Poly(vinylidene fluoride-co-hexafluoropropylene)/polyaniline blends assisted by phosphonium – Based ionic liquid: Dielectric properties and β-phase formation

Highlights

• Ionic liquid increases conductivity of PVDF-co-HFP/polyaniline blends.

• The formation of β-crystalline phase of PVDF-co-HFP was enhanced.

• Higher EMI shielding effectiveness was reached for blends containing ionic liquid.

Abstract

In this paper, the ionic liquid denoted tributyl(tetradecyl)phosphonium dodecylbenzenesulfonate (IL) was used for the first time to improve the processability of blends based on polyaniline and a fluorinated polymer matrix during the melting process. In this case, polymer blends constituted by poly(vinylidene fluoride)-co-(hexafluoropropylene) (PVDF-co-HFP) and polyaniline doped with dodecylbenzene sulfonic acid (PAni.DBSA) were prepared at different compositions with the presence of IL. Thus, the use of 5 wt% of IL exerts a strong influence on the melt viscosity, mainly for blends containing 10 wt% of PAni.DBSA. In addition, improved electrical conductivity and dielectric properties (permittivity and dielectric loss), as well as electromagnetic interference shielding effectiveness (EMI SE) in the X-band frequency range are achieved by adding IL to the conducting blends. The presence of IL favors the formation of conducting pathways of PAni.DBSA in the thermoplastic matrix as observed by scanning electron microscopy (SEM). Moreover, the β-crystalline phase fraction of PVDF–HFP is significantly enhanced by the presence of IL, as indicated by X-ray diffraction.

Introduction

Conducting polymer composites constituted by an insulating matrix and a conducting filler have been extensively studied with the aim of developing new electronic devices with different applications as sensors, capacitors, electromagnetic shielding materials, anti-static coatings, etc. For such applications, poly(vinylidene fluoride) (PVDF) and its copolymers appear as promising candidates because of their high dielectric permittivity and good piezoelectric/pyroelectric properties, associated with excellent mechanical properties, thermal stability and chemical resistance [1]. Such thermoplastics are semi-crystalline and present five crystalline phases, known as α, β, γ, δ and ε [2], [3]. The α-phase is the most stable but is electrically inactive. The β-phase is more polar and responsible for the pyro-, piezo-, and ferroelectric properties of PVDF and its copolymer but is hard to attain [4], [5]. Different ways were investigated in the literature to promote the formation of the β-phase in PVDF and its copolymers: (i) the application of the strain [6], (ii) the use of electric field [7], (iii) the introduction of inorganic fillers [8] and (iv) the growth from a PVDF solution. Conducting composites constituted by PVDF or its copolymers and conducting particles such as carbon nanotubes [9], [10], [11], carbon nanofibers [12], short carbon fiber [13], graphite [14], [15] and graphene [16], [17], [18] have been extensively studied. Intrinsic conducting polymer as polypyrrole [19], [20] and polyaniline [21], [22], [23], [24], [25], [26], [27], [28] have also been experienced as conducting component in PVDF – based blends. Polyaniline (PAni) is considered one of the most popular conducting polymer due to its unique characteristics which combine environmental stability and good electrical conductivity with low cost of the raw materials and easy synthesis. Additionally the conductivity and morphology of PAni are easily tuned by appropriate choice of protonating agent and synthetic procedure, as well as the use of surfactants, also known as “soft template” [29]. The use of protonic acids with large n-alkyl groups, as dodecylbenzene sulfonic acid (DBSA), also impart solubility of PAni in conventional organic solvents and its compatibility with some insulating polymers. Polyaniline and other conducting polymers display electromagnetic wave absorbing characteristics, which constitute a great advantage for electro-electronic applications.

Blending PAni with PVDF and its copolymers has been mostly prepared by solution process or by in situ polymerization of aniline in the presence of the PVDF solution. Some of those papers discuss the ability of PAni in inducing the formation of the β-phase in the PVDF matrix [22], [25], [28]. In spite of the outstanding electrical conductivity, the solution process is not friendly from an environmental point of view due to the use of large amount of solvent. On the other hand, the preparation of PVDF/PAni blends by the melting process has rarely been reported in the literature, probably because of the high temperature used for the PVDF processing, which could promote some degradation of the PAni component. Ray et al. [30] have reported the melt blending of PVDF with poly(methyl methacrylate) and PAni doped with DBSA. They have observed the presence of large aggregates of PAni.DBSA, confirming the poor dispersion of PAni.DBSA in the PVDF matrix. Martins et al. [27] have investigated the rheological behavior of PVDF/PAni.DBSA binary blends and found an increase of both storage and loss moduli as the amount of PAni.DBSA increases. Thus, the incorporation of PAni in polymeric matrices is not an easy route because of the very poor processability and dispersability of PAni.

Recently the incorporation of ionic liquid (IL) in polymer systems has received enormous interest because of their ability in providing better processability and ionic conductivity to polymeric materials. Ionic liquids are organic salts which present some important characteristics such as low vapor pressure at room temperature, high ionic conductivity and non-flammability [31]. In this context, the modification of PVDF and PVDF – based copolymer matrices with imidazolium – based ionic liquids has been reported mainly with the aim of developing new polymeric gel electrolyte membranes for proton exchange fuel cell [32] and for energy storage applications [33], [34], [35]. Okada et al. [36] have also reported the ability of ionic liquid in inducing the β-phase formation of PVDF. ILs have also been successfully employed to improve the dispersion of carbon nanotubes within the PVDF matrix [37]. Regarding PVDF/PAni blends, the use of ionic liquids as dispersing agent for PAni in polymer systems was not documented. Nah and Kumar [38] have reported some results related to the dedoped polyaniline nanorods in blends constituted by PVDF and 1-butyl-3-methyl imidazolium bromide. The blends were prepared in solution to obtain electrolyte film with good ionic conductivity by using 5% of PAni dispersed in a matrix constituted by PVDF/IL in a proportion corresponding to 1:1 by weight. The presence of PAni decreases the crystallinity of the material but no mention related the β-phase formation has been cited.

The aim of the present paper is to evaluate the effect of small amounts of phosphonium – based ionic liquid (up to 5 wt% related to the thermoplastic matrix) on the ability of dispersing PAni in the poly(vinylidene fluoride)-co-hexafluoropropylene (PVDF-co-HFP) matrix during the melt processing and characterize the electrical, dielectric and rheological properties of the corresponding blends. PVDF-co-HFP was employed as the matrix because of the lower crystallinity of this copolymer compared to PVDF homopolymer, which endows better processability to the blend [1]. PAni was doped with dodecylbenzene sulfonic acid (PAni.DBSA) to improve the compatibility with the PVDF-co-HFP matrix. Also the ionic liquid used in this work is tributyl(tetradecyl)phosphonium dodecylbenzenesulfonate, which contains similar anion as the PAni.DBSA and presents an excellent thermal stability [39]. The effect of the ionic liquid on the structural and morphological characteristics of the blends was also investigated by X-ray diffraction measurements and scanning electron microscopy, aiming to elucidate the outstanding electrical properties achieved by using the ionic liquid. The electromagnetic interference shielding effectiveness of the corresponding blends was also evaluated in the range of 8–12 GHz. The motivation of using ionic liquid in PVDF-co-HFP/PAni.DBSA blends is based on the well known ability of ionic liquids in improving the dispersability of inorganic and hybrid particles and decreasing the viscosity of the polymer matrices, which constitutes an interesting approach for the development the blends by melt mixing process.