Noncovalent Method for Improving the Interaction between Reduced

Article pubs.acs.org/IECR

Noncovalent Method for Improving the Interaction between Reduced Graphene Oxide and Poly(ε-caprolactone) Bingjie Wang,† Yujie Zhang,† Jianqiang Zhang,† Huyan Li,† Peng Chen,† Zongbao Wang,*,‡ and Qun Gu*,† †

Ningbo Key Laboratory of Polymer Materials, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, People’s Republic of China. ‡ Faculty of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, People’s Republic of China ABSTRACT: Epitaxial crystallization of poly(ε-caprolactone) (PCL) on reduced graphene oxide (RGO) was investigated by melt and solution crystallization. RGO and graphene oxide (GO) provided an opportunity to investigate the influence of surface functional groups on epitaxial crystallization with lattice matching. After annealing treatment, PCL/RGO composites showed an extra melting peak, which implied that epitaxial crystallization of PCL on RGO could form thicker lamellae. An analogous procedure of solution crystallization was used to confirm the epitaxy morphology and orientation of polymer chains. The results showed that PCL chains existed along the ⟨2110⟩ direction of the RGO (0001) plane forming edge-on lamellae, which implied strong interaction between PCL crystals and the RGO surface. Tensile test results showed that the yield strength and Young’s modulus of PCL/RGO composites with epitaxial interaction were improved by about 34.2 and 53.2%, respectively, compared with neat PCL. disturbing the sp2 conformation.16−20 Despite promising reports that have shown graphene can significantly improve the mechanical properties of a polymer matrix, the investigation of interfacial interactions between semicrystalline polymers and graphene are limited. Interfacial crystallization offers another possible method to improve semicrystalline polymer/graphene interfacial interaction. In this paper, a noncovalent method of polymer epitaxial crystallization on reduced graphene oxide (RGO) was studied to improve the interfacial interaction between RGO and PCL, and the influence of epitaxial crystallization on the mechanical properties of polymer matrix was investigated. For the purpose of comparison, graphene oxide (GO) was used to investigate the influence of the damage of graphene structure on the surface-induced epitaxial crystallization.

1. INTRODUCTION Graphene is a two-dimensional, atomically thick sheet composed of sp2 carbon atoms arranged in a honeycomb structure,1 and it is the strongest material found up to now (ultimate strength, 130 GPa; Young’s modulus, 1 TPa).2 Therefore, graphene has great potential for improving the mechanical properties of polymers.3−5 It has been reported that graphene could significantly improve the physical properties of polyesters such as poly(ε-caprolactone) (PCL), poly(3hydroxybutyrate) (PHB), poly(L-lactic acid) (PLA), and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV).6−9 It is well-known that the mechanical properties of polymer/ nanofiller composites not only depend on the mechanical properties and dispersion of nanofiller in the polymer matrix but also strongly depend on the interaction between polymer and nanofiller. Great effort has been devoted to improving the interaction between graphene and polymer matrixes. It has been reported that graphene can be modified by surface chemical functionalization with functional groups, such as small molecules10,11 and polymer chains.12,13 In this method, functional groups are covalently linked to graphene, which can improve the solubility and processability and enhance the interaction with polymers.14 However, the chemical reaction results in the break of sp2 conformation of the carbon atom in graphene. Therefore, the conjugation of the graphene sheet is disrupted, and the mechanical properties of the chemically modified graphene decreased dramatically.15 Thus it is very essential to find a physical way with no damage to the structure of graphene in polymer/graphene composites. Noncovalent functionalization with different organic compounds by πinteractions has been used to make graphene soluble in common solvents and thereby avoid stacking. This method is an attractive production method, because it offers the possibility of attaching functional groups to graphene without © 2013 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Materials. PCL was purchased from Shanghai Yizhu Co., Ltd. (Shanghai, China), with average weight Mn = 42 500 g·mol−1 and polydispersity index Mw/Mn = 1.5. Natural flake graphite was purchased from Qingdao Jiuyi graphite Co., Ltd. (Shandong, China) with a mean particle size of 50 μm. Hydrochloric acid (HCl, 37%), sulfuric acid (H2SO4, 98%), potassium nitrate (KNO3), potassium permanganate (KMnO4), hydrogen peroxide (H2O2, 35%), 1-hexanol, and chloroform were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All reagents were used as received without further purification. Received: Revised: Accepted: Published: 15824

July 1, 2013 October 17, 2013 October 17, 2013 October 17, 2013 dx.doi.org/10.1021/ie402062j | Ind. Eng. Chem. Res. 2013, 52, 15824−15828

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Figure 1. (a) Full XPS spectra and the deconvolutions of C 1s signal into its constituent contributions for (b) GO and (c) RGO.

mm/min. The standard bars of 50 mm × 3 mm × 2 mm in size were prepared with a HAAKE MiniJet II microinjection molding machine. At least five samples were measured and the average values were reported.

2.2. Sample Preparation. GO was exfoliated by ultrasonication from graphite oxide which was produced by a modified Hummers method.21 RGO was prepared by thermal exfoliation and reduction of GO.22 PCL/1.0 wt % RGO composites were prepared by a solution coagulation method. A 5 mg sample of RGO was added to 50 mL of ethanol by ultrasonication for 1 h, and 0.5 g of PCL was completely dissolved in 20 mL of chloroform. By dropping the predispersed ethanol/RGO suspension into the chloroform/ PCL mixture, PCL/RGO composites were precipitated. PCL/ 1.0 wt % GO composites and neat PCL were prepared with the same procedure in order to eliminate experimental error. All samples used for differential scanning calorimetry (DSC) measurements were first heated to 90 °C at a heating rate of 10 °C/min and equilibrated at 90 °C for 3 min to remove the thermal history. Subsequently, the samples were cooled to 48 °C at a cooling rate of 10 °C/min and kept at this temperature for 30 min for epitaxial crystallization. Finally, the samples were cooled to room temperature. Additionally, the standard bars used for tensile testing were prepared with a HAAKE MiniJet II microinjection molding machine. Neat PCL, PCL/GO composites, and PCL/RGO composites were prepared without annealing treatment. PCL/RGO composites with annealing treatment (for epitaxial crystallization) were prepared by keeping the bars at 48 °C for 30 min during the injection molding. Epitaxial crystallization of PCL/RGO in solution was carried out with the following procedure: RGO (1 mg) was first dispersed in 10 mL of 1-hexanol by sonication for 2 h to form uniform dispersed single-layer or few-layer RGO nanosheets. Subsequently, the suspension was mixed with 10 mL of 0.05 wt % predissolved PCL/1-hexanol solution at 100 °C and kept at this temperature for 10 min. Then the mixture was quenched to 48 °C and crystallized for 1 h. The relatively higher temperature, which is higher than the crystallization temperature (Tc) of neat PCL in 1-hexanol, was chosen. 2.3. Analytical Methods. X-ray photoelectron spectroscopy (XPS) experiments were carried out using an AXIS UTLTRADLD (Shimadzu Corporation, Japan), operated at 15 kV under a current of 30 mA. Differential scanning calorimetry (DSC) measurements were performed with a Mettler Toledo DSC under nitrogen atmosphere. Wide angle X-ray diffraction (WAXD) patterns were recorded on a Bruker D8 diffractometer, using Ni-filtered Cu Kα radiation at 40 kV and 30 mA at room temperature in an angle ranging from 5 to 40° at a rate of 3.5°/min. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) images were obtained on an FEI Tecnai G2 F20 transmission electron microscope with an accelerating voltage of 200 kV. Tensile testing was performed on an Instron 5567 at a tensile rate of 20

3. RESULTS AND DISCUSSION It is well-known that the intrinsic surface chemical of graphene has a significant effect on the performance of polymer/ graphene composites. First, XPS was used to confirm the oxidation degree of graphene. The full XPS spectra for GO and RGO are shown in Figure 1a. The signals observed around binding energies of 532 and 286 eV correspond to the 1s orbital electrons of O and C, respectively. The deconvolution of the C 1s signals of GO and RGO are shown in parts b and c of Figure 1. The peak intensities of C−O bands and O−CO bands in GO are much more distinctive compared with those in RGO. The C/O atomic ratio for GO is 2.4, while that for RGO is 13.1, calculated from XPS results. Those results show that GO and RGO have difference surface properties. The occurrence of a small amount of O elements in RGO is because these functional groups cannot be reduced completely in the thermally exfoliated process, while such an amount of groups with O elements does not significantly affect the investigation of epitaxial crystallization in polymer/RGO composites. Epitaxial crystallization of PCL/RGO composites was first investigated by DSC. PCL was prepared with the same procedure as PCL/RGO composites for comparison. Figure 2

Figure 2. DSC heating scans of neat PCL, PCL/GO composites, and PCL/RGO composites.

shows DSC heating scans of PCL, PCL/GO composites, and PCL/RGO composites. Both composites exhibit a low temperature endothermic peak (Tm1) at about 56.5 °C, which is close to the melting point of neat PCL. These peaks are unambiguously attributed to the melting of lamellae formed during the nonisothermal crystallization process. For the PCL/ 15825

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morphology and the orientation of polymer chains. In order to observe the morphologies more clearly, analogous epitaxial crystallization of PCL on RGO by a controlled solution crystallization procedure was carried out. Figure 4a presents the SAED pattern of RGO. A typical hexagonal symmetry of two dominant reflections, which is representative for RGO sheet, is observed. The innermost hexagon and the next one correspond to indices (1110) (2.13 Å spacing) and (2110) (1.23 Å spacing), respectively.25,26 Figure 4b shows the TEM image of RGO with PCL. Rodlike crystals are observed on the surface of RGO, which can be observed throughout the sample. The length of the rodlike crystals is 50−400 nm, and the width is 25−55 nm. Figure 4c shows the SAED pattern of the rodlike crystals, corresponding to the circled area indicated in Figure 4b. In the diffraction pattern, the bright spots are diffractions contributed by RGO and the arcs with weaker intensity, which are superimposed on the bright spots, are diffractions contributed by PCL crystals. This can be explained in terms of geometric matching. At the interface, the lattice matching between epitaxial and substratal polymer is important and 10− 15% disregistry is considered as the upper limit.27 The orthorhombic unit cell parameters of PCL determined from X-ray data are a = 7.48 Å, b = 4.98 Å, and c = 17.26 Å.24 The crystallographic parameter of ⟨2100⟩ RGO spacing is 2.46 Å.26 In this case, the c-axis of PCL lamellae is almost 7 times that of the ⟨2100⟩ spacing of RGO (17.26 Å = 7(2.46 Å)), which means that perfect lattice matching exists between PCL crystals and RGO. As shown in Figure 4c, PCL (0014) reflection arcs overlap the RGO (2110) reflection spots, because the interatomic space along the PCL chains axis is 1.23 Å,28 which is the same as the RGO (2110) spacing (1.23 Å). The dspacing of PCL (310) is 2.23 Å, approaching the RGO (1110) spacing (2.13 Å),24 so the PCL (310) reflection arcs are superposed with RGO (1110) diffraction spots. These results imply that the PCL chains exist along the ⟨2100⟩ direction of the (0001) RGO surface forming edge-on lamellae. Except for the effect of lattice matching, the PCL molecular chains could easily adsorb onto the RGO surface and further crystallize due to its planar zigzag confirmation.24 Comparing with the results of RGO, GO has similar lattice parameters, but it cannot induce PCL epitaxial crystallization (Figure 4d). This is attributed to that the surface of GO has a larger content of oxygencontaining functional groups, resulting in that the PCL chains could not diffuse and grow further on the surface. Because the

RGO composites, an extra melting peak (Tm2) can be clearly seen at the relatively higher temperature of about 59.6 °C, which is not observed in neat PCL and PCL/GO composites. This peak is certainly attributed to the melting of crystals formed during the isothermal crystallization process. The reason may be that PCL chains epitaxially crystallize on the RGO surface, forming thicker lamellae. The enthalpy of fusion (ΔHm) for PCL, PCL/GO composites, and PCL/RGO composites is 54.1, 50.8, and 67.2 J/g, respectively. The reduced ΔHm of the PCL/GO composites could be ascribed to the interfacial interactions between GO and PCL molecular chains which reduced the chain flexibility and suppressed the crystallization process.23 For PCL/RGO composites, the ΔHm increases significantly. This result implied that RGO could enhance PCL crystallization in the annealing treatment. WAXD was used to study the effect of RGO on the crystal structure of PCL matrix in the composites. Figure 3 shows the

Figure 3. WAXD patterns of neat PCL, PCL/GO composites, and PCL/RGO composites.

WAXD patterns of neat PCL, PCL/GO composites, and PCL/ RGO composites. Three typical diffraction peaks at 2θ = 21.41, 22.03, and 23.71°, which correspond to (110), (111), and (200) crystal planes of PCL,24 are observed in both neat PCL and its composites. Zhang et al. reported that RGO did not change the crystal structure of PCL.6 This result indicated that the crystal structure of PCL could be kept with the incorporation of RGO and GO. To confirm the epitaxial crystallization of PCL on RGO, TEM and SAED were used to investigate the epitaxial

Figure 4. (a) SAED pattern of RGO. (b) TEM image of PCL crystals on RGO and (c) the corresponding SAED pattern of the circled region. (d) TEM image of PCL/GO composites and the corresponding SAED pattern of the circled region. 15826

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experimental Tc of PCL/RGO solution is higher than the Tc of neat PCL, homogeneous nucleation of PCL was prohibited, and all of the PCL crystals grown at this condition were induced by a heteroepitaxial mechanism. Tensile testing was employed for investigating the influence of epitaxial crystallization on the mechanical properties of a polymer matrix. Figure 5 shows the stress−strain curves of neat

Figure 6. Illustration of the structure of PCL/RGO composites. (a) Orientation of PCL chains on the RGO surface according to lattice matching. (b) Interlinking of PCL crystal lamellae and amorphous parts by RGO.

RGO or GO after destruction of the interfacial interaction between PCL and nanofiller. For PCL/RGO composites with annealing treatment, interfacial crystallization could improve the interfacial interaction between PCL and RGO, which is responsible for the improvement in mechanical properties of the composites.

Figure 5. Stress−strain curves of (a) neat PCL, (b) PCL/GO composites, and PCL/RGO composites: (c) without annealing treatment and (d) with annealing at 48 °C for 30 min.

PCL, PCL/GO composites, and PCL/RGO composites, and the inset shows the magnification of the yield point. Compared to neat PCL, yield strength increases from 16.1 to 18.9 MPa and Young’s modulus increases from 258.1 to 337.7 MPa for PCL/RGO composites without annealing treatment. The improvement in mechanical properties is attributed to the natural excellent strength of graphene.6,9,29 For the PCL/RGO composites with annealing treatment, the yield strength and Young’s modulus increase further to 21.6 MPa and 395.4 MPa, respectively. This enhancement is attributed to epitaxial interaction between RGO and PCL as the results exhibited above. For PCL/GO composites, yield strength and Young’s modulus are 18.1 MPa and 313.5 MPa, which are close to those of the PCL/RGO composites without annealing treatment. The functional groups on GO could decrease the mechanical properties of graphene.15 However, compared to RGO, the polar groups in GO may have interaction with the esters in PCL, which may be the reason why PCL/RGO and PCL/GO composites have similar mechanical properties. Petermann et al. has reported that epitaxial interfaces in semicrystalline polymers have strong adherence, which was responsible for the improvement in mechanical properties (Young’s modulus, fracture stress).30−32 It has been reported that effective interfacial interaction between the polymer and nanofiller could improve the mechanical properties of a matrix.33 Figure 6 shows an illustration of the improvement mechanism of PCL/RGO composites with annealing treatment. For these composites, PCL chains exist along the ⟨2100⟩ direction of the (0001) RGO surface because of the lattice matching between PCL and RGO (Figure 6a). RGO could act as nuclei and induce PCL lamellae grow on its surface. Therefore, the crystalline parts of the PCL are interlinked by RGO sheets (Figure 6b). The appearance of epitaxial crystallization favors a strong interfacial adhesion and high load transfer efficiency between PCL matrix and RGO during the stretching tension. For PCL/RGO or GO composites, fracture occurred following neck formation and limited drawing in the tensile direction. The PCL matrix was removed from

4. CONCLUSIONS To conclude, surface-induced epitaxial crystallization of PCL on RGO by melt and solution crystallization were achieved. PCL chains were parallel to the surface of RGO, resulting in thicker edge-on lamellae. The space lattice matching of polymer and RGO improves the interaction between RGO and PCL matrix. In contrast, GO cannot induce PCL epitaxial crystallization despite the lattice matching. Epitaxial crystallization enhanced the interfacial adhesion of RGO and PCL matrix, resulting in the improvement of mechanical properties. This research is expected to be helpful for understanding the application of epitaxial crystallization, and it offers a noncovalent method for improving the mechanical properties of polymer/RGO composites.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (51273210 and 51003117). REFERENCES

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