Mechanical Reinforcement of Polybenzoxazole by Carbon Nanotubes

May 6, 2013 - ABSTRACT: Polybenzoxazole (PBO) reinforced with carbon nanotubes (CNTs) is of great interest in the field of polymer composites because ...
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Mechanical Reinforcement of Polybenzoxazole by Carbon Nanotubes through Noncovalent Functionalization Takahiro Fukumaru,† Tsuyohiko Fujigaya,*,†,‡ and Naotoshi Nakashima*,†,‡,§ †

Department of Applied Chemistry, Graduate School of Engineering, Kyushu University, 744 Motooka Nishi-ku, Fukuoka 819-0395, Japan ‡ International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, Fukuoka 819-0395, Japan § JST-CREST, 5 Sanbancho, Chiyoda-ku, Tokyo 102-0075, Japan S Supporting Information *

ABSTRACT: Polybenzoxazole (PBO) reinforced with carbon nanotubes (CNTs) is of great interest in the field of polymer composites because PBO and CNTs possess the highest-level mechanical properties among polymers and filler materials, respectively. However, the insolubility of PBO and the CNTs makes it difficult to develop a CNT/PBO composite in a solvent except for strong acids. In this study, we describe a novel strategy to design and prepare a CNT/PBO composite; namely, a method using a PBO precursor having an excellent solubility in organic solvents. We discovered that a PBO precursor wraps CNTs though a strong interfacial interaction between the CNT surfaces and the PBO precursor that enables the dispersion of the CNTs in organic solvents. By this method, a CNT/ PBO precursor composite film was easily obtained from the CNT/PBO composite solution, and by heating to 400 °C, we obtained a CNT/PBO composite film. Mechanical property measurements revealed that the CNT/PBO films exhibited a high tensile strength (151 MPa) and Young’s modulus (8.1 GPa) at a CNT loading of 1.7 wt %, corresponding to 130% and 179% increase compared to those of the PBO film, respectively. These values are the highest values ever reported. Wrapping of the CNTs with PBO and the nice dispersion of the CNTs into a PBO matrix are considered to be important steps for the observed reinforcement. Indeed, scanning electron microscopic observations of the cross section of the CNT/PBO film indicated the homogeneous distribution of the isolated CNTs in the PBO film. This is the first example for the preparation of a CNT/PBO precursor film in an organic solvent based on a noncovalent CNT-functionalization.

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INTRODUCTION Polymer reinforcement using a guest material (filler) is one of the most significant targets in plastic engineering and industry because it significantly expands the applications of polymeric materials. Carbon nanotubes (CNTs), 1D-conducting nanomaterials with very high aspect ratios, are considered to be ideal guest materials due to their outstanding mechanical, thermal, and electrical properties together with their lightweight.1−4 Such attractive features enable the reinforcement of polymers without increasing their weight, which is essential from the viewpoint of material weight savings. Among the many polymeric materials, poly(p-phenylenebenzobisoxazole) (PBO)5−7 has been one of the most important materials because PBO is known to provide the toughest polymeric fibers (Young’s modulus = ∼270 GPa),7 but the polymer is not soluble except in strong acids. Thus, a “soluble PBO” is a challenging target for polymer reinforcement. To date, to prepare CNT/PBO composites, strong acids such as, polyphosphoric acid, were utilized to overcome the insolubility.8−18 However, the mechanical reinforcements of the CNT/PBO composites were far lower than theoretical expectation probably due to the poor dispersion of the CNTs in the PBO. It is well recognized that the poor dispersion of CNTs due to the weak interfacial interaction between the © 2013 American Chemical Society

CNTs and polymer matrix prevents an effective reinforcement.19,20 In addition, the use of a strong acid is known to oxidize the CNTs and sometimes reduce their lengths,21 thus decreasing the reinforcement efficiency.22 An acid-free process is strongly required from the industry side as well. Our strategy to prepare a CNT/PBO composite is to utilize a precursor polymer of PBO soluble to an organic solvent as the host for CNT dispersion and to obtain a CNT/PBO composite from the CNT/(PBO precursor) composite. We recently developed a soluble PBO precursor having an aromatic polyamide structure with a tert-butyl dimethylsilyl (TBS) group responsible for the good solubility (TBS-prePBO) and reported the successful conversion to PBO by a thermal treatment.23 We have reported that the one-dimensional polyaromatic structure is advantageous for the effective physical adsorption through a π−π interaction onto the one-dimensional CNT surface by maximizing the overwrapping area.24 Several examples of CNT dispersions with polyaromatic structure, such as polyimides,25,26 polybenzimidazoles,27 and polythiophene,28−30 through a noncovalent interaction also Received: February 26, 2013 Revised: April 20, 2013 Published: May 6, 2013 4034

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off-white powder (MWNT/TBS-prePBO hybrid) was obtained in high yield (>90%) after drying at 80 °C under vacuum for 12 h. Fabrication of MWNT/PBO Films. One-Pot Method. To the MWNTs (0−5.0 mg) dispersion in the DMAc (5.0 mL) solution of the TBS-prePBO (20 mg), DMAc (2.0 mL) solution of TBS-prePBO (75−80 mg) was mixed with stirring for 1 h. The homogeneous solution was cast onto glass substrates followed by heating at 90 °C for 1 h, 100 °C for 1 h, 110 °C for 1 h, and then 120 °C for 1 h. The obtained films on glass were immersed in water to remove them by peeling from the substrate. The obtained free-standing films were dried at 80 °C under vacuum for 8 h and heated at 300 °C for 1 h, 350 °C for 1 h, and 400 °C for 1 h under vacuum to give the MWNT/PBO films. Precoating Method. The MWNT/TBS-prePBO hybrid (0−25 mg) and TBS-prePBO (100−80 mg) were added to DMAc (2.0 mL) and cast on a glass substrate, followed by drying at 90 °C for 1 h, 100 °C for 1 h, 110 °C for 1 h, and then 120 °C for 1 h. The obtained films were immersed in water for 1 h in order to remove them from the substrate and evaporate the residual solvent. The obtained freestanding films were dried at 80 °C under vacuum for 8 h and the obtained film was sandwiched between two glass plates, followed by heating at 300 °C for 1 h, 350 °C for 1 h, and 400 °C for 1 h under vacuum. Theoretical Model: Halpin−Tsai Model.31 The Halpin−Tsai model has been widely used as a prediction model for polymer/filler composites. This model assumes the shape of the fibrous filler and perfect adhesion between the filler and matrix. This model is represented by eq 1:

suggest the effective interaction of TBS-prePBO onto the CNT surfaces as well. In this report, we describe the noncovalent modification of CNTs using TBS-prePBO in an organic solvent and fabrication of the CNT/PBO composite film from the CNT/TBS-prePBO composite by a thermal treatment (Figure 1). The dispersion of

Figure 1. Schematic illustration for the preparation of a CNT/PBO film using a soluble PBO precursor as the CNT solubilizer as well as the polymer matrix.

the pristine CNT with TBS-prePBO, their interfacial interaction, and mechanical properties of the CNT/PBO at different CNT loadings in conjunction with the CNT dispersion inside the film are reported.

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EXPERIMENTAL SECTION

Materials. N,N-Dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), and N-methylpyrrolidone (NMP), and methanesulfonic acid were purchased from Wako Pure Chemical and used as received. As-produced single-walled carbon nanotubes (SWNTs) (HiPco; lot ATP-029, the length and diameter of the pristine SWNTs are 1−10 μm and 0.8−1.2 nm, respectively) were purchased from Carbon Nanotechnologies, Inc., and used as received. The multiwalled carbon nanotubes (MWNTs) (diameter: ∼20 nm) were kindly provided by the Nikkiso Co. TBS-prePBO was synthesized according to our previous report23 and the synthetic scheme is shown in Scheme 1. The TBS group of TBS-prePBO was partly deprotected during the polymerization in NMP at room temperature and the final protection ratio determined from the 1H NMR spectra was around 50%. The inherent viscosity of the polymer was 1.23 dL g−1 measured at the concentration at 0.5 g dL−1 in NMP at 30 °C. The intrinsic viscosity of the obtained PBO from the TBS-prePBO was 2.8 dL g−1 in methanesulfonic acid at 30 °C. Preparation of SWNT Dispersion. The SWNTs (1.0 mg) were added to the TBS-prePBO (8.0 mg) solution in DMAc (8.0 mL) and sonicated by a bath-type ultrasonic cleaner (Branson 2210) for 1 h, followed by centrifugation at 10000 g for 1 h (Hitachi Himac CS 100 GL instrument). Preparation of MWNT/TBS-prePBO Hybrid. The MWNTs (5.0 mg) dispersion in the DMAc (20 mL) solution of TBS-prePBO (20.0 mg) prepared by sonication as described above was poured into water (200 mL) and the obtained precipitate was collected by filtration. An

Ec 3 ⎡ 1 + 2(l /d)ηLVCNT ⎤ 5 ⎡ 1 + 2ηT VCNT ⎤ ⎥+ ⎢ ⎥ = ⎢ Em 8 ⎢⎣ 1 − ηLVCNT ⎥⎦ 8 ⎣⎢ 1 − ηT VCNT ⎥⎦

ηL =

ECNT /Em − 1 ECNT /Em + 2(l /d)

ηT =

ECNT /Em − 1 ECNT /Em + 2

(1)

where Ec is Young’s modulus of the composite, Em is Young’s modulus of the matrix, ECNT is Young’s modulus of the CNTs, l is the length of the CNTs, and d is the diameter of the CNTs. In this study, 1 TPa was used as ECNT from the literature.32 Here, 30 nm and 30 μm were used as d and l, respectively, from the SEM image (data not shown). The volume fraction of the CNTs can be calculated according to WCNT

VCNT = WCNT +

( ) − ( )W ρCNT

ρCNT

ρm

ρm

CNT

(2)

where WCNT is the weight fraction of the CNTs, and ρCNT and ρm are the densities of the CNTs and the polymer matrix, respectively.33,34 We assumed that the density of the MWNTs was 2.035 and the density of the PBO was 1.56.23

Scheme 1. Synthesis of Soluble PBO Precursor (TBS-prePBO)

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aromatic moiety, the π−π interaction between the SWNTs and the compound plays an important role for the effective polymer wrapping.38 In the UV−vis−NIR absorption spectrum of the solution (black line in Figure 2c), characteristic peaks at around 400−600 nm and 1200−1400 nm originating from well-isolated metallic and semiconducting SWNTs, respectively, were clearly observed.38 In addition, the intense photoluminescence (PL) from the solution guarantees the individual isolation of the SWNTs (Figure 2d) since PL is observed only from the individually isolated semiconducting SWNTs.38 It is noteworthy that a nearly identical absorption spectrum (red line in Figure 2c) and PL signals (see Supporting Information, Figure S1) were observed even after one month. All the above results strongly suggest the stable and strong interaction of the TBSprePBO onto the SWNT surfaces. To further study the interaction between the SWNTs and TBS-prePBO, the composite obtained after the filtration of the solution followed by a vigorous washing with DMAc (10 mL, 10times) to remove any unbound and weakly bound TBS-prePBO (denoted as SWNT/TBS-prePBO) was characterized (a photograph of the composite on the filter, see Supporting Information, Figure S2). The characteristic peaks ascribable to TBS-prePBO in the IR spectrum indicated the presence of the TBS-prePBO in the composite (see Supporting Information, Figure S3). Raman spectroscopy of the composite was conducted to study the interaction between the SWNT surfaces and adsorbent (Figure 3).39,40 Compared to the Raman spectrum of the as-received

Measurements. The UV−vis−NIR absorption spectra were measured using a V-670 (JASCO) spectrophotometer. The photoluminescence (PL) spectra were measured using a Fluorolog-3 (Horiba-Jobin Yvon) spectrophotometer equipped with a liquidnitrogen-cooled InGaAs near-IR detector. The excitation and emission wavelengths were in the ranges of 500−900 nm and 900−1300 nm, respectively. The Raman spectra were measured by a Raman RXN System (Kaiser Optical Systems) at an excitation of 785 nm (laser power: 10 mW) by a 10-ms exposure (10 accumulations). The FT-IR spectra were measured using a Spectrum 65 FT-IR (Perkin-Elmer) spectrometer equipped with an ATR apparatus. The mechanical properties of the films were measured at 25 °C using an EZ-S (Shimadzu) at a displacement rate of 1.0 mm min−1. The typical size of the specimen was 20 mm, 10 mm and 20 μm in length, width and thickness, respectively. Photographs of the samples were taken by a digital camera (Nikon, D50). The TGA curves were measured using an SSC 5200 (SII) operated in conditioned nitrogen at a heating rate of 10 °C min−1.

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RESULTS AND DISCUSSION

Solubilization of CNTs using PBO Precursor Polymer. The PBO precursor polymer (TBS-prePBO) exhibits an excellent solubility in amide solvents, such as DMAc, DMF, and NMP. To study the interaction between TBS-prePBO and the CNT surfaces, a simple dispersion experiment was carried out using DMAc as the solvent. The SWNTs were used since the SWNTs provide useful spectroscopic information about how the dispersion works in the solution.36,37 Figure 2a (left) shows a photograph of the supernatant solution of the SWNTs dispersed with TBS-prePBO in DMAc after the centrifugation. The clear contrast of the SWNT dispersion in the absence of the TBS-prePBO showing a light black color (Figure 2a (right)) indicates the successful dispersion of the SWNTs with the aid of the TBS-prePBO through polymer wrapping. Since the TBS-prePBO bears the

Figure 3. Raman spectra of the SWNT/TBS-prePBO (red line) and the SWNTs (black line) in the range of (a) 2450−2750 and (b) 150− 3200 cm−1.

SWNTs, a 7 cm−1 upshift was observed in the 2D band as shown in Figure 3a, clearly suggesting the existence of an interaction between the SWNTs and wrapped polymer.41 It was also found that the D/G (defect/graphite) ratio of the composite (D/G = 0.082) was almost identical to that (D/G = 0.073) of the original SWNTs, which clearly indicated that the wrapping process is free from any oxidative damage to the SWNTs (Figure 3b). Notably, the obtained powder provided a homogeneous dispersion even after the short sonication in the DMAc (blue line in Figure 2c; also see Supporting Information, Figure S4), indicating that the TBS-prePBO was wrapped onto the SWNT surfaces. Fabrication of MWNT/PBO Film. For the fabrication of the CNT/PBO composite, the MWNTs were selected since the MWNTs are regarded as a practical host material in various applications because of the cost. We confirmed the dispersion of MWNTs in a similar manner with dispersion of the SWNTs by TBS-prePBO. After sonication of the MWNTs dispersion in the presence of TBS-prePBO in DMAc (4 mg mL−1), the

Figure 2. (a) Photo images of the supernatant solution of SWNTs in DMAc with (left) and without (right) TBS-prePBO after centrifugation at 10000g for 1 h. (b) Schematic illustration of the CNT dispersion. (c) Absorption spectra of the SWNT solution in DMAc with TBS-prePBO after centrifugation (black line), after one-month (red line), and after redispersion procedure (blue line), and (d) PL mapping of the SWNT solution in DMAc with TBS-prePBO after centrifugation at 10000 g for 1 h. 4036

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expected, the MWNTs were quickly dispersed by simple stirring due to the TBS-prePBO coated on MWNTs. The viscous MWNT/TBS-prePBO solutions were then cast on a glass substrate, and heated at 90 °C for 1 h, 100 °C for 1 h, 110 °C for 1 h, and then at 120 °C for 1 h to remove the solvent. The free-standing MWNT/TBS-prePBO film removed from the substrate was heated at 300 °C for 1 h, 350 °C for 1 h, and 400 °C for 1 h under vacuum. The photographs of the MWNT/TBS-prePBO film (Figure 4b) and the film after the thermal treatments (Figure 4c) are shown, which show homogeneous films without any visible aggregates in all the films with the given MWNT contents. After the conversion from the MWNT/TBS-prePBO to the MWNT/PBO, large weight-reduction (∼38 wt %) of the polymer was recognized due to the removal of water together with elimination of the TBS moiety on the ring closer reaction. As a consequence, the film thickness decreased by 33% compared to that of the MWNT/TBS-prePBO. As shown in Figure 4d, in the IR spectra, the characteristic CN peak at around 1625 cm−1 attributable to the oxazole ring42 newly appeared after the thermal treatment, indicating successful conversion to the PBO structure as we reported in the previous paper.23 Figure 4e shows the Raman spectra of the MWNT/PBO powder measured at 785-nm excitation. The baseline of the spectra is tilted due to the effect of the PBO fluorescence.43 No significant change in the D/G ratio of the hybrid was observed, which clearly suggests that the process is free from the oxidative damage of the pristine MWNT structure. Our method obviously differs from the typical procedures that almost always accompanies the oxidization of the CNTs.44 Mechanical Properties of MWNT/PBO Composite Films. Figure 5 shows the strain−stress curves of the films of

MWNTs were homogeneously dispersed similar to the SWNTs (see Supporting Information, Figure S5). To prepare a fine film, the higher concentrated polymer solution (14 mg mL−1) was used and the MWNTs (0−5 mg) was added to prepare the MWNT/TBS-prePBO composite with MWNT loading at 0−5 wt % (One-pot method). In this process, however, a dramatic increase in the viscosity was found during the sonication especially with the higher MWNT content (concentration: 0.6−1.0 mg mL−1), which makes a homogeneous dispersion of the MWNTs rather difficult. As a result, the error range of the mechanical properties of the films become rather high which would be due to the poor homogeneity of the MWNTs in the polymer matrix. To overcome this problem and to prepare a homogeneous dispersion of the MWNTs for a wide range of MWNTs contents, we developed a new approach; namely, the employment of MWNTs wrapped with TBS-prePBO for the preparation of the nanotube-hybrids. By delaminating and coating the MWNTs with TBS-prePBO prior to the hybrid preparation, the dispersibility of the MWNTs is expected to dramatically improve. MWNTs coated with TBS-prePBO were prepared by precipitation of the MWNT dispersion into a poor solvent, such as water, as described in the Experimental Section. Scanning electron microscopy (SEM) measurements revealed that the obtained MWNT/TBS-prePBO hybrid possessed bumpy surfaces, while the surfaces of the MWNT were very smooth, which clearly indicated the MWNT-coating by TBSprePBO (see Supporting Information, Figure S6). Since the average diameter of the MWNT/TBS-prePBO (∼34 nm) was similar to that of the MWNT (∼31 nm), the wrapping-polymer thickness was found to be very thin. The preparation scheme of the composite is outlined in Figure 4a (precoating method). First, the MWNT/TBS-prePBO hybrid and TBS-prePBO were added to DMAc (polymer concentration: 50 mg mL−1). As

Figure 5. Strain−stress curves for the films of the PBO and MWNT/ PBO with specified MWNT-loading (wt %) shown in the figure.

the PBO and MWNT/PBO with different amounts of the MWNTs. The mechanical properties of the MWNT/PBO together with those of MWNT/TBS-prePBO are summarized in Table 1. The MWNT/TBS-prePBO elongated with increasing the content of the MWNTs. It has been reported that the elongation of polymer/CNT composites occurs when the effective stress transfer was recognized in the polymer/ CNT interface.46 In such a case, the CNTs would act as a bridge to prevent the break in the film, which is important for elongation. Taking the weight reduction upon PBO formation, the content of the MWNTs in the five different MWNT/PBO films were 0.3, 0.7, 1.7, 5.5, and 8.6 wt %, respectively, which are larger than those (0.2, 0.4, 1.1, 3.2, and 5.3 wt % respectively) for the five different MWNT/TBS-prePBO films. The weight ratios of the MWNTs in films significantly

Figure 4. (a) Schematic illustration of the preparation of MWNT/ PBO by the precoating method. (b and c) Photo images of the film of the MWNT/TBS-prePBO before (b) and after (c) the heat treatment. (d) IR spectra of the MWNT/PBO film (red line) and PBO film (black line). (e) Raman spectra of the MWNT/PBO powder (red line) and MWNTs (black line). 4037

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Table 1. Mechanical Properties of the MWNT/TBS-prePBO and MWNT/PBO Films MWNT/TBS-prePBO sample

MWNT [wt %]

1 2 3 4 5 6

0 0.2 0.4 1.1 3.2 5.3

tensile strength [MPa] 67 99 88 74 89 115

± ± ± ± ± ±

7 16 10 14 7 14

MWNT/PBO

Young’s modulus [GPa] 1.5 3.1 2.3 2.2 1.5 2.9

± ± ± ± ± ±

0.4 0.3 0.3 0.1 0.6 0.4

elongation [%]

MWNT [wt %]

± ± ± ± ± ±

0 0.3 0.7 1.7 5.5 8.6

15 54 52 23 46 58

2 7 21 7 7 9

tensile strength [MPa] 64 92 130 151 145 70

± ± ± ± ± ±

7 9 3 11 14 8

Young’s modulus [GPa] 2.9 5.2 7.7 8.1 8.6 5.6

± ± ± ± ± ±

1.0 0.6 0.8 1.3 0.7 0.9

elongation [%] 2.8 2.0 1.9 2.2 1.9 1.5

± ± ± ± ± ±

0.7 0.5 0.4 0.6 0.1 0.6

increased in each sample after the PBO formation since the TBS group was removed from the composites. For all the samples, we found that the tensile strength and Young’s modulus were increased by the addition of the MWNTs. Quite interestingly, dramatic enhancements in the mechanical properties were found by converting the TBS-prePBO to PBO for each corresponding sample. We assumed that the effective interfacial interactions between the TBS-prePBO and CNT surfaces were further reinforced for the PBO/CNT interfaces. SEM observations of the fracture surface after the tensile testing revealed that many holes were formed in the MWNT/TBSprePBO (Figure 6, indicated by the arrows) with the different

Figure 7. SEM images of the cross-section of the fracture surfaces of the MWNT/PBO film with MWNT-loadings at 0.3 wt % (a and b), 1.7 wt % (c and d), and 8.6 wt % (e and f). Black circles show junction structures of the MWNTs in the film.

In the series of the MWNT/PBO, by 1.7-wt % addition of the MWNTs, the tensile strength and Young’s modulus reached 151 MPa and 8.1 GPa, respectively. This result well agreed with the theoretical Halpin−Tsai model31 when 2.9 GPa was used for Em, Young’s modulus of PBO (Table 1), as shown in Figure 8 (dotted line) (also see Experimental Section), suggesting the ideal reinforcement of PBO with the MWNTs. As a matter of fact, we hardly observed the MWNTs, which were observed in Figure 6. SEM images of the cross-section of the fracture surfaces of the MWNT/TBS-prePBO film with MWNT-loading at 0.2 wt % (a and b), 1.1 wt % (c and d), and 5.3 wt % (e and f). Black arrows show holes in the film.

MWNT contents, while such a structure was hardly observed for the MWNT/PBO (Figure 7). It has been reported that such holes are formed after pulling out the CNTs from the matrices due to slippage during the stretching tension, and the phenomena suggest the weak interfacial interaction between the CNTs and matrices.45 Compared to the structure of the TBS-prePBO, PBO’s higher planar structure than that of TBSprePBO would result in effective overwrapping onto the CNT surfaces to achieve a stronger interfacial interaction.

Figure 8. Plots of Young’s modulus of the MWNT/PBO films for different volume fractions of MWNTs (red line) and theoretical line calculated by the theoretical Halpin−Tsai model (blue line). 4038

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Table 2. Mechanical Properties of the CNT/PBO sample

CNT type

film film film

pristine MWNT oxidized MWNT polymera graft MWNT oxidized MWNT SWNTb PBO graft MWNT

fiber fiber fiber a

MWNT content [wt %]

tensile strength [MPa]

reinforcement factor (%)

Young’s modulus [GPa]

reinforcement factor (%)

ref

1.7 5.0 0.54

151 119 1480

130 75 25

8.1 5.2 70.2

179 44 5

this study 8 9

1300 4200 2170

33 62 60

− 167 89.4

− 21 38

17 12 18

5.0 10.0 1.5

Poly[(hydroxyphenyl)amide] (PHA). bSingle-walled carbon nanotubes.

PBO-reinforced by CNTs in acid or with oxidized CNTs. We assumed that such a high reinforcement is due to the effective interfacial interaction between the PBO and CNTs as well as a homogeneous dispersion of the CNTs in a PBO matrix, which maximizes the area of the load transfer between the CNT and PBO. This is the first example showing a remarkable reinforcement of PBO by the aid of CNTs by a noncovalent modification. We believe that the present approach is sufficient for the effective reinforcement of many polymers. Further optimization, such as the modification of a precursor structure, blending condition, etc., would increase the mechanical properties of the CNT−hybrid materials. The presented material possessing such excellent mechanical and thermal properties is a promising metal substitute for superlight highperformance polymers.

the corresponding composite samples before the addition of the MWNTs, in the fracture surfaces with this range as shown in Figure 7a. This fact clearly indicates the load transfer from the MWNTs to PBO is so effective that the composite behaves as a single phase. However, for 1.7−5.5-wt % addition of the MWNTs, the mechanical reinforcement were almost the same level, and further addition up to 8.6 wt % caused a decrease in the value. At this range, we found junction structures (Figure 7f, indicated by circles) probably formed by the insufficient delamination of the MWNTs due to their high viscosity of the dispersion. We assumed that the contact structure observed in the SEM images caused the decrease in the mechanical values because the increase in the contacting areas would decrease the load transfer between the CNTs and PBO. Table 2 summarizes the mechanical properties of the CNT/ PBO films8 and fibers9,12,17,18 already reported and our values. The mechanical properties of the PBO fibers are very high even without the addition of the CNTs because the PBO fibers are spun from a lyotropic liquid crystal phase, thus the high anisotropy of the PBO main-chain along with the fiber axis is achieved (5.8 and 270 GPa for the tensile strength and Young’s modulus, respectively7). We calculated the reinforcement factor based on the mechanical properties of PBO by dividing the CNT/PBO values by those of PBO to extract the effect of the reinforcement upon the addition of the MWNTs. As shown in Table 2, our method shows the highest value of the reinforcement factor among the various composition methods.8,9,12,17,18 The obtained results clearly indicate that the hybrid preparation based on the noncovalent interaction is a powerful approach to utilize the CNT’s intrinsic properties to PBO matrices. We would like to emphasize that the PBO film prepared from the TBS-prePBO possesses the highest thermal resistivity among the reported organic polymer materials.23 Such an outstanding thermal property was retained even after the addition of the CNTs (see Supporting Information, Figure S7).

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ASSOCIATED CONTENT

S Supporting Information *

PL mapping of the TBS-prePBO-dispersed SWNT in DMAc after 1 month, photo image, and PL mapping of the SWNT/ TBS-prePBO in DMAc after the redispersion procedure, a photo image and absorption spectrum of the TBS-prePBOdispersed MWNT in DMAc after sonication, the SEM images of the MWNTs and MWNT/TBS-prePBO hybrid, and TGA curves of PBO and MWNT/PBO. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +81-92-802-2840. Fax: +81-92-802-2840. E-mail: [email protected] (N.N.), [email protected] (T.F.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. Nobuhide Uda at Kyushu University for helpful discussion on the interpretation of fracture surfaces after the tensile testing. This work was supported by the Low-Carbon Research Network (LCnet), Nanotechnology Platform Project and Grant-in-aid for JSPS Research Fellow (No. 231617) (for T.F.) funded by the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan.

CONCLUSION In conclusion, we described a strategy for the mechanical reinforcement of PBO by employing CNTs as a guest material. A soluble PBO precursor, TBS-prePBO, was found to enable the delamination and stable dispersion of the CNTs in organic solvents as well as a PBO film matrix through an effective interfacial interaction. A free-standing MWNT/PBO film obtained from the MWNT/TBS-prePBO film by thermal treatment showed an excellent tensile strength (151 MPa) and Young’s modulus (8.1 GPa) with a 1.7 wt % CNT loading, which corresponded to ∼130% and 180% reinforcement compared to the original PBO film, respectively. These reinforcement ratios are the highest to date among the various

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REFERENCES

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dx.doi.org/10.1021/ma4004117 | Macromolecules 2013, 46, 4034−4040