2017, Vol. 28 
Polymer nanocomposites containing carbon nanofiller have attracted tremendous attention due to their excellent mechanical, electrical, and thermal properties [1-5]. The mechanical properties of composites are affected not only by the dispersion state of the fillers but also by the interactions between the filler and polymer matrix. Most carbon additives are proved to have great reinforcement effect. In order to meet the demands of a variety of requirements, the hybrid nanomaterials [6, 7], composed of two fillers with different dimensions and properties, were used to prepare multifunctional composites. Among these hybrid systems, graphene/carbon nanotubes (GE/CNTs) hybrid was treated as one of the most potential enhancement candidates because of its excellent mechanical properties [8, 9]. Compared to other fillers, GE/CNTs hybrid possessed many advantages. For instance, the π-π stacking interactions between GE and CNTs particles can create bridges between the isolated graphene sheets, which is in favor of efficient load transfer between the fillers and polymer, and therefore has a better reinforcement effect in comparison to single CNTs or GE due to the synergistic interaction of the two kinds of nanofillers [10]. The dispersion and stability of Ge and CNTs are vital. Nevertheless, it is very difficult to disperse them well in solvent or polymer matrix due to their strong interactions [11, 12], and the filler aggregates will deteriorate the electrical and mechanical properties of composites [13]. Under most conditions, it is important to modify these nanofillers, especially GE, because their sheet morphology is highly in favor of agglomeration. The dispersion of graphene can be improved by using dispersants. However, the remaining surfactants may greatly limit the subsequent practical applications. Some prominent reports have shown that the dispersion of pristine CNTs can be developed by using graphene oxide (GO) by π-π stacking interactions between the two nanofillers, and the mechanical properties of polymer composites have a certain improvement [14, 15]. However, in order to make full use of the properties of graphene in composites, graphene is a better filler rather than GO. Some workers prepared polymer/(GE-CNT) composite films by using hydrazine to reduce GO in polymer/GO solution [16, 17]. It should be pointed out that graphene is easy to re-agglomerate during reduction. Hence, seeking proper modification method for preparing GE/CNTs complexes or hybrids, which are able to stabilize graphene sheets in water and endow graphene sheets with additional performance, still remains a challenge.
Preparation of anisotropic nanoparticle or micelle with hierarchical structure by taking the advantage of crystallizationdriven self-assembly of crystalline-coil copolymer is a powerful technique and a significant advance in nanotechnology [18]. In our previous work, we demonstrated that di-block copolymer of poly(p-dioxanone) and polyethylene glycol end-capped with pyrene moieties (Py-PPDO-b-PEG) exhibited very different crystallization and micellization behavior compared to PPDO-b-PEG without pyrene moieties [19]. Combined with π-π stacking interactions and crystallization-induced self-assemble, the Py- PPDO-b-PEG copolymer can form hybrid nano-aggregates together with CNTs. The pyrene moieties may induce interactions with the carbon nanoparticles, crystallization can induce self-assemble into anisotropic nano-aggregates which have unique properties in the environment, the corresponding physiological activity comparing to spherical nano-aggregate [20]. In this work, in order to further increase stability of the hybrid nano-aggregates, a triblock copolymer of PPDO and PEG, (Py-PPDO)2-b-PEG, was synthesized and used for preparing hybrid nano-aggregates because it had more pyrene moieties and stronger interaction with CNTs than those of di-block copolymer. The GE/MWCNTs hybrid using (Py- PPDO)2-b-PEG as modifier was therefore used as hybrid nanofiller for preparing PCL nanocomposites with excellent nanofiller dispersity and load transfer efficiency.
2. Experimental 2.1. Materials and samples preparationGE were purchased from Xfnano, Inc. MWCNTs (purity 95%) were purchased from Sigma-Aldrich with a diameter 6-9 nm and a length 5 mm, and purified with 3 M dilute HNO3 solution. Poly (ecaprolactone) (PCL) was purchased from Jinan Daigang Biomaterial Co., Ltd, with a molecular weight of 1.5 × 105 g/mol. 1-Pyrenemethanol was purchased from Alfa Aesar. Poly(ethylene glycol) ether (PEG, degree of polymerization = 90, Mn = 4 kDa) was purchased from Sigma-Aldrich and dried under vacuum at 40 ℃ overnight before use. p-Dioxanone (PDO) (99.9%) was provided by National Engineering Laboratory of Ecofriendly Polymeric Material (Chengdu, China), and distilled under reduced pressure just before use. All other reagents were analytical grade from Bodi Chemical Factory (China) and used as received without further purification. The synthesis route of (Py-PPDO)2-b-PEG triblock copolymer was shown in Scheme 1. 1H-NMR (CDCl3, 400 MHz): the resonances at 4.16 ppm (s, -COCH2OCH2CH2O-), 3.78 ppm (t, -COCH2OCH2CH2O-), and 4.34 ppm(t, -COCH2OCH2CH2O-) corresponded to the protons of three different methylene groups of the PPDO blocks; the proton resonances at 2.65 ppm (m, -OCOCH2CH2OCO-) and 3.64 ppm (m, -OCH2CH2O-) were attributed to carboxyl terminated PEG blocks; the resonances at 8.16-8.29 ppm (m, CHCHC or CHCHCH) and 5.93 ppm (s, CCH2O) corresponded to the protons of the esterified Pyrene-1-methanol residue of the PPDO blocks. The averagemolecularweights ofPPDOandPEGblocks of (Py-PPDO)2-b- PEG copolymer triblock copolymer studied in this work, calculated from 1H-NMR spectrum of Py-PPDO-OH and PEG precursors, are 1100 and 4000 g/mol, respectively. The 1H-NMR spectra were obtained by using an Avance Bruke-II NMR spectrometer under 400 MHz (Bruker, Germany) using CDCl3 as solvent and tetramethyl silane (TMS) as internal reference.
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| Scheme1. Synthesis of (Py-PPDO)2-b-PEG triblock copolymer. | |
2.2. Preparation of (Py-PPDO)2-b-PEG@MWCNTs nano-aggregates and GE/(Py-PPDO)2-b-PEG@MWCNTs hybrid
(Py-PPDO)2-b-PEG copolymers were dissolved in DMF at temperature (95 ℃). The MWCNTs were then added to the copolymer solution. The homogeneous dark ink-like dispersion obtained after sonication was then cooled quickly to 40 ℃. The feed ratio of (Py-PPDO)2-b-PEG to MWCNTs is 6:1 in weight. After sufficient crystallization of PPDO blocks, the obtained (Py-PPDO)2- b-PEG@MWCNTs hybrid nanoaggregates were precipitated by ether, and redispersed in THF solution by highly speed stirring after vacuum drying. The mixture of PCL, GE, and (Py-PPDO)2-b- PEG@MWCNTs in THF were then cast into a film at room temperature.
2.3. CharacterizationRaman spectra were collected using an Avalon Instruments Raman Station using a 632 nm HeeNe laser. The microstructure was characterized using TecnaiG2 F20 S-TWIN electron microscope (FEI Co., ) bright-field transmission electron microscopy (TEM). A tensile testing machine SANS CMT4104 (SANS Group, China) was used to measure the tensile properties of the composite films at a cross-head speed of 30 mm/min. Each sample did six Parallel.
3. Results and discussion 3.1. The microstructure of GE/MWCNTs in solution and polymer matrixThe stability of the dispersions ofGEsheets inDMFwas tested by a sedimentation experiment, as shown in Fig. 1. The digital photos were taken after two days of sedimentation. Irreversible agglomerates formed in solvent for both GE (Fig. 1A) and GE/MWCNTs (Fig. 1B) hybrid owing to their intrinsic poor dispersity. The (Py- PPDO)2-b-PEG is able to self-assembly and wrap on MWCNTs to form very stable shish-kebab like nano-aggregates ((Py-PPDO)2-b- PEG@MWCNTs) (Fig. 1D and E) in selective solvent [20]. However, similar result was not observed for (Py-PPDO)2-b-PEG and GE (Fig. 1C), the stability of their mixture did not show obvious improvement compared to neat GE, which suggested that the selfassemblies of (Py-PPDO)2-b-PEG cannot wrap on GE probably because of the nano-sheet morphology of GE. When incorporated GE with (Py-PPDO)2-b-PEG@MWCNTs nano-aggregates, a stable dark ink-like dispersion of GE/(Py-PPDO)2-b-PEG@MWCNTs with relatively high stability was easily obtained after sonication in DMF (Fig. 1F-I), which indicated that the addition of (Py-PPDO)2-b- PEG@MWCNTs nano-aggregates provides the GE with good dispersible properties owing to the inter-particle interactions between these two kinds of nanoparticles. The crystallizationinduced self-assembly of (Py-PPDO)2-b-PEG lead to a partially wrapped morphology of the (Py-PPDO)2-b-PEG@MWCNTs (Fig. 1E), which have some bare sections and can therefore induce π-π interactions with GE. The feed ratio of GE to MWCNTs in weight could vary from 1:9 to 4:6. The stability of GE/(Py-PPDO)2-b- PEG@MWCNTs decreased with increasing the feed ratio because the strong π-π stacking interactions of GE were gradually superior to that of between (Py-PPDO)2-b-PEG@MWCNTs and GE, and resulted in obvious agglomeration (5:5, Fig. 1I).
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| Figure 1. Digital photos of solution dispersion of (A) GE; (B) GE/MWCNTs; (C) GE/(Py- PPDO)2-b-PEG; (D) Digital photo and (E) TEM image of (Py-PPDO)2-b- PEG@MWCNTs; Digital photos of GE/(Py-PPDO)2-b-PEG@MWCNTs with different feed ratio of GE to MWCNTs (F) 1:9, (G) 2:8, (H) 3:7, (I) 4:6, (J) 5:5. | |
Raman spectroscopy is the most useful method to evaluate the changes in the electronic structure of specimens. As shown in Fig. 2, MWCNTs (Fig. 2a) which have been purified with dilute HNO3 solution display bands at 1324 and 1578 cm-1, corresponding to the well-documented D and G bands, respectively [21, 22]. However, compared to the MWCNTs, the D band of the GE/(Py- PPDO)2-b-PEG@MWCNTs hybrid (Fig. 2c) at 1341 cm-1 was enhanced and broadened, which indicated the increase of structural defects and lattice distortions. The G bands up-shift from 1578 cm-1 for MWCNTs to 1588 cm-1 for GE/(Py-PPDO)2-b- PEG@MWCNTs. This result further improved the π-π interactions between GE and MWCNTs, which was responsible for the improvement in stability of GE in DMF.
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| Figure 2. Raman spectra (excitation at 632.8 nm) of (a) MWCNTs, (b) (Py-PPDO)2-b- PEG@MWCNTs, and (c) GE/(Py-PPDO)2-b-PEG@MWCNTs hybrid. | |
The GE/(Py-PPDO)2-b-PEG@MWCNTs hybrid nanofillers with good dispersity and stability were therefore used for preparing PCL based nanocomposites. For comparison, control samples of PCL/GE and PCL/(Py-PPDO)2-b-PEG@MWCNTs were also prepared by same procedure. Fig. 3 shows the dispersity and microstructure of MWCNTsand/or GE in PCL matrix. The GE without any modification tended to aggregation in the polymer matrix (Fig. 3A) owing to its intrinsic poor dispersity. Comparably, the MWCNTs modified by (Py-PPDO)2-b-PEG exhibited very good dispersity (Fig. 3B) which can be attributed to the compatibilizing effect of (Py-PPDO)2-b-PEG nano-aggregates wrapped on the MWCNTs surface. When incorporated GE with (Py-PPDO)2-b-PEG@MWCNTs hybrid nano-aggregates, the dispersity of GE was much improved. As shown in Fig. 3C, very thin graphene sheets (marked by black arrow) and MWCNTs with very good dispersity in PCL matrix was observed. Moreover, there were some MWCNTs (marked by white arrow) adsorbed on GE were also observed, which provide evidence of formation of GE/ (Py-PPDO)2-b-PEG@MWCNTs hybrid as well as the interaction between GE and (Py-PPDO)2-b-PEG@MWCNTs. As illustrated in Fig. 3E, the bare sections ofMWCNTswhich have not been wrapped by (Py-PPDO)2-b-PEG may induce π-π stacking interactions with GE sheets and resulted in stable hybrid nano-aggregates. However, when the feed ratio of GE to MWCNTs increase to 5:5, obviously aggregation of GE were clearly seen in the TEM image (Fig. 3D), suggested that the content of (Py-PPDO)2-b-PEG@MWCNTs nanoaggregates is too low to prevent the agglomeration of graphene (as illustrated in Fig. 3F).
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| Figure 3. TEM images of composites films of PCL/GE (A); PCL/(Py-PPDO)2-b- PEG@MWCNTs (B); PCL/GE/(Py-PPDO)2-b-PEG@MWCNTs with different GE/MWCNTs weight ratios (C) 2:8; (D) 5:5, the weight content of nanofiller is 0.06 wt%. Schematic illustration of interconnection of (Py-PPDO)2-b- PEG@MWCNTs and GE with different ratio of GE to MWCNTs (E) 2:8, (F) 5:5. | |
3.2. Tensile property
The mechanical properties of neat PCL and its composites with different feed ratio of GE to MWCNTs are shown in Fig. 4A (the total contents of GE and MWCNTs are 0.01 wt%). For PCL/GE nanocomposite, the tensile strength increased 9% which may be ascribed to the aggregation of GE. The aggregation of graphene will act as the stress concentration site which leads to a weakening effect for the reinforcement of nanocomposite materials. For PCL/ (Py-PPDO)2-b-PEG@MWCNTs nanocomposite, the tensile strength increased only 4% because the well dispersion of (Py-PPDO)2-b- PEG @MWCNTs in PCL matrix was not in favor of the load transfer from matrix to fillers. Compared to the composites using GE or (Py- PPDO)2-b-PEG@MWCNTs as nanofillers individually, all the composites containing GE/(Py-PPDO)2-b-PEG@MWCNTs hybrid nanofiller have a pronounced enhancement especially the feed ratio of GE to MWCNTs is 2:8, the tensile strength reached 58.1 MPa which corresponded to about 163% reinforcement compared to the PCL film suggested that there is synergistic reinforcing effect between GE sheets and (Py-PPDO)2-b- PEG@MWCNTs in the PCL matrix. This phenomenon can be explained by the formation of GE/(Py-PPDO)2-b-PEG@MWCNTs hybrid nanoparticles, which not only improved the dispersity of GE, but also induced effective load transfer. Meanwhile, the reinforcement effect of hybrid nanofiller decreased with increasing the feed ratio of GE/(Py-PPDO)2-b-PEG@MWCNTs because the agglomeration of GE may occurred gradually with the decrease of (Py-PPDO)2-b-PEG@MWCNTs. When the feed ratio of GE to MWCNTs was fixed at 2:8, with addition of the nanofiller, the tensile strength of the composites reached the maximum value at 0.01 wt% nanofiller content and then decreased when more hybrid nanofiller were added (Fig. 4B). This phenomenon may be attributed to the aggregation of nanofiller with relatively large addition amount.
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| Figure 4. Tensile strength of PCL/GE/(Py-PPDO)2-b-PEG@MWCNTs composite films (A) at 0.01 wt% nanofiller content with different GE/MWCNTs feed ratio; (B) with different nanofiller content (the feed ratio of GE to MWCNTs is 2:8) . | |
4. Conclusion
In summary, triblock copolymer of (Py-PPDO)2-b-PEG was synthesized and used for preparing partially wrapped MWCNTs (Py-PPDO)2-b-PEG@MWCNTs). The stability and dispersity of GE in solvent and polymer matrix were obviously improved by incorporating with (Py-PPDO)2-b-PEG@MWCNTs nano-aggregates owing to the interaction between MWCNTs and GE. Compared to pristine PCL, the PCL nanocomposite films using GE/(Py-PPDO)2-b- PEG@MWCNTs as hybrid nanofillers exhibited much improved mechanical properties at very low nanofiller content. With the incorporation of only 0.01 wt% of GE/MWCNTs, the tensile strength and the elongation at break significantly increased by 163 and 17%, respectively. TEM analysis of the composite films suggested that the (Py-PPDO)2-b-PEG@MWCNTs not only prevent the aggregation of GE but also promote efficient load transfer between polymer matrix and nanofillers. The reinforced nanocomposite materials of biopolymers have a great potential in myriads of applications including flexible electronic and biomedical devices.
AcknowledgmentThis work was financially supported by the National Natural Science Foundation of China (No. 21474066) and the Foundation for Young Scientists of State Key Laboratory of Polymer Materials Engineering (No. sklpme2014-3-09) . The Analytical and Testing Center of Sichuan University provided TEM analysis.
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