Highly strong and highly tough electrospun polyimide/polyimide composite nanofibers from binary blend of polyamic acids

Abstract

Electrospun blend-polyimide (blend-PI) nanofibers with high tensile strength and toughness are highlighted in this article. The blend-PI nanofibers were prepared by electrospinning the binary blend of rigid and flexible polyamic acids, followed by thermal imidization. The method is simple and can be extended to other kinds of polyamic acids. The morphologies and structures of the blend-PI nanofibers were investigated by scanning electron microscopy (SEM) and wide-angle X-ray diffraction (XRD). The mechanical properties, thermal properties and miscibility of the blend-PI nanofibers were studied by a tensile test, thermogravimetric analysis (TGA) and dynamic mechanical analysis (DMA). The mechanical properties of the blend-PI nanofibers, including tensile strength, modulus, elongation at break and toughness, could be well-tuned by modifying the molar ratio of the rigid component (B-PI) and the flexible component (O-PI). The blend-PI nanofibers with B-PI/O-PI molar ratio of 4/6 had an ultra-high strength of 1.3 GPa with an excellent toughness of 82 J g⁻¹. All the blend-PI nanofibers showed thermal stability to above 500 °C. The presence of only one glass transition temperature (T_g) suggested the good miscibility of the binary PIs in the blend-PI nanofibers. This study would provide completely new opportunities for modifying the properties of electrospun PI nanofibers.

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1. Introduction

Polyimides (PIs) comprise a promising class of high performance polymers with excellent thermal stability and mechanical properties, outstanding chemical and radiant resistance, long-term stability and superior electrical properties. They have been broadly applied in composites, coatings, adhesives, filtrations, insulators and dielectrics in the form of fibers, films and particles.^1–6 Fibers or electrospun nanofibers based on polyimides exhibit high performance and are highly desired for applications in protective clothing, filtration, gas separation, and battery separators, and composites.^5,7–11 Conventional as-spun PI fibers showed tensile strength in the range from 0.22 to 0.74 GPa, and fibers treated by hot-stretching in the range from 0.8 to 2.7 GPa.¹² Russian scientists have reported as-spun polyimide fibers with an ultra-high ultimate tensile strength of 2.9–3.3 GPa, which is the highest tensile strength of PI fibers reported to date.^13,14 Their studies revealed that the fibers made from co-PI had a considerably higher tensile strength than those made from the corresponding homo-PI.

During the past few decades, our group has paid considerable attention on improving the mechanical properties of electrospun nanofibers.^9,15–19 We have found that the electrospun nanofibers made of polyimides have excellent mechanical properties.^18–20 The aligned electrospun PI nanofiber membranes obtained from rigid 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA)/p-phenylene diamine (PDA) homo-polyimide had ultra-high tensile strength and modulus;²⁰ the aligned nanofiber membranes made of a flexible 2,2-bis[(4-(4-amino-phenoxy) phenyl)] hexafluoropropane (6F-BAPP)/BPDA homo-polyimide had a high toughness;¹⁸ and the aligned electrospun co-polyimide nanofiber membranes based on BPDA/4,4′-oxydianiline (ODA) and 4,4′-diaminobiphenyl (BPA) were the strongest aligned electrospun polymer nanofiber membranes reported to date, which displayed a tensile strength of 1.1 GPa,¹⁹ double than that of the homo-PI BPDA/BPA and homo-PI BPDA/PDA.

Although co-polymerization significantly improves the mechanical properties of the fiber, the chemical structures achieved by copolymerization are random and unfixed, which leads to an uncertainty regarding the resulting mechanical properties. Nevertheless, since a long time, the polymer blends have been recognized as an efficient method for improving the mechanical properties.^21,22 Compared with the development of entirely new polyimides and co-polyimides, blending existing polyimides is a simple, economic and low-cost process. Yokota and many other researchers have shown that the mechanical properties of blended-polyimides were not only higher than those of the corresponding homo-polyimides, but also higher than those of the corresponding co-polyimides.^8,23–25

To obtain a higher performance from electrospun polymer nanofibers with fixed chemical structures by a simple and low-cost process, in this study, a series of blend-polyimide nanofiber membranes (polyimide/polyimide composite nanofibers) with different molar ratios of rigid BPDA/BPA polyimide (B-PI) and flexible BPDA/ODA polyimide (O-PI) were prepared using electrospinning techniques. The morphology, thermal and mechanical properties of the as-prepared electrospun blend-PI nanofibers were studied in detail. The results demonstrate that the electrospun blend-polyimide nanofibers have enhanced mechanical and thermal properties depending on the molar blending ratio of the rigid and flexible polyimides.

2. Experimental

2.1. Materials

3,3′,4,4′-Biphenyltetracarboxylic dianhydride (BPDA) (Hebei Jida Plastic Products Co.), 4,4′-diaminobiphenyl (BPA, Quzhou Kaiyuan Fine Chem. Co) and 4,4′-oxydianiline (ODA, Quzhou Kaiyuan Fine Chem. Co) were purified by sublimation prior to use. N,N-Dimethylacetamide (DMAc) (99%, Tianjin Fu Chen Chemical Reagent Factory) was used as received.

2.2. Synthesis of PAA and blending of PAA precursors

The precursors of polyimide, polyamic acids (PAAs) of BPDA/ODA (O-PAA) and BPDA/BPA (B-PAA) were first synthesized from BPDA dianhydride with two diamines (ODA and BPA), respectively. The polycondensation was performed in DMAc at –5 to –0 °C for 24 h, and the solid contents of PAA solutions were maintained at 10 wt%. The intrinsic viscosities of B-PAA and O-PAA were 5.2 dL g⁻¹ and 5.8 dL g⁻¹, respectively. Different molar ratios of O-PAA and B-PAA in the blend-PAA solutions were prepared by adjusting the weight of O-PAA and B-PAA solutions. The molar ratios of O-PAA and B-PAA were fixed from 0/10, 1/9, 2/8, 3/7, 4/6, 6/4, 7/3, 8/2, 9/1 to 10/0. The molar ratios of O-PAA and B-PAA were calculated according to the equation
where R is the molar ratio of O-PAA to B-PAA; m_O, C_O and M_O are the weight of O-PAA solution, the concentration (wt%) of O-PAA solution and the molar mass of the repeating unit of O-PAA, respectively; and m_B, C_B and M_B are the weight of B-PAA solution, the concentration (wt%) of B-PAA solution and the molar mass of the repeating unit of B-PAA, respectively. Because the repeating unit of O-PAA is derived from one molecule of BPDA/ODA and the repeating unit of B-PAA from one molecule of BPDA/BPA, the molar ratios of O-PAA/B-PAA are equivalent to the molar ratios of ODA/BPA and the molar ratios of O-PI/B-PI.

2.3. Preparation of aligned electrospun PI nanofiber membranes

The precursor (polyamic acid) nanofiber membranes were produced by electrospinning the blended polymer solutions. The absolute viscosity of the solutions was adjusted to about 6 Pa s in DMAc, and the electrical conductivity was adjusted to about 40 μS m⁻¹ by the addition of 0.2 wt% dodecylethyldimethylammonium bromide (DEDAB). The electrospinning process was performed by applying a positive voltage (15 kV) to the spinneret and a negative voltage (−5 kV) to the collector. The collecting distance of the spinneret to collector was 25 cm. The feeding rate of the solution was 0.8 mL h⁻¹, controlled by a digital syringe pump. The laboratory-built rotating disc was used as the collector, which rotated at a surface linear speed of 24 m s⁻¹. All the blend-PAA nanofiber membranes were dried at 60 °C in a vacuum oven for 4 h to remove the residual solvent, and then thermally imidized in a high-temperature furnace under an N₂ atmosphere. The imidization process was performed using the following protocol: (1) rapidly heating to 250 °C at a rate of 10 °C min⁻¹ and annealing for 30 min; (2) slowly heating to 370 °C at a rate of 2 °C min⁻¹ and annealing at 370 °C for 60 min to complete the imidization. The preparation of co-polyimide was similar to that of the blended polyimide,¹⁹ and the specific schematic diagrams of blend-polyimide and co-polyimide are shown in Fig. 1.

Fig. 1 Specific schematic diagrams of Blend-PI and Co-PI.

2.4. Characterizations

A scanning electron microscope (SEM, FEI Quanta 200 FEG) was used to observe the blend-PI nanofiber membranes. Mechanical properties of the blend-PI nanofiber membranes were investigated using a computer-controlled electromechanical universal testing machine (SANS, CMT 8102, Shenzhen, China) at a tensile speed of 5 mm min⁻¹. The thickness of the samples was calculated according to our previous reports.¹⁹ The density (ρ) of PI was calculated from the weight (m) and the volume (V) of the PI films. Thermogravimetric analysis (TGA) was used to explore the thermal stability of the nanofiber membranes under a nitrogen atmosphere at a heating rate of 10 °C min⁻¹. The dynamic mechanical analyses (DMA) of the samples were performed on a Perkin-Elmer diamond analyzer in tensile mode under a nitrogen atmosphere at a heating rate of 3 °C min⁻¹ in the range of 30 °C–450 °C. The load frequency and amplitude were 1 Hz and 20 μm, respectively. X-ray diffraction (XRD) patterns were recorded on a Siemens D5000 X-ray diffractometer using crystal monochromated Cu Kα radiation with the 2θ angles ranging from 5° to 40° at an operating voltage of 40 kV and a current of 20 mA.

3. Results and discussion

3.1. Morphology of polyimide nanofibers

The morphology of the nanofibers is very important for the mechanical performance of nanofiber belts. Beads or other drawbacks on the nanofibers would significantly decrease the mechanical properties.²⁶ To study the blend effect on the mechanical properties of the blend-PI, it is necessary to prepare all the nanofibers homogeneously, without beads and other drawbacks. Fig. 2 shows the morphologies of the aligned homo- and blend-PI nanofibers produced in this study. The electrospun nanofibers were aligned in the membranes with diameters in the range of 200–300 nm. No beads and/or beaded nanofibers were found by the SEM images. No significant morphological differences were observed between the homo PI (B-PI and O-PI) and blend-PI nanofibers.

Fig. 2 SEM images of the aligned nanofibers of (a) B-PI, (b) O-PI and (c) blend-PI with B-PI/O-PI molar ratio of 4/6.

3.2. Mechanical properties of blend-PI nanofiber membranes

Depending on the molecular structure, electrospun polyimide nanofibers not only have a high tensile strength, but also have a high toughness. For example, the aligned nanofiber mats based on rigid homo-PI (BPDA/PDA) has a tensile strength of 660 MPa and modulus of 15 GPa, considerably higher than that of other types of polymer nanofiber mats,²⁰ whereas the 6F-BAPP/BPDA-based homo-polyimide nanofiber membrane has a tensile strength of 308 MPa and a toughness of 365 MPa because of the flexible PI molecular backbone.¹⁸ In this study, a series of blend-PI nanofiber membranes were prepared to investigate the effect of moiety of rigid and flexible components on the mechanical properties. The typical stress–strain curves of the nanofiber membranes are shown in Fig. 3, and the corresponding mechanical properties are summarized in Table 1. The mechanical properties of the blend-PIs showed a trend similar to that of co-PIs with the same monomers, and they could be tuned by changing the molar ratio of the rigid component (B-PI) and flexible component (O-PI).¹⁹ As expected, the increased molar ratio of flexible components led to the increase of elongation from 2.59% to 27.08% and a decrease of modulus from 12.12 to 2.54 GPa. As the molar ratio of rigid B-PI decreased, the tensile strength of the blend-PI nanofibers initially increased and then decreased. At the 4/6 molar ratio of B-PI and O-PI, the blend-PI nanofiber membranes showed the highest tensile strength of 1299 MPa and a Young's modulus of 7.68 GPa with an elongation at break of 16.03%. As compared to the nanofiber membranes of rigid B-PI with high modulus and the flexible O-PI with high elongation, the blend-PI nanofibers (at 4/6 molar ratio of B-PI and O-PI) not only possessed a higher strength but also a higher elongation.

Fig. 3 Typical stress–strain curves homo- and blend-PI nanofiber membranes with different molar ratios of B-PI and O-PI.

Table 1 Mechanical properties of electrospun homo- and blend-PI nanofiber membranes

Molar ratio (B-PI/O-PI)	Tensile strength (MPa)	Elongation at break (%)	Modulus (GPa)	Toughness (J g^−1)
100/0 (B-PI)	415.64	2.59	12.12	3.34
80/20	668.17	4.68	10.77	9.86
60/40	919.44	7.84	8.31	24.53
40/60	1299.43	15.80	7.68	81.79
20/80	1061.95	22.62	5.50	94.25
0/100 (O-PI)	458.26	27.08	3.54	57.62

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Toughness is an important parameter to assess the flexibility of the materials. The toughness is defined by dividing the integral area under the stress–strain curves by the densities of the materials.^27–32 Therefore, the toughness was dependent on both the tensile strength and elongation at break. The toughness of material can be significantly enhanced by introducing carbon nanotubes (CNTs), electrospun nanofibers or ultra-flexible materials such as thermoplastic polyurethane (TPU). Blond et al. reported that the toughness of single-walled CNTs reinforced polyvinyl alcohol (PVA) electrospun non-wovens was 16 J g⁻¹, nearly 7 times that of the PVA film (2.5 J g⁻¹).²⁷ When TPU was incorporated into polystyrene (PS) by coaxial and triaxial electrospinning, the toughness of the single coaxial and triaxial composite fibers was 18.6 J g⁻¹ and 274.5 J g⁻¹, respectively, considerably higher than that of pure PS single fiber (0.4 J g⁻¹).²⁸ The electrospun nylon-6 nanofiber mat had a toughness of 21.8 J g⁻¹.²⁹ It was used to enhance the toughness of the brittle melamine-formaldehyde (MF).²⁹ The nylon-6 nanofibers-reinforced MF composites showed toughness in the range of 3.6–27 J g⁻¹. A synergistic effect on toughness was observed when nylon-6 nanofiber was used to reinforce TPU.³¹ The nylon-6/TPU composite exhibited a considerably higher toughness in the range of 152–274 J g⁻¹ than pure TPU (111 J g⁻¹) and nylon-6 nanofiber mat (21.8 J g⁻¹).^29–31 In this study, the toughness of the blend-PI nanofiber membranes could be tuned by changing the molar ratio of the rigid and flexible moieties. The homo B-PI exhibited a toughness of 3.34 J g⁻¹, whereas O-PI achieved a toughness of 57.62 J g⁻¹, which was 60% higher than that of PI/MWCNTs composites (0.5–5 vol% MWCNTs, 36 J g⁻¹).³² When the molar ratio of O-PI and B-PI was increased to 6/4 and 8/2, the toughness of the blend-PI increased to 81.79 and 94.25 J g⁻¹, respectively, only 26% and 15% smaller than the toughness of the well-known ultra-flexible TPU (111 J g⁻¹).³¹

In our previous report, the mechanical properties of co-PI was controlled by modifying the molar ratio of the monomer diamines BPA and ODA; the highest tensile strength of 1.1 GPa was obtained at a 4/6 molar ratio of BPA and ODA.¹⁹ Fig. 4 shows a comparison between the tensile strength and modulus of blend-PI and co-PI nanofiber membranes with the different amounts of BPA (mol%). Both the blend-PI and co-PI nanofibers exhibited the same trend of tensile strength and modulus, depending on the amount of BPA, and they had a higher strength than that of homo-PI. The strength of co-polyimide and blend-polyimide initially increased and then decreased, while the modulus always increased with the increasing ratio of BPA. However, a slight difference of tensile strength and modulus between blend-PI and co-PI is observed in Fig. 4. Blend-PI nanofibers showed a higher tensile strength and modulus than those of the corresponding co-PI. The highest strength of blend-PI is 1.29 GPa, slightly higher than the 1.1 GPa¹⁹ of the co-PI at the 4/6 molar ratio of BPA/ODA.

Fig. 4 Effect of BPA amount (mol%) on the tensile strength (a) and modulus (b) of blend-PI and co-PI nanofiber membranes.

The excellent mechanical properties of the co-PIs were attributed to the flexible and rigid microblock ordered regions in the nanofibers.¹⁹ However, the reason for the extraordinary mechanical properties of binary blend was much more complicated. The blend-PI was prepared from two types of polyamic acids (B-PAA and O-PAA), and the intrinsic viscosities of B-PAA and O-PAA were 5.2 dL g⁻¹ and 5.8 dL g⁻¹, respectively. This super-high viscosity of PAA implies an ultra-high molecular weight, which guaranteed the high performance of the nanofibers.¹⁶ During the blending of the two PAAs, there were re-equilibration and recombination of different molecular fragments,³³ which might result in a molecular structure that is similar to that of the co-PAA, which would be imidized into co-PI. Another explanation of the excellent mechanical properties of blend-PI could be the special superstructures. Zhang et al. demonstrated that the space around the paracrystalline domains or between the paracrystalline lamellae was filled with amorphous segments of the blend-PI, based on the studies of wide- and small-angle X-ray and dynamic mechanical analysis of blend-PI film.³⁴ Fig. 5 shows the wide-angle X-ray diffraction (XRD) patterns of the O-PI, B-PI and blend-PI nanofiber membranes. Three weak diffraction peaks were observed at about 14.26°, 16.22° and 18.60° for the flexible O-PI nanofiber membranes, indicating small, partially crystalline domain. Both the rigid B-PI and blend-PI nanofiber membranes showed a strong diffraction peak at 14.26° and a middle strong peak at 23.94°, implying the existence of paracrystalline domains. No new lattice parameters or new structural information was found from the X-ray patterns of blend-PI (Fig. 5). Based on the recombination phenomenon of the blend-PAAs³³ and the molecular alignment along the nanofiber axis during electrospinning,¹⁵ we believe that there are microblock structures, interlamellar and interfibrillar segregations of B-PI in the blend nanofibers (Fig. 6).³⁵ These structures were uniformly distributed along the nanofibers, and they introduced a strong intermolecular interaction, i.e., a state of aggregation of the polymer chains via charge-transfer complex formation.^36,37 Therefore, the blend nanofibers exhibited not only high strength, but also high toughness. Moreover, these multiple superstructures could also be used to explain the higher tensile strength and modulus of blend-PI than co-PI nanofibers having only microblock structures.

Fig. 5 Wide-angle X-ray diffraction (XRD) patterns of the B-PI, O-PI and blend-PI nanofiber membranes with different molar ratios of B-PI/O-PI.

Fig. 6 Schematic representation of the microstructure of blend-PI nanofiber with rigid and flexible components.

3.3. Thermal properties and miscibility of blend-PI nanofiber membranes

It is well-known that aromatic polyimides have outstanding thermal stability. Fig. 7 shows the TGA curves and Table 2 summarizes the thermal properties of pure B-PI, O-PI and blend-PI nanofiber membranes. All the samples showed a high thermal stability upto more than 500 °C because of the aromatic backbones. As expected, the thermal stability of the blend-PI nanofiber membranes fluctuated between that of O-PI and B-PI nanofiber membranes. The B-PI backbone was composed of benzene ring structures and imide ring structures. The molecules of this type of rigid-rod-like polyimide (B-PI) possessed high temperature resistance performance. Compared with B-PI, the flexible –O– ether groups on the O-PI reduced the rigidity of the backbone and decomposed the polymer at a lower temperature. Therefore, as the molar ratio of BPA/ODA increased, the 5% weight loss temperature (T_5%) gradually increased from 530 to 587 °C and the decomposition temperature (T_d) increased from 482 to 535 °C because of the inherent oxidation from the increased number of –O– ether groups.

Fig. 7 TGA curves of B-PI, O-PI and Blend-PI nanofiber membranes.

Table 2 Thermal properties of electrospun PI nanofiber membranes

Molar ratio (BPA/ODA)	T5% (°C)	Td (°C)	Tg (°C)
0/100 (O-PI)	530	482	275
2/8	536	506	287
4/6	545	508	296
6/4	555	513	299
8/2	584	532	—
100/0 (B-PI)	587	535	—

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Compared to the differential scanning calorimetry (DSC), the dynamic mechanical analysis (DMA) is an effective and sensitive method to determine the glass transition temperature (T_g) of polymers with rigid-rod-like molecular chains. The DMA curves (tan δ) of B-PI, O-PI and blend-PI nanofibers are shown in Fig. 8; the corresponding T_g is listed in Table 2. The flexible O-PI nanofiber membranes showed a prominent α transition peak at 275 °C, which is usually associated with the relaxation of the main backbone chain. With the increasing amount of B-PI, there were more rigid components in the blend and more energy was needed for the movement of molecular chains. Therefore, an increase of T_g and a decrease of tan δ intensity were observed. When a higher molar ratio of the B-PI was applied, e.g., 8/2, almost no α transition could be observed. In this case, the motion of molecules was restricted by the rigid component in the polymer blend system. However, a medium strong and broad β transition was found from the tan δ curves, which could be attributed to the secondary transition of the movement of aromatic moieties on the backbones.

Fig. 8 tan δ of electrospun B-PI, O-PI and blend-PI nanofiber membranes.

Compatibility or miscibility of the two components is important for the excellent physical properties of the blend. One feasible way to check the miscibility of the blend is to measure the glass transition temperature (T_g) by DSC or DMA. Generally, due to the absence of molecular interactions between the two components, the immiscible blend would show two glass transition temperatures, each coming from the respective component.²⁸ However, the miscible blend usually showed only one glass transition temperature due to the strong molecular interactions between the chains of each component.^8,36 In this study, the blend-PI nanofibers showed only one glass transition temperature (Table 2 and Fig. 8), which suggested the good miscibility of the blend-PI due to the strong intermolecular charge-transfer interaction between B-PI and O-PI molecules.

4. Conclusions

High strength and high toughness PI nanofibers were prepared by electrospinning a binary blend of polyamic acids, followed by a thermal imidization process. The thermal and mechanical properties can be controlled by changing the molar ratio of the rigid B-PI component and flexible O-PI component. The glass transition temperature and decomposition temperature of the blend-PI gradually increase with increasing amount of B-PI. The blend-PI nanofibers possess better mechanical properties than the corresponding co-PI. When the molar ratio of B-PI/O-PI is 4/6, the blend-PI has the highest tensile strength of 1.3 GPa, which is 200 MPa higher than that of the corresponding co-PI and almost 3 times that of homo- O-PI and B-PI. The blend-PI nanofibers with B-PI/O-PI molar ratio of 4/6 and 2/8 show high toughness at the same level of 82 J g⁻¹ and 94 J g⁻¹, respectively, slightly smaller than that of the well-known ultra-flexible thermoplastic polyurethane (111 J g⁻¹). All the blend-PI nanofibers exhibited complete miscibility over the entire range of composition. These excellent properties of blend-PI nanofibers can be probably attributed to the multiple superstructures and the strong intermolecular charge-transfer interaction between B-PI and O-PI molecules. This blend method of making blend-PI is very simple and can be significantly extended to modify the properties of PIs. Such high strength and high toughness of the blend-PI nanofibers with excellent thermal stabilities promise particular applications in the field of high-temperature composites.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (Grants no. 21174058 and 21374044), the Major Special Projects of Jiangxi Provincial Department of Science and Technology (Grant no. 20114ABF05100) and the Technology Plan Landing Project of Jiangxi Provincial Department of Education.

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