Preparation and properties of PIPD nanofibers made by a swelling and ultrasonic stripping process

Abstract

Poly{2,6-dimidazo[4,5-b:4′,5′-e]pridinylene-1,4(2,5-dihydroxy)phenylene} (PIPD) nanofibers were prepared from an as-spun PIPD fiber by a simple solvent swelling and ultrasonic stripping process for the first time. Briefly, with the help of heating and supersonic treatment, a PIPD fiber was made into nanofibers with diameters of 20–500 nm. Due to the nanoscale of the PIPD nanofibers and abundant polarized –OH groups on the surface of the nanofibers, these PIPD nanofibers show excellent solubility and stability in DMSO. It is easy to construct porous PIPD films with these nanofibers and the membrane shows excellent mechanical properties and heat resistance. Moreover, the mechanical performance of the PIPD membrane can be improved greatly after cross-linking with glutaraldehyde.

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1 Introduction

The interest in separation by the use of microfiltration membranes has rapidly increased during the last two decades.¹ Generally, microfiltration means using a pressure-driven flow through a microporous membrane to separate and recover micron or sub-micron-sized particles from blends. The characteristics of a microfiltration membrane include high porosity, low operating pressure, thin thickness, high separation efficiency, uniformity and continuum, which cause microfiltration membranes to be widely used in the pharmaceutical industry, food industry, biotechnology and many other industries.²

Up to now, materials used for microporous filtration membranes can be divided into two distinct classes of material. The first one is polymers consisting of organic materials such as: cellulose acetate (CA), polyamide (PA), polysulfone (PS) and polyvinylidene difluoride (PVDF) amongst others. Another one is inorganic materials, such as metals and ceramic materials.³ Compared with inorganic membranes, organic membranes, often composed of polymer materials, show better flexibility and chemical resistance, which make them attract much more attention.

Briefly, varied methods can be used for the preparation of organic microfiltration membrane, such as physically stretching, phase inversion, track etching, radiation curable and dissolution.⁴ As time goes on, the applications advance higher performance requirements of microfiltration membranes, such as high heat resistance, environmental stability and solvent resistance. Meanwhile, it is advantageous to have the thinnest thickness if possible. In practice, considerations such as mechanical strength usually determine the lower limit of membrane thickness. All the requirements promote the development of microfiltration membrane made of high-performance polymer material. However, the structures of high-performance polymers are often rigid and not soluble in common solvents, which make it difficult to prepare porous membrane by themselves. To solve this problem, people developed a method called nanofiber film, including electrostatic spinning and fiber nanocrystallization.⁵ Due to their nanoscale structure and high surface-to-volume ratios, properties of nanofibers including electrical, optical, dimensional and mechanical characteristics are much different from those of microfibers.⁶ In previous study, K. Q. Cao et al. prepared Kevlar nanofibers by basic degradation of Kevlar fiber in dimethyl sulphoxide (DMSO) and showed that it is easy to form porous structure of the Kevlar membrane with Kevlar nanofiber/DMSO dispersion.⁷ Similarly, Mahiar M. Hamedi et al. prepared porous cellulose membrane with nanocellulose made by oxidizing and mechanical disintegration.⁸ Recently, S. Ifuku et al. reported poly(p-phenylene benzobisoxazole) (PBO) membrane made of PBO nanofibers that prepared by a downsizing process, during which PBO fibers were treated by basic hydrolysis.⁹ However, due to the use of strong oxidizing agent, acid and alkali, degradation of macromolecular chains occurred in the process of exfoliation, which affected the performance and reduced the yield of nanofibers. In addition, nanofibers prepared were irregular and the fibril surface structure was damaged by oxidization and basic hydrolysis.

In this study, inspired by the fibril formation phenomena of organic fiber under strong strain,¹⁰ we proposed a novel method composed of solvent swelling and ultrasonic stripping to convert PIPD fiber into nanofibers. Also, we studied PIPD microfiltration membrane made of the nanofibers. It is expected that this method can be used for many other high-performance nanofibers such as PBO, aramid, ultra-high-molecular-weight polyethylene fiber (UHMWPE) that have a similar fibril structure with PIPD.

2 Experimental

2.1 Materials

PIPD macromolecules were synthesized with the established protocols.¹¹ PIPD fiber is obtained by dry-jet wet spinning from the polymer solution dissolved in poly-phosphoric acid (PPA). DMSO was purchased from Harbin Chenxin Chem. Ltd, China.

2.2 Method

Firstly, PIPD macromolecules were synthesized and long PIPD fiber with a diameter of 45 μm were made with dry-jet wet spinning technology. And then, the long PIPD fiber was kept in DMSO at 155 °C for 3 h and stripped at 95 °C under ultrasonic treatment for another 2 h. After that, PIPD nanofiber/DMSO dispersion was obtained and then concentrated by centrifugation. Lastly, porous PIPD membrane was prepared through filtration of concentrated PIPD nanofiber/DMSO dispersion.

2.3 Characterization

The water permeability of PIPD membrane was studied by using filtration under vacuum of 3 Pa. Then, the water permeability, J₀, was calculated.
in which V: volume of permeated water (L), A: the testing area of membrane (m²), t: testing time.

The relative viscosities of PIPD macromolecules were measured in methanesulfonic acid (MSA) at 25 °C. The morphologies and gold distribution of the membranes were observed by SEM (S-4800, Hitachi Instrument, Inc. Japan) equipped with an Energy Dispersive Spectrometer (EDS). FT-IR spectra of the samples were recorded on an AVATAR 360 ESP FT-IR spectrometer (Nicolet, USA) equipped with a MKII Golden Gate. The X-ray diffraction (XRD) patterns of the samples were recorded on a Rigaku D/Max-2200/PC X-ray diffractometer. Thermal stability of the samples were examined on a TGA 2050 Thermogravimetric Analyzer (TA Instruments Inc., USA.) with a heating rate of 10 °C min⁻¹ under air atmosphere. All samples were dried under vacuum at 120 °C for 24 h prior to TGA measurements.

3 Results and discussion

As we know, PIPD macromolecules have many polar side groups –OH and skeleton groups –NH–, which form intermolecular hydrogen bonding network inside and make the macromolecular interaction in the PIPD fiber very strong, as shown in Scheme 1.¹²

Scheme 1 Chemical structure PIPD macromolecule and the intermolecular hydrogen bonding network inside the PIPD fiber.

The hydrogen bond network structure formed by the hydroxyl groups of the rigid polymer provides the strongest tensile strength, thermal stability, resistance to compression performance, distortion performance and interface performance. However, like most of the organic fibers made by dry-jet wet spinning technology, PIPD fiber show microcrystal and fibrillar structure under strong shearing and compressive loads,¹³ especially for the as-spun PIPD fiber that contain a lot of water and residual solvent, polyphosphoric acid. In the case of as-spun PIPD fiber, the hydrogen bonding network inside is not constructed well and many defect vacancy, such as bubble and water, exist among the parallel fibrils in the as-spun PIPD fiber. These make the interaction among the fibrils very weak and susceptible to be attacked and destroyed. Therefore, as-spun PIPD fiber is an ideal material to be made into micro or nano fibers.

To separate fibrils from the construction, the PIPD fibers was immersed in DMSO and treated by ultrasonic at ordinary temperature. We noted that in dimethyl sulfoxide (DMSO), the as-spun PIPD fiber can be swelled at 155 °C. Under strong supersonic oscillation and shearing at 95 °C, the swelled PIPD fibers were stripped layer by layer into fibrils with micro or nano dimensions. And the diameter of the fibrils decreased with treatment time. Finally, PIPD nanofibers were obtained after a short treatment of 2 h. In our understanding, this is the first time to synthesize PIPD nanofibers with such a simple technology. The technological process and material state evolution were shown as Scheme 2 and Fig. 1, respectively.

Scheme 2 Preparation schematic of PIPD nanofiber.

Fig. 1 Material state evolution of PIPD fiber during the treatment. (a) PIPD fiber immersed in DMSO, (b) PIPD nanofibers dispersed in DMSO and (c) membrane made of PIPD nanofibers (the yellow area inside was caused by the gold coating).

The micro morphology changes of as-spun PIPD fiber during the treatment are determined by SEM, shown as Fig. 2. At the beginning, PIPD fiber showed a compact structure and smooth surface (Fig. 2a). After a period of treatment, the diameter of PIPD fiber increased and fibrils started to peel from the fiber and form micro fibers (Fig. 2b and c). The swelling ratio of PIPD fiber during DMSO treatment, SW, was calculated from volume changes. Briefly, , while V₁ = W₁/ρ₁, V₂ = W₂/ρ₂. In which, V₁ and V₂ are the volume of as-spun PIPD fiber before and after DMSO treatment, respectively. W₁ and W₂ are the weight of as-spun PIPD fiber before and after DMSO treatment, respectively. ρ₁ and ρ₂ are the density of as-spun PIPD fiber before and after DMSO treatment, respectively. They were measured with a density balance. It is found that the as-spun PIPD fiber were swelled by about 4.5% in radius direction after treated with DMSO solvent under 155 °C for 3 h, which is caused by the DMSO molecules that took up the space of residual solvent and swelled among fibrils inside PIPD fibers. And then, fibril of the as-spun PIPD fiber exfoliated into micro fiber and was dispersed into the DMSO solvent (Fig. 2d). When the strong supersonic oscillation was continued, the micro fiber was refined into nanofiber (Fig. 2e). PIPD nanofibers with different diameters were obtained by changing the stripping time. For the PIPD nanofibers, they showed an excellent dispersion in DMSO solvent and had a red color due to the conjugating PIPD macromolecules and scattering system of PIPD nanofiber (Fig. 1b). We found that the diameters of the PIPD nanofibers were homogeneous and can be modified between 20 and 500 nm. It is also found that the supersonic oscillation frequency, treatment time and temperature strongly affected the structure of PIPD nanofibers.

Fig. 2 SEM images of PDIP fiber under different treatment time. (a) PIPD fiber, (b) PIPD fiber swelled by DMSO, (c) PIPD fiber peeled off, (d) and (e) PIPD nanofibers.

And the influence of treating time and ultrasound power on the nanofiber size were studied in details. The ultrasound power was changed from 28–1000 kHz and the treating time was controlled between 0.5 and 2 h. However, it is difficult to study the statistical diameters of PIPD nanofiber. Then, the water permeability of the PIPD nanofiber membrane was tested to investigate the diameter of the nanofiber. As we know, the permeability of the membrane made of nanofibers increased with diameter of the nanofiber. The smaller the nanofiber, the greater obstruction of the membrane to water. The results were shown as Table 1. The membrane made of PIPD nanofiber that treated with power of 1000 kHz for 2 h was 24 161 L m⁻² h⁻¹. And it can be seen that the water permeability of the membranes decreased with the treating time and ultrasound power, which means increasing treating time and power are beneficial to the preparation of PIPD nanofiber. Reasonably, the increase of power could enhance the amplitude of vibration of PIPD fibril, and the degree of stripping. We found that no nanofiber was obtained when the PIPD fiber was treated less than 1 h under 28 kHz. Moreover, it was found that the influence of treating time is very important in the later period of the ultrasonic treatment, namely the change from PIPD microfiber to nanofiber.

Table 1 The influence of treating time and power on the water permeability of obtained PIPD membrane

Number	Treating time (h)	Ultrasound power (kHz)	Water permeability (L m^−2 h^−1)
1	0.5	28	—
2	0.5	40	—
3	0.5	1000	35 685
4	1	28	—
5	1	40	67 584
6	1	1000	28 712
7	2	28	68 733
8	2	40	62 899
9	2	1000	24 161

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Compared with the preparation technology of Kevlar nanofibers reported in the previous study,¹⁴ the technology reported here have many advantages. Firstly, the solvent used in this technology can be recycled easily by low pressure evaporation and reused without any treatment. Therefore, this technology is environmental friendly. Secondly, no potassium hydroxide was used in this technology and the molecular weights of PIPD macromolecules showed a decrease from 21.5 ± 0.1 dL g⁻¹ to 21.2 ± 0.3 dL g⁻¹ in MSA after ultrasound treatment. Meanwhile, the product yield of PIPD nanofibers reached 96%. Moreover, in this technology, PIPD nanofibers can be obtained in 5 h while that of Kevlar nanofibers is more than 1 week. In conclusion, this technology is simple, environmental friendly, high yield and efficient.

To learn the effects of heating and supersonic treatment on the chemical structure of PIPD macromolecules and crystal structure, FT-IR spectra of original PIPD fiber and PIPD nanofibers were collected. Before the collection, all the PIPD nanofibers were immersed in water for 72 h to make enough absorption of water. And then, the PIPD nanofibers with water inside were dried at 80 °C for 30 min, 150 °C for 1 h, 250 °C for 2 h. After each step of heating treatment, the FT-IR spectrum was collected immediately, as shown in Fig. 3. Obviously, free water in the PIPD nanofiber caused the strong absorbance at 3010–3150 cm⁻¹, which disappeared after the heat treatment. At the same time, the absorption peak at 3460 cm⁻¹ corresponded to the stretching vibration of the association hydroxyl among molecular chains. The absorption peak at 1400 cm⁻¹ was assigned to the stretching vibration of C–N bond and the stretching vibration of C=N bond was showed at 1640 cm⁻¹.¹⁵ Meanwhile, the bending vibration corresponding to N–H and bending vibration corresponding to O–H was appeared at 1550 and 1005 cm⁻¹, respectively. Moreover, these absorption peaks became sharper when the PIPD fiber were made into nanofibers. It indicates that most of the N–H and O–H inside the crystals of PIPD fiber were released from hydrogen networks, which made the vibration of N–H and O–H stronger.¹⁶

Fig. 3 FTIR spectra of (a) PIPD fiber, (b) PIPD nanofiber dried at 80 °C for 30 min, (c) PIPD nanofiber dried for another 1 h at 150 °C, and (d) PIPD nanofiber dried for another 2 h at 250 °C.

Also, the effects of treatment on the crystalline structure of PIPD fibers were determined by using XRD. The diffraction patterns of PIPD fiber and nanofibers are showed as Fig. 4. The diffraction peaks of 200, and 110 (11̄0) planes from original PIPD fiber (black line) were observed at 10.6° and 26.15°,¹⁷ respectively, which were poignant showing the good crystal construction. After treating, the peaks became obtuse and the width of peak increased, which revealed the decline of crystallinity and the increase of the distance of crystal planes. The information above indicated that the treatment with DMSO and supersonic stripping partly destroyed the crystal structure of PIPD fiber.

Fig. 4 XRD patterns of PIPD fiber and PIPD nanofibers.

As expected, PIPD nanofibers showed lower thermal stability than original PIPD fiber. From the TGA thermograms under air condition (Fig. 5), we can see that the initial degradation of PIPD nanofibers, namely weight loss of 5%, happened at 375 °C while the 50% of weight loss happened at 583 °C. Compared with that of original PIPD fiber (5% at 510 °C and 50% at 603 °C),¹⁸ we can see that the heat resistance of PIPD nanofiber declined. There are two main reasons for the decrease of thermal stability. Firstly, the –OH and –NH₂ groups on the surface of PIPD nanofibers are greater than the PIPD fiber, which degraded at a lower temperature than the skeleton. Secondly, decreased densification and regularity also benefit the reduction of thermal stability.

Fig. 5 TGA thermogram of PIPD nanofibers. PIPD nanofibers were heated at 10 °C min⁻¹ under air atmosphere.

Then, PIPD porous membrane was made by filtration of PIPD/DMSO dispersion. It is expected to prepare a porous PIPD membrane for micro filtration. As we know, the specific surface area of the porous film increases sharply with the decrease of diameter and the mass transport of water in the membrane decreased greatly with the decreasing diameter of nanofiber. Taking the specific surface area and mass transport into account, PIPD nanofibers with average diameter of 250 nm were chosen for the porous membrane in this study. The morphology of PIPD nanofibers membrane, including surface and cross section, were performed with SEM and presented in Fig. 6. It can be seen that the PIPD nanofibers had a well-defined morphology and distributed uniformly to form the porous membrane. The porous membrane was paper like and has irregular holes with tens to hundreds of nanometers. Meanwhile, nanofibers stick together to form locally physical entanglement point.

Fig. 6 SEM images of PIPD nanofiber membrane. (a) Surface and (b) section.

Nitrogen adsorption–desorption isotherms from Brunauer–Emmett–Teller (BET) surface area analysis measurements are shown in Fig. 7. The curves suggest a type IV-like isotherm for PIPD membrane.¹⁹ The adsorption started straight indicating a single layer of nitrogen adsorption on the material surfaces, followed by the capillary condensation of nitrogen as the relative pressure (P/P₀) increased past 0.82, which could be attributed to the nitrogen condensed in the porous structures of PIPD nanofibers. BET surface area analysis indicates the specific surface area of nanofiber membrane is 44.3 m² g⁻¹. Meanwhile, the pore size distribution is calculated according to the DFT N₂ Tarazona NLDFT model, and it is found that most of pores of PIPD membrane are in the range of 400–1100 nm.

Fig. 7 BET surface area analysis. (a) N₂ adsorption–desorption isotherms, and (b) the pore size distribution of PIPD membrane.

Due to the hydrogen bonds between these entanglement points, this porous membrane showed fracture strength of 20.60 MPa and break elongation of 3.62%, shown as Fig. 8a. Throughout the testing process, the stress increases linearly with the strain and the slope decreases gradually with stress, indicating the plastic property of PIPD membrane. As we know, PIPD fiber shows excellent compressive strength with the help of the hydrogen bond network structure.^12,13 Similarly, the hydrogen bond network in PIPD membrane also played an important role for its high strength. Theoretically, when stress is applied to the membrane, the hydrogen bonds worked like glue and made the membrane show excellent tensile strength. From the section morphology we can see that the membrane had a spongy structure, which makes the film have filtering capabilities and fully modified space.

Fig. 8 Stress–strain properties of (a) PIPD membrane, (b) PIPD membrane crosslinked by glutaraldehyde.

To improve the mechanical properties of porous PIPD membranes, the membranes were treated with glutaraldehyde. Similar to the structure reported in the previous study,²⁰ glutaraldehyde worked as a coupling agent for PIPD nanofibers and caused chemical crosslink structures in the membrane. The end group –NH₂ and side groups –OH of PIPD macromolecules on the surface of PIPD nanofiber could react with end groups –CHO of glutaraldehyde when the PIPD membrane was treated with aqueous glutaraldehyde. As a result, the nanofiber could be chemically crosslinked by glutaraldehyde chains that reacted with different PIPD nanofibers, shown as Scheme 3. As expected, the fracture strength of membranes was enhanced, shown as Fig. 8b. The shapes of stress–strain curves of PIPD membrane before and after treatment were almost same, showing that the membranes have similar mechanical behavior. After treated with glutaraldehyde, the tensile strength reached the amazing 31.1 MPa because the hydrogen bonds between PIPD fibers were strengthened by the coupling reaction.⁷ Meanwhile, the elongation at break decreased to 2.88%, which means the membrane become more rigid after crosslinking.

Scheme 3 The GDA crosslinking reaction between PIPD nanofibers.

In summary, the porous PIPD membranes show high thermal resistance and mechanical properties, which make its potential applications in micro filtration under high temperature, such as the filter for particles of PM 2.5 in auto exhaust.

4 Conclusions

In this study, we proposed a new way to prepare polymer nanofibers. It is the first report of production of PIPD nanofibers by a solvent swelling process. The PIPD membrane prepared by nanofibers has excellent mechanical properties and heat resistance. This facile procession not only simplify the process of nanofibers but also maintain the excellent properties of nanofibers, which will advance the use of high-performance nanofibers in novel composites. In conclusion, this approach is simple, environmental friendly, high yield and efficient. We believe it can be applied to many other high-performance fibers with micro-fiber structures.

Acknowledgements

The research reported in this paper was partly supported by Research Fund for the CASC-HIT Technology Innovation Program 13-1C02, Doctoral Program of Higher Education of China (20122302120038), Special Foundation of China Postdoctoral Science (2013T60379) and Postdoctoral Science Foundation (2013M541392).

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