Abstract
Polyacrylonitrile (PAN) nanofibers are very important to achieve high performance carbon nanofibers. In this work, co-polyacrylonitriles (co-PANs) with different molecular weights were synthesized by a simple free-radical polymerization. The effect of the initiator amount on the molecular weight of co-PAN was investigated. The co-PANs with different molecular weight were electrospun into aligned nanofibers by adjusting the absolute viscosity of co-PAN solution into ~1.0 Pa·s. All the co-PAN nanofibers showed smooth surfaces and homogeneous fiber diameters of ~450 nm. Tensile tests were applied to evaluate the mechanical properties of electrospun aligned co-PAN nanofibers. The results indicated that higher molecular weight led to better mechanical performance of electrospun aligned co-PAN nanofibers. When the molecular weight was 2.3×105, the highest strength of 153 MPa, strain of 0.148, and toughness of 16.0 J/g were obtained. These electrospun aligned co-PAN nanofibers could be good candidates for the preparation of high performance carbon nanofibers.
1 Introduction
Carbon nanofibers (CNFs) are ideal candidates for high performance composites because of their excellent mechanical properties and thermal stabilities (1), (2), (3), (4). CNFs can be simply produced by electrospinning followed by thermal treatments from different kinds of polymer precursors, such as polyacrylonitrile (PAN) and its copolymers (5), (6), (7), (8), (9), poly(vinyl alcohol) (10), (11), polybenzimidazole (PBI) (12), (13), polyamic acid (14), (15), (16), polybisbenzimidazobenzophenanthroline-dione (BBB) (17), etc. Among these precursors, PAN and its copolymers (co-PAN) are widely investigated due to their mechanical inheritance to CNFs. Therefore, many researchers have devoted their efforts to study the mechanical performance of electrospun PAN or co-PAN nanofibers. In general, the mechanical properties could be affected by the following factors: (a) fiber alignment (5), (18); (b) addition of oxygen-containing co-monomer (8); (c) pre-oxidation temperature (19); (d) fiber diameter (20); (e) post stretching (21), (22); (f) molecular weight (23); and (g) addition of other reinforcements (24), (25), (26), (27). Among these factors, higher molecular weight has been proved to effectively increase the mechanical properties. Further investigation indicated that the larger molecular weight could lead to better physical properties of fibers. For example, Termonia et al. theoretically disclosed the effect of molecular weight on the tensile strength of polymer fibers (28). Their theoretical simulation by a stochastic Monte Carlo approach, proved that the higher molecular weight of polymers would lead to better molecular orientation in fibers and therefore enhance the mechanical properties. In another study, Lyoo et al. showed that electrospun PVA nanofibers with higher molecular weight possessed better thermal stabilities, crystalline properties and mechanical properties than those with lower molecular weight (29). However, there are only a few countable reports on the effects of molecular weight on the mechanical properties of electrospun PAN or co-PAN nanofibers. Liu et al. studied the electrospinning of PAN fibers from strain-hardening PAN solutions by adding ultrahigh-molecular-weight PAN (UHMWPAN) into medium-molecular-weight PAN (MMWPAN) solutions (30). However, due to the too small addition of UHMWPAN, it is difficult to observe obvious mechanical improvement on electrospun PAN nanofibers. Therefore, it is still highly desired to systematically study the effect of molecular weight on the mechanical properties of electrospun PAN or co-PAN nanofibers.
In this work, co-PANs with different molecular weights were prepared by changing the amount of initiator. Then co-PAN nanofibers with uniform fiber diameters were obtained from co-APN solutions with similar absolute viscosity of 1.0 Pa·s by electrospinning. The effects of molecular weight on mechanical properties of electrospun co-PAN nanofibers were investigated.
2 Experimental part
2.1 Materials
Acrylonitrile (99%, Shanghai SSS Reagent Co., Ltd., Shanghai, China), dimethyl sulfoxide (DMSO, 99.9%, Xilong Chemical Co., Ltd., Shaanxi, China), n-butyl acrylate (BA, 99.9%, Yonghua Chemical Technology Co., Ltd., Jiangsu, China) were purified by vacuum distillation. Azodiisobutyromitrile (AIBN, 99%, Shanghai No.4 Reagent, Shanghai, China) was recrystallized by absolute methanol. Itaconic acid (IA, 99.6%, Hanerchem, Guanzhou, China) and N, N-dimethyl formamide (DMF, 99%, Aldrich) were used as received.
2.2 Preparation of co-PAN nanofibers
Series copolymers, poly(acrylonitrile-co-IA-co-BA) (co-PAN), were prepared by radical polymerization in DMSO at 50°C for 24 h in N2 with different amounts of AIBN initiator. The concentration of co-PAN/DMSO was fixed at 25 wt% and the weight ratio of AN/IA/BA was set as 93/5/2. The amount of AIBN was 0.31, 0.35, 0.48, 0.55 and 0.60 wt% to the weight of AN. The obtained co-PAN solutions were diluted into absolute viscosity of ~1.0 Pa·s by DMF for electrospinning with an electric field of 100 kV/m. The flow rate was 0.6 ml/h. The applied humidity was 10–25%, which was achieved by an air conditioner with a dehumidification mode. The aligned nanofibers were collected by a highly rotating disc (diameter of 30 cm) with a linear speed of 24 m/s for 5 h.
2.3 Characterization
The intrinsic viscosity ([η]) of co-PAN solutions was characterized using a Ubbelohde viscometer (0.705 mm, 50°C, DMSO). The number-average molar mass (Mn) could be calculated by the following Mark-Houwink equation (31):
The absolute viscosity was determined by an NDS-8S digital absolute viscometer at 25°C. The nanofiber morphology was monitored by a scanning electron microscopy (SEM, TESCAN vega3). Tensile tests were performed on a computer-controlled electromechanical testing machine (CMT-8102, Shenzhen, China) at a stretching rate of 5 mm/min. The thickness of samples was measured by a screw micrometer. The samples for mechanical testing were prepared by cutting the electrospun nanofiber mat into rectangle shape with a size of 40×5 mm.
3 Results and discussion
The amount of AIBN has a significant effect on [η] of co-PAN solutions (Figure 1). When increasing the amount of AIBN, [η] first increases and then decreases. When the amount of AIBN was 0.48 wt%, [η] reached to the largest value of 3.30 dl/g. It is well-known that the number-average molar mass (Mn) could be calculated using the Mark-Houwink equation. In this work, Mn of co-PAN were summarized in Table 1. It is obvious that Mn showed a trend of first increase and then decrease. The reaction for the preparation of co-PAN is a radical polymerization. A lower amount of initiator AIBN led to a smaller amount of active sites and therefore produced a smaller amount of radicals and finally decreased the reaction rate. However, an excess amount of initiator of AIBN also led to the decrease of Mn because too many radicals from the initiator would lead to the fast chain termination. Thus, the highest Mn of 2.3×105 was achieved when the addition of AIBN was 0.48 wt%.
Effect of amount of AIBN on the intrinsic viscosity of co-PAN solutions.
Summary of amount of AIBN, [η], Mn, solution concentration (C), sample thickness (T), absolute viscosity (ηab) of co-PAN solutions for electrospinning and mechanical properties of electrospun aligned co-PAN nanofibers.
| AIBN (wt%) | [η] (dl/g) | Mn (105) | C (wt%) | Thickness (μm) | ηab (Pa·s) | Strength (MPa) | Strain | Toughness (J/g) |
|---|---|---|---|---|---|---|---|---|
| 0.31 | 0.70 | 0.3 | 14.5 | 326 | 0.992 | 94 | 0.072 | 4.8 |
| 0.35 | 1.82 | 1.1 | 9.93 | 208 | 1.013 | 107 | 0.127 | 10.2 |
| 0.48 | 3.30 | 2.3 | 4.64 | 93 | 0.971 | 125 | 0.129 | 11.3 |
| 0.55 | 3.06 | 2.1 | 6.61 | 152 | 1.086 | 137 | 0.142 | 14.1 |
| 0.60 | 2.88 | 1.9 | 7.13 | 169 | 1.130 | 153 | 0.148 | 16.0 |
The diameter and surface morphology of nanofibers play a crucial effect on their mechanical properties (20), (32), (33). To rule out the effects from these two parameters, it is necessary to optimize the electrospinning parameters. In this work, the same electric field of 100 kV/m, a flow rate of 0.6 ml/h and collector rotating speed of 24 m/s were applied for all co-PAN solutions. Previous reports showed that it was helpful to achieve similar fiber diameters via keeping ηab the same (29), (30), (31), (32). Therefore, ηab of co-PAN solutions were adjusted to ~1 Pa·s by DMF. With 1 Pa·s of ηab, all the co-PAN nanofibers were very homogeneous, smooth, and highly aligned along the fiber collecting direction, and showed a uniform diameter of ~450 nm (Figure 2). In detail, the corresponding average fiber diameters were 433±39 nm, 464±51 nm, 449±35 nm, 475±68 nm and 454±57 nm, for samples with Mn of 0.3×105, 1.1×105, 2.3×105, 2.1×105 and 1.9×105, respectively. These fiber morphologies could avoid the effect of size and defects on mechanical properties.
SEM images of electrospun aligned co-PAN nanofibers from co-PAN with different Mn.
Because of the inheritance mechanical performance, it is necessary to prepare PAN nanofibers also with excellent mechanical performance. In this work, via optimizing the synthesis process, the co-PAN with different molecular weights were electrospun into aligned nanofibers, and their mechanical properties were characterized by tensile tests. As shown in Figure 3 and Table 1, when the molecular weight was 0.3×105, the tensile strength, elongation at break and toughness of co-PAN electrospun aligned nanofibers were 94 MPa, 0.072, and 4.8 J/g, respectively. When the molecular weight increase to 2.3×105, the corresponding values were increased to 153 MPa, 0.148, and 16.0 J/g, which were 162%, 206% and 333% of the sample from molecular weight of 0.3×105, respectively. The strength of 153 MPa is much larger than ~50 MPa of aligned electrospun nanofibers of co-PAN with weight ratio of 93/5.3/1.7 AN/methyl acrylate/IA and average molecular weight of 100,000 g/mol (34), (35), and also larger than 15 MPa of aligned electrospun bundles of co-PAN with a weight ratio of 92.8/1.2/6.0 of AN/IA/methyl acrylate (21). The possible reasons for these mechanical differences could be the much better fiber alignment in this work than the above two examples. In particular, the toughness of 16.0 J/g was much lower than the thermoplastic polyurethane (TPU, 111 J/g) (36), but comparable to the carbon nanotubes reinforced PVA nonwovens (16 J/g) (37), and even much higher than the PVA casted films (2.5 J/g) (37). Such high toughness of electrospun co-PAN nanofibers could be a good candidate for the fabrication of carbon nanofibers with much better flexibility.
Stress-strain curves of electrospun aligned co-PAN nanofibers.
Previous research indicated that the fiber diameter could significantly affect the mechanical properties of electrospun fibers, which was called the “size effect” on mechanical performance (20). However, in this work, the influence from the fiber diameter was excluded because of the homogeneous fiber diameter of ~450 nm for all samples. Other reports pointed out that higher molecular weight would lead to higher molecular orientation along the fiber axis, which was confirmed by polarized FT-IR spectroscopy and polarized Raman spectroscopy (38), (39). Therefore, the co-PAN nanofibers with higher molecular weight could present better mechanical properties and could be promising precursors for high performance carbon nanofibers.
4 Conclusions
Co-PANs with different molecular weights have been successfully synthesized by adjusting the additional amount of initiator. The highest molecular weight of 2.3×105 could be achieved by adding 0.48 wt% of AIBN. This polymer also possessed the highest intrinsic viscosity of 3.30 dl/g. Homogeneous co-PAN nanofibers with an average fiber diameter of ~450 nm could be fabricated when adjusting the absolute viscosity to ~1 Pa·s. A higher molecular weight would lead to better mechanical properties of electrospun aligned co-PAN nanofibers. The best mechanical properties with strength/strain/toughness of 153 MPa/0.148/16.0 J/g could be achieved from the co-PAN (2.3×105), which could be due to the high molecular orientation along the fiber axis. These high mechanical performance co-PAN nanofibers could be good candidates for the high performance electrospun carbon nanofibers.
Acknowledgments
This research was funded by Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD); National Natural Science Foundation of China (Grants No.: 21574060 and 21374044); Major Special Projects of Jiangxi Provincial Department of Science and Technology (Grant No.: 20114ABF05100); Technology Plan Landing Project of Jiangxi Provincial Department of Education (GCJ2011-24).
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