Elsevier

Applied Surface Science

Volume 448, 1 August 2018, Pages 320-330
Applied Surface Science

Full Length Article
Enhancement in electroactive crystalline phase and dielectric performance of novel PEG-graphene/PVDF composites

https://doi.org/10.1016/j.apsusc.2018.04.144Get rights and content

Highlights

  • Novel PEG-graphene/PVDF composites were prepared by the solution-blending method.

  • The PEG-graphene/PVDF composites had high content of electroactive crystalline phase.

  • The PEG-graphene/PVDF composites had good dielectric performance.

Abstract

In this work, polyethylene glycol grafted graphene (PEG-graphene) was obtained from the amidation reaction between graphene oxide and methoxypolyethylene glycol amine followed by the NaHB4 reduction and characterized by transmission electron microscope, Fourier transform infrared spectroscope, wide-angle X-ray diffraction, Raman spectroscope, X-ray photoelectron spectroscope, and thermogravimetric analysis. The PEG-graphene was incorporated into polyvinylidene fluoride (PVDF) to form novel PEG-graphene/PVDF composites by the solution blending method. The well dispersion of PEG-graphene in the PEG-graphene/PVDF composite was confirmed by scanning electron microscopy. Based on the results of FTIR, WAXD, differential scanning calorimetry, and XPS, it was found that the presence of PEG-graphene effectively enhanced the electroactive crystalline content of PVDF from 24.6% for the pure PVDF to 90.5% for the PEG-graphene (15 wt.%)/PVDF composite by the interfacial interaction. Moreover, the PEG-graphene (10 wt.%)/PVDF composite near the percolation threshold possessed a much higher dielectric constant of 53.3 compared to the pure PVDF (8.2), and a relatively low dielectric loss of 0.265 at 1000 Hz.

Introduction

Poly(vinylidene fluoride) (PVDF) and its copolymers, such as poly(vinylidene fluoride-co-hexafluoropropylene), poly(vinylidene fluoride-co-trifluoroethylene), poly(vinylidene fluoride-co-chlorotrifluoroethylene), poly(vinylidene fluoride-ter-trifluoroethylene-ter-chlorotrifluoroethylene), and poly(vinylidene fluoride-ter-trifluoroethylene-ter-chlorofluoroethylene), have many advantages including easy processing, low density, as well as good flexibility, toughness, and durability. More importantly, due to the outstanding pyroelectric, piezoelectric, ferroelectric, and dielectric properties, PVDF and its copolymers have been regarded as promising materials for versatile applications in transducers, sensors, actuators, embedded capacitors, electrical-energy-storage devices, and so on, [1], [2], [3], [4].

As a semicrystalline polymer, PVDF has different crystal forms including α, β, γ, δ, and ε crystalline phases. Among these five crystal forms, the non-polar α crystalline phase is the most thermodynamically stable one for PVDF, but it does not exhibit any electroactivity. Although the polar β and γ crystalline phases of PVDF have the electroactivity, they cannot be easily obtained. The effective achievement of electroactive β or γ phase in PVDF is still a challenge until now [2]. On the other hand, as the dielectric constant of PVDF is only about 10, it is highly demanding to further enhance the dielectric performance for various applications [1], [5].

It is well known that the incorporation of functional fillers especially carbon-based materials with large dimensional aspect ratio into the PVDF matrix can not only induce the electroactive crystalline phase of PVDF by interfacial interaction [2], but also improve the dielectric constant of resultant composites based on the percolation theory [6], [7]. Graphene has been considered as one of the ideal functional fillers because of its high electrical and thermal conductivities, good strength and modulus, large aspect ratio as well as large specific surface area [7], [8], [9], [10], [11], [12], [13], [14], [15]. However, owing to the poor compatibility, carboneous graphene has a tendency to form aggregates in the organic PVDF matrix owing to the poor compatibility, which unavoidably declines its reinforcing effect for the resultant composites [9], [12], [16]. Moreover, graphene cannot effectively induce the formation of electroactive crystalline phase because the interfacial interaction between PVDF and graphene is the weak dipole-π electron interaction [17]. Therefore, the homogeneous dispersion of graphene in PVDF and the strong interfacial adhesion between graphene and PVDF are two key factors to achieve advanced graphene/PVDF composites. The grafting of polymer possessing strong interfacial interaction with PVDF onto the graphene surface is one of the efficient ways to solve the aforementioned problems [10]. For example, Dang et al. covalently functionalized GO with poly(vinyl alcohol) (PVA) by esterification using N,N-dicyclohexylcarbodiimide and 4-dimethylaminopyridine as the coupling agents, dimethyl sulfoxide as the reaction medium, and toxic hydrazine as the reducing agent to obtain the PVA-modified graphene [10]. The resultant PVA grafted graphene was added into PVDF to form a composite with high dielectric constant. Nevertheless, the whole preparation process of PVA-graphene was not environmental friendly with the utilization of organic solvent and toxic hydrazine. Polyethylene glycol (PEG), as an oxygen-atom-rich water-soluble polymer, can form stable intermolecular hydrogen bonds with the hydrogen atoms of PVDF. Hence, in this work, PEG grafted graphene (PEG-graphene) was obtained from the amidation reaction using water as a solvent and non-toxic NaHB4 reduction in sequence, and then incorporated into PVDF to form novel PEG-graphene/PVDF composites by a simple solution blending method. Furthermore, the crystalline behavior and dielectric performance of resultant graphene/PVDF composites were investigated.

Section snippets

Materials and sample preparation

PVDF (Solef 6008) was bought from Solvay Ltd. Co. (Shanghai, China). Natural graphite flakes was purchased from Qingdao Longjun Graphite Company in Shandong, China. Graphene oxide (GO) was prepared according to our previous works [8], [15]. 1-ethyl-3-(3-dimethyaminopropyl) carbodiimide (EDC), N-hydroxysuccinnimide (NHS), methoxypolyethylene glycol amine (PEG-NH2, MW = 20,000) were bought from Aladdin Chemical Company (Shanghai, China). NaHB4, N,N-dimethylformamide (DMF), KMnO4, ethanol, H2SO4,

Results and discussion

GO, containing a large amount of functional groups such as epoxy, hydroxyl, and carboxyl, was used as a precursor for the preparation of PEG-graphene. As illustrated in Fig. 1, the PEG-GO was obtained through the amidation reaction between the carboxyl groups of GO and the amino groups of PEG-NH2 with EDC and NHS as the coupling agents in an aqueous solution, and then the resultant PEG-GO was further reduced by NaHB4 in water to prepare PEG-graphene [15]. The preparation process of PEG-graphene

Conclusions

The grafting of PEG onto the graphene surface was achieved by the amidation reaction between GO and PEG-NH2 followed by the NaHB4 reduction. The PEG-graphene/PVDF composites were prepared by the solution-blending method, and their properties were confirmed by TEM, FTIR, WAXD, Raman spectroscopy, XPS, TGA, and SEM. The PEG-graphene could effectively facilitate the useful electroactive β and γ phases. Particularly, the electroactive crystalline content of PVDF for the PEG-graphene (15 wt.%)/PVDF

Acknowledgements

The authors would like to acknowledge the support of the Guangdong Province Natural Science Foundation, China (No. 2017A030313268 and No. 2014A030307037), the Guangdong Common University Special Innovation Project Foundation, China (No. 641618), the Guangdong Yangfan Project, China (No. 915028), the Guangdong Provincial Science and Technology Project, China (No. 2016B020211002) and the Hong Kong Polytechnic University, Hong Kong (No. 1-ZVGH).

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