Styrenic block copolymer-based nanocomposites: Implications of nanostructuration and nanofiller tailored dispersion on the dielectric properties

Highlights

• Increased AC short-term breakdown strength up to 45% upon orientation of nanoclays.

• Reduced dielectric loss upon orientation of nanoclays up to 2 orders of magnitude.

• Reduced breakdown strength upon orientation of styrene phase of the block copolymer.

• Higher breakdown strength in block copolymer lamellar structure compared to hexagonal.

• Improved breakdown strength due to increased nanoclay/polymer interfacial area.

Abstract

In this work, the effect of controlling the morphology on the dielectric properties of triblock copolymers and their clay-containing nanocomposites was evaluated. Two different copolymers: polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene (SEBS) and SEBS grafted with maleic anhydride were used for that purpose. Morphologies with different degrees of intercalation, exfoliation and orientation were obtained and tested. At the highest state of dispersion, achieved at a clay loading equal to 5 wt%, 50% of clay nanoplatelets were individually dispersed and located within the soft domain of the block copolymer and a maximum of interfacial polarization and a minimum of dynamic mechanical damping factor were respectively exhibited. When the nanoclays were oriented, the dielectric loss due to nanoclays conductivity contribution was reduced up to 2 orders of magnitude at high temperatures and low frequencies and the AC short-term breakdown strength increased up to 45%.

Introduction

“Nanodielectrics” or nanocomposite dielectrics are a new generation of dielectric materials containing fillers that have at least one dimension less than 100 nm [1]. This class of materials is gaining a lot of interest aiming at developing dielectrics with distinct properties originating from the intrinsic properties of the nanofillers and mostly from the important interfacial region introduced due to the huge surface area of the nanofillers [2], [3]. Several publications over the last decade proved the efficiency of nanofillers at improving several aspects related to the dielectric performance of insulating polymers in applications such as cable insulation [4], [5], high energy storage capacitors [6] and dielectric elastomer actuators [7]. In these studies, different types of nanofillers and polymers have been tested.

In terms of nanofillers, nanoclays were the most common nanoparticles used due to their relatively low cost. More specifically, organically-modified nanoclays have been the nanoparticles of choice since their surface modification made them more compatible with organic polymer media [8], [9]. These nanoparticles have proved their positive role in improving the mechanical properties of polymers and in few studies in improving their dielectric properties. In effect, it was demonstrated that nanoclay addition and orientation has an influence on the high field properties of polyolefins such as the short-term breakdown strength [10], [11], [12], [13]. In this context, David et al. [10] reported an increase up to 15% of the breakdown strength of low density polyethylene (LDPE) matrix filled with a low content of nanoclay equal to 3 wt%. This improvement was combined with an increase of the dielectric loss by roughly two orders of magnitude. They attributed the increase of the breakdown strength to the exfoliated structure of nanoclays and the high dielectric loss to the high conductivity along the clay nanoplatelets. Also, Tomer et al. [11] reported an increase of the breakdown strength by 20% upon the alignment of nanoparticles in LDPE containing 6 wt% nanoclay whereas they observed a deterioration in the breakdown strength and consequently in the maximum recoverable energy when nanoclays are randomly oriented. The authors reported as well an increase of the dielectric loss by two orders of magnitude in both types of nanocomposites with either random or aligned nanoclays.

In terms of polymers, styrenic block copolymers constitute interesting candidates which represent an important part of the current market of thermoplastic elastomers. In fact, these materials exhibit attractive mechanical properties due to their combination of a soft elastomer phase and a hard polystyrene phase. Furthermore, as block copolymers, this class of materials presents the advantage of being self-assembled at the nanoscale in unique morphologies such as lamellar, cylindrical and spherical hard domains distributed within the elastomer domains. The shape of these ordered structures depends on the chemical composition, molar mass of the blocks, affinity between the blocks and processing method [14]. In the field of nanocomposites, this ability to control their spatial organization makes styrenic block copolymers attractive as promising template matrices for selective dispersion of nanoparticles with competitive mechanical properties [15], [16], [17]. In this context, a great number of recent publications were related to block copolymer-based nanocomposites with tailored dispersion of nanofillers. Among them, the studies performed by Carastan et al. [18], [19] show that selective dispersion and orientation of nanoclays were successfully achieved in a triblock copolymer. The published researches treated various applications such as polymer solar cells [21], [22] and block copolymer electrolytes [23]. They demonstrated that the orientation as well as the location of nanoparticles seems to affect the resulting properties. To the best of our knowledge, only few of these studies addressed the field of dielectric applications [24], [25], [26] in spite of the fact that thermoplastic elastomers could represent promising candidates for dielectric elastomer actuators [26] or styrenic block copolymer/polyolefin blends for high voltage insulation [27].

In this paper, the dielectric properties of polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene (SEBS), an interesting thermoplastic elastomer widely used [28], [29], [30], [31], [32], [33], and its nanocomposites containing clay nanoparticles were investigated as function of various parameters related at times to the nanofillers and other times to the SEBS structure. In this context, the effect of clay dispersion (intercalated vs. exfoliated), clay location (within the hard or soft phase), clay concentration and clay orientation were evaluated. Besides, the effect of SEBS morphology was studied. In particular, two types of SEBS structures, namely a hexagonal structure where polystyrene cylinders are distributed within the rubber phase and a lamellar structure formed by alternating polystyrene and rubber layers, were evaluated. The structures of SEBS nanocomposites studied were characterized by different tools such as TEM, SAXS and DMA. The complex dielectric permittivity and breakdown strength of these nanocomposites were also evaluated and correlated to the different structures.