Poly(urethane-co-vinyl imidazole)/Graphene Nanocomposites
Zhixin Cai, Li Liu, Zhen Zheng, Xinling Wang State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China
Poly(urethane-co-vinyl imidazole) (PUVI)/graphene nanocomposites were facilely prepared by a kind of noncovalent way. Herein, the 1-vinylimidazole acted as dispersion agent as well as monomer, graphene was uniformly dispersed in the copolymer matrix without obviously agglomeration. A significant enhancement of mechanical and thermal properties of the PUVI/graphene nanocomposites were obtained at low graphene loading; specifically, a 147% improvement of tensile strength, a nearly 10 times increase of elastic modulus and a 128C enhancement of thermal decomposition temperature were achieved at a graphene loading of 1.5 wt%. Moreover, the volume resistivity of the PUVI/graphene nanocomposites decreased by an order of magnitude after adding 0.5 wt% graphene, demonstrating an obvious change in the electrical property of the nanocomposites prepared. POLYM. COMPOS., 33:459–466, 2012. ª 2012 Society of Plastics Engineers
INTRODUCTION Graphene, a two-dimensional material, discovered in 2004 [1], is composed of several planar sheets of sp2-bonded carbon atoms. It is regarded as the ‘‘thinnest material in the universe’’ with tremendous application potential [2, 3]. Typically important properties of graphene are extreme mobility of charge carriers ( 2 3 105 cm2/V/s), high values of Young’s modulus (1.0 6 0.1 Tpa), thermal conductivity (5,000 W/m/K), fracture strength (125 GPa), specific surface area and outstanding transport phenomena such as the quantum Hall effect [4–8]. These special properties have great potential applications in many technological fields, such as lithium ion batteries, ultrasensitive sensors, supercapacitors, and solar cells [9–12]. Recently, graphene nanocomposites with various polymer matrices have been explored. [13, 14]. Polymer/graCorrespondence to: Xinling Wang; e-mail:
[email protected] or Zhen Zheng e-mail:
[email protected] Contract grant sponsor: Shanghai Leading Academic Discipline Project; contract grant number: B202. DOI 10.1002/pc.22168 Published online in Wiley Online Library (wileyonlinelibrary.com). C 2012 Society of Plastics Engineers V
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phene nanocomposites show superior mechanical, thermal, gas barrier, electrical, and flame retardant properties compared with the neat polymer [15–17]. The improvement in mechanical and electrical properties of graphene-based polymer nanocomposites is much better in comparison with that of clay or other carbon filler-based polymer nanocomposites [15, 18–20]. For example, Ramanathan et al. [16] prepared polymer nanocomposites, which obviously increase the glass transition, strength, and thermal stability, with low-filled content of functionalized graphene sheets. When embedded in an epoxy matrix, the graphite nanoplatelets provided a thermal conductivity enhancement of more than 3,000% (loading of 25 vol%), and a thermal conductivity surpassed the performance of conventional fillers that require a loading of 70 vol% to achieve these values [21]. However, the improvement in the physicochemical properties of the nanocomposites depends on the dispersion of graphene layers in the polymer matrix. Because of its high specific surface area, graphene tends to form irreversible agglomerates or even restack by means of van der Waals interactions. Hence, the chemical functionalization of nanomaterials is a usual method to solve the problem because the chemical groups can improve the solubility and processability as well as enhance the interactions with organic polymers [22, 23]. Stankovich et al. [24] treated graphene oxide with organic isocyanates, and these isocyanate-treated graphene oxides could then exfoliated into functionalized graphene oxide nanoplatelets, exhibiting a stable dispersion in polar aprotic solvents. Liu et al. [25] presented ionic–liquid-treated graphite sheets via one-step electrochemical, which could exfoliated into functionalized graphene nanosheets, and then synthesized graphene nanosheet/polystyrene composites. Another method of dispersing graphene sheets is developed by noncovalent functionalization through ‘‘p––p’’ interactions. Unlike chemical functionalization, noncovalent functionalization preserves the structure of graphene. For example, Xu et al. [26] demonstrated the pyrenebutyric acid could be used to noncovalently functionalize graphene sheets via strong ‘‘p––p’’ interactions between pyrenyl rings and basal planes of graphene sheets. Mu¨llen, Feng, and coworkers [27] described a method to disperse
FIG. 1.
1
H-NMR spectrum of PU.
graphene sheets in organic solvent supported by ionic interactions. Zhou et al. [28] reported that dispersion of graphene sheets in ionic liquid was successfully achieved with the aid of a polymerized ionic liquid. Consequently, there is a constant demand for developing a facile and efficient approach to disperse graphene sheets for exploring the polymer/nanocomposites. In this article, poly(urethane-co-vinyl imidazole) (PUVI)/graphene nanocomposites prepared by incorporating graphene into PUVI matrix will be studied in detail. Graphene was successfully fabricated into the PUVI matrix by noncovalnet functionalization, rather than with strong oxidation and complicated chemical functionalization process. Unlike other noncovalent methods, this method did not require a surfactant because 1-vinylimidazole was both a dispersion agent and monomer. EXPERIMENTAL Materials Graphite with an average particle size of 30 lm and a purity of [99% was supplied from Qiaodao RuiCheng Graphite Co., Ltd. Concentrated sulfuric acid (H2SO4), hydrochlo460 POLYMER COMPOSITES—-2012
ric acid (37% HCl), potassium permanganate (KMnO4), hydrogen peroxide (30% H2O2), hydrazine hydrate (85%), and dibutyltin dilaurate (DBTDL) were purchased from Shanghai Regents Co., Ltd. 1-Vinylimidazole was purchased from Sigma-Aldrich Co., Ltd. Polypropylene glycol (PPG, Mn ¼ 1,000) provided by Gaoqiao Chemical Co., which needed to be purified in vacuum oven at 1008C for 4 h. Toluene diisocyanate (TDI) was supplied from Bayer MaterialSci. Co., Ltd. Hydroxyethyl methacrylate (HEMA) was purchased from TCI (Shanghai) Development Co., Ltd. Toluene was purchased from Sinopharm Chemical Regent Co., which was dried with CaH2 and purified by distillation at room temperature. Benzoyl peroxide (BPO) was purchased from Shanghai Regents Co., which was recrystallized before use as an initiator for radical polymerization. The other chemicals were used as received.
Synthesis of PU TDI (17.416 g) and toluene (17 ml) were loaded in a three-necked round-bottomed flask. Then, PPG1000 (50 g), toluene (50 ml), and DBTEL (0.02 g) were slowly injected into the flask. The mixture was stirred in N2 atmosphere and reacted at 708C for about 4 h. The blockDOI 10.1002/pc
FIG. 2. The typical AFM tapping-mode images of GO (A) and graphene (B), which deposited onto a mica substrate from an aqueous dispersion (0.1 mg/ml). The image indicated that thickness of GO sheet was 1 nm, and thickness of graphene was 0.5 nm. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
ing reaction was carried out with HEMA (13.014 g), toluene (14 ml), and DBTEL (0.02 g) at 708C for 5–6 h to synthesize the PU (The 1HNMR spectrum of PU was shown in Fig. 1). Synthesis of PUVI Calculated amount of 1-vinylimidazole and BPO (dissolved in acetone) fed into the PU; then, the mixture was quickly stirred for about 10 min; and then vacuumized by a vacuum pump at 308C to remove most of the solvent in it. Finally, the viscous mixture was cast on a tetrafluoroethylene plate and initiated at 808C for 8 h, 1008C for 2 h under N2 atmosphere to completely cure the PUVI composites. Synthesis of Graphite Oxide and Graphene Nanosheets Graphite oxide (GO) was synthesized from natural graphite by oxidation with KMnO4 in concentrated H2SO4 according to Hummers method as originally presented by Kovtyukhova et al. [29, 30]. Concentrated H2SO4 (46 ml) was poured into the 250 ml three-necked flask and stirred in an ice bath until the temperature dropped to 0 6 38C. Natural graphite (2 g) was added and stirred uniformly. KMnO4 (6 g) was gradually added with stirring and cooling to keep the temperature below 208C. The solution was heated to 35 6 38C and maintained for 2 h. Then, DOI 10.1002/pc
distilled water was slowly added and the temperature was controlled lower than 1008C. After 15 min, this reaction was terminated by addition of a large amount of distilled water and 30% H2O2 solution (10 ml). The mixture was filtered and washed with 5% HCl aqueous solution and water. The sample of GO was obtained after drying. In a typical procedure for chemical conversion of GO to graphene, 0.2 g GO was dispersed with 400 ml distilled water to create a yellow-brown dispersion. Exfoliation of GO was achieved by ultrasonication of the dispersion for 60 min. Then the resulting homogeneous dispersion was mixed with 280 ll hydrazine solution as a reducing agent, and 2.8 ml ammonia solution was added to adjust pH to around 10. After stirred for a few minutes, the mixed solution was heated at 958C for 1.5 h [31, 32]. The solution was washed by water, methanol, and acetone several times. After removing most of solvents, graphene nanosheets, that is, the reduced graphene oxide (r-GO) were obtained (The AFM images of GO and graphene were shown in Fig. 2). Synthesis of PUVI/Graphene Nanocomposites The typical synthesis route of the PUVI/graphene nanocomposites was shown in Scheme 1. The graphene was prepared in the 1-vinylimidazole by ultrasonication. Then, the graphene suspension and BPO (dissolved in acetone) were fed into the PU. The mixture POLYMER COMPOSITES—-2012 461
SCHEME 1. Illustration for preparation of the PUVI/graphene nanocomposites (a) and chemical structure of PUVI (b) (R ¼ 1-vinylimidazole).
was quickly stirred for about 10 min and then vacuumized by a vacuum pump at 308C to remove most of the solvents in it. Finally, the viscous mixture was cast on a tetrafluoroethylene plate and cured at 808C for 8 h and then at 1008C for 2 h under N2 atmosphere.
Characterization The FTIR spectra were recorded on a Perkin-Elmer 1000 FTIR spectrometer. The structure of PU was confirmed by 1H-NMR spectra (Avance-400, Bruker Switzerland). Scanning electron microscopy (SEM) micrographs
FIG. 3. Vials containing dispersions (1 mg/ml) of graphene in DMF (A), DMF: 1-vinylimidazole (v: v) ¼ 1:1 (B), 1-vinylimidazole (C), PU: 1-vinylimidazole (w:w) ¼ 1:1 (D). [(1) dispersions 2 h, (2) dispersions 12 h, (3) dispersions 24 h, and (4) dispersions one week]. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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speed of 500 mm/min for 75 3 4 3 1 mm3 dumb-bell specimens according to Chinese National Standard GB/T 528-1998. The resistance of nanocomposites was measured by digital multimeter (EST 121). RESULTS AND DISCUSSION
FIG. 4. FTIR spectra region from 3,700 cm21 to 2,500 cm21 of PU (a), PUVI (b), and the PUVI/graphene nanocomposite with 1 wt% grapheme (c). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
were attained by a Hitachi S-5200 field-emission scanning electron microscope. The cross-sections were obtained by brittle fracture of the composites in liquid nitrogen, and sputter coated with 10 nm gold. The gold-coated samples were mounted on an aluminum stub using electric adhesive tape. The morphology and phase were observed by a Veeco-bioscope atomic force microscopy (AFM) (Veeco Instruments Inc.). Dynamic mechanical analysis (DMA), applying a frequency of 1 Hz, a temperature ramp of 3.08C/min and a scanning range from 2808C to 1208C conducted on Netzsch 242C. The size of specimens was about 30 3 5 3 1 mm3. Thermogravimetric analysis (TGA) was investigated by a Q5000 (TA Instrument Inc., USA) from 40 to 5008C at heating rate of 208C/min under N2 flow. Tensile properties were carried out on the INSTRON 4465 Universal Testing System with a crosshead
Figure 3 showed the dispersions of the graphene in DMF (A), DMF: 1-vinylimidazole (v:v) ¼1:1 (B), 1vinylimidazole (C), PU:1-vinylimidazole (w:w) ¼ 1:1 (D), at 1 mg/ml concentration. The vials with graphene in DMF and DMF/1-vinylimidazole contained visible precipitates, indicating poor dispersion (after 12 h). The dispersion of graphene in 1-vinylimidazole solvent contained partly precipitates, while that of graphene in PU/1-vinylimidazole almost had no precipitates and was stable for weeks. We speculated that the ‘‘p––p’’ interactions, which formed between imidazole ring and graphene, might be one of the reason to improve the dispersions of graphene. Another one might be attributed to the viscosity of dissolved PU, which could also assist graphene to be dispersed uniformly stability. Figure 4 showed the FTIR spectra region from 3,700 cm21 to 2,500 cm21of PU, PUVI, and PUVI/graphene nanocomposite with 1 wt% graphene. A shoulder peak around 3,313 cm21 (the characteristic peak of NH of PU) in the FTIR spectra of PUVI was broadened and shifted to lower frequency (3,244 cm21), which indicated the hydrogen bonding interaction in PUVI were increased. Another peak around 3,117 cm21 was the characteristic peak of CH of imidazole ring, which displayed the successful synthesis of PUVI. However, the peak of CH of imidazole ring in the FTIR spectra of PUVI/graphene nanocomposite became much smaller, while the shoulder peak of NH remained practically unchanged. The reason might be due to the ‘‘p––p’’ interactions between imidazole ring and graphene tended to impair the hydrogen bonding interactions in PUVI. The micromorphological structures of PU, PUVI, and PUVI/graphene nanocomposite were observed by AFM
FIG. 5. AFM images of PUVI (A), PU (B), PUVI/graphene nanocomposites (C). (The scale sizes are 500 nm. The dark region was assigned to the ‘‘hard’’ segment, whereas the bright to the ‘‘soft’’ segment.). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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FIG. 6. FESEM image of cross section of PUVI (A) and the PUVI/graphene nanocomposite with 1 wt% graphene (B).
(Fig. 5). The microphase-separated structure of polyurethane was shown in Fig. 5B. The ‘‘soft’’ and ‘‘hard’’ segments formed a phase-separated morphology that was uniformly distributed. However, the separated degree of phase structure was enhanced in PUVI (Fig. 5A). It was possible that the ‘‘hard’’ segments tended to stacked tightly by the increased hydrogen bonding interactions. Hence, the domains of them became much clearer. The domains of the PUVI/graphene nanocomposite also increased comparing with PU (Fig. 5C). The reason might be that the ‘‘p––p’’ interactions between imidazole ring and graphene led the ‘‘hard’’ blocks to stack tightly further. Figure 6 showed FESEM images of PUVI and PUVI/ graphene nanocomposite with 1 wt% graphene. The arrows pointed to the graphene dispersed in the PUVI ma-
trix. The images could be demonstrated that the graphene were well dispersed in the PUVI matrix. The storage modulus (E0 ) and loss tangent (tan d) of PUVL and PUVL/graphene nanocomposites were showed in Figs. 7 and 8, respectively. The PUVI/graphene nanocomposites displayed higher storage modulus because of the graphene as the reinforced filler. Because of the microphase-separated structure, the PUVI/graphene nanocomposites had two glass transition temperatures, Tg1 of ‘‘soft’’ segments and Tg2 of ‘‘hard’’ segments. The Tg2 of PUVI/graphene nanocomposites increased in the presence of graphene, while Tg1 had no obviously change. The results could be the interactions between graphene and ‘‘hard’’ segments of polymer matrix to the properties of nanocomposites.
FIG. 7. Storage modulus of PUVL and PUVL/graphene nanocomposites. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
FIG. 8. Loss factor of PUVL and PUVL/graphene nanocomposites. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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FIG. 10. Electrical property of PUVL/graphene nanocomposites. FIG. 9. Tensile strength of PUVL and PUVL/graphene nanocomposites. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
Figure 9 showed the relationship between graphene nanofillers and the tensile strength of the nanocomposites. The addition of graphene into the polymer matrix significantly influenced the mechanical behavior. The tensile strength enhanced with increasing graphene content, and the 1.5 wt% sample was 147% stronger than the neat polymer. And Table 1 also showed a nearly 10 times increase of elastic modulus. Besides, the low contents of graphene nanosheets would have less influence on the elongation at break of the nanocomposite. The electrical properties of PUVI and PUVI/graphene nanocomposites were presented in Fig. 10. The volume resistivity of the nanocomposites rapidly decreased by an order of magnitude after adding 0.5 wt% graphene. However, an increase in graphene sheet loading above 0.5 wt% yielded a slow growth in volume resistivity. It was possible that the phenomenon of graphene restacking together occurred due to van der waals force of the nanosheets. Zhao et al. also reported this phenomenon [32]. In summary, the increase of loading had a gradual improvement on the electrical property, which provided a new method to prepare conductive nanocomposites by noncovalent ways. Figure 11 presented the thermal properties of PUVL and PUVL/graphene nanocomposites, the initial decomposition temperature of them were 279, 280, 280, and 2848C, respectively. It is indicated that graphene filler was improved the heat resistance of PUVL matrix.
CONCLUSIONS The PUVL/graphene nanocomposites with good dispersion were successfully prepared by noncovalent ways. The result might be the ‘‘p––p’’ interaction formed between the imidazole ring and graphene. With the help of 1-vinylimidazole solvent, which played as the roles of both dispersion agent and monomer, the method of incorporating graphene into PU matrix was simplified. With low-filled content of graphene, the PUVI/graphene nanocomposites exhibited great improvements on modulus, tensile strength, and heat resistance, which could be owing to the enhanced distribution of graphene layers in the polymer matrix and the strengthen molecular interactions. Moreover, the electrical conductivity of the PUVI/ graphene nanocomposites tended to gradually increase with low load of graphene. Dispersion of the bulk quantity graphene nanosheets and further improvement of physicochemical properties were presently under investigation.
TABLE 1. Mechanical properties of PUVL/graphene nanocomposites. Graphene content (wt%) 0.0 0.5 1.0 1.5
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Tensile strength (MPa)
Elongation at break (%)
Elastic modulus (MPa)
5.7 8.0 10.2 14.1
63.0 61.4 61.2 60.0
12.7 22.2 51.4 129.1
FIG. 11. TGA curves of PUVI and PUVL/graphene nanocomposites. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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