Article pubs.acs.org/IECR
A Strategy To Functionalize the Carbon Nanotubes and the Nanocomposites Based on Poly(L‑lactide) Yihui Xu, Qifang Li, Da Sun, Wenjing Zhang, and Guang-Xin Chen* Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China ABSTRACT: A strategy to achieve balanced properties using functionalized carbon nanotubes has been investigated, and its nanocomposite exhibited good performance in mechanical and electrical properties. Polybutyl acrylate (PBA) has been utilized to functionalize multiwalled carbon nanotubes (MWNTs) by in situ atom transfer radical polymerization, resulting in a shell of molecular weight controlled on MWNTs (MWNT-PBA). Poly(L-lactide) (PLLA), which has been pitched on acting as the matrix of the polymers that are compatible with PBA such as poly(methyl methacrylate) (PMMA) and poly(vinylidene fluoride) (PVDF). A series of PLLA/MWNT-PBA nanocomposites were prepared by solution blending. Morphology, mechanical properties, and electrical properties have been tested, and the results showed that the volume electrical resistance of PLLA/ MWNT-PBA nanocomposites decreased by 10 orders of magnitude as the content of MWNTs arrived at its percolation threshold, which is just 1.51 wt %, while the elastic modulus had increased 94.7%, compared to neat PLLA when the content of MWNTs is 2.98 wt %. Optical microscope images exhibited the spherulite morphologies of the nanocomposites while differential scanning calorimetry (DSC) measures showed that the temperature of cold crystallization increased when the content of MWNT-PBA increased.
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INTRODUCTION After the discovery of carbon nanotubes in 1991,1 plenty of researchers have been attracted to them, making attempts to have them functionalized or take advantage of their mechanical and electrical properties to modify polymers promptly.2 As known to all, carbon nanotubes possess high flexibility, low mass density, and large aspect ratio (typically >1000), whereas they present extraordinary high tensile strength and modulus, together with excellent electrical properties.3 However, the aggregation of carbon nanotubes in polymer composites makes it the major drawback, which may prevent efficient stress from transferring to individual nanotubes and have a bad influence on demonstrating its function.4 And as yet, it has not been shown that their potential properties act as fillers in composites. Therefore, it is a critical point to make the efficient dispersion of individual nanotubes and the establishment of a strong chemical affinity (covalent or noncovalent) with the surrounding polymer matrix; thus, thousands of works have been performed to achieve the goal.5−8 There shows a widespread phenomenon that researchers preferred modifying carbon nanotubes with matrix polymer itself in order to get a better dispersion.9−16 However, an alternate strategy could be more efficiently and widely promoted that we could use a polymer that has good compatibility with the matrix to functionalize carbon nanotubes, thus taking advantage of the properties of both the polymer shell on tubes and carbon nanotubes that would be devoted to the performance of the final composites. Poly(L-lactide) (PLLA) has been selected to serve as the matrix polymer, because of its excellent biological performance, biorenewable origins, benign degradation, and obvious brittleness.17−20 The polymer has properties similar to most petroleum-based materials, such as polyethylene, polypropylene, and poly(ethylene terephthalate). It is biocompatible, easy to process by extrusion or injection molding, and has been © 2012 American Chemical Society
widely used in the areas of biomedicine, package material, and etc. Nevertheless, the widespread acceptance of polylactide is limited by its brittle nature.21 Compared with traditional petroleum-based polymers, achieving a property balance between mechanical properties and maintaining biodegradability are still arduous tasks. Therefore, its value would be greatly enhanced if the restriction of the inherent brittleness of PLLA has been settled. One method is through the introduction of a rubbery phase in block copolymer.22 Another method is blending PLLA with tough polymer.23 Both methods would improve the toughness of PLLA Bioplastic. However, high content of rubber phase would affect other mechanical properties, such as elastic strength and modulus.24 Therefore, it is difficult to have a good balance of stiffness−-toughness properties. In this study, different from our previous PLLA-based nanocomposites, which were formed using PLLA-modified carbon nanotubes,14,15 polybutyl acrylate (PBA) which is partly compatible with PLLA, was chosen to modify multiwalled carbon nanotubes (MWNTs), it is quite ideal to act as the third component to modify PLLA, not only because the interaction of the ester groups of both, make PBA compatible with PLLA,25 but the low glass-transition temperature of PBA makes it a soft segment in thermoplastics, acting as a rubber phase, which could enhance the mechanical property of PLLA. Under the effect of PBA, the interfacial tension between MWNTs and PLLA is significantly reduced, which improves the dispersion of MWNTs. The well-distributed filler results in full use of Received: Revised: Accepted: Published: 13648
April 16, 2012 August 20, 2012 September 27, 2012 September 27, 2012 dx.doi.org/10.1021/ie300989w | Ind. Eng. Chem. Res. 2012, 51, 13648−13654
Industrial & Engineering Chemistry Research
Article
was separated by rotary evaporation, and residual SOCl2 was collected when heated under vacuum. The acyl chloridefunctionalized MWNTs (MWNT-COCl) were added into the solution of glycol (100 mL) quickly to avoid the acyl chloride group being hydrolyzed readily into carboxylate ions in air due to its high reactivity. The reaction then was stirred for 48 h at 120 °C to get hydroxyl group-functionalized MWNTs (MWNT-OH). In the third step, MWNT-OH (0.4g), anhydrous CHCl3 (10.0 mL), DMAP (0.04 g), and triethylamine (0.4 mL) were sonicated for 30 min to get a relatively uniform distribution and evacuated and thrice filled with N2. A mixture of bromoisobutyrate bromide (0.2 mL) and CHCl3 (5 mL) then was added dropwise in an ice bath for 1 h. The reaction continued to be stirred for another 48 h at room temperature, after the ice bath melt. The production would be washed adequately by CHCl3 to get one of the initiators MWNT-Br for polymerization. Polymerization of BA from MWNT-Br by using ATRP. The resulting MWNT-Br (0.4910g) was mixed with dioxane (11.24 mL), then PMDETA (0.1606 g), CuBr (0.18 g), and BA (13 mL) were added into the mixture; the system was evacuated and thrice filled with N2, undergoing stirring at 80 °C for 50 h. After the reaction, the final product was dissolved in CHCl3, then the solvent was removed using a rotary evaporator; the solid was antiprecipitated with methanol several times until the supernatant appeared. Finally, the product of polymerization was collected, after being dried overnight. Preparation of PLLA/MWNT-PBA Nanocomposites. Different weights of MWNT-PBA were blended with PLLA (4 g) in CHCl3. MWNT-PBA and PLLA with a CHCl3 volume of 40 mL were packed separately in a 100-mL flask, which is covered with parafilm stirred at room temperature for 4 h. The MWNT-PBA solution then was poured into the PLLA solution and stirred for another 4 h. After that, the mixture was put in a mold and dehydrated by evaporating up CHCl3, then the nanocomposite material was made. The final product was dried under vacuum at 70 °C overnight. Similarly, different ratios of nanocomposite was made. Table 1 shows the component content of all the samples.
mechanical and electrical properties of MWNTs and the toughness feature of PBA phase to achieve the equilibrium mentioned above. In order to fabricate well-controlled thicknesses of polymer layers onto MWNTs and find the most suitable condition for the dispersion of nanotubes, atom transfer radical polymerization26 (ATRP) has been used to graft butyl acrylate from MWNTs to get an average length of graft polymer chains. As we expected, MWNTs encapsulated with PBA are easily dispersed in CHCl3 solution involving PLLA. Finally, solution blending has been used to process the nanocomposites, and the blend films were characterized using various techniques, to study the influence of polybutyl acrylatemodified carbon nanotubes (MWNT-PBAs) on the various properties of PLLA.
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EXPERIMENTAL SECTION Materials. Polylactide (Mw = 167 000 g/mol, PDI = 1.63) was purchased from Zhejiang Hisun Pharmaceuticals Co., Ltd. Multiwalled carbon nanotubes (MWNTs) (diameter = 10−20 nm, length = 10−15 μm) were provided by Carbon Nanotechnologies, Inc. Butyl acrylate (BA, Beijing Chemical Works) was washed three times with a 10% aqueous solution of sodium hydroxide and distilled water to remove the inhibitor hydroquinone, dried with anhydrous sodium sulfate, then vacuum distilled. Nitrate (65%−68%, HNO3), sulfuric acid (98%, H2SO4), dimethylaminopyridine (DMAP), bromoisobutyrate bromide, pentamethyl diethylenetriamine (PMDTA) were all purchased from Aladdin Co. (Shanghai, China) and were used without further purification. Glycol (HOCH2− CH2OH), tetrahydrofuran (THF), chloroform (CHCl3), triethylamine, dioxane, and methanol (Beijing Chemical Works) were used after being refined under the method of solvent handbook and kept with 4A molecular sieves. Synthesizing the Initiator MWNT-Br for ATRP. Scheme 1 illustrates the ATRP synthesis route to wrap MWNTs with Scheme 1. Grafting Polybutyl Acrylate (PBA) from Chemically Modified MWNTs via Atom Transfer Radical Polymerization (ATRP)
Table 1. Component Content of All of the Nanocomposites sample 1
sample 2
MWNT loading (wt %)
PLLA/MWNT-PBA-1 PLLA/MWNT-PBA-2 PLLA/MWNT-PBA-3 PLLA/MWNT-PBA-4 PLLA/MWNT-PBA-5 PLLA/MWNT-PBA-6
PLLA/MWNT-1 PLLA/MWNT-2 PLLA/MWNT-3 PLLA/MWNT-4 PLLA/MWNT-5 PLLA/MWNT-6
0.59 1.19 1.49 1.79 2.38 2.98
Characterizations. Fourier transform infrared (FTIR) spectra (collected from a Bruker Tensor 27 FTIR system) were used to characterize the molecular structure. The samples of nanocomposites were imbedded in KBr disks. Raman spectroscopy (Renishaw inVia) was used to confirm the structure of MWNTs operating at 514 nm with a resolution of 1.5 cm−1. Thermogravimetric analysis (TGA, HCT-2) was conducted in nitrogen atmosphere. MWNT and MWNT-PBA were heated at a heating rate of 10 °C/min from 50 °C to 500 °C, to determine the graft content of PBA. The characterization of the coated polymer was carried out by transmission electron microscopy (TEM) (Hitachi, Model 800), the samples was soluble in THF, dropped in the micro grid. Field-emission
PBA. First, the mixture of pure MWNTs (2 g) and HNO3 (100 mL) aqueous was sonicated (SB25-12DT, 600 W) in a bath of 16 L of water for 20 h at 25 °C, and then stirred overnight under reflux at 68−70 °C to generate carboxyls. After cooling to room temperature, the solution was diluted by deionized (DI) water twice the volume of itself and vacuum-filtered through a 0.1 μm polytetrafluoroethylene membrane. The solid was washed by DI water several times to reach pH 7 and dried under vacuum at 60 °C for 24 h. Second, dried acid-treated MWNT (MWNT-COOH, 0.5 g) was suspended in SOCl2 (40 mL) and stirred at 65 °C for 24 h under reflux, the solid−liquid 13649
dx.doi.org/10.1021/ie300989w | Ind. Eng. Chem. Res. 2012, 51, 13648−13654
Industrial & Engineering Chemistry Research
Article
scanning electron microscopy (FESEM) (Hitachi, Model S4700) was used to characterize the cross section; the nanocomposite samples were heated to melt, and rapidly quenched in liquid nitrogen for 5 min, to inhibit the crystallization of PLLA. The universal mechanical tester (Lloyd, Model LR30K Plus) was operated at a stretching rate of 10 mm/min, and all of the samples were cut into dimensions of 50 mm × 4 mm, according to a national standard; in order to reduce the error, each sample has been tested three times. A resistance meter (Model PC68) was used to measure the volume resistance of nanocomposites, and each sample was test at three different voltages250, 500, and 1000 Vfive times repeatedly, to minimize the error. Thermal properties of the nanocomposites and pure polylactide were measured by differential scanning calorimetry (DSC) (Netzsch, Model TASC414/4) under the nitrogen atmosphere, and the samples were heated to 180 °C at the heating rate of 10 °C/min after melt-quench. Optical microscope (OM, Motic 3.2) was served to observe the crystallization of all nanocomposites films, the samples were heated over the melt temperature and cooled to the crystallization temperature.
similar absorption peaks, which had been marked in the graph. All of this information supports the functionalization. Raman Spectroscopy. In the Raman spectra of MWNTs and MWNT-PBA (Figure 2), there were some differences that
RESULTS AND DISCUSSION FTIR. Figure 1 presents the FTIR spectra of pure MWNTs, MWNT-OH, MWNT-COOH, MWNT-PBA, and PBA. The
Figure 2. Raman spectra of MWNTs and MWNT-PBA.
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could identify MWNTs that had been modified with polymer. MWNTs show the tangential band spectra (G-band) at 1564 cm−1 which was a characteristic feature of the graphite layers and the disorder band spectra (D-band) at 1343.4 cm−1 (this was a typical sign for defective graphite structures).27 After being functionalized with PBA, the G-band and D-band had shifted 18.2 cm−1 and 6.0 cm−1, respectively, to the high frequency, which may due to the strong chemical reaction between polymer chains and MWNTs.28 Otherwise, as marked in the graph, the ID/IG values (intensities of the two peaks) of MWNTs had increased from 0.74 to 1.39, which means a mass of new functional group had been grafted onto MWNTs.29,30 TEM Analysis. A further certification of grafting PBA onto MWNT was determined by transmission electron microscopy (TEM) in Figure 3. Figures 3A and 3C clearly show that the surface morphology of MWNT-PBAs is significant different, by contrasting with MWNTs. The diameter is ∼30 nm, which is marked in the picture. Both of them possess a tune inside and multisimilar structure of graphene on the surface; furthermore, after grafting PBA from MWNTs, a polymer layer can be easily distinguished, with a thickness of ∼5.53 nm. When the graph was done in lower multiples, it could be seen that a homogeneous shell had formed on all of the surfaces of MWNTs. Significantly, the coating will improve the solubility of MWNTs and the chemical affinity with PLLA. Furthermore, the original length and pattern of MWNTs remain intact after a series of chemical treatments, which can be identified in Figure 3D, so that it can dramatically contribute to the properties of PLLA/MWNT-PBA nanocomposites for long fibers and thin flakes, which are the best choices for fillers to reduce the percolation threshold.31 Thermogravimetric Analysis (TGA). To evaluate the content of grafted PBA, thermogravimetric analysis (TGA) was performed with MWNTs and MWNT-PBA in Figure 4. For pure MWNTs, 99.5 wt % was retained when heated over a temperature range of 50−500 °C; over this range, almost no weight loss can be observed. However, MWNT-PBA gave a one-step thermal degradation up to 70.2 wt % at ∼370 °C, as determined by the derivative (DTG), this is approximately the
Figure 1. Fourier transform infrared (FTIR) spectra of MWNTs (spectrum a), MWNT-COOH (spectrum b), MWNT-OH (spectrum c), MWNT-PBA (spectrum d), and PBA (spectrum e).
spectra of all four samples that contain MWNTs exhibit a typical weak peak at 1550 cm−1, which belongs to the vibration of the aromatic ring, and a peak at 1630 cm−1, which represents the CC stretching vibration, indicating that the structure of MWNTs is still preserved after chemical modification. As shown in the spectrogram for MWNT-COOH, an absorption peak at 1730 cm−1 appeared, which is characteristic of the C O group, because of the oxidation of MWNT when being sonicated in strong acid. After polymerization, MWNT-PBA shows the feature of PBA apparently when compared with spectra of the other three, namely the strong absorption peak at 1730 cm−1, which associate with the ester-carbonyl group of PBA, while the two absorption peak at 1250 cm−1 and 1120 cm−1, related to C−O−C stretching vibration of the ester group of PBA. In addition, the spectrum of PBA exhibited 13650
dx.doi.org/10.1021/ie300989w | Ind. Eng. Chem. Res. 2012, 51, 13648−13654
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Figure 3. TEM images of MWNT-PBA at different magnifications: (A) pure MWNT (low magnification), (B) pure MWNT (high magnification), (C) MWNT-PBA (low magnification), and (D) MWNT-PBA (high magnification).
Figure 5. FESEM images of (A) PLLA/MWNT-PBA-1, (B) PLLA/ MWNT-PBA-2, (C) PLLA/MWNT-PBA-3 (bar = 20 μm), and (D) PLLA/MWNT-PBA-1, (E) PLLA/MWNT-PBA-2, (F) PLLA/ MWNT-PBA-3 (bar = 5 μm).
such as Figure 5A, it can be easy distinguished that most of the white points in the area, which are indicated by red arrows, are tubes and they are well-dispersed. The single MWNTs separated with another as pointed by arrows. Thereafter, a similar phenomenon was observed in Figures 5B and 5C; when the concentration of MWNTs reaches 1.49 wt %, no obvious agglomerations are found in the polymer matrix. When the images were taken under a higher magnification (Figures 5D ,5E, 5F), truncated cross sections can also be identified from the picture and most of the MWNT-PBA was dispersed separately. Hence, the FESEM micrographs gave further evidence for MWNT-PBAs forming a well dispersion in PLLA matrix. Mechanical Properties. The fracture toughness and tensile strength of the nanocomposites were measured, and the data are summarized in Table 2. Clearly, the tensile modulus of PLLA/MWNT-PBA-6 improves by 94.7%, compared to that of pure PLLA, which is ∼1490 MPa; yet, the relative content of MWNTs is just ∼2.98 wt %. Yoon et al. have discussed the influences of PLLA grafted MWNTs on PLLA; for this material, the tensile module of PLLA/MWNT-PLLA had improved ∼31.8% when the content of MWNTs reached 5 wt %.33 The addition of MWNT-PBA is unquestionably a factor of the strengthened performance, and the enhancement has contributed to testify the designation that functionalized reinforcement with polymer that is compatible with the matrix could manifest properties well and even better than that of modified by the matrix itself. With the increase of MWNT-PBA content in PLLA, the improvement of elongation at break can also be observed. The elongation at break can approximately represent toughness, because the area under the stress−strain curve represents the energy to failure. In contrast, the nanocomposites with increasing content of pure MWNTs
Figure 4. Thermogravimetric analysis (TGA) of pure MWNTs (spectrum a) and MWNT-PBA (spctrum b).
temperature of degradation of PBA.32 In addition, the relatively narrow temperature range revealed that ATRP was effectively carried out, which is helpful for the homogeneous dispersion of MWNT-PBA in a PLLA matrix. FESEM. Obviously, study of the morphology of individualized MWNTs and clusters dispersed among a polymer matrix is necessary at the micrometer scale via FESEM; it directly reflects the dispersion of MWNTs with different level of modification. In general, better dispersions of MWNTs means stronger bonding, more uniform chemical affinity, more stable network structures which assign to the performance of mechanical properties as discussed previously. FESEM micrographs of PLLA/MWNT-PBA at various levels of magnification are shown in Figure 5. All the samples were via the melt-quench process, and spraying gold before imaging. In lower multiples, 13651
dx.doi.org/10.1021/ie300989w | Ind. Eng. Chem. Res. 2012, 51, 13648−13654
Industrial & Engineering Chemistry Research
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Table 2. Tesile Properties of Pure PLLA and PLLA/MWNT Nanocomposites sample PLLA PLLA/MWNTPBA-1 PLLA/MWNTPBA-2 PLLA/MWNTPBA-3 PLLA/MWNTPBA-4 PLLA/MWNTPBA-5 PLLA/MWNTPBA-6 PLLA/MWNT-1 PLLA/MWNT-2 PLLA/MWNT-3 PLLA/MWNT-4 PLLA/MWNT-5 PLLA/MWNT-6
tensile modulus (MPa)
tensile strength (MPa)
elongation (%)
1490.4 ± 68 1492.6 ± 56
48.10 ± 3.50 52.85 ± 5.19
2.45 ± 0.18 3.23 ± 0.14
1817.9 ± 62
52.31 ± 1.56
3.35 ± 0.21
1937.8 ± 42
50.46 ± 1.24
3.87 ± 0.13
2459.0 ± 48
51.17 ± 0.98
3.83 ± 0.10
2754.1 ± 40
50.39 ± 2.81
3.61 ± 0.06
2902.0 ± 55
51.92 ± 7.32
3.73 ± 0.21
2110.1 2063.6 2097.7 2307.2 2278.3 2310.4
± ± ± ± ± ±
172 92 98 89 47 84
38.02 44.81 34.80 32.20 32.72 30.80
± ± ± ± ± ±
4.24 3.52 5.77 4.12 4.93 10.78
2.08 2.86 1.87 1.65 1.53 1.47
± ± ± ± ± ±
0.24 0.14 0.16 0.21 0.25 0.15
Figure 6. Relationship between the resistivity of nanocomposites and the content of MWNT-PBA and MWNTs.
MWNTs (2.11 wt %) when the differential is done. The phenomenon illustrates that compatibility of PBA and PLLA has improved the distribution of MWNTs-PBA efficiently. Crystallization Behavior. Optical microscopy and differential scanning calorimetry (DSC) were used to characterize the crystallization behavior of the nanocomposites. Figure 7A
show an obvious decrease in elongation percentage, which can be attributed to the aggregation of MWNTs in the PLLA matrix, as can be seen in the SEM analysis. Grafting MWNTs with PBA via ATRP improves the chemical affinity between the filler and the matrix, which leads to homogeneous dispersion, resulting in devotion to the toughness properties of nanocomposites. The experimental elongation data supports our assumption that the advantage properties of the polymer shell, which is compatible with the matrix, can be utilized with that of MWNTs simultaneously. In addition, another easy arises: the tensile strength of PLLA/MWNT-PBA is ∼52 MPa, while that of PLLA/MWNTs drops from 38.02 MPa to 30.80 MPa. The critical difference can also explain why MWNT-PBA is welldispersed in the matrix, because the agglomeration of pure MWNTs could make PLLA much more brittle. Therefore, after treatment with the design idea mentioned at the beginning of the article, not only is the stiffness of PLLA improved significantly, but the process also gave the material relatively good toughness, despite the fact that a small amount of filler was added. Electrical Properties. Figure 6 shows the effect of different MWNTs and MWNT-PBA content on the volume resistance of the nanocomposites with specific values. Pure PLLA is insulating; its resistivity is beyond the range of the test instrument (1016 Ω m), so the conductive mechanism of nanocomposite can be due to the electron transfer in the path of the MWNTs. For PLLA/MWNT-PBA, the resistivity maintains a value over 1014 Ω m when the percentage of MWNTs is
