ARTICLE pubs.acs.org/IECR
Morphology, Crystallization Behavior, and Dynamic Mechanical Properties of Biodegradable Poly(ε-caprolactone)/Thermally Reduced Graphene Nanocomposites Jinbing Zhang† and Zhaobin Qiu*,† †
State Key Laboratory of Chemical Resource Engineering, Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, China ABSTRACT: Biodegradable poly(ε-caprolactone) (PCL)/thermally reduced graphene (TRG) nanocomposites were prepared via a solution mixing method at low TRG loadings in this work. Transmission electron microscopy and high resolution transmission electron microscopy observations reveal that a fine dispersion of TRG has been achieved throughout the PCL matrix. Scanning electron microscopy observation shows not only a nice dispersion of TRG but also a strong interfacial adhesion between TRG and the matrix, as evidenced by the presence of some TRG nanosheets embedded in the matrix. Nonisothermal melt crystallization behavior, isothermal melt crystallization kinetics, spherulitic morphology, and crystal structure of neat PCL and the PCL/TRG nanocomposites were studied in detail with various techniques. The experimental results indicate that both nonisothermal and isothermal melt crystallization of PCL have been enhanced significantly by the presence of TRG in the nanocomposites due to the heterogeneous nucleation effect; however, the crystallization mechanism and crystal structure of PCL do not change. Dynamic mechanical analysis study shows that the storage modulus of the nanocomposites has been greatly improved by about 203% and 292%, respectively, with incorporating only 0.5 and 2.0 wt % TRG at 80 °C as compared with neat PCL.
’ INTRODUCTION As a novel two-dimensional material, graphene has received considerable attention since its first discovery in 2004 because of its superior thermal, electronic, and mechanical properties.1,2 One of the most promising methods for scale production of graphene is the exfoliation and reduction of graphite oxide (GO), through which chemically reduced graphene (CRG) or thermally reduced graphene (TRG) may be achieved.36 As a kind of new nanofiller, graphene may significantly improve the physical properties of host polymers at very low loading.716 Macosko, Lee, and Ruoff have reviewed the fabrication, morphology, structure, and properties of graphene based polymer nanocomposites recently, respectively.1719 As a biodegradable and biocompatible polyester, poly(ε-caprolactone) (PCL) has recently attracted considerable attention from both academic and industrial viewpoints;2023 however, its wider practical application has been restricted by some disadvantages such as slow crystallization rate and poor mechanical properties. The incorporation of nanofillers, such as carbon nanotubes, layered silicate, polyhedral oligomeric silsesquioxanes, and graphite oxide has provided an effective method for improving the physical properties of PCL.2433 By this means, only a small amount of nanofillers are required to achieve a high performance of the PCL matrix, which is very important for its practical application. However, to our knowledge, biodegradable PCL/graphene nanocomposites have not been reported so far in the literature. In the present work, we prepared the PCL/TRG nanocomposites at low TRG loading via a solution mixing method and studied the influence of TRG on the nonisothermal melt crystallization behavior, isothermal melt crystallization kinetics, spherulitic r 2011 American Chemical Society
morphology, crystalline structure, and dynamic mechanical properties of PCL in the nanocomposites with various techniques in detail. The research reported herein is expected to be of great interest for a better understanding of the relationship between structure and properties of biodegradable polymer nanocomposites.
’ EXPERIMENTAL SECTION Materials. PCL (Mn = 80,000) was purchased from SigmaAldrich (Shanghai) Trading Co., Ltd. The natural graphite powder (about 48 μm) was kindly provided by Prof. Zhongzhen Yu of Beijing University of Chemical Technology. Sulfuric acid and nitric acid were purchased from Beijing Modern Eastern Fine Chemical Company. Hydrochloric acid and chloroform were purchased from Beijing Chemical Works. Potassium chlorate was purchased from Tianjin Fuchen Chemical Reagent Works. All reagents were used as received without further purification. Preparation of TRG. GO was prepared by modifying the method as described by Staudenmaier et al.34 Generally, natural graphite powder (5 g) was added into a 250 mL flask containing nitric acid (45 mL) and sulfuric acid (87.5 mL), the mixture was stirred for 15 min in an ice bath, and then potassium chlorate (55 g) was added into the solution slowly to avoid the temperature of the mixture from exceeding 20 °C. After all potassium chlorate was added, the mixture was then maintained at room Received: September 16, 2011 Accepted: November 8, 2011 Revised: November 7, 2011 Published: November 08, 2011 13885
dx.doi.org/10.1021/ie202132m | Ind. Eng. Chem. Res. 2011, 50, 13885–13891
Industrial & Engineering Chemistry Research
ARTICLE
Figure 1. (a) TEM, (b) HR-TEM, and (c) SEM images of PCL/TRG-2.0.
temperature for at least 96 h. After that, deionized water (2 L) was added into the solution until GO was precipitated completely. The mixture was first washed three times with 5% hydrochloric acid to remove the sulfuric ions. GO was further washed five times with deionized water until pH = 7 is reached, and then the suspension was centrifugalized with an Anke TDL-40B centrifuge at 3500 rpm for 20 min. At last, GO was obtained by sonicating with a JY98-IIID ultrasonic generator for 20 min, dried at room temperature for 2 days, and then further dried at 50 °C in vacuum for 3 days. The dried GO powder was quickly inserted into a muffle furnace preheated to 1050 °C and held for 30 s to obtain TRG nanosheets. Preparation of PCL/TRG Nanocomposites. PCL/TRG nanocomposites were prepared via a solution mixing method with chloroform being a mutual solvent. For the preparation of PCL/ TRG nanocomposite with 2.0 wt.% TRG, 0.06 g of TRG was added into 60 mL of chloroform, and then the mixture was sonicated with a KQ-700DE ultrasonic generator for 2 h at 630 W to afford a uniformly dispersed suspension. 2.94 g of PCL was also placed into 80 mL of chloroform and stirred for 1 h to dissolve PCL completely. The PCL solution was then put into the TRG suspension, followed by stirring vigorously for 2.5 h at room temperature. The mixture was poured into a dish to evaporate the solvent for one day. The obtained film was further dried in vacuum at 50 °C for 3 days to remove the solvent completely. In this way, PCL was mixed with the addition of 0.5 and 2.0 wt.% TRG, respectively. For brevity, they were abbreviated as PCL/TRG-0.5 and PCL/TRG-2.0, respectively, in this work from now on. Characterizations. Morphology of the PCL/TRG nanocomposites was examined using a Hitachi H-800 transmission electron microscopy (TEM) under an accelerating voltage of 200 kV. The as-prepared nanocomposites were cryogenically cut into ultrathin sections thinner than 100 nm for the TEM observations using a Leica EM FC6 ultramicrotome under cryogenic conditions (80 °C). In addition, high resolutionTEM (HR-TEM) (JEM-3010) was also used to show the dispersion of TRG in the PCL matrix. Scanning electron microscopy (SEM) observation of the PCL nanocomposites was observed with a Hitachi S-4700 scanning electron microscope at an accelerating voltage of 20 kV. All the samples were fractured in liquid nitrogen, and the fractured sections were coated with gold before examination. Thermal analysis was performed using a TA Instruments differential scanning calorimetry (DSC) Q100 with a Universal Analysis 2000 software. All operations were performed under nitrogen purge, and the weight of the samples varied between 4 and 6 mg. In the present work, both nonisothermal and isothermal melt crystallization behaviors of neat PCL and its nanocomposites were investigated. In the case of nonisothermal
melt crystallization, the sample was heated to 100 at 10 °C/min, held there for 3 min to erase any thermal history, and then cooled to 0 °C at different cooling rates ranging from 5 to 20 °C/min. The crystallization peak temperature (Tp) was obtained from the cooling traces. In the case of isothermal melt crystallization, the sample was heated to 100 at 10 °C/min, held for 3 min to erase any thermal history, and then cooled to the chosen crystallization temperature (Tc) at 60 °C/min. The sample was held at the chosen Tc for a period of time until the crystallization was complete. Tcs chosen in this work were from 37 to 51 °C. Wide angle X-ray diffraction (WAXD) patterns were recorded on a Rigaku D/Max 2500 VB2t/PC X-ray diffractometer at 40 kV and 200 mA at room temperature in an angle ranged from 5 to 40° at 5 o/min. The samples were first pressed into films with a thickness of 0.5 mm on a hot stage at 100 °C for about 3 min and then transferred into a vacuum oven at 44 °C for 56 h. Spherulitic morphology of neat PCL and its nanocomposites was observed with polarized optical microscopy (POM) (Olympus BX51) equipped with a Linkam THMS 600 hot stage. The samples were first heated to 100 at 40 °C/min, held there for 3 min to erase any thermal history, and then cooled to 45 at 60 °C/min. Dynamic mechanical analysis (DMA) was performed on the samples of 42 mm 7 mm 0.2 mm in size with a dynamic mechanical analyzer from Rheometric Scientific Company under tension film mode in a temperature range of 90 to 40 °C at a frequency of 1 Hz with a heating rate of 3 °C/min.
’ RESULTS AND DISCUSSION Morphology of PCL/TRG Nanocomposites. It is well-known that the homogeneous dispersion of graphene in the polymer matrix plays an important role in influencing the physical properties of biodegradable polymers. Figure 1a shows the TEM image of PCL/TRG-2.0, from which it can be seen that TRG nanosheets are randomly dispersed in the PCL matrix without any apparent aggregation. It should be noted that functionalization and defects may distort flat graphene into highly wrinkled sheets during thermal exfoliation, thereby resulting in the wrinkled nature of TRG.17 As shown in Figure 1a, curved thin sheets of TRG can be observed in the PCL matrix, indicative of the wrinkled nature of TRG. Figure 1b illustrates the HR-TEM image of PCL/TRG-2.0. It is clear from Figure 1b that TRG sheets show a characteristic worm-like morphology in the PCL matrix. It should be noted that TEM is relatively less useful than SEM in providing the information regarding the interfacial interaction between nanofillers and the polymer matrix, especially in carbon nanotubes based polymer nanocomposites.35 Therefore, SEM was further used in the present work to reveal the interfacial interaction between TRG nanosheets and the PCL matrix. Figure 1c shows the SEM image of PCL/TRG-2.0, which 13886
dx.doi.org/10.1021/ie202132m |Ind. Eng. Chem. Res. 2011, 50, 13885–13891
Industrial & Engineering Chemistry Research
ARTICLE
Figure 2. (a) DSC cooling traces at 10 °C/min and (b) variation of Tp with different cooling rate for neat PCL and its nanocomposites.
Figure 3. Development of relative crystallinity with crystallization time for neat PCL and its nanocomposites at different Tcs: (a) neat PCL and (b) PCL/TRG-0.5.
reveals not only a nice dispersion of TRG but also a strong interfacial adhesion between TRG and the matrix, as evidenced by the presence of some TRG nanosheets embedded in the matrix.17 Such strong interfacial adhesion is usually responsible for the significant enhancement of the mechanical properties of the polymer nanocomposites.35 Crystallization Behavior of PCL/TRG Nanocomposites. Much more attention should be paid to the crystallization study because it affects not only the crystalline structure and morphology of semicrystalline polymers but also the final physical properties and biodegradability of biodegradable polymers. Therefore, nonisothermal melt crystallization behavior, overall isothermal melt crystallization kinetics, spherulitic morphology, and crystal structure studies of neat PCL and its nanocomposites at low TRG contents were investigated in detail with various techniques for a better understanding of the structure and properties of biodegradable polymer nanocomposites. As introduced in the Experimental Section, nonisothermal melt crystallization behavior of neat PCL and its nanocomposites was studied first with DSC at different cooling rates. Figure 2a shows the DSC cooling traces for neat PCL and its two nanocomposites during the nonisothermal melt crystallization at 10 °C/min. From Figure 2a, Tp is around 25.1 °C for neat PCL; however, Tps shifts upward to 33.6 and 35.8 °C for PCL/ TRG-0.5 and PCL/TRG-2.0, respectively, indicating that the presence of TRG has enhanced significantly the nonisothermal melt crystallization of PCL in the nanocomposites relative to neat PCL. The effect of cooling rate on the variation of Tp for neat PCL and its two nanocomposites was further investigated. Figure 2b summarizes the variation of Tp with cooling rate for neat PCL and the PCL/TRG nanocomposites. The effects of
both cooling rate and the TRG loading on the variation of Tp may be obtained from Figure 2b. First, Tp shifts to low temperature range with increasing cooling for all the three samples despite the TRG loading, because the samples do not have enough time to crystallize at high temperature range at faster cooling rate. Second, Tps are higher in the nanocomposites than in neat PCL and increases with the TRG loading irrespective of cooling rate. Such variation of Tp with the TRG loading indicates that TRG actually acts as nucleating agent for the crystallization of PCL. It is of interest to note that the difference in Tps between neat PCL and PCL/TRG-0.5 is around 8 °C; indicating that incorporation of a very small amount of TRG may enhance significantly the nonisothermal melt crystallization of PCL. However, with further increasing the TRG loading from 0.5 to 2 wt %, the difference in Tps is only around 2 °C, suggesting that the enhancement effect of nonisothermal melt crystallization behavior of PCL is more pronounced at low TRG loading. The aforementioned results indicate that the upward shift in Tp is more apparent at low TRG loading because of the heterogeneous nucleating effect; however, such upward shift in Tp may level off at a plateau value with further increasing the TRG loading because of the saturation of the nucleating effect at higher TRG loading. Similar results have also been found for the isotactic polypropylene/carbon nanotubes nanocomposites.36 In the above section, nonisothermal melt crystallization behaviors of neat PCL and the PCL/TRG nanocomposites with different TRG loadings were studied with DSC at different cooling rates. In this section, isothermal melt crystallization kinetics of neat PCL and its nanocomposites was further studied in a wide range of crystallization temperatures. Figure 3 shows the variation of relative crystallinity with crystallization time for 13887
dx.doi.org/10.1021/ie202132m |Ind. Eng. Chem. Res. 2011, 50, 13885–13891
Industrial & Engineering Chemistry Research
ARTICLE
Figure 4. Avrami plots of neat PCL and its nanocomposites: (a) neat PCL and (b) PCL/TRG-0.5.
Table 1. Summary of Parameters of Isothermal Crystallization Kinetics for Neat PCL and Its Nanocomposites at Different Crystallization Temperatures samples neat PCL
Tc (°C) 37 39 41 43 45
PCL/TRG-0.5
k (minn)
2.39 3.14 102 2.39 1.05 10
2
2.38 1.50 10
3
2.52 2.45 10
4
2.48 3.84 10
5
3.64
2.74 101
5.75
1.74 101
13.23
7.56 102
23.45
4.26 102
52.21
1.91 102
2.34 8.02 10 2.47 9.51 103
2.51 5.67
3.98 101 1.76 101
47
2.42 9.40 104
15.36
6.51 102
32.61
3.06 102
92.24
1.08 102
1.94
5.15 101
3.85
2.59 101
10.06
9.94 102
23.5 71.93
4.25 102 1.39 102
51 43 45 47 49 51
2
t0.5 (min) 1/t0.5 (min1)
43 45 49
PCL/TRG-2.0
n
1.14 10
4
2.76 2.65 10
6
2.49 1.33 10
1
2.45 2.56 10
2
2.47 2.31 10
3
2.5
4
2.29 5.06 10 2.55 1.28 105
neat PCL and PCL/TRG-0.5 at different Tcs. All these curves illustrate the similar sigmoid shape; moreover, the crystallization time prolongs with increasing Tc for both neat PCL and PCL/ TRG-0.5. It is also of interest to find that the corresponding crystallization time for the PCL/TRG nanocomposites becomes shorter relative to neat PCL at the same crystallization temperature. For instance, it took around 138 min for neat PCL while only 14 and 8 min for PCL/TRG-0.5 and PCL/TRG-2.0 to finish the crystallization at 45 °C, which indicates that TRG acts as an effective nucleating agent and provides more sites for PCL to nucleate, thereby accelerating dramatically the crystallization process of PCL. The overall isothermal melt crystallization kinetics of both neat PCL and its nanocomposites was analyzed by the wellknown Avrami equation, which assumes that Xt develops as a function of t as follows 1 Xt ¼ expð kt n Þ
ð1Þ
where Xt is the relative crystallinity at time t, k is the crystallization rate constant, and n is the Avrami exponent depending on the nature of nucleation and growth geometry of the crystals.37,38 Figure 4 shows the Avrami plots for neat PCL and PCL/TRG-0.5 at different Tcs, from which the Avrami parameters n and k are obtained from the slopes and intercepts of the
Avrami plots, respectively. As shown in Figure 4, a series of almost parallel lines are obtained for both samples at various Tcs, indicating that the Avrami equation may describe well the isothermal melt crystallization process of neat PCL and its nanocomposites in this work. Table 1 lists the related crystallization kinetics parameters for the overall isothermal melt crystallization of neat PCL and its nanocomposites at various Tcs. From Table 1, the average value of n is around 2.4 for neat PCL and 2.5 for its nanocomposites. The slight variation of n indicates that the crystallization mechanism of PCL may not change within the investigated crystallization temperature range despite the addition of TRG in the nanocomposites. The values of n are between 2 and 3 for neat PCL and its nanocomposites at different Tcs, indicating that the crystallization mechanism of PCL may correspond to a threedimensional truncated growth with heterogeneous nucleation.39 It should be noted that it is not suitable to compare the overall crystallization rate directly from the k values with the unit of minn if n is not completely constant. Therefore, the crystallization half-life time (t0.5), the time required to achieve 50% of the final crystallinity of the samples, is an important parameter for the discussion of crystallization kinetics. Thus, the crystallization rate can be easily described by the reciprocal of t0.5. The value of t0.5 is calculated by the following equation t0:5 ¼
ln 2 k
1=n
ð2Þ
The values of t0.5 and 1/t0.5 are also summarized in Table 1. It is obvious from Table 1 that 1/t0.5 decreases with increasing Tc for all the three samples, indicative of a slow-down of the overall isothermal crystallization rate at higher Tc. The isothermal melt crystallization investigated in this work is dominated by the nucleation process because of low supercooling; moreover, the nucleation becomes more difficult with increasing Tc, thereby resulting in the reduction of the overall crystallization rate at higher Tc. Moreover, the 1/t0.5 values are greater in the nanocomposites than in neat PCL at a given Tc, indicating again that the isothermal melt crystallization of PCL has been accelerated by the presence of TRG in the nanocomposites. Both nonisothermal and isothermal melt crystallization studies suggest that TRG has enhanced the crystallization of PCL in the nanocomposites relative to neat PCL, which may be attributed to the efficient heterogeneous nucleating agent effect of TRG. Spherulitic morphology of neat PCL and its nanocomposites was further studied with POM. As an example, Figure 5 shows the spherulitic morphology of neat PCL and PCL/TRG-0.5 13888
dx.doi.org/10.1021/ie202132m |Ind. Eng. Chem. Res. 2011, 50, 13885–13891
Industrial & Engineering Chemistry Research
ARTICLE
Figure 5. Optical micrographs of spherulite morphology of neat PCL and its nanocomposite at 45 °C: (a) neat PCL and (b) PCL/TRG-0.5.
Figure 6. WAXD patterns for neat PCL and its nanocomposites.
Figure 7. DMA curves for neat PCL and its nanocomposites.
isothermally crystallized at 45 °C. It is clear from Figure 5a that the well-developed spherulites of neat PCL grew to a size of about 150 μm in diameter with relatively clear boundaries. However, as shown in Figure 5b, the size of PCL spherulites is apparently reduced to be less than 100 μm for PCL/TRG-0.5; moreover, the PCL spherulites boundaries become obscurer relative to neat PCL. In the case of PCL/TRG-2.0, the PCL spherulites become much smaller than those in PCL/TRG-0.5, because the higher TRG loading provides more sites for PCL to nucleate. For brevity, the result is not shown here. The number of PCL spherulites is significantly greater in the nanocomposites than in neat PCL, indicating that the nucleation density of PCL spherulites has been enhanced apparently in the PCL/TRG nanocomposites. In brief, TRG acts as an efficient nucleating agent during the crystallization process of PCL, thereby influencing both the spherulitic morphology and the overall crystallization process of PCL in the PCL/TRG nanocomposites. It is of interest to study the effect of TRG on the crystal structure of PCL in the PCL/TRG nanocomposites. Figure 6 illustrates the WAXD patterns of neat PCL and its two nanocomposites at low TRG loadings. Neat PCL presents three main diffraction peaks at 2θ = 21.27°, 21.87°, and 23.55°, corresponding to (110), (111), and (200) planes, respectively.20 Moreover, as shown in Figure 6, both the PCL/TRG nanocomposites show the similar diffraction peaks at 2θ = 21.35°, 21.93°, and 23.60°, respectively, for PCL/TRG-0.5 and 2θ = 21.34°, 21.96°, and 23.56°, respectively, for PCL/TRG-2.0, suggesting that the crystal structures of PCL remain unchanged despite the presence of TRG in the PCL/TRG nanocomposites.
Dynamic Mechanical Properties of PCL/TRG Nanocomposites. The addition of graphene may result in an improvement in
mechanical properties of polymer matrix because of the ultimate strength of graphene. Figure 7 shows the temperature dependence of storage modulus (E0 ) for both neat PCL and its nanocomposites. It can be seen from Figure 7 that relative to neat PCL, a considerable increase of E0 is found for the PCL/ TRG nanocomposites in the presence of TRG within a wide temperature range of 90 to 40 °C, indicating that the addition of TRG nanosheets induces a reinforcement effect. For example, the values of E0 at 80 °C increases from around 714 MPa for neat PCL to 2164 and 2801 MPa for PCL/TRG-0.5 and PCL/ TRG-2.0, respectively. Compared with neat PCL, the E0 values of the nanocomposites are significantly improved by about 203% and 292%, respectively, with incorporating only 0.5 and 2.0 wt % TRG. The improvement in storage modulus of the nanocomposites may be attributed to the fine dispersion of TRG and the strong interfacial interaction between TRG and the PCL matrix, indicating that TRG is an effective filler to enhance the mechanical properties of polymer.1719
’ CONCLUSIONS Biodegradable PCL/TRG nanocomposites have been prepared in this work via a solution mixing method at low TRG loadings. Both TEM and HR-TEM observations reveal a fine dispersion of TRG throughout the PCL matrix; moreover, SEM observation shows not only a homogeneous dispersion of TRG but also a strong interfacial adhesion between TRG and the 13889
dx.doi.org/10.1021/ie202132m |Ind. Eng. Chem. Res. 2011, 50, 13885–13891
Industrial & Engineering Chemistry Research matrix. The effect of TRG on the nonisothermal melt crystallization, isothermal melt crystallization kinetics, spherulitic morphology, and crystal structure of PCL in the PCL/TRG nanocomposites was investigated with DSC, POM, and WAXD in detail. The experimental results indicate that TRG act as a nucleation agent for the crystallization of PCL in the PCL/TRG nanocomposites; therefore, both nonisothermal and isothermal melt crystallization of PCL have been enhanced significantly in the nanocomposites relative to neat PCL. It should be noted that the presence of TRG does not change the crystallization mechanism and crystal structure of PCL. Mechanical properties of the nanocomposites were probed by dynamic mechanical analysis, showing that the storage modulus of the nanocomposites has been significantly improved by about 203% and 292%, respectively, with incorporating only 0.5 and 2.0 wt % TRG at 80 °C as compared with neat PCL.
’ AUTHOR INFORMATION Corresponding Author
*Fax: +86-10-64413161. E-mail:
[email protected].
’ REFERENCES (1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666–609. (2) Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6, 183–191. (3) Park, S.; Ruoff, R. S. Chemical Methods for the Production of Graphenes. Nat. Nanotechnol. 2009, 4, 217–224. (4) McAllister, M.; Li, J.; Adamson, D.; Schniepp, H.; Abdala, A.; Liu, J.; Herrera-Alonso, M.; Milius, D.; Car, R.; Prud’homme, R.; Aksay, I. Single Sheet Functionalized Graphene by Oxidation and Thermal Expansion of Graphite. Chem. Mater. 2007, 19, 4396–4404. (5) Dreyer, D.; Park, S.; Bielawski, C.; Ruoff, R. The Chemistry of Graphene Oxide. Chem. Soc. Rev. 2010, 39, 228–240. (6) Allen, M.; Tung, V.; Kaner, R. Honeycomb Carbon: A Review of Graphene. Chem. Rev. 2010, 110, 132–145. (7) Ramanathan, T.; Abdala, A.; Stankovich, S.; Dikin, D.; HerreraAlonso, M.; Piner, R.; Adamson, D.; Schniepp, H.; Chen, X.; Ruoff, R.; Nguyen, S.; Aksay, I.; Prud’homme, R.; Brinson, L. Functionalized Graphene Sheets for Polymer Nanocomposites. Nat. Nanotechnol. 2008, 3, 327–331. (8) Liang, J.; Huang, Y.; Zhang, L.; Wang, Y.; Ma, Y.; Guo, T.; Chen, Y. Molecular-level Dispersion of Graphene into Poly(vinyl alcohol) and Effective Reinforcement of Their Nanocomposites. Adv. Funct. Mater. 2009, 19, 1–6. (9) Kim, H.; Miura, Y.; Macosko, C. Graphene/Polyurethane Nanocomposites for Improved Gas Barrier and Electrical Conductivity. Chem. Mater. 2010, 22, 3441–3450. (10) Zhao, X.; Zhang, Q.; Chen, D. Enhanced mechanical properties of graphene-based poly(vinyl alcohol) composites. Macromolecules 2010, 43, 2357–2363. (11) Ansari, S.; Giannelis, E. Functionalized graphene sheet poly(vinylidene fluoride) conductive nanocomposites. J. Polym. Sci. Polym. Phys. 2009, 47, 888–897. (12) Yoonessi, M.; Gaier, J. Highly Conductive Multifunctional Graphene Polycarbonate Nanocomposites. ACS Nano 2010, 4, 7211–7220. (13) Xu, Z.; Gao, C. In stiu Polymerization Approach to GrapheneReinforced Nylon-6 Composites. Macromolecules 2010, 43, 6716–6723. (14) Nguyen, D.; Lee, Y.; Raghu, A.; Jeong, H.; Shin, C.; Kim, B. Morphological and Physical Properties of a Thermoplastic Polyurethane Reinforced with Functionalized Graphene Sheet. Polym. Int. 2009, 58, 412–417.
ARTICLE
(15) Kim, I.; Jeong, Y. Polylactide/Exfoliated Graphite Nanocomposites with Enhanced Thermal Stability, Mechanical Modulus, and Electrical conductivity. J. Polym. Sci. Polym. Phys. 2010, 48, 850–858. (16) Xu, J.; Chen, T.; Yang, C.; Li, Z.; Mao, Y.; Zeng, B.; Hsiao, B. Isothermal Crystallization of Poly(L-lactide) Induced by Graphene Nanosheets and Carbon Nanotubes: A Comparative Study. Macromolecules 2010, 43, 5000–5008. (17) Kim, H.; Abdala, A.; Macosko, C. Graphene/Polymer Nanocomposites. Macromolecules 2010, 43, 6515–6530. (18) Kuilla, T.; Bhadra, S.; Yao, D.; Kim, N.; Bose, S.; Lee, J. Recent Advances in Graphene Based Polymer Composites. Prog. Polym. Sci. 2010, 35, 1350–1375. (19) Potts, J.; Dreyer, D.; Bielawski, C.; Ruoff, R. Graphene-based Polymer Nanocomposites. Polymer 2011, 52, 5–25. (20) Eastmond, G. Poly(ε-caprolactone) Blends. Adv. Polym. Sci. 1999, 149, 59–223. (21) Qiu, Z.; Ikehara, T.; Nishi, T. Miscibility and Crystallization of Poly(ethylene oxide) and Poly(ε-caprolactone) Blends. Polymer 2003, 44, 3101–3106. (22) Qiu, Z.; Komura, M.; Ikehara, T.; Nishi, T. Miscibility and Crystallization Behaviour of Biodegradable Blends of Two Aliphatic Polyesters. Poly(butylene succinate) and Poly(ε-caprolactone) Blends. Polymer 2003, 44, 7749–7756. (23) Qiu, Z.; Yang, W.; Ikehara, T.; Nishi, T. Miscibility and Crystallization Behavior of Biodegradable Blends of Two Aliphatic Polyesters. Poly(3-hydroxylbutyrate-co-hydroxyvalerate) and Poly(ε-caprolactone). Polymer 2005, 46, 11814–11819. (24) Qiu, Z.; Wang, H.; Xu, C. Crystallization, Mechanical Properties, and Controlled Enzymatic Degradation of Biodegradable Poly(εcaprolactone)/Multi-Walled Carbon Nanotubes Nanocomposites. J. Nanosci. Nanotechnol. 2011, 11, 7884–7893. (25) Zeng, H.; Gao, C.; Yan, D. Poly(ε-caprolactone)-Functionalized Carbon Nanotubes and Their Biodegradation Properties. Adv. Funct. Mater. 2006, 16, 812–818. (26) Chen, E.; Wu, T. Isothermal Crystallization Kinetics and Thermal Behavior of Poly(ε-caprolactone)/Multi-walled Carbon Nanotube Composites. Polym. Degrad. Stab. 2007, 92, 1009–1015. (27) Chrissafis, K.; Antoniadis, G.; Paraskevopoulos, K.; Vassiliou, A.; Bikiaris, D. Comparative Study of the Effect of Different Nanoparticles on the Mechanical Properties and Thermal Degradation Mechanism of in Situ Prepared Poly(ε-caprolactone) Nanocomposites. Compos. Sci. Technol. 2007, 67, 2165–2174. (28) Liu, Y.; Yang, X.; Zhang, W.; Zheng, S. Star-shaped Poly(εcaprolactone) with Polyhedral Oligomeric Silsesquioxane Core. Polymer 2006, 47, 6814–6825. (29) Pan, H.; Yu, J.; Qiu, Z. Crystallization and Morphology Studies of Biodegradable Poly(ε-caprolactone)/Polyhedral Oligomeric Silsesquioxanes Nanocomposites. Polym. Eng. Sci. 2011, 51, 2159–2165. (30) Hua, L.; Kai, W.; Inoue, Y. Synthesis and Characterization of Poly(ε-caprolactone)-Graphite Oxide Composites. J. Appl. Polym. Sci. 2007, 106, 1880–1884. (31) Hua, L.; Kai, W.; Inoue, Y. Crystallization Behavior of Poly(εcaprolactone)/Graphite Oxide Composites. J. Appl. Polym. Sci. 2007, 106, 4225–4232. (32) Kai, W.; Hirota, Y.; Hua, L.; Inoue, Y. Thermal and Mechanical Properties of a Poly(ε-caprolactone)/Graphite Oxide Composite. J. Appl. Polym. Sci. 2008, 107, 1395–1400. (33) Lei, H.; Kai, W.; Liang, Z.; Inoue, Y. Polyester/OrganoGraphite Oxide Composite: Effect of Organically Surface Modified Layered Graphite on Structure and Physical Properties of Poly(εcaprolactone). J. Polym. Sci. Polym. Phys 2010, 48, 294–301. (34) Staudenmaier, L. Verfahren Zur Darstellung Der Graphits€aure. Ber. Dtsch. Chem. Ges. 1898, 34, 1481–1487. (35) Liu, T.; Phang, I.; Shen, L.; Chow, S.; Zhang, W. Morphology and Mechanical Properties of Multiwalled Carbon Nanotubes Reinforced Nylon-6 Composites. Macromolecules 2004, 37, 7214–7222. (36) Lu, K.; Grossiord, N.; Koning, C.; Miltner, H.; Van Mele, B.; Loos, J. Carbon Nanotube/Isotactic Polypropylene Composites 13890
dx.doi.org/10.1021/ie202132m |Ind. Eng. Chem. Res. 2011, 50, 13885–13891
Industrial & Engineering Chemistry Research
ARTICLE
Prepared by Latex Technology: Morphology Analysis of CNT-Induced Nucleation. Macromolecules 2008, 41, 8081–8085. (37) Avrami, M. Kinetics of Phase Change. II Transformation-Time Relations for Random Distribution of Nuclei. J. Chem. Phys. 1940, 8, 212–224. (38) Avrami, M. Granulation, Phase Change, and Microstructure Kinetics of Phase Change. III. J. Chem. Phys. 1941, 9, 177–184. (39) Wunderlich, B. Macromolecular Physics; Academic Press: New York, 1976; Vol. 2, p 147.
13891
dx.doi.org/10.1021/ie202132m |Ind. Eng. Chem. Res. 2011, 50, 13885–13891
