Solution Casting of Polyvinyl Alcohol–Functionalized Graphene Nanocomposites

Solution Casting of Polyvinyl Alcohol–Functionalized Graphene Nanocomposites

Available online at www.sciencedirect.com ScienceDirect Materials Today: Proceedings 17 (2019) 640–645 www.materialstoday.com/proceedings RAMM 2018...

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Available online at www.sciencedirect.com

ScienceDirect Materials Today: Proceedings 17 (2019) 640–645

www.materialstoday.com/proceedings

RAMM 2018

Solution Casting of Polyvinyl Alcohol–Functionalized Graphene Nanocomposites Y. Kamal a,* T. Li Sana, I. Zulhelmib, A. Abu Hannifaa a

Faculty of Chemical & Natural Resources Engineering, Universiti Malaysia Pahang, Lebuhraya Tun Razak, 26300 Gambang Pahang b Faculty of Manufacturing Engineering, Universiti Malaysia Pahang, 26600, Pekan Pahang

Abstract Graphene materials have attracted enormous academics and industries in different fields, ranging from nanocomposites to medical applications, to explore due to its outstanding thermal, barrier and mechanical properties. In this work, the solutioncasted polyvinyl alcohol (PVA) film reinforced with functionalized sonication exfoliated graphene and its properties such as sub surface mechanical behavior i.e hardness and modulus were studied. The compatibility between graphene and PVA was enhanced by the functionalization of graphene with a gum arabic. The decoration of the graphene surface allowed better interfacial interactions with PVA. As been observed during morphological study by scanning electron microscopy (SEM) and xray diffraction (XRD), agglomeration of graphene sheets was detected at highest graphene loading (0.10 wt.%). Improvement of mechanical properties was observed by nanoindentation for sample with lowest graphene contents (0.05 wt%). The enhancement of modulus up to 130% was contributed by homogeneously distributed graphene in the matrix of PVA. However, the present of higher graphene content seem to reduce of the hardness properties of PVA/graphene nanocomposites. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 6th International Conference on Recent Advances in Materials, Minerals & Environment (RAMM) 2018. Keywords: Graphene; Polyvinyl alcohol; Solution casting; Nanoindentation

* Corresponding author. Tel.: +609-549-2901; fax: +609-549-2889. E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 6th International Conference on Recent Advances in Materials, Minerals & Environment (RAMM) 2018.

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1. Introduction Graphene is a two-dimensional version of graphite with layer thickness that ranged from few layers to a single layer of atoms. Pristine graphene is theoretically a mono atomically structured whilst graphene with few-layers (n <10) is considered as few-layers graphene. At its finest form, pristine graphene is not only exhibits excellent mechanical properties1, but it is also possesses high impermeability against gas2. Combined with extreme conductivity for electrical due to its super rate of electron mobility (2.5 × 105 cm2 V-1 s-1) 3 and thermal (3000 W m K-1)4, graphene is regarded as a strong candidate for precursor in coating5, composite6, electronics device7 and nanofluids8. In the production of graphene, there have been a countless number of graphene synthesis techniques that have been developed ever since the discovery of graphene in 2004. The available techniques fundamentally can be divided into two categories which are known as the bottom-up and top-down approaches. A bottomup approach which is used for the preparation of defect-free graphene includes methods such as pyrolysis9, epitaxial growth10, and chemical vapor disposition11. Mechanical cleavage12, chemical exfoliation13 and liquid-phase exfoliation14 meanwhile are production methods of graphene that belong to the top-down approach and are commonly employed for the production of liquid or powdered graphene. In contrast to the limited scalability of a bottom-up approach, a top-down approach is more scalable and flexible for future mass production of graphene. Of all the presented methods, liquid phase exfoliation of graphite that can be accomplished either by sonication or shear exfoliation is considered the most facile method for production of low-defect graphene flakes. Polyvinyl alcohol (PVA) meanwhile is a non-toxic polymer and due to its strong hydrophilicity characteristic, PVA is easily dissolved in water. It is commonly used as coating 15-16 and adhesive17 for electronics and furniture. However, the thermal and mechanical properties of PVA are relatively low as compared to other synthetic polymer such as polyamide 6 or polycarbonate. To improve the thermal and mechanical properties of PVA, incorporation of graphene in the matrix was proposed in this research. Herein, shear-exfoliated graphene was used as filler in the preparation of PVA/graphene nanocomposites. The compatibility between graphene and hydrophilic PVA was provided by the functionalization of graphene with gum Arabic component. As non-ionic surfactant18, adsorption of gum Arabic on the graphene surface allows stabilisation in water after exfoliation stage. 2. Experimental 2.1. Materials In this work, Sigma Aldrich (Malaysia) supplied 25 g of powdered PVA for the preparation of PVA/graphene film. Based on the given data by the supplier, the molecular weight of PVA was quoted between 89000 and 98000 in value. For the exfoliation of graphene, 2.5 g of graphite (332461) and 0.5 g of gum Arabic (G9752) were also obtained from Sigma Aldrich (Malaysia). The dispersion of PVA and gum Arabic were carried out in ultrapure water (Millipore). 2.2. Preparation of Gum Arabic functionalized graphene by shear mixer In the preparation of gum Arabic based graphene, 5mg/ml of graphite was dispersed into 400ml volume of gum Arabic solution (5mg/ml) using mixer (HR 2096). Total operation hour was 7h with blade speed of 16krpm. After exfoliation, centrifugation was carried out for separation of gum Arabic modified graphene (GGA) from the original dispersion volume. Washing through filtration was conducted in multiple cycles for removal of excessive gum Arabic from the graphene sheets. 2.3. Solution casting for PVA/Graphene films 1.5g of PVA powder was first dissolved in 30ml of water at the temperature of 80 ºC for 60 min. Then, the PVA solution was leaved for 30 min in ambient temperature for cooling. To study the concentration effect of GGA on PVA, a PVA/graphene solution was prepared at different concentration percentage (0wt%, 0.05wt.%, 0.075wt. %,

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0.1wt.%, 0.15 wt.%) with stirring duration of 24 hours. The resulting PVA/graphene solution then was poured into Petri dish and leaved in drying oven at 60ºC for a day. The solid PVA/graphene film then was peeled of Petri Dish and stored in desiccators. 2.4. Characterization of PVA/Graphene films Fractured surface of PVA strips from mechanical tensile test were studied using scanning emission microscopy (SEM). Each of the sample surfaces was coated with platinum for production of clearer micrographs during imaging by JEOL JSM 7800F. Low intensity of electron beam was used to prevent thermal degradation of sample. The crystal structure for graphene and polymer/graphene film meanwhile was determined using x-ray diffraction (XRD). XRD spectrum of suspended film of graphene and PVA nanocomposites were obtained using XRD Rigaku Miniflex II. For nanoindentation, rectangle shape of polymer/graphene film with dimension of 75 × 10 × 0.15 was prepared from PVA/graphene nanocomposites film. Then, the film was mounted on metal cylinder with a diameter of 50 mm using thin layer of PVC gum. Nanoindenter (Micro Indenter) was calibrated first by using Si as sample. 5mN of loading was used during the nanoindentation for PVA with a loading rate of 0.5 mN/s. A Berkovich tip was used to indent the sample surface with holding time of 10s. A total of 10 indentations were performed on each sample of PVA/graphene films. 3. Results & Discussion 3.1. Morphological structure after dispersion All shear exfoliated graphene was dispersed in PVA using solution casting method. Presence of hydroxyl groups in PVA allows high solubility of PVA in water 18. It was expected that co-dispersion of gum Arabic based graphene would be influenced by amphiphilic proteins on the graphene surface. The protruding polysaccharide blocks into PVA/water provide the interfacial pathways for hydroxyls through H-bonding mechanism. As the mechanical properties of prepared PVA/graphene nanocomposites film can be affected by distribution quality of graphene in the matrix, the morphological structure of PVA/graphene was extensively studied from scanning electron microscopy (SEM) and x-ray diffraction (XRD). The micrographs of fractured surfaces from tensile test were presented as series of images (A, B and C) in Fig. 1. The smoothness of fractured surface was used as indicator for homogenous distribution of graphene in PVA/graphene nanocomposites. In principle, the extreme low content of graphene (0.05 wt. %) allows a better dispersion behavior as compared to low graphene content in PVA. As shown in Fig. 1B, smooth surface was obtained for 0.05 wt. % GGA whilst rough surface for 0.15 wt. %GGA in Fig. 1C indicating poor dispersion of graphene in the matrix. For further investigation on the morphological state of prepared PVA/graphene film, x-ray diffraction (XRD) measurement was performed on 0.05 wt.% and 0.15 wt.% samples (see Fig. 1D). The red shift of intense graphene associated peak for 0.15 wt.% sample verified the agglomeration presence in PVA. The loss peak at 2θ = 26.9º for 0.05 wt.% mean whilst is indicating a good dispersion of graphene in PVA matrix. Based on the XRD result alone, it was observed that concentration threshold of graphene in PVA was 0.05 wt.%, which surprisingly was supremely low as compared to previous published work on PVA/graphene nanocomposites 19-20. Increase of van der Waals forces of graphene sheets at higher loading was attributed to reduce distance (r2) between sheets at confined space 21. As the majority graphene produced in this work was measured at hundredth nanometer size range, the risk of agglomeration was further increased due to high surface area of graphene.

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Fig. 1: (A) Fractured surface observed under SEM for pure PVA, (B) lowest concentration of 0.05 wt.% GGA, (C) highest concentration of 0.15 wt.% GGA, and (D) stacked XRD spectrum for GGA, 0.05wt.% and 0.15 wt.% PVA/graphene nanocomposites films were presented

3.2 Mechanical Properties – Nanoindentation Mechanical reinforcement of PVA by graphene was investigated further from nanoindentation study on the film samples. The plotted nanoindentation curves for PVA, 0.05 wt.% GGA and 0.15 wt.% GGA were illustrated in Fig. 2A. The shift of 0.15 wt.% GGA corresponding curve towards higher displacement as compared to original PVA indicates the hardness loss due to agglomeration presence. Meanwhile, the increase of PVA hardness at 0.05 wt.% of GGA was extremely minor and addition of more graphene contents softened the material further. The change of nanoindentation modulus (E) showed the negative effect of graphene at higher concentration, where the presence of more graphene lowered the E further. In further investigation of graphene effect on the nanoindentation properties of PVA, the normalized values of hardness (H) and modulus (E) were computed and are presented in Fig. 2B. Impressively, the addition of GGA at 0.05wt.% of concentration allow the increase of E at about 130% improvement. However, the values of E were significantly reduced after incorporation of more graphene. As discussed earlier, the drop of modulus was caused by the reaggregation of GGA in the matrix. Tendency of graphene towards agglomeration was originated from the high surface are of single sheet22. In discussing the effect of GGA on the hardness of PVA, the improvement was very minor although the reduction of hardness was also observed for higher presence of graphene in PVA.

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Fig. 2: (A) Nanoindentation curves of pure PVA and PVA after addition of 0.05 wt.% GGA and 0.15 wt.% GGA were illustrated, (B) Normalized hardness and reduced modulus value of PVA and PVA/graphene nanocomposites

4. Conclusion In this work, the mechanical reinforcement effect of gum Arabic based graphene (GGA) was observed for PVA at extremely low concentration of graphene. Addition of 0.05wt.% of GGA allows the improvement of hardness and modulus of PVA. Based on the SEM and XRD observations, it was proposed that the increase of PVA mechanistic at 0.05wt.% of graphene could be attributed to the well dispersed GGA in the matrix. Higher graphene presence in the matrix meanwhile was found to influence the hardness and modulus of PVA negatively as been verified from SEM and XRD of sample with 0.15 wt.% of GGA. Since mechanical reinforcement effect of graphene in this work was only observed for PVA with extremely low content of GGA, future work therefore is required for development of dispersion method that allow incorporation of high content graphene into PVA. Acknowledgements This research was supported by a fundamental research grant scheme (FRGS) by Ministry of Higher Education Malaysia under grant agreement no RDU160149. References [1] Lee, C.; Wei, X.; Kysar, J. W.; Hone, J. Science 2008, 321, 385. [2] Kim, H. W.; Yoon, H. W.; Yoon, S.-M.; Yoo, B. M.; Ahn, B. K.; Cho, Y. H.; Shin, H. J.; Yang, H.; Paik, U.; Kwon, S. Science 2013, 342, 91. [3] Bolotin, K. I.; Sikes, K.; Jiang, Z.; Klima, M.; Fudenberg, G.; Hone, J.; Kim, P.; Stormer, H. Solid State Communications 2008, 146, 351. [4] Balandin, A. A.; Ghosh, S.; Bao, W.; Calizo, I.; Teweldebrhan, D.; Miao, F.; Lau, C. N. Nano letters 2008, 8, 902. [5] Li, N.; Zheng, M.; Lu, H.; Hu, Z.; Shen, C.; Chang, X.; Ji, G.; Cao, J.; Shi, Y. Chemical Communications 2012, 48, 4106. [6] Abdullah, AH,; Ismail, Z.; Abidin, AZS.; Yusoh, K. J.Matchemphys 2019, 222, 11. [7] Zeng, J.-J.; Tsai, C.-L.; Lin, Y.-J. Synthetic Metals 2012, 162, 1411. [8] Hadadian, M.; Goharshadi, E. K.; Youssefi, A. Journal of Nanoparticle Research 2014, 16, 1. [9] Lin, Z., et al., Advanced Energy Materials 2012, 2, 884. [10] Xu, X., et al., Science Bulletin, 2017 62, 1074. [11] Li, X., L. Colombo, and R.S. Ruoff, Advanced Materials, 2016, 28, 6247. [12] Jayasena, B. and S. Subbiah, Nanoscale Research Letters, 2011, 6, 1. [13] Voiry, D., et al., Science, 2016, 353, 1413. [14] Harvey, A., et al., 2D Materials, 2017, 4, 025054 [15] Ismail, Z.; Abdullah, A.H.; Abidin, A.S.Z.; Yusoh, K.Appl. Nanosci 2017, 7, 317. [16] Venugopalan, T.; Yeo, T. L.; Sun, T.; Grattan, K. T. IEEE Sensors Journal 2008, 8, 1093.

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