High-Dielectric PVP@PANI/PDMS Composites Fabricated via an Electric Field-Assisted Approach

Abstract

Polymer-based composite films with multiple properties, such as low dielectric loss tangent, high dielectric constant, and low cost are promising materials in the area of electronics and electric industries. In this study, flexible dielectric films were fabricated via an electric field-assisted method. Polyaniline (PANI) was modified by polyvinylpyrrolidone (PVP) to form a core–shell structure to serve as functional particles and silicone rubber polydimethylsiloxane (PDMS) served as the matrix. The dielectric constant of the composites prepared under electric fields was improved by the micro-structures formed by external electric fields. With the addition of 2.5 wt% PVP@PANI, the dielectric constant could be significantly enhanced, up to 23; the dielectric loss tangent is only 1, which is lower than that of the aligned PANI samples. This new processing technology provides important insights for aligning fillers in polymer matrix to form composites with enhanced dielectric properties.

1. Introduction

With the development of electronic technology, the preparation of electronic materials with high dielectric property has attracted lots of attention. Compared to traditional ceramic-based dielectric materials, polymer composites have the advantages of easy processing, good flexibility, light weight, and low cost, and have become the mainstream material of the microelectronics industry in recent years [1,2,3,4]. Adding high-dielectric fillers to polymers can significantly increase the dielectric constant of the composites; however, high filler loading leads to higher dielectric loss tangent, which limits the application of composites [5,6,7,8]. Therefore, the preparation of polymer composite materials with high dielectric constant and low dielectric loss tangent is still a challenge [9,10,11,12,13].

Incorporating high-dielectric ceramic particles into the polymer matrix is a classical composite with enhanced dielectric property. In order to restrain the dielectric loss tangent caused by the addition of fillers, a transition layer is usually employed on the filler surface. For instance, dopamine-coated barium titanate particles (DP-BT) were introduced into silicone rubber (SR) to form high-dielectric composites [14]. The composite of SR/DP-BT exhibits a dielectric constant as high as approximately 7.9 at 1 kHz when the filler content is 40 wt%. Gall et al. [15] explored the electrical properties of lead magnesium niobate–lead titanate/silicone elastomer (PMN-PT/PDMS) composites which were prepared by heat curing. The dielectric constant of 30 wt% PMN-PT/PDMS composites increased from 8 to 32 at 10 Hz. The problem with the aforementioned strategy is that the dielectric constants of the composites only effectively increase when the addition of dielectric ceramic fillers in composite materials is very high (up to 40–50 vol%). Excessive addition is likely to cause holes and defects in the composite material, which leads to difficulty in processing and reduction of mechanical strength, and at the same time, the cost is high, which is not conducive to the practical application of these types of materials. Another strategy is to disperse conductive particles into a polymer matrix. Cameron et al. [16] added conductive graphite to polyurethane and observed high dielectric constant (4400) at a volume fraction of 18.76% graphite loading. Chen et al. [17,18,19,20,21] also prepared a series of modified graphite/polymer matrix composites with excellent dielectric properties with high dielectric constant and low dielectric loss tangent. The mechanism of these conductive filler/polymer matrix composites is that the dielectric constant of polymer composites can be significantly increased as the loading of conductive fillers increases around the permeation threshold. However, when the conductive particles exceed the permeation threshold in the matrix, which means the conductive fillers are interconnected to form a continuous conductive path or network, the polymer material undergoes an insulator–conductor transition and the dielectric loss tangent increases rapidly, which causes considerable difficulties and challenges in obtaining reproducible and stable products for practical applications [22,23].

Conductor–insulator core–shell particles have been used as ideal high-k filler particles in polymer composites. Due to the interface polarization, the conductor is used to increase the dielectric constant, while the insulating shell acts as a dielectric interlayer, which can effectively reduce the dielectric loss tangent by blocking electron transfer between adjacent conductors. For instance, Zhang et al. investigated polyaniline-coated calcium copper titanate (CCTO@PANI) core–shell particles as additives within PDMS to prepare CCTO@PANI/PDMS composites. The experimental results showed that the introduction of core–shell structured fillers increased the dielectric constant and suppressed the increasement of dielectric loss tangent [24]. Dang et al. [25] prepared Ag@ TiO₂ /PVDF nanocomposite films by solvent casting method. The Ag@TiO₂ core@shell nanoparticles were synthesized via a water-thermal meting route. A certain amount of Ag@TiO₂ nanoparticles and PVDF were ultrasonically dispersed in the organic solvent DMF. Afterwards, the solution was heated to completely evaporate the solvent, and subsequently molded by hot-pressing. The dielectric constant increases significantly with increasing filler content since the silver forms innumerable tiny micro-capacitors in the matrix. In a study by Silakaew et al., Ag@BaTiO₃ and RuO₂@BaTiO₃ particles were prepared by surface adsorption deposition to promote the dielectric response of polyvinylidene fluoride (PVDF). The dielectric constant of the composites increased with the particle volume fraction, while the loss tangent was effectively suppressed. A higher dielectric constant was obtained, while a low loss tangent was obtained [26,27]. In a study by Plattke et al., a novel dielectric core-satellite BT-gold (Au) nanoparticle (NP) was prepared by a surface chemical reaction method for use as a filler in PVDF to improve the dielectric constant, energy density, and efficiency of composites while reducing dielectric loss [28]. Nevertheless, the high modulus of the insulating shell leads to defects in the composites, which inevitably limits the properties of the composites. Given these shortcomings, all-organic fillers and external field assistance have been developed in recent years. The use of all-organic fillers and external field assistance has the following two advantages: (i) the insulating polymer has a low modulus on the basis of insulating properties, is compatible with the polymer matrix, and is easy to disperse; and (ii) the external field-assisted orientation makes it easier for the composites to form anisotropic micro-structures, which can enhance the dielectric properties and reduce the filler loading of the composites [29].

In this study, high dielectric polymer composites fabricated by a combination of conductive–insulative core–shell structure design and electric field-assisted self-assembly approach. PVP@PANI fillers were prepared by using conductive polyaniline (PANI) as the core and insulative polyvinylpyrrolidone (PVP) as the layer, then assembled into micro-structures to further enhance the dielectric property of the composite. The dielectric constant of the composites prepared under electric fields was improved by the micro-structures formed by external electric fields. With the addition of 2.5 wt% PVP@PANI, the dielectric constant could be significantly enhanced up to 23.

2. Experimental Section

2.1. Materials

PVP K30 purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). PANI with an average particle size of around 30 microns was provided by Anhui Kuer Biological Engineering Co., Ltd (Hefei, China). The polydimethylsiloxane (PDMS) base and curing agent (Sylgard 184) were supplied by Dow Corning (Midland, Michigan, USA). The PDMS and curing agent were mixed in a mass ratio of 10:1, and then cured at 90 °C for 1 h. The indium tin oxide (ITO)-coated glass substrate was purchased from ** the dielectric loss tangent at a very low value. For samples containing PVP@PANI, the dielectric loss tangent is only 1 when adding 2.5 wt% PVP@PANI, lower than that of aligned samples PANI. This is because the presence of PVP between conductive PANI in aligned channels can block the formation of conductive paths and minimize the increase in dielectric loss tangent caused by the leakage of current. The insulated PVP shell reduces the dielectric loss tangent, which acts as a dielectric layer to suppress the current leakage between PANI in direct contact [40,41,42,43].

4. Conclusions

In conclusion, we have developed a new strategy for facilitating particles in forming a network structure within a matrix under an applied electric field, and fabricated high dielectric constant polymer composites. By incorporating PVP@PANI particles into PDMS and forming a network through an electric field, polymer composites with high dielectric constant and low dielectric loss tangent can be obtained. The dielectric constant can be significantly increased to 23 after adding 2.5 wt% PVP@PANI.