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TrGO nanocomposites prepared by in situ ring-opening polymerization of cyclic butylene terephthalate
Polymer 54 (2013) 1603e1611
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Effect of thermally reduced graphite oxide (TrGO) on the polymerization kinetics of poly(butylene terephthalate) (pCBT)/TrGO nanocomposites prepared by in situ ring-opening polymerization of cyclic butylene terephthalate Hongliang Chen, Chongwen Huang, Wei Yu*, Chixing Zhou Advanced Rheology Institute, Department of Polymer Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 October 2012 Received in revised form 16 January 2013 Accepted 22 January 2013 Available online 29 January 2013
Thermally reduced graphite oxide (TrGO) was prepared by thermal exfoliation and reduction of highly oxidized graphite. The pCBT/TrGO nanocomposites were prepared by in situ ring-opening polymerization (ROP) of cyclic butylene terephthalate (CBT). The polymerization kinetics of pCBT/TrGO was monitored by dynamic time sweep in a parallel-plate rheometer. It was found that the increasing TrGO content depressed the rate and degree of CBT polymerization, which is ascribed to the reaction between the growing pCBT chains terminated with carboxyl groups and TrGO surface groups such as hydroxyl and epoxy groups at the initial polymerization stage. The grafted pCBT chains were confirmed by X-ray photoelectron spectroscopy (XPS), nuclear magnetic resonance (NMR) and thermogravimetric analysis (TGA) measurements, and the grafting content was up to 53 wt%. Small amplitude oscillation shear (SAOS) was applied to investigate the rheological properties of pCBT/TrGO and the critical loading to form percolation network was determined as 0.47 vol%, which confirmed the good dispersion of TrGO in matrix. The grafting reaction was also justified from nonlinear rheology and the fractal dimension analysis. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Cyclic butylene terephthalate Thermally reduced graphite oxide Ring-opening polymerization
1. Introduction Graphene, a monolayer of sp2-hybridized carbon atoms arranged in a two-dimensional lattice, has attracted tremendous attention in recent years because of its exceptional electronic [1], thermal [2] and mechanical [3] properties and the discovery of methods for their production. There has been several techniques for preparing graphene nanosheets, including mechanical cleavage of graphite [4], chemical vapor deposition [5e7], epitaxial growth on SiC [8e10], alkali metals intercalation and expansion [11], chemical reduction of graphite oxide (GO) [12e15] and thermal exfoliation of GO [16,17] and so on. Among them chemical reduction and thermal exfoliation are suitable for large scale production. One of the most promising applications of this material is in polymer nanocomposites. Polymer nanocomposites show substantial property enhancements at much lower loadings than polymer composites with conventional micron-scale fillers (such as glass or carbon fibers), which ultimately results in lower component weight. Moreover, the multifunctional property enhancements
* Corresponding author. Tel.: þ86 21 5474 3275. E-mail address: [email protected] (W. Yu). 0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2013.01.036
made it possible to create new applications of polymers nanocomposites. As far as graphene is concerned, its nanocomposites can provide combined benefits of both layered silicates and CNT. Unlike layered silicates, it has exceptional electrical and thermal transport properties. Unlike CNT, it can be derived from natural graphite at smaller cost. Among the possible routes of preparation of polymer-graphene nanocomposites reviewed by Macosko et al. [18], melt blending is the most economically attractive and scalable method. However, melt blending also presents several severe problems due to the high melt viscosity of most polymers, such as impregnation, flow of resin and removal of bubbles. These problems can be overcome by using cyclic butylene terephthalate oligomers (CBT) which has extremely low melt viscosity before polymerization. CBT can quickly polymerized to the corresponding linear poly(butylene terephthalate) (referred to as pCBT to distinguish from conventional poly(butylene terephthalate) (PBT) obtained by polycondensation reaction) via entropically driven ring-opening polymerization (ROP) in the presence of suitable initiator [19]. Because the ROP is entropically driven, the reaction is athermal and no chemical emission. Unlike conventional thermoplastic resins, cyclic oligomers have an initial water-like melt viscosity prior to polymerization and thus can be employed in processes, such as resin
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transfer molding (RTM), which are typically used for thermosets. Due to the athermal characteristic of polymerization, there are no hot spots in the mold which lead to thermal degradation and no byproducts need to remove during the molding process. All of these favorable advantages allow CBT to be a promising material to produce pCBT nanocomposites with perfect impregnation and excellent dispersion of fillers [20e22] using various processing methods [23,24]. Research on in situ polymerized graphene nanocomposites should not only analyze the effect of the fillers in the polymer matrix morphology and final properties but also in the polymerization reaction or curing reaction. A study on this subject has revealed a decrease in the rate of the polymerization of PDMS foams by the addition of thermally exfoliated graphene and a change in the dynamic evolution of the reaction as compared to the effect of CNT [25]. Changes in molecular weight of thermoplastic polyurethane (TPU) due to the presence of graphene have also been reported [26]. However, the fundamental mechanisms for graphene effect on polymerization have not been completely demonstrated yet. To the best of our knowledge, there have been no publications on the PBT/graphene nanocomposites, let alone the pCBT/thermally reduced graphite oxide (TrGO) nanocomposites prepared by in situ ring-opening polymerization. In our previous work, we successfully used the rheological method to investigate the polymerization kinetics of CBT [27]. In this study, rheological method was still used to monitor the polymerization process to determine the TrGO effect on polymerization kinetics. A possible mechanism was proposed to illustrate the decreased rate and degree of polymerization. It is also believed that the rheological properties are sensitive to the internal microstructures of nanocomposites, including the dispersion state of graphene nanosheets and its confinement on the motion of polymer chains. The response of nanocomposites to linear and nonlinear deformation were both investigated, which can provide important information for understanding the relationship of its rheological behavior and the nature of its microstructures. 2. Experimental 2.1. Materials Cyclic butylene terephthalate oligomers (CBT160, Mw ¼ 220n, n ¼ 2e7) in granule form obtained from Cyclics company. CBT160 already contained a pre-mixed initiator butylchlorotin dihydroxide (Fastcat 4101, Mw ¼ 245.29 g/mol, C4H9Sn(OH)2Cl) which could initiate CBT to polymerize into high molecular weight poly(butylene terephthalate) termed as pCBT. Hydrophilic fumed silica nanoparticles (SiO2) with specific surface area 160 20 m2/g and average particle diameter 30 nm was purchased from Hang Zhou Wan Jing New Material Co., Ltd. Multiwalled carbon tubes (CNT) with diameter 10e20 nm and length 5e15 um were purchased from the Shenzhen Nanotech Port Co., Ltd. Graphite powder (30 mm), sulfuric acid (98%), potassium persulfate, phosphorus pentoxide, potassium permanganate, hydrogen peroxide (30%) and hydrochloric acid (37%) and other solvents were obtained from Sinopharm Chemical Reagent Co., Ltd. 2.2. Preparation of thermally reduced graphite oxide (TrGO) Graphite oxide (GO) was synthesized from natural graphite powder by a modified Hummers method [28,29]. The synthesis procedure consisted of two-step oxidation. In the peroxidation step, graphite powder (3 g) was put into an 80 C solution of concentrated H2SO4 (12 mL), K2S2O8 (2.5 g) and P2O5 (2.5 g). The mixture was kept at 80 C for 4.5 h and then cooled down to room
temperature, diluted with 0.5 L of deionized water and left overnight. Then, the mixture was filtered and washed with deionized water using a 0.2 micron Nylon Millipore filter to remove the residual acid. The product was dried under ambient condition overnight. In the second oxidation step, the pretreated graphite powder was put into cold (0 C) concentrated H2SO4 (120 mL). Then, KMnO4 (15 g) was added gradually under stirring and the temperature of the mixture was kept to be below 20 C. Successively, the mixture was stirred at 35 C for 2 h, and then diluted with deionized water (250 mL). The addition of water was carried out in an ice bath to keep the temperature below 50 C. After adding all of the 250 mL of deionized water, the mixture was stirred for 2 h, and then additional 0.7 L of deionized water was added. Shortly after the dilution with 0.7 L of water, 20 mL of 30% H2O2 was added to the mixture, and the color of mixture changed into brilliant yellow along with bubbling. The mixture was filtered and washed with 10% HCl aqueous solution (1 L) to remove metal ions. The precipitate mixture was repeated washing and centrifugation successively with deionized water at least three centrifugation cycles until the decantate became neutral. The GO slurry was dried at 40 C for one week. Finally, TrGO was obtained by the thermal treatment of GO at 550 C for 15 min under argon atmosphere. 2.3. Preparation of pCBT/TrGO nanocomposites A series of pCBT/TrGO nanocomposites were prepared by solution blending. 20 g CBT160 were added into 500 mL THF and stirred for 24 h to obtain a suspension. Varying amount of TrGO was dispersed in the CBT160 suspension and the mixture was sonicated for 30 min. Most of the THF was evaporated at room temperature with magnetic stirring, and then, the final mixture was dried in vacuum at 80 C for 48 h to remove trace amount of THF. When the mixture was heated to more than 200 C, the CBT will polymerize to pCBT and the nanocomposites were obtained. 2.4. Characterization of graphite, GO, TrGO and pCBT/TrGO The structural order of graphite, GO and TrGO was investigated by adopting X-ray diffraction method. X-ray diffraction patterns were obtained with a Rigaku D/max-2200/PC diffractometer with Cu Ka radiation at a scanning rate of 4 /min. X-ray photoelectron spectroscopy (Kratos Axis Ultra DLD) was also used to evaluate atomic compositions in the TrGO. In order to investigate the dispersion of TrGO in pCBT/TrGO, Transmission electron microscopy (TEM) was performed with a JEM-2100 electron microscope operating at an accelerating voltage of 200 kV. Thermogravimetric analysis (TGA) was performed on a TA Instruments Q5000IR to investigate the thermal stability of TrGO, neat pCBT and TrGO-g-pCBT. Samples of about 10 mg were heated from room temperature to 700 C at a rate of 20 C/min under a nitrogen atmosphere. 2.5. Rheological measurements Rheological measurements were all performed on a rotational rheometer (Gemini 200HR, Bohlin Instruments, UK) equipped with a parallel-plate geometry using 25 mm diameter plates. The dried CBT mixture was directly loaded onto the plate and tested without compressing into circular sheet (thickness z 1 mm) in order to minimize the influence of pre-polymerization. Since the low viscosity of melt CBT, there were no air bubbles in the samples after polymerization. After sample loading, a dynamic time sweep was conducted at 1 rad/s at 230 C using small amplitude strain until no change in complex viscosity was observed which indicated the completed polymerization. Subsequently, using a dynamic strain sweep at 6.28 rad/s, the critical strain, gcrit, where G0 drops to 90% of
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its limiting low strain value was recorded. Small amplitude oscillatory shear frequency sweeps were carried out at frequency of 0.1e100 rad/s at 230 C with 1% strain for all products. It is noted that all rheological measurements were performed in the linear viscoelastic regime for all samples. 3. Results and discussion 3.1. Characterization Fig. 1 shows the XRD spectrum of pristine graphite, GO and TrGO. For graphite, the strong and sharp diffraction peak was detected at 2q ¼ 26.7, which corresponds to the interlayer spacing of 0.33 nm. The peak of graphite disappeared after oxidation and instead a new peak at 2q ¼ 9.4 appeared, indicating the highly oxidization of graphite [17,26,30,31]. After thermal reduction of GO, there was no apparent diffraction peak detected, which means the periodic structure of GO was eliminated and graphene nanosheets were formed. XPS was employed to analyze atomic compositions in the TrGO. The C 1s XPS spectrum of TrGO (Fig. 2) confirmed the existence of oxygen functionalities on the TrGO surface. There are four kinds of carbon atoms that correspondingly exist in different functional groups: the non-oxygenated ring carbon atoms (284.8 eV), the carbon atoms in carboneoxygen bonds (286.2 eV, corresponding to hydroxyl and epoxy groups), the carbonyl carbon (287.6 eV), and the carboxylate carbon (289.0 eV, corresponding to carboxyl group) [12,32e34]. After the thermal reduction at 550 C for 15 min, the carbon content of TrGO reaches 83.4 wt%. While the carbon content of graphite is 97.8%, which is much higher than TrGO. TEM was used to further evaluate the exfoliation and dispersion quality of TrGO within pCBT. From the TEM image of the pCBT/TrGO nanocomposites with 3.0 wt% TrGO (Fig. 3a), it can be seen that well exfoliated graphene nanosheets are evenly distributed in the matrix and there are almost no large agglomerates. The good dispersion of TrGO is ascribed to the interaction between the oxygen and hydroxyl functional groups on the surface of TrGO and the polar groups of pCBT [31,35]. In addition, a compact continues network throughout the matrix is also observed (Fig. 3a). The high resolution TEM image (Fig. 3b) reveals that graphene exists as single-layer or few-layers nanosheets. The thickness of TrGO sheets from TEM images is nearly 2 nm for thinner one and 4 nm for thicker one prepared in this study. It is also seen that graphene sheets are highly winkled, which are in agreement with other observations [26,36,37]. This is due to the structural defects caused by the loss of CO2 during the thermally reduced treatment process
Fig. 1. X-ray scattering intensity profiles of graphite, GO and TrGO.
Fig. 2. C 1s XPS spectrum of TrGO.
and the thermodynamically instability of perfect two-dimensional graphene crystal [38]. High magnification image (Fig. 3b) indicates that the network is composed of abundant thin stacks of a few sheets of monolayer graphene (black curly thin lines), which are bridged by the crumpled or overlapped graphene sheets (slightly thicker lines) (Fig. 3b). These wrinkled and fewer overlapped graphene sheets can effectively link individual graphene sheets and carry high density of current, resulting in high electrical conductivity [39]. In addition to the TEM images of the nanocomposites, XRD will also be useful to evaluate the dispersion of TrGO sheets in pCBT matrix as shown in Fig. 4. It is easy to note that there are no strong and sharp diffraction peaks for all the samples, which indicates that the CBT polymerization almost has no effect on the dispersion and aggregation of TrGO sheets. It also confirms the good dispersion of TrGO in the matrix. 3.2. Effect of TrGO on the polymerization kinetics of CBT Brunelle [40] has discussed in detail the mechanism of polymerization of CBT, which involves initiation to form an active chain end, then followed by propagation reaction continuing until all of the cyclic oligomers are depleted and the ring-chain equilibration becomes degenerate as shown in Fig. 5. During the polymerization, the initiator becomes built into the polymer, and it is not terminated unless quenched, which indicates that the ultimate molecular weight depends on the concentration of initiator. Since the polymerization of CBT is athermal, the conventional method such as DSC cannot be used to investigate the reaction dynamics. Polymerization study was carried out under isothermal conditions in the rheometer. Using the complex viscosity and modulus of the melt as a measure of the extent of polymerization allows to monitor the changes in the molecular weight, or degree of polymerization, as a function of time [27]. Dynamic time sweep was conducted at 1 rad/s at 230 C using small strain amplitude to investigate the polymerization of CBT with different TrGO contents. The complex viscosity increased sharply with time and ultimately reached a constant value which implied that the polymerization was completed. It was also found that the modulus evolved in a similar trend like complex viscosity during the polymerization process of CBT. Therefore, it’s reasonable to use the rheological data such as complex viscosity as an indirect tool to characterize the polymerization of CBT with TrGO. The function of (h(t) h0)/(hN h0) was used to denote the degree of polymerization, where h(t) is the complex viscosity as a function of time, hN is the ultimate complex viscosity when polymerization is finished, and h0 is the initial
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Fig. 3. TEM images of pCBT/TrGO with 3 wt% TrGO under low (a) and (b) high magnifications.
complex viscosity before polymerization. The slope of (h(t) h0)/ (hN h0)versus time can be used to represent the polymerization rate. The evolutions of (h(t) h0)/(hN h0) with time for samples with different TrGO contents are shown in Fig. 6. With the increase in TrGO content, the polymerization rate decreases sharply and it takes longer time to finish whole reaction. The time for polymerization to finish (tend) is about 50 min in CBT/3 wt% TrGO system, which is over three times longer than that of pure CBT. The effect of TrGO content on the reaction time (tend) is shown in the inset of Fig. 6. At low concentration of TrGO, the reaction time is only slightly increased, while the reaction is retarded greatly at higher TrGO content. The transition appears at about 0.7e0.8% of TrGO content. In order to interpret how the TrGO inhibits the reaction, different fillers, such as graphite, carbon nanotube and SiO2, were selected to prepare composites via solution blending used for pCBT/TrGO depicted in Experimental part. The dynamic time sweeps were also conducted at the same condition as TrGO system to compare the effect of different fillers on the polymerization of CBT. The relative viscosities for different systems are shown in Fig. 7. It is seen that the fillers have different impact on the CBT polymerization. Samples with graphite and CNT have similar polymerization rate as pure CBT, which only needs about 15 min to complete the whole reaction. The CNT has no effect on the CBT polymerization, although it has large specific surface area with respect to graphite. Note that the surfaces of graphite and CNT are clean and there have almost no specific functional groups on them. Those with TrGO and SiO2 extend the polymerization time greatly. The sample with SiO2 has the smallest polymerization rate. In contrast to all the others, SiO2 system has a long induction time (over 500 s) before the increase in complex viscosity. It is important to know that the SiO2 nanoparticles have large amount hydroxyl functional groups on the surface. C 1s XPS spectrum of TrGO (Fig. 2) quantified the amount of oxygen on the
surface and also identified the types of carboneoxygen bonds. So it is possible that the functional groups on the fillers surface affect the CBT polymerization. It was reported that the chain extension reaction between (growing) pCBT chains and epoxy resin happened by esterification of carboxyl end groups of polyester and glycidyl functional groups of the epoxy resin [41]. Furthermore, Tripathy et al. synthesized a series of copolyesters of CBT and tetrabromobisphenol A (TBBPA), bisphenol A diglycidyl ether (BPADGE) and Carbinol PDMS respectively based on the reaction between the carboxyl end groups of pCBT chains and the epoxy groups and hydroxyl groups of TBBPA, BPADGE and Carbinol PDMS [42]. These literature reports confirmed the assumption that the epoxy and hydroxyl groups on the TrGO surface affect the polymerization of CBT. The inhibition of SiO2 to the polymerization of CBT in Fig. 7 is probably attributed to the large amount of hydroxyl groups on the surface. To further confirm our assumption, the effect of functional groups content on polymerization was determined. According to the above analysis, the CBT should have higher polymerization rate incorporation with TrGO with lower epoxy and hydroxyl groups content on the surface. With additional thermal treatment at high temperature at 600 C and 700 C for 30 min, respectively, the epoxy and hydroxyl groups content of TrGO should decrease as compared to the one without treatment [16,32]. According to the C 1s XPS spectrum results, the oxygen contents of TrGO decreased down to 11.6 and 7.2 wt%, respectively. Fig. 8 shows the polymerization of CBT with 2 wt% TrGO treated with different high temperature. Compared to the sample with TrGO without additional thermal treatment, sample with TrGO treated at 600 C can finish reaction within 30 min, and the one treated at 700 C has similar polymerization rate as pure CBT. These results suggest the important role of surface functional group on TrGO nanosheets in the ring-open polymerization of CBT.
Fig. 4. X-ray scattering intensity profiles of pCBT and pCBT/TrGO nanocomposites.
Fig. 5. Mechanism for polymerization of CBT160.
H. Chen et al. / Polymer 54 (2013) 1603e1611
Fig. 6. Variation of (h(t) h0)/(hN h0) with time for CBT polymerization at 230 C with varying amount TrGO. The effect of TrGO content on the end time of CBT polymerization is shown in the inset.
According to the above investigation, we proposed a mechanism for effect of TrGO on the polymerization of CBT, which is shown in Fig. 9. The mechanism of the polymerization of CBT is analogous to the conventional ROP, with initiation and propagation steps. In the presence of initiator, a carbonyl of CBT is activated via complexation, and then transferring an alkoxide ligand to form an active species, where one chain end is terminated with a functional group originating from the initiator and the other is terminated with carboxyl group. Propagation is thought to proceed by coordination of the monomer to the active species, followed by insertion of the monomer into the metaleoxygen bond by rearrangement of the electrons. The process is repeated as many more monomers are successively added to propagate the active species to form linear polymers. As far as CBT polymerization with TrGO is concerned, many growing pCBT chains with low molecular weight terminated with carboxyl group may react with epoxy and hydroxyl groups of TrGO at the initial stage and as a result some pCBT chains are possibly grafted to TrGO sheets. As the reaction continues, the pCBT which is in the melt matrix at early stage may still have the chance to react with the epoxy and hydroxyl groups remained on the surface of TrGO. The initiator and active species remained in the matrix can proceed to form high molecular weight pCBT as pure CBT sample. While the polymerization reactivity of pCBT grafted to the TrGO decreased sharply due to the restriction of TrGO which is very large for a single pCBT chain. The TrGO sheets may restrict the motion of pCBT chains to lower the reactivity of the active species. Such effect seems to be more significant when the TrGO content is
Fig. 7. Variation of (h(t) h0)/(hN h0) with time for CBT polymerization at 230 C with 2 wt% graphite, TrGO, CNT, SiO2 respectively.
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Fig. 8. Variation of (h(t) h0)/(hN h0) with time for CBT polymerization with 2 wt% TrGO with additional thermal treatment at different temperature.
over 0.7e0.8%. The ultimate complex viscosity of pCBT with TrGO varying from 0.1 to 3 wt% is smaller than that of pure pCBT sample since the pCBT chains grafted to the TrGO surface has lower molecular weight than the pCBT in the matrix. Therefore, the rationality of such mechanism relies on the existence of the grafted pCBT on the TrGO surfaces. To prove the graft of pCBT on TrGO sheet, the pCBT/TrGO nanocomposites with 2 wt% TrGO were dissolved in a mixture of phenol/tetrachloroethane at 110 C for 1 h and recovered by filtration and then washed with methanol. The dispersing, filtering, and washing cycle was repeated several times to remove any possible ungraft polymer or cyclic oligomers. The final black samples were named as TrGO-g-pCBT. It is found that the TrGO-g-pCBT can be well dispersed in trifluoroacetic acid. The suspension was stable and uniform. While for the pure TrGO, they settled at the bottom of the bottle as shown in Fig. 10. It implies that certain modification of
Fig. 9. Mechanism for effect of TrGO on polymerization of CBT.
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Fig. 12. 1H NMR spectrum of TrGO-g-pCBT in CF3COOD.
Fig. 10. The comparison of TrGO (a) and TrGO-g-pCBT (b) in CF3COOD.
TrGO sheet might happen after the polymerization of pCBT, possibly due to the graft of pCBT chains onto the TrGO surface. Firstly, XPS was employed to determine the surface composition of TrGO-g-pCBT. As shown in Fig. 11, the binding energy for the Sn 3d is clearly displayed in the spectrum of TrGO-g-pCBT, indicating that the pCBT chains were covalently attached to the surface of TrGO. Since the content of Sn is very low, the peak of Sn 3d is very weak compared to O 1s and C 1s. Moreover, NMR was chosen to confirm the molecular structure. The 1H NMR spectrum of TrGO-gpCBT was obtained using deuterated trifluoroacetic acid as a solvent. In Fig. 12, the corresponding proton peaks of the graft pCBT chains appeared in the 1H NMR spectrum of TrGO-g-pCBT, peaks at 8.15 ppm are due to aromatic protons, and 4.54 and 2.07 ppm are assigned to ester and methylene groups, respectively, and the peaks ration is almost 1:1:1, which indicated that the pCBT chains were present in the sample. To determine quantitative information of the pCBT chains on the surface of TrGO, TGA analysis was performed on the resulting products as shown in Fig. 13. A gradual weight loss appeared above 500 C for TrGO due to the decomposition of functional groups on the TrGO surface. There was almost 15 wt% weight loss for TrGO at 700 C. For pCBT, the degradation started at about 350 C and finished at nearly 475 C. As far as the TrGO-g-pCBT is concerned, the obvious degradation process with 53 wt% weight loss occurred between 350 C and 475 C which agrees with pure pCBT. So the
Fig. 11. XPS spectrum of TrGO and TrGO-g-pCBT.
pCBT chains account for 53 wt% of the TrGO-g-pCBT. As the temperature increases, the degradation of remained groups on TrGO surface resulted in about 4% weight loss which was less that of pure TrGO owing to the reaction between the functional groups of TrGO and pCBT. Since the oxygen content of TrGO is 16.6 wt%, the mole ratio of C/O is about 7:1. If 10% percent of all of the hydroxyl and epoxy groups can react with pCBT chains, the mole ration of carbon to grafting pCBT chains would be 70:1. Although the pCBT chains are not so long, the average molecular weight is 894 g/mol (including the initiator) for CBT oligomers which are the shortest. So the final mass ratio of carbon to grafting pCBT chains is about 1:1.06, and the weight fraction of the grafting content is about 51 wt%, which is quite close to the TGA result (53 wt%). Actually, since the polymerization degrees of the grafted pCBT chains are expected to be larger than 1, only a small amount of hydroxyl and epoxy groups react with pCBT. 3.3. Rheological behavior of pCBT/TrGO nanocomposites After performing the time sweep until no significant change in complex viscosity was observed, strain sweep tests at 230 C were subsequently carried out to find the limit of linear viscoelasticity. Fig. 14 shows the dependence of dynamic modulus on the strain amplitude for pCBT and its hybrids with different TrGO contents, where the real part of dynamic modulus under fundamental frequency is normalized by that in linear regime. From Fig. 14, the critical strain (gcrit) decreases significantly with increase in filler content. The appearance nonlinear behavior in pCBT is ascribed to the chain orientation and disentanglement under larger oscillatory shear, while in composites is ascribed to the breakdown of the
Fig. 13. TGA curves of pCBT, TrGO and TrGO-g-pCBT.
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Fig. 14. Dynamic strain sweeps of pCBT/TrGO melts at frequency of 1 Hz at 230 C. Critical strains are determined by G0 =G00 ¼ 0.9. All TrGO are treated under 550 C unless otherwise stated.
network of TrGO sheets. This is similar to that has been observed in polymer/clay nanocomposites [43]. The nonlinear moduli of pCBT/TrGO with 1 wt% TrGO reduced at different temperatures (550 C and 900 C) are also compared in Fig. 14. It is interesting to find that the sample with TrGO reduced at 550 C has a broader linear viscoelastic range than the other one. As mentioned above, the critical strain is closely related to the breakup of the connected network of TrGO sheets. For TrGO reduced at higher temperature, less pCBT chain is grafted on the TrGO sheets, and the network of nanosheets is only stabilized by the frictions between sheets. However, the sample with TrGO reduced at 550 C has more grafted pCBT on the surface, the network of TrGO sheets can be stabilized by the additional entanglements between the grafted chains and the bulk chains, which results in more stable network. Linear viscoelasticity has been studied by dynamic frequency sweeps at g < gcrit for different composites. Complex viscosity and dynamic moduli of pCBT/TrGO melts at 230 C are shown in Fig. 15 as a function of frequency. From Fig. 15a, the frequency dependence of complex viscosity for pCBT/TrGO with 0.1e1.0 wt% is quite similar to that of pure pCBT matrix, and very weak frequency dependence of complex viscosity is observed. There are no Newtonian viscosity plateaus for samples with 2.0e3.0 wt% TrGO. Usually the low-frequency data are sensitive to the effect of filler, while high frequency data are mainly dominated by the matrix polymer. Apparently, the complex viscosity at high frequency decreases with the increase in TrGO content, which denotes the decrease in the molecular weight of pCBT as TrGO content
Fig. 15. Dynamic frequency sweep of pCBT/TrGO melts at 230 C.
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increases. This is the direct evidence to show the effect of TrGO on the molecular weight of the pCBT. According to the theory of linear viscoelastic liquid, the slopes of log G0 w log u and log G" w log u are 2 and 1, respectively, in the low frequency region. Such behavior is seen in pure pCBT and pCBT/TrGO composite with TrGO content less than 0.1% (Fig. 15b and c). As the TrGO content increases, the dependence of dynamic moduli on frequency decreases sharply in the terminal zone. The deviation from linear viscoelastic liquid is more evident in storage modulus, and starts in composites with TrGO content as low as 0.5 wt%. The significant influence of TrGO on the storage modulus at such low TrGO content is ascribed to the good compatibility between TrGO and pCBT as some pCBT chains grafted to the TrGO surface and good dispersion of graphene sheets. The storage modulus of pCBT/TrGO system is shown as a function of TrGO content in Fig. 16 at different frequencies. Power-law behaviors ðG0 ffm Þ are seen from the logelog plot. At low TrGO concentration, the power-law exponent is smaller and frequency dependent, while it becomes larger and frequency independent at higher TrGO concentration. The storage modulus of the percolated colloidal suspension can be expressed near the percolation threshold by a power-law correlation on the difference between volume fraction of particles f and the threshold value fc [44e46].
G0 fðf fc Þn
(1)
Assuming the shear modulus of pCBT dispersed with TrGO follows this power-law relationship, the percolation threshold fc and exponent n can be evaluated by applying Eq. (1) to G0 at different frequencies. Least-squares regressions were carried out until optimum fc and n were found that best fit the data. Then, the onset of percolation concentration is 0.47 vol% (0.8 wt%), which is close to the point where the critical strain starts to drop more greatly (1 wt %) from Fig. 14. The percolation concentration is also quite close to the transition concentration that polymerization reaction rate starts to decrease substantially (see inset of Fig. 6). As stated above, the constraint of TrGO nanosheets on the reactivity of grafted pCBT chains is the main reason for the slowdown of polymerization at low TrGO concentration. The agreement of the percolation concentration and the transition concentration in Fig. 6 implies that the constraint on the diffusion of reactive species from the network of nanosheets is the main reason for the substantial decrease in the polymerization rate at higher TrGO concentration. We also could evaluate the percolation threshold of nanocomposites solids from electrical conductivity, as electrical conductivity obeys the following power laws:
Fig. 16. The dependence of the low-frequency storage modulus measured at 0.1, 0.158 and 0.251 rad/s on TrGO content.
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sc fðp pc Þm
(2)
where sc is the electrical conductivity, p is the volume fraction of particles, pc is the threshold value and m is a constant. As shown in Fig. 17, a percolation threshold value of 0.95 vol% (1.6 wt%) was obtained. This threshold value was two times larger than the one from rheological method, which is ascribed to the different kinds of network that is detected by rheology and electrical conductivity. The contact network from the gyration radius of nanosheets can contribute to the rheological behavior, while it is not sufficient to have a direct contact for electrical conductivity. That is why rheological threshold is often smaller than the electrical one especially for polymer composites filled by fillers with high aspect ratio. The lower percolation threshold of TrGO is strong evidence for better dispersion. Ren and co-workers showed that a relationship can be constructed between the percolation threshold and the aspect ratio, Af of a tactoid [47]. Supposing imaginary spheres surrounding each tactoid, the expression for the fraction of particle diameter 2r to thickness h can be written as
Af ¼
3fsphere 2r ¼ 2fc h
(3)
Percolation of interpenetrating, randomly packed sphere occurs at fsphere ¼ 0.29 [48,49]. Using the onset of percolation of TrGO from melt rheology, Eq. (3) gives aspect ratio of 92.6. The aspect ratio of TrGO evaluated from the TEM images is 73.3, which is slightly less than the value from rheological measurement. The rheological behaviors shown above of the pCBT/TrGO nanocomposites are quite similar to flocculated suspensions. It has been suggested that the relationship between viscoelastic properties of flocculated system and particle concentration can be described by the scaling law proposed by Shih and co-workers [50]. Assuming the elastic network composed of TrGO particles can be treated as fractal aggregates of graphene flocs, their storage modulus (G0 ), and limits of linearity (gcrit), above the gelation threshold can be scaled with the volume fraction of the flocs. When the bonds between flocs are stronger than the links within each floc, the G0 and gcrit can be written as
G0 ffð3þxÞ=ð3df Þ ð1þxÞ=ð3df Þ
gcrit ff
Fig. 18. Scaling of G0 at 0.1 rad/s for pCBT/TrGO melts at 230 C as a function of TrGO volume fraction.
storage modulus and critical strains above percolation threshold were chosen as shown in Fig. 18. Both G0 and gcrit show a power-law behavior with respect to TrGO volume fraction. Using Eqs. (4) and (5), we obtained df y 0.37 and x y 5.26, which are unreasonable for our system. The exponent 3.14 for G0 is significantly lower than the scaling exponent reported for G0 of graphene in PEN melts, w5.16 [36]. However, for pCBT/TrGO melts where the TrGO was treated at 900 C to minimize the TrGO influence to polymerization, The scaling exponent for G0 increases to 4.28, and we obtained df y 1.94 and x y 1.55 which are close to the values of PEN/graphene system reported by Kim [36]. The success and failure of fractal dimension analysis in different pCBT/TrGO systems imply the inherent difference in the microstructures. TrGO that reduced at lower temperature reduces the molecular weight of matrix pCBT and induces the graft of pCBT chain on the nanosheets. The change in the matrix molecular weight actually makes the nanocomposites with different TrGO concentration not comparable in the sense of the same matrix polymer. Moreover, the entanglement between the grafted pCBT chains on the nanosheets and the pCBT chains in bulk make the nanosheets network not the ideal one with only particleeparticle interactions. This result again justifies the effect of TrGO on the polymerization of CBT. 4. Conclusions
(4) (5)
where df is the fractal dimension of the aggregates and x is the fractal dimension of the backbone in the aggregate which is responsible for elasticity. In order to apply this model to our system,
Graphene sheets were prepared by chemical oxidation of pristine graphite flakes followed by thermal exfoliation and reduction of graphite oxide. The pCBT/TrGO nanocomposites were successfully prepared by ROP of CBT. TrGO was found to depress the polymerization rate as well as the degree of polymerization as observed from rheology. It was confirmed by XPS, NMR and TGA measurements that the growing pCBT chains could graft onto the surface of TrGO nanosheets by reaction between carboxyl groups of pCBT and hydroxyl and epoxy groups of TrGO with the grafting content up to 53 wt%. The grafting reaction was further justified from nonlinear rheology and the fractal dimension analysis. The constraint effect of nanosheets on the reactivity of grafted pCBT chain is the main reason for the slowdown in the polymerization rate at low TrGO concentration, while the network formation at higher TrGO concentration and its constraints on the diffusion of reactive species will further decrease the polymerization rate. Acknowledgment
Fig. 17. Electrical conductivity of pCBT/TrGO nanocomposites.
The authors would like to thank the support from National Basic Research Program of China (973 Program) 2012CB025901 and the National Natural Science Foundation of China (No. 50930002). W. Yu is supported by the Program for New Century Excellent Talents in University and the SMC project of Shanghai Jiao Tong University.
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Effect of thermally reduced graphite oxide (TrGO) on the polymerization kinetics of poly(butylene terephthalate) (pCBT)/TrGO nanocomposites prepared by in situ ring-opening polymerization of cyclic butylene terephthalate Hongliang Chen, Chongwen Huang, Wei Yu*, Chixing Zhou Advanced Rheology Institute, Department of Polymer Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 October 2012 Received in revised form 16 January 2013 Accepted 22 January 2013 Available online 29 January 2013
Thermally reduced graphite oxide (TrGO) was prepared by thermal exfoliation and reduction of highly oxidized graphite. The pCBT/TrGO nanocomposites were prepared by in situ ring-opening polymerization (ROP) of cyclic butylene terephthalate (CBT). The polymerization kinetics of pCBT/TrGO was monitored by dynamic time sweep in a parallel-plate rheometer. It was found that the increasing TrGO content depressed the rate and degree of CBT polymerization, which is ascribed to the reaction between the growing pCBT chains terminated with carboxyl groups and TrGO surface groups such as hydroxyl and epoxy groups at the initial polymerization stage. The grafted pCBT chains were confirmed by X-ray photoelectron spectroscopy (XPS), nuclear magnetic resonance (NMR) and thermogravimetric analysis (TGA) measurements, and the grafting content was up to 53 wt%. Small amplitude oscillation shear (SAOS) was applied to investigate the rheological properties of pCBT/TrGO and the critical loading to form percolation network was determined as 0.47 vol%, which confirmed the good dispersion of TrGO in matrix. The grafting reaction was also justified from nonlinear rheology and the fractal dimension analysis. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Cyclic butylene terephthalate Thermally reduced graphite oxide Ring-opening polymerization
1. Introduction Graphene, a monolayer of sp2-hybridized carbon atoms arranged in a two-dimensional lattice, has attracted tremendous attention in recent years because of its exceptional electronic [1], thermal [2] and mechanical [3] properties and the discovery of methods for their production. There has been several techniques for preparing graphene nanosheets, including mechanical cleavage of graphite [4], chemical vapor deposition [5e7], epitaxial growth on SiC [8e10], alkali metals intercalation and expansion [11], chemical reduction of graphite oxide (GO) [12e15] and thermal exfoliation of GO [16,17] and so on. Among them chemical reduction and thermal exfoliation are suitable for large scale production. One of the most promising applications of this material is in polymer nanocomposites. Polymer nanocomposites show substantial property enhancements at much lower loadings than polymer composites with conventional micron-scale fillers (such as glass or carbon fibers), which ultimately results in lower component weight. Moreover, the multifunctional property enhancements
* Corresponding author. Tel.: þ86 21 5474 3275. E-mail address: [email protected] (W. Yu). 0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2013.01.036
made it possible to create new applications of polymers nanocomposites. As far as graphene is concerned, its nanocomposites can provide combined benefits of both layered silicates and CNT. Unlike layered silicates, it has exceptional electrical and thermal transport properties. Unlike CNT, it can be derived from natural graphite at smaller cost. Among the possible routes of preparation of polymer-graphene nanocomposites reviewed by Macosko et al. [18], melt blending is the most economically attractive and scalable method. However, melt blending also presents several severe problems due to the high melt viscosity of most polymers, such as impregnation, flow of resin and removal of bubbles. These problems can be overcome by using cyclic butylene terephthalate oligomers (CBT) which has extremely low melt viscosity before polymerization. CBT can quickly polymerized to the corresponding linear poly(butylene terephthalate) (referred to as pCBT to distinguish from conventional poly(butylene terephthalate) (PBT) obtained by polycondensation reaction) via entropically driven ring-opening polymerization (ROP) in the presence of suitable initiator [19]. Because the ROP is entropically driven, the reaction is athermal and no chemical emission. Unlike conventional thermoplastic resins, cyclic oligomers have an initial water-like melt viscosity prior to polymerization and thus can be employed in processes, such as resin
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transfer molding (RTM), which are typically used for thermosets. Due to the athermal characteristic of polymerization, there are no hot spots in the mold which lead to thermal degradation and no byproducts need to remove during the molding process. All of these favorable advantages allow CBT to be a promising material to produce pCBT nanocomposites with perfect impregnation and excellent dispersion of fillers [20e22] using various processing methods [23,24]. Research on in situ polymerized graphene nanocomposites should not only analyze the effect of the fillers in the polymer matrix morphology and final properties but also in the polymerization reaction or curing reaction. A study on this subject has revealed a decrease in the rate of the polymerization of PDMS foams by the addition of thermally exfoliated graphene and a change in the dynamic evolution of the reaction as compared to the effect of CNT [25]. Changes in molecular weight of thermoplastic polyurethane (TPU) due to the presence of graphene have also been reported [26]. However, the fundamental mechanisms for graphene effect on polymerization have not been completely demonstrated yet. To the best of our knowledge, there have been no publications on the PBT/graphene nanocomposites, let alone the pCBT/thermally reduced graphite oxide (TrGO) nanocomposites prepared by in situ ring-opening polymerization. In our previous work, we successfully used the rheological method to investigate the polymerization kinetics of CBT [27]. In this study, rheological method was still used to monitor the polymerization process to determine the TrGO effect on polymerization kinetics. A possible mechanism was proposed to illustrate the decreased rate and degree of polymerization. It is also believed that the rheological properties are sensitive to the internal microstructures of nanocomposites, including the dispersion state of graphene nanosheets and its confinement on the motion of polymer chains. The response of nanocomposites to linear and nonlinear deformation were both investigated, which can provide important information for understanding the relationship of its rheological behavior and the nature of its microstructures. 2. Experimental 2.1. Materials Cyclic butylene terephthalate oligomers (CBT160, Mw ¼ 220n, n ¼ 2e7) in granule form obtained from Cyclics company. CBT160 already contained a pre-mixed initiator butylchlorotin dihydroxide (Fastcat 4101, Mw ¼ 245.29 g/mol, C4H9Sn(OH)2Cl) which could initiate CBT to polymerize into high molecular weight poly(butylene terephthalate) termed as pCBT. Hydrophilic fumed silica nanoparticles (SiO2) with specific surface area 160 20 m2/g and average particle diameter 30 nm was purchased from Hang Zhou Wan Jing New Material Co., Ltd. Multiwalled carbon tubes (CNT) with diameter 10e20 nm and length 5e15 um were purchased from the Shenzhen Nanotech Port Co., Ltd. Graphite powder (30 mm), sulfuric acid (98%), potassium persulfate, phosphorus pentoxide, potassium permanganate, hydrogen peroxide (30%) and hydrochloric acid (37%) and other solvents were obtained from Sinopharm Chemical Reagent Co., Ltd. 2.2. Preparation of thermally reduced graphite oxide (TrGO) Graphite oxide (GO) was synthesized from natural graphite powder by a modified Hummers method [28,29]. The synthesis procedure consisted of two-step oxidation. In the peroxidation step, graphite powder (3 g) was put into an 80 C solution of concentrated H2SO4 (12 mL), K2S2O8 (2.5 g) and P2O5 (2.5 g). The mixture was kept at 80 C for 4.5 h and then cooled down to room
temperature, diluted with 0.5 L of deionized water and left overnight. Then, the mixture was filtered and washed with deionized water using a 0.2 micron Nylon Millipore filter to remove the residual acid. The product was dried under ambient condition overnight. In the second oxidation step, the pretreated graphite powder was put into cold (0 C) concentrated H2SO4 (120 mL). Then, KMnO4 (15 g) was added gradually under stirring and the temperature of the mixture was kept to be below 20 C. Successively, the mixture was stirred at 35 C for 2 h, and then diluted with deionized water (250 mL). The addition of water was carried out in an ice bath to keep the temperature below 50 C. After adding all of the 250 mL of deionized water, the mixture was stirred for 2 h, and then additional 0.7 L of deionized water was added. Shortly after the dilution with 0.7 L of water, 20 mL of 30% H2O2 was added to the mixture, and the color of mixture changed into brilliant yellow along with bubbling. The mixture was filtered and washed with 10% HCl aqueous solution (1 L) to remove metal ions. The precipitate mixture was repeated washing and centrifugation successively with deionized water at least three centrifugation cycles until the decantate became neutral. The GO slurry was dried at 40 C for one week. Finally, TrGO was obtained by the thermal treatment of GO at 550 C for 15 min under argon atmosphere. 2.3. Preparation of pCBT/TrGO nanocomposites A series of pCBT/TrGO nanocomposites were prepared by solution blending. 20 g CBT160 were added into 500 mL THF and stirred for 24 h to obtain a suspension. Varying amount of TrGO was dispersed in the CBT160 suspension and the mixture was sonicated for 30 min. Most of the THF was evaporated at room temperature with magnetic stirring, and then, the final mixture was dried in vacuum at 80 C for 48 h to remove trace amount of THF. When the mixture was heated to more than 200 C, the CBT will polymerize to pCBT and the nanocomposites were obtained. 2.4. Characterization of graphite, GO, TrGO and pCBT/TrGO The structural order of graphite, GO and TrGO was investigated by adopting X-ray diffraction method. X-ray diffraction patterns were obtained with a Rigaku D/max-2200/PC diffractometer with Cu Ka radiation at a scanning rate of 4 /min. X-ray photoelectron spectroscopy (Kratos Axis Ultra DLD) was also used to evaluate atomic compositions in the TrGO. In order to investigate the dispersion of TrGO in pCBT/TrGO, Transmission electron microscopy (TEM) was performed with a JEM-2100 electron microscope operating at an accelerating voltage of 200 kV. Thermogravimetric analysis (TGA) was performed on a TA Instruments Q5000IR to investigate the thermal stability of TrGO, neat pCBT and TrGO-g-pCBT. Samples of about 10 mg were heated from room temperature to 700 C at a rate of 20 C/min under a nitrogen atmosphere. 2.5. Rheological measurements Rheological measurements were all performed on a rotational rheometer (Gemini 200HR, Bohlin Instruments, UK) equipped with a parallel-plate geometry using 25 mm diameter plates. The dried CBT mixture was directly loaded onto the plate and tested without compressing into circular sheet (thickness z 1 mm) in order to minimize the influence of pre-polymerization. Since the low viscosity of melt CBT, there were no air bubbles in the samples after polymerization. After sample loading, a dynamic time sweep was conducted at 1 rad/s at 230 C using small amplitude strain until no change in complex viscosity was observed which indicated the completed polymerization. Subsequently, using a dynamic strain sweep at 6.28 rad/s, the critical strain, gcrit, where G0 drops to 90% of
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its limiting low strain value was recorded. Small amplitude oscillatory shear frequency sweeps were carried out at frequency of 0.1e100 rad/s at 230 C with 1% strain for all products. It is noted that all rheological measurements were performed in the linear viscoelastic regime for all samples. 3. Results and discussion 3.1. Characterization Fig. 1 shows the XRD spectrum of pristine graphite, GO and TrGO. For graphite, the strong and sharp diffraction peak was detected at 2q ¼ 26.7, which corresponds to the interlayer spacing of 0.33 nm. The peak of graphite disappeared after oxidation and instead a new peak at 2q ¼ 9.4 appeared, indicating the highly oxidization of graphite [17,26,30,31]. After thermal reduction of GO, there was no apparent diffraction peak detected, which means the periodic structure of GO was eliminated and graphene nanosheets were formed. XPS was employed to analyze atomic compositions in the TrGO. The C 1s XPS spectrum of TrGO (Fig. 2) confirmed the existence of oxygen functionalities on the TrGO surface. There are four kinds of carbon atoms that correspondingly exist in different functional groups: the non-oxygenated ring carbon atoms (284.8 eV), the carbon atoms in carboneoxygen bonds (286.2 eV, corresponding to hydroxyl and epoxy groups), the carbonyl carbon (287.6 eV), and the carboxylate carbon (289.0 eV, corresponding to carboxyl group) [12,32e34]. After the thermal reduction at 550 C for 15 min, the carbon content of TrGO reaches 83.4 wt%. While the carbon content of graphite is 97.8%, which is much higher than TrGO. TEM was used to further evaluate the exfoliation and dispersion quality of TrGO within pCBT. From the TEM image of the pCBT/TrGO nanocomposites with 3.0 wt% TrGO (Fig. 3a), it can be seen that well exfoliated graphene nanosheets are evenly distributed in the matrix and there are almost no large agglomerates. The good dispersion of TrGO is ascribed to the interaction between the oxygen and hydroxyl functional groups on the surface of TrGO and the polar groups of pCBT [31,35]. In addition, a compact continues network throughout the matrix is also observed (Fig. 3a). The high resolution TEM image (Fig. 3b) reveals that graphene exists as single-layer or few-layers nanosheets. The thickness of TrGO sheets from TEM images is nearly 2 nm for thinner one and 4 nm for thicker one prepared in this study. It is also seen that graphene sheets are highly winkled, which are in agreement with other observations [26,36,37]. This is due to the structural defects caused by the loss of CO2 during the thermally reduced treatment process
Fig. 1. X-ray scattering intensity profiles of graphite, GO and TrGO.
Fig. 2. C 1s XPS spectrum of TrGO.
and the thermodynamically instability of perfect two-dimensional graphene crystal [38]. High magnification image (Fig. 3b) indicates that the network is composed of abundant thin stacks of a few sheets of monolayer graphene (black curly thin lines), which are bridged by the crumpled or overlapped graphene sheets (slightly thicker lines) (Fig. 3b). These wrinkled and fewer overlapped graphene sheets can effectively link individual graphene sheets and carry high density of current, resulting in high electrical conductivity [39]. In addition to the TEM images of the nanocomposites, XRD will also be useful to evaluate the dispersion of TrGO sheets in pCBT matrix as shown in Fig. 4. It is easy to note that there are no strong and sharp diffraction peaks for all the samples, which indicates that the CBT polymerization almost has no effect on the dispersion and aggregation of TrGO sheets. It also confirms the good dispersion of TrGO in the matrix. 3.2. Effect of TrGO on the polymerization kinetics of CBT Brunelle [40] has discussed in detail the mechanism of polymerization of CBT, which involves initiation to form an active chain end, then followed by propagation reaction continuing until all of the cyclic oligomers are depleted and the ring-chain equilibration becomes degenerate as shown in Fig. 5. During the polymerization, the initiator becomes built into the polymer, and it is not terminated unless quenched, which indicates that the ultimate molecular weight depends on the concentration of initiator. Since the polymerization of CBT is athermal, the conventional method such as DSC cannot be used to investigate the reaction dynamics. Polymerization study was carried out under isothermal conditions in the rheometer. Using the complex viscosity and modulus of the melt as a measure of the extent of polymerization allows to monitor the changes in the molecular weight, or degree of polymerization, as a function of time [27]. Dynamic time sweep was conducted at 1 rad/s at 230 C using small strain amplitude to investigate the polymerization of CBT with different TrGO contents. The complex viscosity increased sharply with time and ultimately reached a constant value which implied that the polymerization was completed. It was also found that the modulus evolved in a similar trend like complex viscosity during the polymerization process of CBT. Therefore, it’s reasonable to use the rheological data such as complex viscosity as an indirect tool to characterize the polymerization of CBT with TrGO. The function of (h(t) h0)/(hN h0) was used to denote the degree of polymerization, where h(t) is the complex viscosity as a function of time, hN is the ultimate complex viscosity when polymerization is finished, and h0 is the initial
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Fig. 3. TEM images of pCBT/TrGO with 3 wt% TrGO under low (a) and (b) high magnifications.
complex viscosity before polymerization. The slope of (h(t) h0)/ (hN h0)versus time can be used to represent the polymerization rate. The evolutions of (h(t) h0)/(hN h0) with time for samples with different TrGO contents are shown in Fig. 6. With the increase in TrGO content, the polymerization rate decreases sharply and it takes longer time to finish whole reaction. The time for polymerization to finish (tend) is about 50 min in CBT/3 wt% TrGO system, which is over three times longer than that of pure CBT. The effect of TrGO content on the reaction time (tend) is shown in the inset of Fig. 6. At low concentration of TrGO, the reaction time is only slightly increased, while the reaction is retarded greatly at higher TrGO content. The transition appears at about 0.7e0.8% of TrGO content. In order to interpret how the TrGO inhibits the reaction, different fillers, such as graphite, carbon nanotube and SiO2, were selected to prepare composites via solution blending used for pCBT/TrGO depicted in Experimental part. The dynamic time sweeps were also conducted at the same condition as TrGO system to compare the effect of different fillers on the polymerization of CBT. The relative viscosities for different systems are shown in Fig. 7. It is seen that the fillers have different impact on the CBT polymerization. Samples with graphite and CNT have similar polymerization rate as pure CBT, which only needs about 15 min to complete the whole reaction. The CNT has no effect on the CBT polymerization, although it has large specific surface area with respect to graphite. Note that the surfaces of graphite and CNT are clean and there have almost no specific functional groups on them. Those with TrGO and SiO2 extend the polymerization time greatly. The sample with SiO2 has the smallest polymerization rate. In contrast to all the others, SiO2 system has a long induction time (over 500 s) before the increase in complex viscosity. It is important to know that the SiO2 nanoparticles have large amount hydroxyl functional groups on the surface. C 1s XPS spectrum of TrGO (Fig. 2) quantified the amount of oxygen on the
surface and also identified the types of carboneoxygen bonds. So it is possible that the functional groups on the fillers surface affect the CBT polymerization. It was reported that the chain extension reaction between (growing) pCBT chains and epoxy resin happened by esterification of carboxyl end groups of polyester and glycidyl functional groups of the epoxy resin [41]. Furthermore, Tripathy et al. synthesized a series of copolyesters of CBT and tetrabromobisphenol A (TBBPA), bisphenol A diglycidyl ether (BPADGE) and Carbinol PDMS respectively based on the reaction between the carboxyl end groups of pCBT chains and the epoxy groups and hydroxyl groups of TBBPA, BPADGE and Carbinol PDMS [42]. These literature reports confirmed the assumption that the epoxy and hydroxyl groups on the TrGO surface affect the polymerization of CBT. The inhibition of SiO2 to the polymerization of CBT in Fig. 7 is probably attributed to the large amount of hydroxyl groups on the surface. To further confirm our assumption, the effect of functional groups content on polymerization was determined. According to the above analysis, the CBT should have higher polymerization rate incorporation with TrGO with lower epoxy and hydroxyl groups content on the surface. With additional thermal treatment at high temperature at 600 C and 700 C for 30 min, respectively, the epoxy and hydroxyl groups content of TrGO should decrease as compared to the one without treatment [16,32]. According to the C 1s XPS spectrum results, the oxygen contents of TrGO decreased down to 11.6 and 7.2 wt%, respectively. Fig. 8 shows the polymerization of CBT with 2 wt% TrGO treated with different high temperature. Compared to the sample with TrGO without additional thermal treatment, sample with TrGO treated at 600 C can finish reaction within 30 min, and the one treated at 700 C has similar polymerization rate as pure CBT. These results suggest the important role of surface functional group on TrGO nanosheets in the ring-open polymerization of CBT.
Fig. 4. X-ray scattering intensity profiles of pCBT and pCBT/TrGO nanocomposites.
Fig. 5. Mechanism for polymerization of CBT160.
H. Chen et al. / Polymer 54 (2013) 1603e1611
Fig. 6. Variation of (h(t) h0)/(hN h0) with time for CBT polymerization at 230 C with varying amount TrGO. The effect of TrGO content on the end time of CBT polymerization is shown in the inset.
According to the above investigation, we proposed a mechanism for effect of TrGO on the polymerization of CBT, which is shown in Fig. 9. The mechanism of the polymerization of CBT is analogous to the conventional ROP, with initiation and propagation steps. In the presence of initiator, a carbonyl of CBT is activated via complexation, and then transferring an alkoxide ligand to form an active species, where one chain end is terminated with a functional group originating from the initiator and the other is terminated with carboxyl group. Propagation is thought to proceed by coordination of the monomer to the active species, followed by insertion of the monomer into the metaleoxygen bond by rearrangement of the electrons. The process is repeated as many more monomers are successively added to propagate the active species to form linear polymers. As far as CBT polymerization with TrGO is concerned, many growing pCBT chains with low molecular weight terminated with carboxyl group may react with epoxy and hydroxyl groups of TrGO at the initial stage and as a result some pCBT chains are possibly grafted to TrGO sheets. As the reaction continues, the pCBT which is in the melt matrix at early stage may still have the chance to react with the epoxy and hydroxyl groups remained on the surface of TrGO. The initiator and active species remained in the matrix can proceed to form high molecular weight pCBT as pure CBT sample. While the polymerization reactivity of pCBT grafted to the TrGO decreased sharply due to the restriction of TrGO which is very large for a single pCBT chain. The TrGO sheets may restrict the motion of pCBT chains to lower the reactivity of the active species. Such effect seems to be more significant when the TrGO content is
Fig. 7. Variation of (h(t) h0)/(hN h0) with time for CBT polymerization at 230 C with 2 wt% graphite, TrGO, CNT, SiO2 respectively.
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Fig. 8. Variation of (h(t) h0)/(hN h0) with time for CBT polymerization with 2 wt% TrGO with additional thermal treatment at different temperature.
over 0.7e0.8%. The ultimate complex viscosity of pCBT with TrGO varying from 0.1 to 3 wt% is smaller than that of pure pCBT sample since the pCBT chains grafted to the TrGO surface has lower molecular weight than the pCBT in the matrix. Therefore, the rationality of such mechanism relies on the existence of the grafted pCBT on the TrGO surfaces. To prove the graft of pCBT on TrGO sheet, the pCBT/TrGO nanocomposites with 2 wt% TrGO were dissolved in a mixture of phenol/tetrachloroethane at 110 C for 1 h and recovered by filtration and then washed with methanol. The dispersing, filtering, and washing cycle was repeated several times to remove any possible ungraft polymer or cyclic oligomers. The final black samples were named as TrGO-g-pCBT. It is found that the TrGO-g-pCBT can be well dispersed in trifluoroacetic acid. The suspension was stable and uniform. While for the pure TrGO, they settled at the bottom of the bottle as shown in Fig. 10. It implies that certain modification of
Fig. 9. Mechanism for effect of TrGO on polymerization of CBT.
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Fig. 12. 1H NMR spectrum of TrGO-g-pCBT in CF3COOD.
Fig. 10. The comparison of TrGO (a) and TrGO-g-pCBT (b) in CF3COOD.
TrGO sheet might happen after the polymerization of pCBT, possibly due to the graft of pCBT chains onto the TrGO surface. Firstly, XPS was employed to determine the surface composition of TrGO-g-pCBT. As shown in Fig. 11, the binding energy for the Sn 3d is clearly displayed in the spectrum of TrGO-g-pCBT, indicating that the pCBT chains were covalently attached to the surface of TrGO. Since the content of Sn is very low, the peak of Sn 3d is very weak compared to O 1s and C 1s. Moreover, NMR was chosen to confirm the molecular structure. The 1H NMR spectrum of TrGO-gpCBT was obtained using deuterated trifluoroacetic acid as a solvent. In Fig. 12, the corresponding proton peaks of the graft pCBT chains appeared in the 1H NMR spectrum of TrGO-g-pCBT, peaks at 8.15 ppm are due to aromatic protons, and 4.54 and 2.07 ppm are assigned to ester and methylene groups, respectively, and the peaks ration is almost 1:1:1, which indicated that the pCBT chains were present in the sample. To determine quantitative information of the pCBT chains on the surface of TrGO, TGA analysis was performed on the resulting products as shown in Fig. 13. A gradual weight loss appeared above 500 C for TrGO due to the decomposition of functional groups on the TrGO surface. There was almost 15 wt% weight loss for TrGO at 700 C. For pCBT, the degradation started at about 350 C and finished at nearly 475 C. As far as the TrGO-g-pCBT is concerned, the obvious degradation process with 53 wt% weight loss occurred between 350 C and 475 C which agrees with pure pCBT. So the
Fig. 11. XPS spectrum of TrGO and TrGO-g-pCBT.
pCBT chains account for 53 wt% of the TrGO-g-pCBT. As the temperature increases, the degradation of remained groups on TrGO surface resulted in about 4% weight loss which was less that of pure TrGO owing to the reaction between the functional groups of TrGO and pCBT. Since the oxygen content of TrGO is 16.6 wt%, the mole ratio of C/O is about 7:1. If 10% percent of all of the hydroxyl and epoxy groups can react with pCBT chains, the mole ration of carbon to grafting pCBT chains would be 70:1. Although the pCBT chains are not so long, the average molecular weight is 894 g/mol (including the initiator) for CBT oligomers which are the shortest. So the final mass ratio of carbon to grafting pCBT chains is about 1:1.06, and the weight fraction of the grafting content is about 51 wt%, which is quite close to the TGA result (53 wt%). Actually, since the polymerization degrees of the grafted pCBT chains are expected to be larger than 1, only a small amount of hydroxyl and epoxy groups react with pCBT. 3.3. Rheological behavior of pCBT/TrGO nanocomposites After performing the time sweep until no significant change in complex viscosity was observed, strain sweep tests at 230 C were subsequently carried out to find the limit of linear viscoelasticity. Fig. 14 shows the dependence of dynamic modulus on the strain amplitude for pCBT and its hybrids with different TrGO contents, where the real part of dynamic modulus under fundamental frequency is normalized by that in linear regime. From Fig. 14, the critical strain (gcrit) decreases significantly with increase in filler content. The appearance nonlinear behavior in pCBT is ascribed to the chain orientation and disentanglement under larger oscillatory shear, while in composites is ascribed to the breakdown of the
Fig. 13. TGA curves of pCBT, TrGO and TrGO-g-pCBT.
H. Chen et al. / Polymer 54 (2013) 1603e1611
Fig. 14. Dynamic strain sweeps of pCBT/TrGO melts at frequency of 1 Hz at 230 C. Critical strains are determined by G0 =G00 ¼ 0.9. All TrGO are treated under 550 C unless otherwise stated.
network of TrGO sheets. This is similar to that has been observed in polymer/clay nanocomposites [43]. The nonlinear moduli of pCBT/TrGO with 1 wt% TrGO reduced at different temperatures (550 C and 900 C) are also compared in Fig. 14. It is interesting to find that the sample with TrGO reduced at 550 C has a broader linear viscoelastic range than the other one. As mentioned above, the critical strain is closely related to the breakup of the connected network of TrGO sheets. For TrGO reduced at higher temperature, less pCBT chain is grafted on the TrGO sheets, and the network of nanosheets is only stabilized by the frictions between sheets. However, the sample with TrGO reduced at 550 C has more grafted pCBT on the surface, the network of TrGO sheets can be stabilized by the additional entanglements between the grafted chains and the bulk chains, which results in more stable network. Linear viscoelasticity has been studied by dynamic frequency sweeps at g < gcrit for different composites. Complex viscosity and dynamic moduli of pCBT/TrGO melts at 230 C are shown in Fig. 15 as a function of frequency. From Fig. 15a, the frequency dependence of complex viscosity for pCBT/TrGO with 0.1e1.0 wt% is quite similar to that of pure pCBT matrix, and very weak frequency dependence of complex viscosity is observed. There are no Newtonian viscosity plateaus for samples with 2.0e3.0 wt% TrGO. Usually the low-frequency data are sensitive to the effect of filler, while high frequency data are mainly dominated by the matrix polymer. Apparently, the complex viscosity at high frequency decreases with the increase in TrGO content, which denotes the decrease in the molecular weight of pCBT as TrGO content
Fig. 15. Dynamic frequency sweep of pCBT/TrGO melts at 230 C.
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increases. This is the direct evidence to show the effect of TrGO on the molecular weight of the pCBT. According to the theory of linear viscoelastic liquid, the slopes of log G0 w log u and log G" w log u are 2 and 1, respectively, in the low frequency region. Such behavior is seen in pure pCBT and pCBT/TrGO composite with TrGO content less than 0.1% (Fig. 15b and c). As the TrGO content increases, the dependence of dynamic moduli on frequency decreases sharply in the terminal zone. The deviation from linear viscoelastic liquid is more evident in storage modulus, and starts in composites with TrGO content as low as 0.5 wt%. The significant influence of TrGO on the storage modulus at such low TrGO content is ascribed to the good compatibility between TrGO and pCBT as some pCBT chains grafted to the TrGO surface and good dispersion of graphene sheets. The storage modulus of pCBT/TrGO system is shown as a function of TrGO content in Fig. 16 at different frequencies. Power-law behaviors ðG0 ffm Þ are seen from the logelog plot. At low TrGO concentration, the power-law exponent is smaller and frequency dependent, while it becomes larger and frequency independent at higher TrGO concentration. The storage modulus of the percolated colloidal suspension can be expressed near the percolation threshold by a power-law correlation on the difference between volume fraction of particles f and the threshold value fc [44e46].
G0 fðf fc Þn
(1)
Assuming the shear modulus of pCBT dispersed with TrGO follows this power-law relationship, the percolation threshold fc and exponent n can be evaluated by applying Eq. (1) to G0 at different frequencies. Least-squares regressions were carried out until optimum fc and n were found that best fit the data. Then, the onset of percolation concentration is 0.47 vol% (0.8 wt%), which is close to the point where the critical strain starts to drop more greatly (1 wt %) from Fig. 14. The percolation concentration is also quite close to the transition concentration that polymerization reaction rate starts to decrease substantially (see inset of Fig. 6). As stated above, the constraint of TrGO nanosheets on the reactivity of grafted pCBT chains is the main reason for the slowdown of polymerization at low TrGO concentration. The agreement of the percolation concentration and the transition concentration in Fig. 6 implies that the constraint on the diffusion of reactive species from the network of nanosheets is the main reason for the substantial decrease in the polymerization rate at higher TrGO concentration. We also could evaluate the percolation threshold of nanocomposites solids from electrical conductivity, as electrical conductivity obeys the following power laws:
Fig. 16. The dependence of the low-frequency storage modulus measured at 0.1, 0.158 and 0.251 rad/s on TrGO content.
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sc fðp pc Þm
(2)
where sc is the electrical conductivity, p is the volume fraction of particles, pc is the threshold value and m is a constant. As shown in Fig. 17, a percolation threshold value of 0.95 vol% (1.6 wt%) was obtained. This threshold value was two times larger than the one from rheological method, which is ascribed to the different kinds of network that is detected by rheology and electrical conductivity. The contact network from the gyration radius of nanosheets can contribute to the rheological behavior, while it is not sufficient to have a direct contact for electrical conductivity. That is why rheological threshold is often smaller than the electrical one especially for polymer composites filled by fillers with high aspect ratio. The lower percolation threshold of TrGO is strong evidence for better dispersion. Ren and co-workers showed that a relationship can be constructed between the percolation threshold and the aspect ratio, Af of a tactoid [47]. Supposing imaginary spheres surrounding each tactoid, the expression for the fraction of particle diameter 2r to thickness h can be written as
Af ¼
3fsphere 2r ¼ 2fc h
(3)
Percolation of interpenetrating, randomly packed sphere occurs at fsphere ¼ 0.29 [48,49]. Using the onset of percolation of TrGO from melt rheology, Eq. (3) gives aspect ratio of 92.6. The aspect ratio of TrGO evaluated from the TEM images is 73.3, which is slightly less than the value from rheological measurement. The rheological behaviors shown above of the pCBT/TrGO nanocomposites are quite similar to flocculated suspensions. It has been suggested that the relationship between viscoelastic properties of flocculated system and particle concentration can be described by the scaling law proposed by Shih and co-workers [50]. Assuming the elastic network composed of TrGO particles can be treated as fractal aggregates of graphene flocs, their storage modulus (G0 ), and limits of linearity (gcrit), above the gelation threshold can be scaled with the volume fraction of the flocs. When the bonds between flocs are stronger than the links within each floc, the G0 and gcrit can be written as
G0 ffð3þxÞ=ð3df Þ ð1þxÞ=ð3df Þ
gcrit ff
Fig. 18. Scaling of G0 at 0.1 rad/s for pCBT/TrGO melts at 230 C as a function of TrGO volume fraction.
storage modulus and critical strains above percolation threshold were chosen as shown in Fig. 18. Both G0 and gcrit show a power-law behavior with respect to TrGO volume fraction. Using Eqs. (4) and (5), we obtained df y 0.37 and x y 5.26, which are unreasonable for our system. The exponent 3.14 for G0 is significantly lower than the scaling exponent reported for G0 of graphene in PEN melts, w5.16 [36]. However, for pCBT/TrGO melts where the TrGO was treated at 900 C to minimize the TrGO influence to polymerization, The scaling exponent for G0 increases to 4.28, and we obtained df y 1.94 and x y 1.55 which are close to the values of PEN/graphene system reported by Kim [36]. The success and failure of fractal dimension analysis in different pCBT/TrGO systems imply the inherent difference in the microstructures. TrGO that reduced at lower temperature reduces the molecular weight of matrix pCBT and induces the graft of pCBT chain on the nanosheets. The change in the matrix molecular weight actually makes the nanocomposites with different TrGO concentration not comparable in the sense of the same matrix polymer. Moreover, the entanglement between the grafted pCBT chains on the nanosheets and the pCBT chains in bulk make the nanosheets network not the ideal one with only particleeparticle interactions. This result again justifies the effect of TrGO on the polymerization of CBT. 4. Conclusions
(4) (5)
where df is the fractal dimension of the aggregates and x is the fractal dimension of the backbone in the aggregate which is responsible for elasticity. In order to apply this model to our system,
Graphene sheets were prepared by chemical oxidation of pristine graphite flakes followed by thermal exfoliation and reduction of graphite oxide. The pCBT/TrGO nanocomposites were successfully prepared by ROP of CBT. TrGO was found to depress the polymerization rate as well as the degree of polymerization as observed from rheology. It was confirmed by XPS, NMR and TGA measurements that the growing pCBT chains could graft onto the surface of TrGO nanosheets by reaction between carboxyl groups of pCBT and hydroxyl and epoxy groups of TrGO with the grafting content up to 53 wt%. The grafting reaction was further justified from nonlinear rheology and the fractal dimension analysis. The constraint effect of nanosheets on the reactivity of grafted pCBT chain is the main reason for the slowdown in the polymerization rate at low TrGO concentration, while the network formation at higher TrGO concentration and its constraints on the diffusion of reactive species will further decrease the polymerization rate. Acknowledgment
Fig. 17. Electrical conductivity of pCBT/TrGO nanocomposites.
The authors would like to thank the support from National Basic Research Program of China (973 Program) 2012CB025901 and the National Natural Science Foundation of China (No. 50930002). W. Yu is supported by the Program for New Century Excellent Talents in University and the SMC project of Shanghai Jiao Tong University.
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