Unzipped Multiwalled Carbon Nanotubes for Mechanical

Nov 3, 2010 - An-Ting Chien , H. Clive Liu , Bradley A. Newcomb , Changsheng Xiang , James M. Tour , and Satish Kumar. ACS Applied Materials ...
2 downloads 0 Views 5MB Size
Download PDF
J. Phys. Chem. C 2010, 114, 19621–19628

19621

Unzipped Multiwalled Carbon Nanotubes for Mechanical Reinforcement of Polymer Composites Yan Wang, ZiXing Shi,* and Jie Yin School of Chemistry and Chemical Technology, State Key Laboratory for Metal Matrix Composite Materials, Shanghai Jiao Tong UniVersity, 200240, Shanghai, People’s Republic of China ReceiVed: July 30, 2010; ReVised Manuscript ReceiVed: September 30, 2010

Multiwalled carbon nanotubes (MWNTs) have been widely used as mechanical reinforcement agents in the past few years. However, the enhancement of mechanical properties of composites has been greatly hampered by its limited available interface area in composites. Toward solving this intrinsic limitation of MWNTs, in this paper, we report the use of unzipped MWNTs (uCNTs) as nanofillers for reinforcement of polymer composites for the first time. The uCNTs were produced by an oxidative unzipping process, involving the lengthwise cutting and opening the walls of MWNTs, and yielded separated ribbonlike graphene layers, thus increasing the surface area of MWNTs. With different amounts of oxidant (KMnO4), uCNTs with different unzipping degrees were obtained. For the surface functional groups that were generated during oxidation treatment, the morphologies and structures of uCNTs with different unzipping degrees were well characterized by several measurements. The mechanical testing of the resultant poly(vinyl alcohol)-based composites confirmed that the uCNTs were more effective than pristine MWNTs in terms of reinforcing polymers in strength and modulus, and the uCNTs oxidized by 400 wt % KMnO4 showed the highest reinforcement effect. The reinforcement effect is correlated with the structure changes of CNTs. This study may provide an important guideline and alternative way to design and fabricate low-cost and high-performance polymer-CNT composites. 1. Introduction Carbon nanotubes (CNTs) have attracted substantial attention in the past decades for their use in various fields of science and technology, since their excellent mechanical, electrical, and thermal properties.1-4 Especially, it is because of their ultrahigh stiffness and strength that CNTs show promise as fiber reinforcing agents for increasing the modulus and strength of polymer composite materials.5-7 CNTs can be viewed as cylinders that consist of seamlessly rolled graphene sheets, according to the number of the rolled graphene sheets, and they are classified into two major types, namely, single-walled carbon nanotubes (SWNTs) and multiwalled carbon nanotubes (MWNTs). It is generally considered that SWNTs are the ideal reinforcing fillers because of their small size, high surface area, high aspect ratio, and wellcrystalline structure.8,9 However, the difficulty in separating SWNTs from bundles into individual nanotubes is still a big challenge for the use of SWNT in real application. Additionally, the high cost and difficulty in purification also limit the largescale production of SWNT.10 On the other hand, for CVD-growth MWNTs, the cost is much lower, and it is relatively easy to disperse MWNTs in solvent or embed them individually into polymer matrix. Therefore, it is accepted that MWNTs are one of the most attractive candidates to reinforce polymer composites.5-7 During the last years, tremendous efforts have been made to realize the full potential of MWNTs in improving the mechanical property of polymers. Some obstacles, such as poor dispersion and lack of interfacial bonding, which limit the reinforcement effect, have been partially addressed by either physical methods * To whom correspondence should be addressed. Tel: +86-21-54743268. Fax: +86-21-54747445. E-mail: [email protected].

(ultrasonication,11 high shear mixing,12 etc.) or chemical methods (covalent13-15 or noncovalent modification16,17 of the surface of MWNT). It is demonstrated that by controlling the dispersion of MWNT in polymer matrix and the interactions between them, good reinforcement can be obtained. However, in addition to the two factors that efforts mainly focused on, another important issue has always been overlooked; that is, the available interface areas (AIA) with the MWNT can interact with the matrix. For MWNT reinforced polymers, because only the outmost wall is exposed to the surrounding matrix, the AIA of MWNT is almost 1 order of magnitude lower than SWNT.18 This difference may elucidate the reason that the reinforcement effect of SWNT is much better than MWNT, since more AIAs provide much more interaction sites between fillers and matrix, which is also confirmed by some recent comparative studies.19,20 In other words, for MWNT reinforced composites, only the outermost walls of MWNTs could bear the stress and the inner walls could not efficiently participate in the stress transferring from the polymer matrix to the MWNTs. This suppresses the reinforcement effect achievable in MWNT polymer composites.21 Unfortunately, traditional strategies are only focused on surface functionalization to improve the interface action between MWNTs and polymer matrix, and almost no reports address this intrinsic limitation of MWNTs as reinforced fillers. This work aims to address the problems of the limited AIA of MWNTs. Herein, we provide a novel concept to increase the reinforcement effect of MWNTs in polymer composites, which involves the use of unzipped MWNTs (uCNT) as reinforced fillers. Recently, various strategies have been reported for longitudinal unzipping of MWNTs, such as oxidative unzipping22,23 and electrically unwrapping.24 The unzipped MWNTs can be exfoliated into ribbonlike graphene

10.1021/jp107151e  2010 American Chemical Society Published on Web 11/03/2010

19622

J. Phys. Chem. C, Vol. 114, No. 46, 2010

layers in solution aided by sonication. In this study, the oxidative unzipping method was chosen to produce uCNT as fillers for several reasons: (1) It provides an economical way for largescale production of uCNT because the reagents involved are all commonly used in laboratory. (2) The unzipping degree can be easily controlled by modulating the amount of oxidant. (3) The oxide-containing groups generated during oxidation enable uCNT to be easily dispersed in several solvents and thus facilitate the fabrication of polymer composites. Other functional moieties can also be introduced into the uCNT through these groups, if necessary. (4) Most importantly, the unzipping process can open and separate the walls of MWNTs; thus, the AIA per volume MWNTs can be greatly increased and both the inner walls and the outermost walls can interact with the polymer matrix. Poly(vinyl alcohol) (PVA) was chosen as a model polymer because it can cause strong interactions with both pristine and oxidized CNT,25,26 and the process for the preparation of PVA composites was simple and environmentally friendly. The unzipping process has been well characterized by several methods. MWNTs with different unzipping degrees were incorporated into PVA, the mechanical testing results showed that the uCNT was more effective in reinforcing polymers, and there exists a maximum reinforcement when the MWNTs are unzipped to a certain degree. 2. Experimental Section 2.1. Materials. MWNTs (purity, >95%; length, ∼10 µm; and diameter, >30 nm) were obtained from Chengdu Organic Chemistry Co. PVA with repeat unit numbers of 2400-2500 (PVA 124; hydrolysis degree, 98-99%) was purchased from Shanghai Chemical Reagents Co. (Shanghai, China). Concentrated sulfuric acid (95-98%), potassium permanganate (KMnO4), hydrogen peroxide (H2O2, 30%), and concentrated hydrochloric acid were purchased from Sinopharm Chemical Reagent Co., Ltd. (SCRC), and used as received. 2.2. Preparation of uCNTs. uCNTs were prepared by an oxidative unzipping method as originally reported by Kosynkin and colleagues.22 First, 0.1 g of MWNTs was dispersed in 50 mL of concentrated sulfuric acid and stirred for 2 h until a visually homogeneous black solution formed. Then, a different amount of KMnO4 (100, 300, 400, and 500 wt % related to the weight of MWNTs) was slowly added to the solution and further stirred for 1 h at room temperature. After that, the temperature was gradually raised up to 70 °C and maintained at that temperature for 1 h. When the reaction was completed, the mixture was poured into 500 mL of deionized water that contained 3 mL of H2O2. The solution was centrifuged and extensively washed with dilute hydrochloric acid (10%) six times and deionized water three times. The obtained uCNTs were dried in vacuo at 80 °C. 2.3. Fabrication of PVA/CNT and PVA/uCNTs Composites. A typical procedure for the synthesis of composites involved the dispersion of the required amount of nanofillers in 5 mL of water and sonication for 30 min to yield a homogeneous solution; meanwhile, 1 g of PVA was dissolved in 10 mL of water. Then, these two solutions were mixed together and sonicated for a further 30 min. Finally, the PVA/ nanofiller solution was poured onto a substrate and dried at 60 °C for 12 h. To completely remove the water, the composite films were further dried in vacuo at 60 °C for 6 h. 2.4. Measurements. Fourier transform infrared (FT-IR) spectra were recorded on a Perkine-Elmer Paragon 1000PC spectrometer. X-ray powder diffraction (XRD) spectra were recorded on a D/max-2200/PC (Japan Rigaku Corp.) using Cu KR radiation. Thermogravimetric analysis (TGA) was performed

Wang et al.

Figure 1. Photograph of aqueous dispersion of pristine CNT and uCNTs at a concentration of 0.1 mg/mL.

in nitrogen with a Perkin-Elmer TGA 2050 instrument at a heating rate of 20 °C/min. Raman spectra were taken with Jobin Yvon Micro-Raman Spectroscopy (RamLab-010), equipped with a holographic grating of 1800 lines/mm and a He-Ne laser (632.8 nm) as an excitation source. The morphology of MWNTs and uCNTs was obtained using a scanning electron microscope (SEM) (JSM-7401F) and transmission electron microscopy (TEM) (JEOL2100F). The tensile property of composite films was measured with an Instron 4465 instrument at room temperature with a humidity of about 50% at a crosshead speed of 5 mm/min and an initial gauge length ) 30 mm, samples were cut into strips of ∼60 mm × 4 mm × 0.05 mm using a razor blade, and four strips were measured for each sample. The tensile fracture surfaces were also characterized by SEM. Optical microscope images of composite films were obtained using Olympus GX51. The degree of crystallinity of samples was measured by a differential scanning calorimeter (DSC), model 6200 (Seiko, Japan). 3. Results and Discussion The oxidized unzipping of MWNTs was first reported by Tour’s group to fabricate graphene nanoribbons (GNR).22 We have not used the term “GNR” to represent our unzipped MWNTs because our produced uCNT was not completely unzipped. Even though 500 wt % KMnO4 was used, which is thought to completely open the MWNTs,22 we could not guarantee that all of the MWNTs were totally unzipped or formed GNR. This is because the raw MWNTs that we used possessed different crystalline structures, and the agglomerations of MWNTs might cause an inhomogeneous reaction with oxidant. Thus, we expressed the resultant uCNTs as CNT-1, CNT-3, CNT-4, and CNT-5 to represent the MWNTs reacted with 100, 300, 400, and 500 wt % KMnO4, respectively. 3.1. Synthesis and Characterization of Unzipped MWNTs. The preparation of uCNT was achieved by dispersing MWNTs in concentrated sulfuric acid followed by treatment with different amounts of KMnO4. The resultant uCNTs are highly soluble in water (Figure 1), suggesting that hydrophilic groups were generated during the unzipping process. In contrast, the pristine CNT precipated from the solution soon after the sonication. Moreover, the solutions of CNT-4 and CNT-5 appear dark brown in color, while CNT-1 and CNT-3 form uniform black solutions. These different color solutions indicate that the destruction of electronic conjugation in CNT-4 and CNT-5 is more significant than that of CNT-1 and CNT-3.27,28 The introduced functional groups were detected by FT-IR. As shown in Figure 2, the intensities of bands centered at 1040

Carbon Nanotubes for Reinforcement of Polymer Composites

J. Phys. Chem. C, Vol. 114, No. 46, 2010 19623

Figure 4. Raman spectra of pristine CNT and uCNTs. Figure 2. FT-IR spectra of pristine CNT and uCNTs.

Figure 5. XRD patterns of pristine CNT and uCNTs.

Figure 3. TGA (a) and DTG (b) curves of pristine CNT and uCNTs.

and 1228 cm-1 increased in CNT-1 and became more significant in CNT-3, CNT-4, and CNT-5. We tentatively assign these groups to hydroxyl and epoxy groups, similar to that of graphite oxide, which was prepared by treatment of graphite with KMnO4.29-31 Additionally, the appearance of a band around 1720 cm-1 in CNT-3, CNT-4, and CNT-5, indicating carbonyl/ carboxyl groups, was introduced in these samples. TGA (Figure 3a) shows that the total weight loss increases with an increase in the amount of KMnO4 during reaction (from 20% in CNT-1 to 46% in CNT-5 at 700 °C), indicating increased oxidation degree from CNT-1 to CNT-5. The DTG curve (Figure 3b) suggests that there exists domain peaks around 180-200 °C in all of the uCNT samples, which can be assigned to the burning of the most labile oxygen functionalities,32 and the integrated area increased with an increase in the oxidant amount. However, a new peak gradually roused around 260 to 270 °C in CNT-3, CNT-4, and CNT-5, which can be ascribed to decomposition of more stable oxygen functional groups, that is, carbonyl/carboxyl groups, since the double bond between

the carbon and the oxygen should be stronger than the single bond between them,33 which is also consistent with the FT-IR results. The intensity of the peak around 260-270 °C increased from CNT-3 to CNT-5, indicating that more carbonyl/carboxyl groups are generated with more KMnO4. Because both simulation and experiment results from the literature suggest that carbonyl/carboxyl groups mainly attached to the edges of graphene sheets,22,34,35 the FT-IR and TGA results indirectly confirm that the MWNTs were indeed cut during oxidation. Raman spectroscopy was also applied to characterize the uCNTs (Figure 4). The CNT shows a D band at 1327 cm-1, which originates from the disordered graphite structure or sp3hybridized carbons of the nanotubes, and a G band at 1571 cm-1, which is attributed to a splitting of the E2g stretching mode of graphite. Meanwhile, a weak shoulder (D′) of G band at 1608 cm-1 is also related to the extent of disorder in the nanotubes. The D/G intensity ratio (R) is often utilized to monitor the functionalization of CNT. In our work, it is found that the R increases with an increasing amount of oxidant (from 0.889 of CNT to 1.302 of CNT-5), indicating that the more oxidant, the more functional groups or defects were generated in CNTs. This result is consistent with the FT-IR and TGA measurements. The intensity of the D′ band was also increased after oxidation. Moreover, the G band of CNT-4 and CNT-5 was shifted to a higher frequency at 1586 cm-1, and it was not able to distinguish the D′ bands in these samples. We assumed that the increased intensity of D′ was the reason for this change. As the intensity of D′ increased, it was gradually merged with the G band, as shown in Figure 4, and this merging between the G and D′ caused the appearance of the shifting of G band. The direct evidence of the structure changes of MWNTs comes from XRD (Figure 5) and microscopic characterization

19624

J. Phys. Chem. C, Vol. 114, No. 46, 2010

Wang et al.

Figure 6. SEM and TEM images of (a and b) prisine CNT, (c and d) CNT-1, (e and f) CNT-3, (g and h) CNT-4, and (i and j) CNT-5.

(Figure 6). In the XRD patterns, the CNT shows a peak at 25.9°, attributed to the (002) plane of the interplanar graphite spacings, corresponding to a d spacing of 0.34 nm. However, in CNT-1, in addition to the domain peak at 25.9°, a new small peak at 13.4° appeared, with d spacing of 0.66 nm, indicating that some of the compacted graphene layers have been loosened by oxidation treatment. However, in CNT-3, CNT-4, and CNT-5, the predominant peaks appeared at 11.5,10.9, and 10.5° for CNT-3, CNT-4, and CNT-5, with d spacing of 0.77, 0.81, and

0.84 nm, respectively, suggesting that most of the MWNT structure was changed. The d spacing increased with an increasing oxidant content due to more functional groups generated between the adjacent layers during the process of oxidation, which were also confirmed by TGA. The peak of MWNT in these samples gradually decreased from CNT-3 to CNT-5, suggesting that the unzipping degree was increased with increasing KMnO4. Moreover, the pattern of CNT-5 was very similar to that of graphite oxide,36 accompanied by the disap-

Carbon Nanotubes for Reinforcement of Polymer Composites pearance of the characteristic peak of MWNT. This indicates that the unzipping process was nearly completed in this sample. The morphologies of uCNTs can be directly observed by electronic microscope. As shown in Figure 6, SEM shows an overview of each sample, while high-resolution TEM provides their individual represented images in detail. As compared to CNT, the CNT-1 was significantly shortened by oxidation from several micrometers to hundreds of nanometers. Figure 6d further reveals that the outer walls of CNT-1 were partially loosened or unwrapped, but the whole MWNTs’ tube structure can be clearly identified. With further oxidation, we can see that most of the MWNT walls possess unzipped structures, as shown in Figure 6e. More interestingly, the length seems to not further be shortened as compared to CNT-1. The TEM image of CNT-3 (Figure 6f) shows that the multiwalls of MWNT were opened to a higher degree with less inner tubes remaining. With a further increase in the KMnO4 content, the length of CNT-4 was not further decreased, comparable with CNT-1 and CNT3. However, the lower contrast in SEM image (Figure 6g) suggests that thinner ribbons were produced, and the increased width indicates a higher exfoliation state due to a higher unzipping degree as compared to CNT-3. This is also supported by TEM image of CNT-4 (Figure 6h), where the tubular structure of MWNT cannot be observed in CNT-4 instead of the appearance of unzipped layer structure. For CNT-5, very thin layers were obtained (Figure 6j) in TEM observation, and most of MWNTs were cut and unzipped into small pieces as shown in SEM image (Figure 6i). Thus, all of the characterizations carried out support that unzipped MWNTs with different unzipping degree were successfully prepared, and the walls of MWNTs were indeed exfoliated. 3.2. Synthesis and Properties of PVA/uCNT Composites. The PVA/uCNT composites were fabricated by a simple solution-casting procedure at a fixed filler content of 0.2 wt %. The low weight fraction was selected to ensure relative uniform dispersion of fillers in matrix. Optical microscope images provide direct evidence for evaluating the quality of the composite films. From Figure 7, we can conclude that all of the composite films are homogeneous and uniform, at least at micrometer-scale resolution. Interestingly, although pristine CNT cannot form a stable dispersion in water, the composite containing CNT is also relatively homogeneous. This may suggest that there exist interactions between CNT and PVA chains, which prevent the aggregation of CNTs in the resultant composite. To further characterize the dispersion and possible reinforcing mechanism, SEM images were taken of the tensile fracture surface of composites as shown in Figure 8. In Figure 8b,c, it is shown that some bright dots and short lines were well distributed in the CNT/PVA and CNT-1/PVA, which was thought to be the broken or pulling-out CNTs, and no cluster was found across the whole fracture surface. However, the CNT in CNT-3/PVA and CNT-4/PVA could hardly be observed. This is not unexpected because from the previous characterization of uCNTs, the CNT-3 and CNT-4 have been unzipped into thin layers rather than tubular structures as in the case of CNT and CNT-1, and the “disappearance” of them in the composites could be direct evidence of the good dispersion of them in the matrix. The relative rough surfaces of CNT-3/PVA and CNT-4/PVA indicate that more energy is needed to break the films, and we assumed that the increased interfacial area of CNT-3 and CNT-4 was largely responsible for this morphology change and influenced the mechanical property of the resultant composite as discussed

J. Phys. Chem. C, Vol. 114, No. 46, 2010 19625

Figure 7. Optical micrographs of pure PVA (a) and composite films containing 0.2 wt % of CNT (b), CNT-1 (c), CNT-3 (d), CNT-4 (e), and CNT-5 (f).

in the following sections. For CNT-5/PVA, this tough fracture surface disappeared, and no fillers can be seen from the SEM images. The typical stress-strain curves for the PVA-based composites are given in Figure 9a, and their mechanical properties are compared in Figure 9b. The results indicate that all types of fillers are able to improve the modulus and stress of PVA with a weight fraction as little as 0.2 wt %, and the reinforcement depends on the treatment of CNT under different oxidant content conditions. The pure PVA shows a modulus of 4.07 GPa and a stress of 83.2 MPa, which were similar to previously reported values.20,37 As compared to CNT/PVA, CNT-1/PVA shows no big difference in mechanical properties as the oxidant content (100 wt %) is low. However, further increasing the oxidant content (above 100 wt %) leads to more pronounced improvement in the mechanical properties of PVA, and the CNT-4/ PVA exhibits the highest modulus of 5.52 GPa and stress of 118.1 MPa, corresponding to an increase of 1.45 GPa (35.6%) and 34.9 MPa (41.9%) as compared to that of pure PVA. A further increase in the oxidant content to 500 wt % leads to decreased mechanical properties in PVA, but its mechanical data are still higher than those of CNT/PVA. Such results seem contrary to the general point of view that extensive oxidation would shorten the CNT and impair its intrinsic mechanical property, and these disadvantages would result in negative effects toward enhancing the mechanical properties of CNT reinforced polymer composites.38-40 However, in our research work, more oxidized degree (below 500 wt %) of CNT, more reinforcement is obtained, which may shed some light on the different effect of oxidative unzipping and commonly utilized oxidative modifying of CNT surface on the reinforcement of polymer composites. As known, PVA is a semicrystalline polymer, and the mechanical properties are strongly dependent on its crystallinity.

19626

J. Phys. Chem. C, Vol. 114, No. 46, 2010

Wang et al.

Figure 8. SEM images of pure PVA (a) and PVA composites containing 0.2 wt % CNT (b), CNT-1 (c), CNT-3 (d), CNT-4 (e), and CNT-5 (f).

Therefore, we use DSC to quantify the PVA crystalline fraction of each composite (Figure 10). The crystallinity (χc) of samples was calculated as follows:

χc )

∆Hm ∆H0

where ∆Hm is the measured melting enthalpy (from DSC) and ∆H0 is the enthalpy of pure PVA crystal (138.6 J/g).19,37 As shown in Table 1, the χc values were slightly higher in CNT/ PVA and CNT-5/PVA, and this could partially explain the higher modulus of CNT/PVA than CNT-1/PVA. No obvious changes in χc was found in other samples as compared to pure PVA. Therefore, the greatly increased strength and modulus in CNT-3/PVA and CNT-4/PVA cannot be attributed to changes in crystallinity as some other reports reported.37 It is considered that the oxidation treatment has a reverse effect on the reinforcement of CNT in composites. Initially, it decreased the aspect ratio of CNT, as revealed by SEM, which could depress the reinforce efficiency. This may be another reason that accounts for the higher modulus of CNT/PVA than CNT-1/PVA. The oxidation treatment may increase the defect density, which lowers the mechanical property of CNT;

however, for most polymers, the polymer-nanotube interfacial stress transfer is low with values typically less than 50 MPa.41,42 Thus, according to Coleman et al., there is no difference between nanotubes with Y ∼ 1 TPa and Y ∼ 0.1 TPa in terms of polymer reinforcement.19 Comparing CNT-3/PVA and CNT-4/PVA with CNT-1/PVA, as the unzipping degree increases, more functional groups were penetrated between adjacent layers of MWNT; therefore, more graphene layers could be slipped off from the CNT, and the key factor that governs the reinforcement changed into the surface areas of fillers. A simple model for illustrating the interface area change is given in Figure 11; as the surface area of uCNTs increases, the AIA between the polymer and the uCNTs also increases, which may result in more polymer-CNT interactions and consequently improved composite properties. Moreover, the increased functional groups further guarantee the homogeneous dispersion of uCNTs and strengthened the interaction between uCNTs and matrix via hydrogen bonds between the oxygen-containing groups and the hydroxyl groups of PVA, which will result in improved stress-strain transfer in the PVA composites. All of these factors result in better mechanical properties of CNT-3/PVA and CNT-4/PVA. However, in the case of CNT-5/PVA, although the CNTs were nearly completely unzipped, the effective aspect ratio was decreased

Carbon Nanotubes for Reinforcement of Polymer Composites

J. Phys. Chem. C, Vol. 114, No. 46, 2010 19627

Figure 9. (a) Typical stress-stain curves of composite containing 0.2 wt % of different fillers. (b) Comparison of Young’s modulus and maximum stress of different composites.

Figure 11. Simple model for illustrating the effect of unzipping of MWNTs on the interface area change in CNT-polymer composites.

Figure 10. DSC curves of pure PVA and different types of PVAbased composites.

TABLE 1: Crystallinity and Melting Enthalpy of Pure PVA and PVA-Based Composites samples

∆Hm (J/g)

χc (%)

pure PVA CNT/PVA CNT-1/PVA CNT-3/PVA CNT-4/PVA CNT-5/PVA

33.0 38.0 32.4 34.1 31.8 39.3

23.8 27.4 23.4 24.6 22.9 28.4

dramatically, which contributes to the limitation of their mechanical improvement in composites. In the meantime, we have also investigated the mechanical properties of composites as a function of the concentration of CNT-4 (Figure 12). It can be seen that the modulus and strength of composite films increase with an increase in the CNT-4

content and reach an ultimate value of 6.10 GPa (almost 50% increasement) in modulus and 126.8 MPa (almost 52% improvement) in strength with 1 wt % CNT-4. It is generally found that the improvement was more significant with low CNT-4 content (