Synthesis of high-k and low dielectric loss polymeric composites from crosslinked divinylbenzene coated carbon nanotubes
Graphical abstract
Introduction
High-permittivity (high-k) polymer materials are widely used in electrical energy storage devices, such as integrated capacitors, actuators, piezoelectric, pyroelectric sensors, and power cable terminals because of their intrinsic advantages of flexibility, durability, easy processing and lightness [1], [2], [3]. However, the very low dielectric constant (ε' = 2 – 3) of most insulating polymer materials hinders their further application.
One approach to enhance the dielectric constant of a polymer is to introduce high-permittivity ceramic powders to it [4], [5]. The resultant ceramic/polymer composites may be expected to possess large dielectric permittivities, coupled with high stiffness and excellent thermal stability. However, high loading levels of ceramic fillers are often necessary to realize adequately high permittivities; in such a case, the composites may exhibit deteriorated physical and processing properties [2]. An alternative approach is to introduce conductive fillers to the polymer matrix to prepare percolative insulator/conductor polymer composites [6], [7], [8], [9], [10], [11], [12], [13], which show the high dielectric constants when the content of conductive filler reachs to percolation threshold. CNT-based polymers are typical insulator/conductor systems that have attracted much attention in recent years because they produce ultrahigh dielectric values at low CNT loadings [14], [15], [16], [17], [18]. However, insulator/conductor materials often present large dielectric loss as a result of high leakage currents, which are caused by direct contact between conductive fillers in composites near the percolation threshold [19].
Several methods have been developed to address this problem. Traditional research has focused on modification of the surface of conductive fillers to improve filler dispersion and interaction with the polymer matrix [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35]. These methods include physical absorption, chemical grafting, and coating, among others. Of the methods developed thus far, creating insulating layers between conductive fillers and matrix is a popular choice. Wu et al. prepared TiO2 nanorod-decorated CNTs/polystyrene (PS) composites [2] and showed that the composite containing of 17.2 vol% CNTs had a permittivity of 37 at 1 kHz, which is 13.7 times higher than that of pure PS (2.7), and a very low dielectric loss of less than 0.11. The same authors fabricated graphene oxide-encapsulated CNT hybrids filled with polyurethane [17]. The dielectric permittivity of composite containing 10% CNTs is up to 204, which is 34-fold higher than that of pure polyurethane (about 5.8). Another method often used to decrease dielectric loss involves decoration of the polymer matrix with special macro-structured layers. Wang et al. used a layer-by-layer casting technique to fabricate a multi-layer MWCNTs/epoxy resin composite and two layer polyethylene-CNTs/cyanate ester resin composites [16]. The dielectric constants of the prepared materials were as high as 465 at 1 Hz and 168 at 10 Hz, but their dielectric losses remained very low. Generally, however, keeping the dielectric loss of composites to less than 0.1 and maintaining high dielectric constants with increasing frequency are difficult.
In our previous work, we reported a strategy to coat MWCNTs with cross-linkable materials by combining Diels−Alder cyclo-addition with atom transfer radical polymerization and introduced a direct radical reaction to achieve controllable surface thickness on electrically conductive CNTs [36], [37], [38]. Afterward, we compounded coated-CNTs with polyvinylidene and successfully prepared composites with a high dielectric constant (110) and low dielectric loss (0.53). Because of the simple operation and widely applicable monomers of radical reaction, many researchers have aimed to use free-radical initiators for CNT surface modification [39], [40], [41], [42].
In this paper, we employed a highly convenient method to synthesize completely coated multiwalled carbon nanotubes (MWCNTs) based on free-radical polymerization of divinylbenzene (DVB) as a monomer in a controllable manner. Because of its π-π conjugated structure [43], DVB tends to be absorbed onto the surface of MWCNTs. Suitable initiators could attack the surface of MWCNTs or DVB, forming MWCNT and monomer-free radicals to realize further reaction. Consequently, cross-linked DVB coated on the MWCNT surface produced a core-shell microstructured MWCNT-centered hybrid (DVB@MWCNTs). We combine traditional chemical modification with special processing procedures to successfully prepare DVB@MWCNTs/PS composites with a high filling content, high dielectric constant (115), and low dielectric loss (0.015) at 1 kHz. The dielectric constant of the composites was maintained at high levels over a wide frequency range, and their dielectric loss remained low. At the same frequency, the dielectric properties of the composites could be tuned by controlling the weight proportion of the MWCNTs. The effect of the coating layer on the dielectric properties of the composites is also discussed.
Section snippets
Materials
Two types of MWCNTs, namely, S-MWCNTs (average outer diameter: 40–60 nm, length: <5 μm, purity: >99.9%) and L-MWCNTs (average outer diameter: 40–60 nm, length: 5–15 μm, purity: >99.9%), were purchased from Shenzhen Nano Port Company, China. DVB (purity: 80% purchased from J&K company) was used as a cross-linking monomer. AIBN was used to initiate free-radical polymerization and organic solvents, such as N,N-dimethylformamide (DMF), toluene, and methanol, were purchased from Beijing Chemical
Core-shell microstructure of DVB@MWCNTs
The FT-IR spectra of pristine MWCNTs and DVB@MWCNTs obtained through in situ free-radical polymerization are shown in Fig. 2. After functionalization, peaks at around 703, 993, and 3018 cm−1, corresponding to CH stretching and hydrogen bending vibrations on the benzene ring of the DVB segment, are observed. A peak indicating CH2 stretching vibrations on the benzene ring appears at around 2923 cm−1. The absorption band ranging from 1466 cm−1 to 1605 cm−1 is characterized as the CC stretching
Conclusions
In this study, MWCNTs are successfully coated with DVB cross-linked polymers by a fairly simple and low-cost approach involving a one-step operation that is environment friendly and energy efficient. The coating thickness of the composites could be controlled from about 15 nm to 70 nm by adjusting the weight ratio of DVB and MWCNTs. The coated MWCNTs are well-dispersed in the polymer matrix and high-k and low dielectric loss polymeric composites are obtained; these composites possess a high
Acknowledgments
The authors gratefully acknowledge financial support of this work coming from National Natural Science Foundation of China (NSFC) (No. 51173009, 51573010).
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