Article pubs.acs.org/IECR
UV-Curable Functionalized Graphene Oxide/Polyurethane Acrylate Nanocomposite Coatings with Enhanced Thermal Stability and Mechanical Properties Bin Yu,†,‡,§ Xin Wang,† Weiyi Xing,† Hongyu Yang,†,‡,§ Lei Song,† and Yuan Hu*,†,‡ †
State Key Laboratory of Fire Science, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, P.R. China ‡ USTC-CityU Joint Advanced Research Centre, Suzhou Key Laboratory of Urban Public Safety, Suzhou Institute for Advanced Study, University of Science and Technology of China, 166 Ren’ai Road Suzhou, Jiangsu 215123, P.R.China § Department of Building and Construction, City University of Hong Kong, Tat Chee Avenue Kowloon, Hong Kong ABSTRACT: Functionalized graphene oxide (FGO) was synthesized and subsequently incorporated into polyurethane acrylate (PUA) by UV curing technology. The structural and morphological features of FGO/PUA nanocomposite coatings were characterized by FTIR, XRD, and TEM. The results showed that FGO sheets were uniformly dispersed into the PUA matrix and formed the strong interfacial adhesion with PUA owing to the formation of the cross-linking networks between FGO and PUA after UV curing. The incorporation of FGO effectively enhanced the thermal stability and mechanical properties of host polymer. The initial degradation temperature of the PUA composite with 1.0 wt % FGO was increased to 316 °C from 299 °C for neat PUA. Meanwhile, the storage modulus and tensile strength of the PUA composite with 1.0 wt % FGO were also improved by 37% and 73%, respectively, compared with those of neat PUA. The slight increase in glass transition temperature (Tg) of the composites was observed upon the incorporation of FGO. By contrast, untreated GO/PUA nanocomposites exhibited relatively low thermal stability and poor mechanical properties than its modified-GO counterpart. The covalent functionalization of graphene oxide presented herein will provide a feasible and effective approach to obtain high-performance UV-curing nanocomposite coatings.
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INTRODUCTION Over the past few decades, there has been increasing interest in UV-curing technology, which is widely used in the fields of paints, coatings, adhesives, inks and composites because of the distinctive advantages over thermal curing, such as low energy consumption, low temperature operation, environmental friendliness (without VOC emissions), and high curing speed.1−3 Among various UV-curable resins, polyurethane acrylate (PUA) shows outstanding adhesion to substrates and excellent flexibility and impact strength. However, the poor thermal stability and low tensile strength of UV-cured PUA coatings probably limit their engineering applications in some fields.4−6 It is thereby essential to fabricate modified PUA composites with enhanced thermal and mechanical properties to broaden their application. Recently, graphene, a two-dimensional (2D) one-atom thick carbon layer material, has attracted considerable attention owing to its superior electronic, thermal, and mechanical properties.7−9 With these fascinating properties, GNS has motivated a tremendous interest to prepare GNS-based polymer composites.10−12 As reported in previous literatures, graphene, graphene oxide (GO) or functionalized graphene have been added into various polymer matrices, which shows remarkably reinforced properties.13−15 Cai et al. prepared polyurethane-GO nanocomposites by an solution processing method. The nanocomposite displayed a ∼900% and ∼327% increment in Young’s modulus and hardness at the GO loading of 4.4%, indicating the promising application of graphene in coatings.11 Wang et al. reported that polyurethane-GNS nanocomposite © 2012 American Chemical Society
prepared by in situ polymerization exhibited a 239% and 202% increase in tensile strength and storage modulus, respectively. Meanwhile, high electrical conductivity and good thermal stability of polyurethane were also achieved.15 As is well-known, the strong intrinsic van der Waals interactions between graphene nanosheets (GNS) make them easily stack and reagglomerate which restricts their application in polymer nanocomposites.16,17 Therefore, uniform dispersion of the GNS in polymer matrix is an important issue of concern. Besides, to achieve efficiently improved properties for composites, the formation of a strong bond between polymer matrices and graphene is another challenge that needs to be resolved.18 To solve these two problems, chemical modification of GNS has provided an effective approach. Graphene oxide, the precursor of graphene, possesses the rich oxygen-containing functional groups (such as epoxy, hydroxyl) on the surface and edge that provide reactive sites for chemical modification.19−24 For example, Stankovich and co-workers functionalized GO with isocyanate derivatives via formation of amides or carbamate esters. The resulting functionalized nanoplatelets were well dispersed in polar aprotic organic solvents.25 Haddon et al. prepared modified GO by converting the edge carboxylic acid groups of GO into octadecylamines, which made GO steadily Received: Revised: Accepted: Published: 14629
May 28, 2012 September 18, 2012 October 9, 2012 October 9, 2012 dx.doi.org/10.1021/ie3013852 | Ind. Eng. Chem. Res. 2012, 51, 14629−14636
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Scheme 1. Preparation Route of Functionalized GO and FGO/PUA Nanocomposites
dispersed in THF, CCl4, and 1,2-dichloroethane.26 Also, reduced GO (rGO) platelets were covalently functionalized with diazonium salts, and the resulting rGO was readily dispersed in several polar organic solvents.27 Besides these modification methods aforementioned, some other approaches have also been developed to improve the dispersion of graphene in organic solvents, like by cycloaddition of nitrenes onto the −CC− double bonds or nucleophilic substitution of epoxy groups.28−30 The chemical functionalization of graphene not only greatly weakens the intermolecular interactions which can prevent the agglomerate of fillers, but also strengthens the interactions between polymer matrix and graphene through covalent linkage or noncovalent bonding (such as hydrogen bonding). Therefore, GNS-based polymer composites with effectively enhanced properties can be achieved. However, the research work aforementioned scarcely involved the application of graphene in UV-curable polymer matrices. To develop high-performance coatings, graphene is one kind of promising nanofiller to reinforce the polymer materials. Herein, we demonstrated an approach to functionalize graphene oxide with an active end-group of acrylate, and then the functionalized graphene oxide was covalently incorporated into PUA by UVcuring technology. The effect of surface functionalization of GO
on the morphology, mechanical, and thermal properties of PUA composites was investigated.
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EXPERIMENTAL SECTION Materials. Polyurethane acrylate (PUA) was purchased from Tianjin Tianjiao Radiation Curable Materials Co. Ltd. (Tianjin, China). 4,4′-Diphenylmethane diisocyanate (MDI) was supplied by Anhui ANLI Artificial Leather Co. Ltd. (Hefei, China). Graphite powder (SP), potassium permanganate (AP), hydrochloric acid (CP), sulfuric acid (98%), sodium nitrate (AP), hydrogen peroxide (30% aq.) N,N-dimethylformamide (DMF, AP), chloroform (AP), and acetone (AP) were all purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Hydroxylethyl acrylate (HEA) was purchased from Dong-fang Chemical Co., Ltd. (Beijing, China), distilled at reduced pressure, and dried over 4 Å molecular sieves before use. 2Hydroxy-2-methyl-1-phenyl-1-propanone (Darocur 1173) was kindly provided by Ciba Specialty Chemicals and used as a photoinitiator. Preparation of Graphene/Polyurethane Acrylate Nanocomposite. The preparation procedure of graphene/ polyurethane acrylate nanocomposite was illustrated in Scheme 1. First, graphene oxide (GO) was synthesized on the basis of the 14630
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Hummer’s method.31 Then, functionalized graphene oxide (FGO) was prepared as follows: 0.6 g of GO was dispersed in 60 mL of DMF with ultrasonication for 30 min. Afterward, the suspension was obtained and introduced into a 250 mL fournecked flask equipped with a nitrogen inlet, a mechanical stirrer, a dropping funnel, and a reflux condenser. After the flask was saturated with nitrogen atmosphere, 14.28 g of MDI predissolved in 10 mL of DMF was added dropwise at 70 °C. After 3 h, the reaction mixture was cooled down to 50 °C. Then 0.1 g of 4methoxyphenol and 14.3 g of HEA were dropped and the temperature was kept at 50 °C for 12 h. Finally, the suspension obtained was filtered and washed with chloroform. The FGO was collected and dried at 35 °C under reduced pressure overnight. A series of UV-cured FGO/PUA coatings with 0.1 wt %, 0.5 wt %, 1.0 wt % of FGO were prepared, which were designated as FGO/PUA-0.1, FGO/PUA-0.5, and FGO/PUA-1.0, respectively. In brief, the preparation procedure was depicted as follows: first, FGO was dispersed in acetone with ultrasonic treatment for 30 min and then added into PUA with stirring to get a uniform mixture. Subsequently, 3 wt % Darocur 1173 was added into the mixture. The mixture was coated on a glass plate with a scraper and put in a oven at 60 °C for 3h to degas and remove the solvent. Finally, the film was cured using the UV irradiation equipment (80 W cm−2, Lantian Co., China). Characterization. Fourier transformed infrared (FTIR) spectra were obtained using a Nicolet 6700 (Nicolet Instrument Company, USA) spectrophotometer with the wavenumber ranging from 500 to 4000 cm−1. Samples were mixed with KBr powders and pressed into tablets for characterization. X-ray photoelectron spectroscopy (XPS) was conducted on a VG Escalab Mark II spectrometer (Thermo-VG Scientific Ltd. UK), using Al Ka excitation radiation (h2ν = 1253.6 eV). X-ray diffraction (XRD) measurements was performed with a Japan Rigaku D/Max-Ra rotating anode X-ray diffractometer equipment using a Cu Kα tube and Ni filter (λ = 0.1542 nm). The scan rate was 4° min−1 and the 2θ scan range was from 3 to 65°. Atomic force microscopy (AFM) observation of GO and FGO was performed on the DI Multimode V in tapping-mode. The samples were dispersed in deionized water with ultrasonic treatment and then dip-coated onto freshly cleaved mica surfaces for characterization. Transmission electron microscopy (TEM) was performed using a JEOL JEM-2100 instrument with an acceleration voltage of 100 kV. The films were cryo-microtomed with a diamond knife into a thickness of 50−70 nm. The ultrathin films were placed on copper grids before observation. Tensile testing was carried out according to the Chinese standard method (GB 13022-91) with a WD-20D electronic universal testing instrument (Changchun Intelligent Instrument Co., Ltd., China) at a crosshead speed of 10 mm min−1. At least five samples were tested to obtain average values. Thermogravimetric analysis (TGA) was carried out on a TGA Q5000IR (TA Instruments, USA) thermo-analyzer instrument. About 3.0 mg of the sample was measured from 30 to 700 °C at a linear heating rate of 10 °C min−1 under nitrogen atmosphere. Dynamic mechanical analysis (DMA) was measured using a DMA Q800 (TA Instruments Inc., USA) from −80 to 150 °C at the heating rate of 5 °C min−1. The frequency of dynamic oscillatory loading was 10 Hz. The transparency of samples was evaluated with a DUV-3700 UV−vis spectrometer (Sahimadzu, Japan). The transmission mode was used and the wavelength ranged from 400 to 700 nm.
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RESULTS AND DISCUSSION Structural and Morphological Characterizations. FTIR Analysis. The FTIR spectra of GO, FGO, FGO/PUA-0.5 are shown in Figure 1. The pristine GO presents some characteristic
Figure 1. FTIR spectra of GO, FGO, and FGO/PUA-0.5.
absorption peaks including 3379 cm−1 (−OH), 1724 cm−1 (C O carboxyl stretching vibration), 1623 cm−1 (CC in aromatic ring), 1226 cm−1 (−COO−), and 1043 cm−1 (C−O−C in epoxide), which is in agreement with the earlier research.32 Compared with GO, FGO shows two new absorption peaks at around 2924 and 2854 cm−1, which originate from the symmetric and asymmetric vibration of −CH2 − groups after the functionalization of GO by HEA and MDI. Moreover, the stretching vibrations of −NH− groups appear at 3393 cm−1, which together with the carbonyl bands at 1705 cm−1 are indicative of the existence of urethane moieties. Additionally, the typical peaks at 1635 cm−1 and 811 cm−1 are attributed to the CC vibration, while the characteristic band at 1412 cm−1 is assigned to the bending vibration of C−H, indicating the covalent bonding of HEA onto the surface of GO sheets via the reaction between −OH/−COOH groups with −NCO groups. This reaction is also supported by the disappearance of the peak of the isocyanate group of MDI at 2264 cm−1. From the FTIR spectra of FGO/PUA-0.5, it can be seen that FGO/PUA-0.5 shows no obvious absorption in the infrared range of 1530−1730 cm−1 after curing, where the CC groups vibration appear, suggesting the complete curing reaction of FGO/PUA nanocomposites. XPS. To further confirm the structure of GO and FGO, XPS analysis was performed to evaluate the surface composition of GO before and after its modification. Figure 2 presents the C1s XPS spectra of (a) GO, (b) FGO, and (c) XPS survey scans of GO and FGO. As shown in Figure 2a, the C1s XPS spectrum of GO can be divided into three peaks, corresponding to carbon atoms in different functional groups. The characteristic peaks at 284.6, 286.6, and 288.7 eV are attributed to unoxidized graphite carbon (C−C), the C atoms in hydroxyl and epoxy/ether groups (C−O), and carboxyl groups (O−CO), respectively.33 After the surface functionalization, the peak intensity at 287.2 eV is drastically reduced, which is due to the reduced hydroxyl groups. Moreover, a new peak at 285.6 eV (C−N) is observed in the C1s spectra of FGO, indicating the successful attachment of MDI to the surface of GO sheets. As expected, the N1s peak with significantly increased intensity at 400 eV appears in the XPS 14631
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Figure 2. C1s XPS spectra of (a) GO, (b) FGO, and (c) XPS survey scans of GO and FGO.
value of 19.4° in the XRD pattern, which is ascribed to the amorphous feature of the cross-linked polymer matrix. The FGO/PUA nanocomposites present the similar XRD patterns as that of virgin PUA, regardless of the addition content of FGO. It is worth noting that, in the XRD patterns of FGO/PUA nanocomposites, no visible FGO characteristic peak is observed, indicating the long-range disorder or full exfoliation of FGO sheets in the PUA matrix owing to the uniform dispersion of FGO within PUA. AFM. Atomic force microscopy (AFM) is utilized to observe the morphology of GO and FGO nanosheets and measure their thickness. Figure 4 gives the tapping-mode AFM images and corresponding height profiles of (a) GO and (b) FGO, respectively. It can be seen from panel a that GO shows the exfoliated sheets with the thickness of about 0.923 nm, which agrees well with the values reported in the previous study.34 After the surface functionalization, the thickness of FGO sheets is increased to 3.157 nm. The increment in thickness of FGO nanosheets is probably caused by the organic compounds grafted on the graphene oxide sheets,35−37 indicating the successful functionalization of GO. TEM. To evaluate the dispersion state of GO and FGO in the PUA matrix, the composites were characterized by TEM, as shown in Figure 5. It can be seen from Figure 5a that the bare GO is dispersed in the PUA matrix mainly with the form of agglomerates, whereas the FGO is dispersed more uniformly (Figure 5b), suggesting much better dispersion of FGO in the PUA matrix. This phenomenon can be explained by that functionalized graphene oxide with some active double bond on the basal planes and edges enables itself to disperse well within PUA and form the strong interfacial adhesion with PUA through covalent linkage.
survey scans of FGO, which further demonstrates the reaction of GO with MDI. XRD. The XRD patterns of the GO, FGO, and FGO/PUA nanocomposites are shown in Figure 3. The GO exhibits a sharp
Figure 3. XRD patterns of (a) GO, (b) FGO, and FGO/PUA nanocomposites: (c) neat PUA, (d) 0.1% FGO, (e) 0.5% FGO, and (f) 1.0% FGO.
diffraction peak at 2θ = 10.0°, corresponding to the (002) reflection of GO, which is consistent with the previous report.34 Moreover, the interlayer spacing of GO is calculated as 0.89 nm according to the Bragg’s equation. In contrast, the XRD trace of FGO presents a small peak at 2θ = 5.4°, giving an interlayer spacing of 1.61 nm. The enlarged d-spacing indicates the modifiers have been successfully bonded into the interlayer of GO. For the pure PUA, there is a broad peak appearing at 2θ 14632
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Figure 4. AFM images and corresponding height profiles of (a) GO and (b) FGO dispersed in deionized water.
Tensile Properties. To investigate the reinforced effect, the tensile properties for PUA nanocomposites were tested. The tensile strength versus filler content curves for the pure PUA, FGO/PUA, and GO/PUA are plotted in Figure 7. It can be observed that an obvious increase in the tensile properties is achieved: the incorporation of 1 wt % FGO leads to a 73% increase in tensile strength. However, at the same loading of GO, PUA composite only displays an 11% increase. The reason why FGO/PUA composite exhibits better tensile strength than its untreated-GO counterpart can be attributed to two aspects: the organic molecular chain grafted on the GO surface can prevent its stacking and aggregation, and thus improve the dispersion state of the GO in the PUA matrix; the acrylate functional groups of FGO are expected to form the chemical bonding to PUA after UV irradiation, consequently, the interfacial interaction between different components becomes stronger. DMA. Graphene or graphene oxide is one of the most effective nanofillers, which can achieve a significant reinforcement in mechanical properties for polymer composites.13,38 Therefore, DMA was carried out to study the influence of FGO on dynamic mechanical properties of UV-cured FGO/PUA films. Figure 8 gives the storage modulus (E′) (left) and loss factor (right) curves of pure PUA and various FGO/PUA composites as a function of temperature. As shown in Figure 8, incorporation of FGO results in an increase of the storage modulus throughout the entire temperature range studied. Compared to the pure PUA, the E′ at −80 °C for FGO/PUA-0.1, FGO/PUA-0.5 and FGO/PUA-1.0 is increased by 19%, 29% and 37%, respectively. The storage modulus values are greatly influenced by the interfacial interactions between the graphene sheets and the PUA
Thermal and Mechanical Properties. TGA. Figure 6 gives the TGA and DTG curves of GO, FGO, PUA, FGO/PUA, and GO/PUA nanocomposites under N2 atmosphere. As can be observed, GO starts to lose weight below 100 °C owing to the release of adsorbed water. As the temperature increases, the main decomposition process of GO takes place in the temperature range of 150−450 °C, resulting from the removal of some oxygen-containing functional groups such as hydroxyl and epoxy. After functionalization, FGO becomes more thermally stable compared to GO. The slight weight loss below 100 °C of FGO is ascribed to the release of adsorbed water, while the second weight loss in the temperature range of 180−250 °C is due to the degradation of organic compounds attached on the surface of GO. The initial degradation temperature (Ti) is defined as the temperature when the mass loss is 5%. As can be seen, the virgin PUA undergoes a two-step decomposition process with the Ti of 299 °C. For the UV-cured FGO/PUA nanocompostie coatings, the incorporation of FGO gives rise to a remarkable improvement in thermal stability. When the loading of FGO is 1.0 wt %, the Ti of the nanocompostie is increased to 316 °C, which is a 17 °C increment than that of pristine PUA. The enhanced thermal stability is considered to be due to the physical barrier effect of graphene, just like other layered materials such as clay and layered double hydroxides, which slows down the escape of pyrolysis products and good dispersion of the nanosheet filler and strong graphene−polymer interactions.38 However, at the loading of 1 wt % GO, the Ti of GO/PUA composite is lower relative to pure PUA, since GO is thermally unstable due to the decomposition of the oxygen-contained functional moieties. 14633
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Figure 5. TEM microphotographs of (a) GO/PUA-1.0 and (b) FGO/ PUA-1.0 nanocomposites. Figure 6. (a) TG and (b) DTG curves of GO, FGO, PUA, GO/PUA, and FGO/PUA nanocomposites under nitrogen atmosphere.
matrix and the degree of dispersion of FGO in composites. The observed results suggest a strong interfacial bonding and well dispersed levels of FGO are obtained, as supported by TEM images. The glass transition temperature (Tg) is determined from the temperature of the tan δ peaks in DMA. There is a slight increase in Tg (about 5 °C) of FGO/PUA-0.1 nanocomposite, indicating that the formation of covalent linkage between FGO and PUA improves the cross-linking density. However, Tg subsequently decreases while adding more FGO. This phenomenon may be attributed to the UV-shielding effect of graphene nanosheets, which leads to the reduced cross-linking density with excessive FGO.39,40 As discussed above, at the high content of FGO fillers, FGO forms a strong interfacial interaction with the PUA matrix and weakens the cross-linking network of UV-cured PUA, thereby increasing the storage modulus and reducing the glass transition temperature of nanocomposite films. Transparency. Figure 9 shows the UV−vis spectra of PUA and FGO/PUA nanocomposite films, and the inserted picture is the digital photos of the nanocomposites. It can be observed that the pure PUA shows high transparency throughout the visible light range of 400−700 nm, while the transmittance values of PUA were decreased gradually along with the incorporation of FGO. The transmittance at 550 nm is about 67% amd 26% for the FGO/PUA-0.1 and FGO/PUA-1.0 nanocomposites, respectively, whereas that of pure PUA is 87%. The significant reduction of the transmittance is probably attributed to the shielding effect of FGO nanosheets.40
Figure 7. The tensile strength versus filler content curves for the pure PUA, FGO/PUA, and GO/PUA.
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CONCLUSION In summary, the functionalized graphene oxide with acrylate was successfully synthesized and confirmed by FTIR, XPS and XRD. The AFM images showed that the thickness of FGO was increased to 3.157 nm as compared to GO. The increase in thickness of FGO suggested that the organic compounds were bonded onto the surface of GO. TEM images revealed that FGO had better a dispersion state in PUA relative to GO, due to the 14634
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CAS and USTC Special Grant for Postgraduate Research, Innovation and Practice.
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Figure 8. Storage modulus (E′) (left) and loss factor (right) curves of pure PUA and various FGO/PUA nanocomposites as a function of temperature.
Figure 9. UV−vis spectra and digital photos of (a) PUA, (b) FGO/ PUA-0.1, (c) FGO/PUA-0.5, and (d) FGO/PUA-1.0 nanocomposites.
strong interfacial adhesion between fillers and polymer matrix. The incorporation of FGO into PUA exhibited superior thermal stability and tensile strength over GO, which was probably attributed to more uniform dispersion state. Meanwhile, the storage modulus was also significantly enhanced due to the strong interfacial interaction between the FGO and PUA matrix, resulting in the efficient load transfer at the interface. However, the addition of FGO had a negative impact on the transmittance of UV-cured PUA coatings, resulting from the UV-shielding effect of FGO sheets. The covalent functionalization of graphene oxide presented herein will provide a feasible and effective approach to obtain high-performance UV-curing nanocomposite coatings.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*Tel./Fax: +86-551-3601664. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The work was financially supported by National Basic Research Program of China (973 Program) (2012CB719701), the joint fund of NSFC and CAAC (No. 61079015), the joint fund of Guangdong province and CAS (No.2010A090100017), and the 14635
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Industrial & Engineering Chemistry Research
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dx.doi.org/10.1021/ie3013852 | Ind. Eng. Chem. Res. 2012, 51, 14629−14636
