polyvinyl alcohol composites

CARBON

5 5 ( 2 0 1 3 ) 3 2 1 –3 2 7

Available at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/carbon

The effect of the ultrasonication pre-treatment of graphene oxide (GO) on the mechanical properties of GO/polyvinyl alcohol composites Yuanqing Li a, Rehan Umer a, Yarjan Abdul Samad a, Lianxi Zheng b, Kin Liao a b

a,*

Aerospace Engineering, Khalifa University of Science Technology & Research, Abu Dhabi 127788, United Arab Emirates School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 639798, Singapore

A R T I C L E I N F O

A B S T R A C T

Article history:

The effects of ultrasonication pre-treatment of graphene oxide (GO) on the mechanical

Received 14 September 2012

properties of GO/poly(vinyl alcohol) (PVA) composites were studied. Results show that

Accepted 21 December 2012

the mechanical properties are sensitive to ultrasonication time (or energy input) and there

Available online 3 January 2013

is an optimum ultrasonication time (OUT) that leads to the largest improvement in mechanical property. If the ultrasonication time is less than OUT, the GO sheets are not fully exfoliated, and only a partial reinforcement effect is achieved. Ultrasonication times longer than OUT cause a reduction of GO sheet size and impair the efficiency of mechanical improvement. The optimized ultrasonication energy input of 15 W h/L may serve as a general guideline for preparation of GO composites.  2013 Elsevier Ltd. All rights reserved.

1.

Introduction

Recently, graphene, a two-dimensional, single-atom-thick structure of sp2-bonded carbon atoms, has attracted tremendous research interest due to its high specific area and unique mechanical, electrical, and thermal properties [1,2]. In the past few years, several methods, such as micromechanical exfoliation [3], chemical vapour deposition [4], chemical reduction of graphene oxide (GO) [5], have been employed to prepare single or multi-layer graphene sheets. Among these methods, chemical reduction of GO is the most promising approach leading to large-scale production of graphene. Although its effectiveness on conversion of GO to restore the intrinsic graphene structure is still in debate, there is an emerging realization on the excellent properties of GO itself for material applications [6]. GO is easily obtainable from natural graphite flakes by strong oxidation and subsequent exfoliation based on Hummers method [7]. Compared to graphene, GO is heavily oxygenated, bearing hydroxyl, carbonyl, and epoxy groups

on the basal planes and edges [8]. Hence, GO is highly hydrophilic and readily exfoliated in water, yielding stable dispersion consisting mostly of single-layered sheets. In addition, GO sheets are mechanically strong with ultimate stress at around 60 GPa and Young’s modulus around 250 GPa [9,10]. The excellent mechanical properties combined with large aspect ratio and inexpensive sources have spurred intensive interests in GO for developing high-performance, cost-effective polymer composites [11]. To date, GO and its derivatives have been applied in various polymers and achieved high reinforcement efficiency [12,13]. Generally, fabrication of GO/polymer composites involves the following four steps: (1) preparation of GO, (2) dispersing GO sheets in a suitable solvent, (3) mixing with the polymer and, (4) fabricating composites with a suitable technique. It is worth noting that the use of ultrasound has become a standard step in fabricating GO/polymer composites [14]. For instance, ultrasonication is required to exfoliate graphite oxide into single-layered GO sheets, and it is also a common measure to break up GO agglomeration in order to achieve

* Corresponding author. E-mail address: [email protected] (K. Liao). 0008-6223/$ - see front matter  2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2012.12.071

322

CARBON

5 5 ( 2 0 1 3 ) 3 2 1 –3 2 7

good GO dispersion in the polymer matrix [15,16]. Although using high-power and/or longer ultrasonication times tends to: (1) decrease the amount of graphite oxide precipitate, (2) fully exfoliate the graphite oxide to single layer GO sheets, and (3) achieve better GO dispersion in the polymer matrix, overexposure to ultrasonication can shatter GO sheets and induce defects [17–20]. Besides material composition, the specific steps in the fabrication process themselves play an important role in determining the structure and the properties of the composite. Despite many earlier studies concerning fabrication and final properties of GO/polymer composites, the effects of ultrasonication pre-treatment of GO on mechanical properties of nanocomposites are rarely addressed. Taking these considerations into account, a better understanding on the effects of the ultrasonication pre-treatment of GO is needed for producing a nanocomposite of better performance, and to this end its effect was investigated systematically. The results show that there exists an optimal ultrasonication time (OUT) that leads to best mechanical improvement.

2.

Experimental

2.1.

Materials

Graphite powder with particle size <20 lm, concentrated sulfuric acid (H2SO4, 98%), potassium persulfate (K2S2O8), phosphorus pentoxide (P2O5), hydrochloric acid (HCl), potassium permanganate (KMnO4), hydrogen peroxide (H2O2, 30%), and poly(vinyl alcohol) (PVA) with molecular weight about 89,000– 98,000, were all obtained from Sigma–Aldrich Co. Ltd. All of the materials were directly used without further purification.

2.2.

Preparation of GO/PVA sheets

GO was synthesized from graphite powder by the modified Hummers method as described in Supporting information [21,22]. The graphite oxide suspension obtained was diluted to 5 mg/ml, and was then ultrasonicated in a WiseClean ultrasonic bath (WUC-D22H, frequency 40 kHz, 300 W) at room temperature (RT). The actual energy input into the graphite oxide suspension is related with the mass of bath water, in this work, the calculated power input into the suspension is 30 W/L (Watt per liter). The ultrasonication time varied from 0 min to 5 h to vary the energy input into the solution (Table 1). During sonication over a prolonged period, the bath water was heated up and considerable evaporation occurs. Whenever the temperature of bath water became 5 C higher than RT, the hot water was replaced by fresh RT water and the water level was kept as constant. PVA (10 g) was dissolved in deionized water (DI) (90 g) at 90 C for 1 h to give a 10 percent by weight (wt.%) solution.

GO solution (0.4 ml) treated with different ultrasonication time and DI water (2.6 ml) was then mixed with PVA solution (2 ml), and stirred for 15 min at room temperature. Finally, GO/PVA solution was casted onto polyethylene terephthalate (PET) sheets and dried at room temperature for 24 h, then at 60 C for another 12 h. The size of the GO/PVA films obtained was 3 · 10 cm, and about 10 lm in thickness.

2.3.

Characterization

Photographs of GO solution were taken by a Canon digital camera (IXUS 70). The GO sheets were characterized by a Veeco Nanoman atomic force microscope (AFM) using tapping mode operated in air. Samples for AFM imaging were prepared by depositing a dispersed GO water solution onto a H2SO4 and H2O2 pretreated Si wafer. XRD analysis of GO films was carried out on a Bruker D8 ADVANCE X-ray diffractome˚ ) in ter at a voltage of 40 kV with Cu Ka radiation (l = 1.5406 A the 2h ranging from 5 to 60. GO films for XRD analysis were prepared by solution casting GO solution on PET films, similar with preparation of GO/PVA composite films. The tensile behavior of neat PVA and GO/PVA composite films were studied using an Instron 8848 Microforce Tester at a loading rate of 2 mm/min with a gauge length of 10 mm. All samples were cut into strips of 30 · 5 mm. For each composition, more than 8 samples were tested, from which the mean and standard deviation were calculated. The fractured surface of neat PVA and PVA composite films, after fractured in liquid nitrogen and sputter-coated with platinum, were examined by a FEI Quanta FEG 250 scanning electron microscopy (SEM).

3.

Results and discussion

The optical images of GO-water solution treated with different ultrasonication times are shown in Fig. 1. Without ultrasonication treatment, most of graphite oxide particles are precipitated at the bottom of the tube with a yellow-colored supernatant containing graphite oxide of limited exfoliation. After exposure to ultrasonication, the precipitate of graphite oxide is diminished significantly due to the exfoliation of graphite oxide into GO sheets. It is obvious that both the amount of graphite oxide precipitation and the transparency of the supernatant decrease with an increase in ultrasonication time. At an ultrasonication time of 30 min (energy input 15 W h/L), no precipitation is immediately visible. After keeping the solution for 7 d, only traces of graphite oxide precipitate were visible. The weight percent of the visible remaining precipitation was less than 2%. However, by further increasing the ultrasonication time beyond 30 min, no obvious reduction in the precipitation amount was observed. The visible remaining precipitation is non-fully-oxidized graphite particles, which cannot be exfoliated by the ultrasonication process.

Table 1 – The energy input of graphite oxide suspension with different ultrasonication time. Sample code Ultrasonication time Energy input per liter suspension (W h/L)

GO-0 min

GO-15 min

GO-30 min

GO-1 h

GO-2 h

GO-5 h

0 min 0

15 min 7.5

30 min 15

1h 30

2h 60

5h 150

CARBON

5 5 (2 0 1 3) 3 2 1–32 7

Fig. 1 – Optical images of GO water solution in a centrifugation tube, treated with different ultrasonication time, stand by 7 d before taking the photos.

To further investigate the degree of exfoliation and size distribution of GO in water, AFM imaging of the GO sheets deposited onto silicon wafer was carried out. Without ultrasonication, as shown in Fig. 2(a), most of the graphite oxide appear as agglomerated particles, and no single layer GO sheet is observed. Fig. 2(b) shows the representative AFM image of GO sheets with 30 min ultrasonication, where all of the GO sheets have uniform thickness with the lateral dimension ranging from several hundred nm to several lm. As indicated by the part marked with red line in Fig. 2(b), the thickness of GO sheet is around 1.2 nm, which is in good agreement with previous reported single-layer GO [5]. This observation leads to a conclusion that a complete exfoliation of GO into individual GO sheets is achieved.

323

As the ultrasonication time was further increased, large GO sheets were shattered into smaller pieces, resulted in a reduction of their lateral dimensions. As shown in Fig. 2(c), after exposure to 5 h of ultrasonication, no GO sheets with lateral dimensions over 500 nm are seen. Furthermore, it is also revealed from the XRD spectrum (Fig. 3) that all the diffraction peaks of GO films appears at 10.90 (d = 0.81 nm), and no typical diffraction peak of graphite at 26.3 (d = 0.34 nm) is observed, indicating that the oxidization and exfoliation process has increased the d-spacing of graphite to 0.81 nm and completely eliminated the 0.34 nm graphite interlayer spacing [23]. Further increase in ultrasonication time shows no effect on the peak position, but obvious changes on the peak intensity and width are seen. The diffraction peak of GO-5h is much weaker and broader than that from GO30min. From the well known Scherrer equation [24], it is known that the crystallite size is a reciprocal ratio to the half peak width at 2h, so the sheet size in GO-5h is much smaller than that in GO-30min. Thus, the XRD results also confirmed that prolonging the ultrasonication time caused the decrease of GO sheets size. To study the effect of ultrasonication pre-treatment of GO on the mechanical properties of GO/polymer composites, GO/ PVA composite films with 1 wt.% GO sheets pre-treated at different ultrasonication times were prepared by a simple solution casting method. The tensile properties of these composite films are shown in Fig. 4. Despite a 12.6% increase in tensile strength compared to neat PVA films, the incorporation of GO-0min into PVA causes a decrease in the Young’s modulus, due to the non-exfoliation of the GO sheets. As shown in Fig. 4, it is clear that ultrasonication pre-treatment

Fig. 2 – AFM images of GO sheets with different ultrasonication times; (a) 0 min, (b) 30 min, and (c) 5 h.

324

CARBON

5 5 ( 2 0 1 3 ) 3 2 1 –3 2 7

(a)

115 110

Failure Strength (MPa)

Intensity (a.u.)

105

30 min 2h 5h

100 95 90 85 80 75 70 65

6

8

10

12

14

16

18

60

20

PVA

0 min 15 min 30 min

2θ (°)

Young's Modulus (GPa)

of GO has a significant influence on the mechanical behavior of GO/PVA composites. Both tensile strength and Young’s modulus of GO/PVA composite films increase with an increase in ultrasonication time, up to 30 min. Compared to neat PVA films (tensile strength 82.5 MPa, Young’s modulus 4.1 GPa), improvement in tensile strength and Young’s modulus by 30.8% and 26.3%, respectively, is observed. The increase of tensile strength and Young’s modulus by extending the ultrasonication time is significant at relatively shorter ultrasonication time. On the other hand, further ultrasonication results in declined tensile strength and Young’s modulus. The tensile strength and Young’s modulus of GO-5h/PVA composite are 94.2 MPa and 4.33 GPa, respectively. Compared with those of GO-30min/PVA composite, a decrease of 12.7% and 16.4% respectively is observed. As shown in Fig. 4(c), the failure strain of the composites decreases compared to neat PVA, and similar results have been reported in a previous study [8,25]. This could be plausibly attributed to the large aspect ratios of GO sheets and the interaction between GO sheets and the polymer matrix that restrict the movement of polymer chains. When the ultrasonication time is within 30 min, no obvious change of failure strain of GO/PVA composites is observed. However, when the ultrasonication time is beyond 30 min, the failure strain of GO/PVA increases with increase of ultrasonication time. It is believed that smaller GO sheets are exerting less restriction to the movement of the polymer chains than the bigger ones. In order to reveal the effect of ultrasonication on the microstructure of GO/PVA composites, the fracture surfaces of the PVA composite films, fractured in liquid nitrogen, were studied by SEM. As shown in Fig. 5(a), the neat PVA fracture surface is characterized with smooth surface. However, the fracture surfaces of GO/PVA composites are remarkably different compare with neat PVA, as shown in (Fig. 5(b–d)). For GO-0min/PVA composite, many micron-sized agglomerates are clearly seen at the fracture surface. These agglomerates are non-exfoliated graphite oxide, with particle size consistent with AFM results obtained earlier. It is clear that pre-treatment of GO with ultrasonication has a significant

(b)

2h

5h

2h

5h

2h

5h

5.5

5.0

4.5

4.0

3.5

3.0

2.5 PVA

0 min

15 min

30 min

1h

Ultrasonication time

(c)

30

25

Failure Strain (%)

Fig. 3 – XRD patterns of GO films with different ultrasonication time.

1h

Ultrasonication time

20

15

10

5

0

PVA

0 min 15 min 30 min

1h

Ultrasonication time Fig. 4 – The effect of ultrasonication time on the tensile properties of GO/PVA composite films, (a) tensile strength, (b) Young’s modulus, and (c) failure strain, the GO content was fixed at 1 wt.%.

influence on the microstructure of the composites. After ultrasonication pre-treatment, the big agglomerates observed

CARBON

5 5 (2 0 1 3) 3 2 1–32 7

325

Fig. 5 – SEM images of fracture surface of neat PVA (a), GO-0 min/PVA (b), GO-15 min/PVA (c), and GO-30 min/PVA (d) composite films.

on fracture surface of GO-0min/PVA have disappeared, due to the exfoliation of the GO particles into the sheets. For GO15min/PVA (Fig. 5(c)), some small particles can still be observed on the fracture surface, which means that 15 min ultrasonication pre-treatment of graphite cannot fully exfoliate the GO into single layer GO sheets. When the ultrasonication pre-treatment time goes up to 30 min (energy input 15 W h/L), no particles is seen on the fracture surface of GO30min/PVA composites. At the same time, compared with neat PVA, the fracture surface of the composite is much rougher, where many small protrusion of polymer coated GO are seen, evincing a strong GO/PVA interfacial interaction [8], which lead to the strong reinforcement efficiency. These visual results agree well with the quantitative results that complete exfoliation of GO into individual GO sheets and good dispersion of GO sheets in polymer matrix can be achieved with 30 min ultrasonication (energy input 15 W h/L) pre-treatment of GO. Prolonging ultrasonication pre-treatment time further, however, results in no obvious change of the fractured surface. Form rule of mixture, the effect of reinforcement in a composite can be quantified by the increase of the composite Young’s modulus, YC, over those of the constituents [8,26]: Y C ¼ ðg0 g1 Y f  Y m ÞVf þ Y m

ð1Þ

where g0, Yf, Ym, Vf and are orientation efficiency factor, the Young’s modulus of filler and matrix, and the volume fraction of filler, respectively. The coefficient g1 is the so called length efficiency factor which reflects the dependence of reinforcement on platelet length and increases from 0 to 1 with increasing platelet aspect ratio (length/thickness). Previous investigations reveal that the value of g1 for graphene flakes with aspect ratio 104 and 103 is 0.9 and 0.3, respectively [26]. This means that, when the lateral dimension of the GO sheets are around several microns, relatively small decrease in flake size can cause obvious reduction in mechanical properties of the composites. Our results reveal that the mechanical properties of the GO/PVA composites are very sensitive to ultrasonication time (or energy input) and optimized mechanical properties of GO/PVA composites can be achieved with an OUT of 30 min (energy input 15 W h/L), qualitatively in agreement with earlier studies. If the ultrasonication time is shorter than 30 min, GO sheets are not fully exfoliated, only partial reinforcement effect of GO is achieved; while further increase the ultrasonication time causes size reduction of GO sheets and mechanically weakening the composite. In the past few years, although many studies about GO/ polymer composites have been carried out, little attention has been paid to the role of ultrasonication. In practice, ultrasonication and centrifugation is common way to obtain fully

326

CARBON

5 5 ( 2 0 1 3 ) 3 2 1 –3 2 7

exfoliated GO sheets. Centrifugation can guarantee the removal of large non-exfoliated graphite oxide particles, but prolonging the ultrasonication time (and/or increasing the energy input) can leads to reduction in the size of GO sheets, and weaken the composite. Although, OUT that produces fully exfoliated graphene oxide sheets with maximum sheet size may vary, depending on different GO preparation methods, concentration, ultrasonication type, etc. It is important to determine the proper ultrasonication energy input to achieve the highest improvement in mechanical property. The optimized ultrasonication energy input of 15 W h/L may serve as a general guideline for preparation of GO composites.

4.

Conclusions

The effects of ultrasonication pre-treatment of GO on GO exfoliation, dispersion, and mechanical properties of GO/ PVA composites were studied. The results show that ultrasonication time (or energy input) has significant effect on the mechanical properties of GO/polymer composites, and there is an OUT leading to highest mechanical properties improvement. GO sheets are not fully exfoliated if the ultrasonication time is shorter than OUT such that only partial reinforcement effect from GO is achieved, while further increase the ultrasonication time beyond OUT causes reduction of GO sheet size and eventually weaken the mechanical properties of the composite. Although OUT (or optimized energy input) that produces fully exfoliated graphene oxide sheets and maximum sheet size certainly varies depending on different GO preparation methods, concentration, ultrasonication type, etc. The optimized ultrasonication energy input of 15 W h/L determined in this study may serve as a general guideline for preparation of GO composites.

Acknowledgments The authors are grateful to the partial financial support by the Agency for Science, Technology and Research (A*STAR) of Singapore (SERC Grant No. 092-137-0012) and the Internal Research Funds of Khalifa University of Science, Technology & Research (No. 21011A).

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.carbon. 2012.12.071.

R E F E R E N C E S

[1] Rafiee MA, Rafiee J, Srivastava I, Wang Z, Song HH, Yu ZZ, et al. Fracture and fatigue in graphene nanocomposites. Small 2010;6:179–83. [2] Huang X, Yin ZY, Wu SX, Qi XY, He QY, Zhang QC, et al. Graphene-based materials: synthesis, characterization, properties, and applications. Small 2011;7(14):1876–902.

[3] Novoselov KS, Jiang D, Schedin F, Booth TJ, Khotkevich VV, Morozov SV, et al. Two-dimensional atomic crystals. Proc Natl Acad Sci USA 2005;102(30):10451–3. [4] Li XS, Cai WW, An JH, Kim S, Nah J, Yang DX, et al. Large-area synthesis of high-quality and uniform graphene films on copper foils. Science 2009;324(5932):1312–4. [5] Stankovich S, Dikin DA, Piner RD, Kohlhaas KA, Kleinhammes A, Jia Y, et al. Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide. Carbon 2007;45(7):1558–65. [6] Tian LL, Anilkumar P, Cao L, Kong CY, Meziani MJ, Qian HJ, et al. Graphene oxides dispersing and hosting graphene sheets for unique nanocomposite materials. ACS Nano 2011;5(4):3052–8. [7] Hummers WS, Offeman RE. Preparation of graphitic oxide. J Am Chem Soc 1958;80(6):1339. [8] Li YQ, Yang TY, Yu T, Zheng LX, Liao K. Synergistic effect of hybrid carbon nantube–graphene oxide as a nanofiller in enhancing the mechanical properties of PVA composites. J Mater Chem 2011;21(29):10844–51. [9] Paci JT, Belytschko T, Schatz GC. Computational studies of the structure, behavior upon heating, and mechanical properties of graphite oxide. J Phys Chem C 2007;111(49):18099–111. [10] Gomez-Navarro C, Burghard M, Kern K. Elastic properties of chemically derived single graphene sheets. Nano Lett 2008;8(7):2045–9. [11] Cao YW, Zhang J, Feng JC, Wu PY. Compatibilization of immiscible polymer blends using graphene oxide sheets. ACS Nano 2011;5(7):5920–7. [12] Zhu YW, Murali S, Cai WW, Li XS, Suk JW, Potts JR, et al. Graphene and graphene oxide: synthesis, properties, and applications. Adv Mater 2010;22(35):3906–24. [13] Huang X, Qi XY, Boey F, Zhang H. Graphene-based composites. Chem Soc Rev 2012;41(2):666–86. [14] Cravotto G, Cintas P. Sonication-assisted fabrication and post-synthetic modifications of graphene-like materials. Chem Eur J 2010;16(18):5246–59. [15] Zou ZG, Yu HJ, Long F, Fan YH. Preparation of graphene oxide by ultrasound-assisted hummers method. Chin J Inorg Chem 2011;27(9):1753–7. [16] Paredes JI, Villar-Rodil S, Martinez-Alonso A, Tascon JMD. Graphene oxide dispersions in organic solvents. Langmuir 2008;24(19):10560–4. [17] Si Y, Samulski ET. Synthesis of water soluble graphene. Nano Lett 2008;8(6):1679–82. [18] Dreyer DR, Park S, Bielawski CW, Ruoff RS. The chemistry of graphene oxide. Chem Soc Rev 2010;39(1):228–40. [19] Zhao X, Zhang QH, Hao YP, Li YZ, Fang Y, Chen DJ. Alternate multilayer films of poly(vinyl alcohol) and exfoliated graphene oxide fabricated via a facial layer-by-layer assembly. Macromolecules 2010;43(22):9411–6. [20] Khan U, O’Neill A, Lotya M, De S, Coleman JN. Highconcentration solvent exfoliation of graphene. Small 2010;6(7):864–71. [21] Geng JX, Jung HT. Porphyrin functionalized graphene sheets in aqueous suspensions: from the preparation of graphene sheets to highly conductive graphene films. J Phys Chem C 2010;114(18):8227–34. [22] Kovtyukhova NI, Ollivier PJ, Martin BJ, Mallouk TE, Chizhik SA, Buzaneva EV, et al. Layer-by-layer assembly of ultrathin composite films from micron-size graphite oxide sheets and polycations. Chem Mater 1999;11:771–8. [23] McAllister MJ, Li JL, Adamson DH, Schniepp HC, Abdala AA, Liu J, et al. Single sheet functionalized graphene by oxidation and thermal expansion of graphite. Chem Mater 2007;19(18):4396–404. [24] Weibel A, Bouchet R, Boulc’h F, Knauth P. The big problem of small particles: a comparison of methods for determination

CARBON

5 5 (2 0 1 3) 3 2 1–32 7

of particle size in nanocrystalline anatase powders. Chem Mater 2005;17(9):2378–85. [25] Liang JJ, Huang Y, Zhang L, Wang Y, Ma YF, Guo TY, et al. Molecular-level dispersion of graphene into poly(vinyl

327

alcohol) and effective reinforcement of their nanocomposites. Adv Funct Mater 2009;19(14):2297–302. [26] May P, Khan U, O’Neill A, Coleman JN. Approaching the theoretical limit for reinforcing polymers with graphene. J Mater Chem 2012;22(4):1278–82.