 The strong intermolecular interaction caused by the hydrogen bond also played an important role in the occurrence of spontaneous orientation.20 In addition, the lower CTE of the PI-a series compared with the PI-b as shown in Figure 5 may result from the configuration differences of the
BPDA). This result should be ascribed to the strong physical interaction between the molecular chains that include hydrogen bonds (CO···N−H and CN···N−H) and electron donation−acceptance interaction, and the interaction was further strengthened after the introduction of bisbenzimidazole as a tectonic unit.44,45 The Tg shows the highest usage temperature of a polymer material. When the temperature exceeds Tg, the mechanical properties of the polymer may deteriorate because of the movement of the polymer segments. The rotational energy of the polymer segments is insufficient to overcome the forces that hold the molecular segments together until Tg, and thus prevent molecular segments from rotation. Robert46 proposed eq 3 to express this assumption: Hc = HR + C ;
HR = 0.5nRTg
(3)
where Hc is the molar cohesive energy, HR is the molar rotational energy, and C is a constant. The rotational energy can be expressed by the term 0.5nRTg, where n is a number that is analogous to the degrees of freedom in the kinetic energy expression. Figure 4 presents the Tg values of the PI-a
Figure 4. Tg of PI-a and PI-b series as a function of DP content.
and PI-b as a function of DP content. The Tg values of the PI-b series were slightly higher than those of the PI-a series. According to eq 3 and Figure 2, the PI-b that is based on iPABZ has a bent and distorted conformation, which results in a more difficult chain rotation than that of the PI-a with a high linearity. Therefore, the molar rotational energy HR of PI-b should be higher than that of PI-a, which led to much higher Tg values for the PI-b series compared with the PI-a series. In Figure 4, the Tg values of the PI-a series showed growth with an increase in DP, whereas the Tg values of the PI-b series decreased after the DP exceeded 20%. This can be explained from three aspects. First, with an introduction of DP, the benzimidazole content increased, which resulted in an increase in hydrogen bonds (CO···N−H and CN···N−H). The hydrogen bonds could restrict chain-segment motion, which may account for the increase in Tg of the PI-a series. Second, the Tg of the PI-b was higher than that of the PI-a because of the higher molar rotational energy in the PI-b. With an increase in DP, the i-PABZ content decreased, and the advantage of a high molar rotational energy disappeared gradually. Third, hydrogen bonds are susceptible to changes in
Figure 5. CTE of PI-a and PI-b series as a function of DP content.
PABZ and i-PABZ in the PI-a and PI-b chains. As demonstrated above, the PI-a has a rigid backbone with a high level of linearity and coplanarity, which benefits the molecular-chain orientation. The DP has a better linearity and coplanarity compared with the monomer i-PABZ; therefore, the CTE of the PI-b series decreases with an increase in DP content. However, for the PI-a series, the copolymerization of DP reduced the ability of the spontaneous in-plane orientation because of the relatively poor linearity of the DP compared with the PABZ, which resulted in a small increase in CTE of the PI-a series with an increase in DP content. Mechanical Properties. As shown in Table 4, the PI films exhibit excellent mechanical properties with a tensile strength from 119.42 to 238.52 MPa, an initial modulus from 3.32 to 5.36 GPa, and an elongation at break from 2.82 to 10.23%. In F
DOI: 10.1021/acs.macromol.8b02282 Macromolecules XXXX, XXX, XXX−XXX
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Table 4. Mechanical Properties of Polyimide Films
0:100 20:80 40:60 50:50 60:40 80:20 100:0
σc (MPa)
Ed (GPa)
238.52 4.69 196.16 4.32 189.22 3.98 166.99 4.00 130.98 4.39 123.71 3.94 119.42 4.28 (b) mechanical properties of PI-b
ASSOCIATED CONTENT
S Supporting Information *
(a) mechanical properties of PI-a m:na
Article
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b02282.
εe (%) 10.23 7.22 6.87 5.35 3.33 3.70 2.82
m:nb
σc (MPa)
Ed (GPa)
εe (%)
0:100 20:80 40:60 50:50 60:40 80:20 100:0
227.16 232.84 187.72 165.60 159.79 139.08 119.42
4.60 5.36 4.18 4.15 4.17 3.32 4.28
9.22 6.00 7.80 5.45 5.52 3.02 2.82
1
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H NMR, 13C NMR, and TOF-MS mass spectra of DP and ATR spectra; TGA, DMA, and TMA curves of PI-a and PI-b films (DOCX)
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Corresponding Author
*E-mail:
[email protected] (Q.L.). ORCID
Xuemin Lu: 0000-0002-2057-6280 Qinghua Lu: 0000-0003-1522-1458 Notes
The authors declare no competing financial interest.
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a
n refers to the diamine PABZ content; m refers to the diamine DP content. bn refers to the diamine i-PABZ content; m refers to the diamine DP content. ctensile strength. dinitial modulus. eelongation at break.
ACKNOWLEDGMENTS This work was supported financially by Shanghai Key Projects of Basic Research (16JC1403900) and 973 Projects (2014CB643605).
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general, the mechanical properties of the polyimide films depend on the chemical structure, molecular weight, and aggregation structure of the polymer chains. A higher molecular weight, crystallinity, and intermolecular interactions enhance the mechanical properties of the polyimides.50,53 The tensile strength and elongation at break decrease slightly with an increase in DP content. This may be ascribed to a slight decrease in PAA molecular weight as shown in Table 1. Another reason for the decreasing trend may be the change in fracture mechanism. Feng et al. had pointed out that the fracture mechanism changed from ductile to brittle fracture with the increase in rigid segments, which affects the improvement of the mechanical properties adversely.54 Nevertheless, the two series of PIs still retained sufficient mechanical properties for application as a flexible substrate. The tensile strength exceeds 100 MPa, and the initial modulus is 2 GPa for the AMOLED application.
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CONCLUSIONS A novel diamine DP with bis-benzimidazole and its copolyimides with PABZ or i-PABZ were synthesized. The experimental results indicated that the introduction of a bisbenzimidazole group can increase the density and packing coefficient, which lead to an excellent thermal stability, including 5% weight-loss temperatures (T5% d ) over 553 °C, extremely high Tg values, even to 480 °C, and low CTEs from 2.68 to 23.90 ppm/K depending on the DP content. The Tg of the PI-b was higher than that of their counterpart PI-a because of the higher molar rotational energy. However, the latter possessed much lower CTE values than those of PI-b and most reported PI materials. All PIs exhibited excellent mechanical properties. In summary, the diamine DP with bis-benzimidazole moieties can be used as a highly effective monomer to improve the Tg significantly and reduce the CTE of the PI films, which provides a new approach to develop AMOLED flexible substrates. G
DOI: 10.1021/acs.macromol.8b02282 Macromolecules XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.macromol.8b02282 Macromolecules XXXX, XXX, XXX−XXX
