Preparation of poly(vinylidene fluoride) films with excellent electric property, improved dielectric property and dominant polar crystalline forms by adding a quaternary phosphorus salt functionalized graphene

Abstract

In our work, a novel modifier, quaternary phosphorus salt, (1-hexadecyl) triphenylphosphonium bromide (HTPB), was introduced for the noncovalent functionalization of graphene for the first time. With it, an excellent dispersion of graphene in organic solvent and later in poly(vinylidene fluoride) (PVDF) matrix has been achieved, e.g., transmission electron microscopy (TEM) shows a single-layer dispersion and multi-layer structure of graphene sheets in PVDF matrix. As a result, the films exhibit outstanding electric property with a very low percolation threshold of 0.662 wt% being observed. Their dielectric property is also improved, the dielectric constant of PVDF/graphene composites at 1000 Hz with a loading lower than 0.86 wt% shows an obvious increase (more than 3 times of that of PVDF at most), while the dielectric loss remain quite low (all lower than 0.07). Even more intriguingly, the quaternary phosphorus salt also induces dominant polar β and γ crystalline forms in the prepared composite films. These PVDF/graphene films with good electric and dielectric property as well as dominant polar β and γ forms promise a wide range of potential applications in electronic devices.

Introduction

In recent years, graphene has attracted great attention of scientific researchers. It is a two-dimensional monolayer graphite with excellent electronic [1], mechanical [2], and thermal properties [3] as well as high specific surface area [4]. With these remarkable characteristics, it is expected to endow polymers with outstanding performances in a variety of areas, such as increasing the bulk conductivity by several orders of magnitude [5], [6], [7], [8], [9], [10], [11], [12], [13] and improving the dielectric property [14], [15], [16], [17], [18], [19] of polymers. So far, there have been several methods to prepare graphene, including chemical vapor deposition [20], epitaxial growth [21], raw graphite exfoliation [22], [23] and graphite oxide (GO) reduction [24], among which chemical reduction of GO is considered to be the most efficient way. However, direct reduction of GO will result in inevitable aggregation of graphene sheets due to the strong Van der Waals’ force. It is well known that the excellent properties of graphene are tightly associated with its individual sheets. Therefore, dispersion of single-layer graphene in polymer matrix is a key factor for preparing high performance polymer/graphene composites. Up to now, covalent [25], [26], [27], [28] and noncovalent [14], [29], [30], [31], [32], [33] functionalization, mainly aimed at single layer dispersion in polymer matrix, have been developed to prevent the aggregation of graphene sheets during reduction. Unfortunately, covalent functionalization inevitably induces defects onto the parallel plane of graphene, which impairs the properties of graphene, and that of the polymer composites as a result. While effectively avoiding defects, noncovalent functionalization is still possible to bring decline to properties of graphene. For example, we have tried using polyvinyl pyrrolidone (PVP) to noncovalently functionalize graphene. Although an excellent dispersion of graphene sheets was achieved, the conductivity was rather poor due to the nonconductive PVP coating on the surface of graphene [34]. Hence, it is very important to explore suitable modifier to achieve an excellent dispersion of graphene in polymer matrix while maintaining its intrinsic properties. Some aromatic molecules captured scientists’ attention, since the π–π interactions between their aromatic rings and graphene sheets [14], [31], [35], are able to facilitate not only the satisfying dispersion but also the electron conduction of graphene.

Poly(vinylidene fluoride) (PVDF) is an interesting polymer with many excellent properties, such as good mechanical property, chemical and weathering resistance, and more remarkably, the unique piezoelectric and ferroelectric properties which are closely associated with its polar crystalline forms. There are mainly five crystalline forms of PVDF: α, β, γ, δ and ɛ. Among them, polar β and γ, especially the β form, make great contributions to the piezoelectric and ferroelectric properties of PVDF [36], [37]. However, PVDF are usually dominated by α form since the conformation of α form is more stable than those of β and γ forms [38]. Therefore, many methods have been employed to induce polar β and γ forms in PVDF matrix, for instance, drawing under certain temperature [39], electrospinning [40], application of high electric filed [41] and Langmuir–Blodgett (LB) deposition [42]. Recently, we found that some quaternary ammonium salts and quaternary phosphorus salts could also induce the formation of polar forms in PVDF [43]. Luckily, some of them with aromatic rings are also ideal modifiers to prevent the aggregation of graphene sheets during or after reduction through π–π interactions. In this way, both the excellent dispersion of PVDF/graphene composites and the polar forms in PVDF is expected to be accomplished simultaneously through one simple agent.

In this work, graphene was noncovalently functionalized with one kind of quaternary phosphorus salts, called (1-hexadecyl) triphenylphosphonium bromide (HTPB), which is indeed capable of preventing the aggregation of graphene sheets after reduction. The functionalized graphene was added into PVDF, and an excellent dispersion (single layer dispersion) and layered structure of graphene sheets within PVDF matrix were observed. The composite films showed excellent electric property and improved dielectric property, as well as dominant polar β and γ forms.