Editorial/prefaceConductive polymer composites with segregated structures
Introduction
Electrically conductive polymer composites (CPCs), which consist of single or hybrid conductive fillers (e.g., carbonaceous, metallic, and conducting polymericparticles) dispersed in a polymer matrix based on a single polymer or a multi-phase blend, have attracted considerable academic and industrial attention for several decades [1], [2], [3], [4], [5]. Their popularity can be demonstrated by the amount of research articles relevant to CPCs: over 12,000 publications was retrieved on 20 May, 2014 on the Institute for Scientific Information (ISI)—Web of Science database, searching the subject “conductive polymer composite”. Owing to their ease of processing, low-cost, and tunable electrical properties in comparison to intrinsic conducting polymers, CPCs have served applications as anti-static materials, electromagnetic interference (EMI) shielding, sensors and conductors [6], [7], [8], [9]. The electrical resistivity of CPCs determines their specific applications, e.g., see Fig. 1. For example, the CPC materials for electrostatic dissipation in plastic fuel tanks typically require an electrical resistivity of ∼106 Ω cm, while EMI shielding requires electrical resistivity values below 10−2 Ω cm.
Because most of the ordinary host polymers are essentially insulating, the electrical performance of CPCs relies solely on continuous conductive networks constructed after incorporating the conductive fillers [10], [11], [12]. When the conductive filler content reaches a critical value, the CPC material will exhibit an insulator/conductor transition; specifically, the electrical conductivity dramatically increases by several orders of magnitude when the initial conducting channels are formed. This critical volume fraction ϕ is defined as the percolation threshold ϕc. As the conductive filler content increases, additional conductive pathways may be established in the polymer matrix, allowing the electrical conductivity to increase gradually until saturation plateau is reached. The electrically conductive behavior of a CPC material may usually be empirically described the power law [11], [12]:where σ represents the CPC electrical conductivity and t is the critical exponent related to the dimensionality of the conductive networks in CPC. In this model, t ≈ 2 and t ≈ 1.3 are for three-dimensional (3D) and two-dimensional (2D) conductive networks, respectively. However, the experimental values of usually deviate from these predicted values [13], [14].
Among the conventional CPC fabrication methods (i.e., solution processing, melt mixing, and in situ polymerization) [1], [2], [4], the melt mixing technologies, such as twin-screw extrusion, internal mixing and injection molding, are the most common approaches used to fabricate commercial CPC materials because these techniques are compatible with current industrial practices. Nonetheless, CPCs fabricated via conventional melt-mixing approaches generally have a relatively high ϕc. Theoretically, the ϕc for spherical, randomly dispersed fillers (e.g., carbon black (CB), metallic particles, and conducting polymer particles) is approximately 10–20 vol%, which is close to the percolation value of ∼16 vol% predicted by the classical percolation theory [15], [16]. Although high-aspect-ratio conductive nano-particles (e.g., carbon nanotube (CNT) or graphene nanosheet (GNS)) have large surface areas able to support well-developed transport networks, their extreme agglomeration behavior during processing in host polymers generates the relatively high ϕc. Unfortunately, the CPCs with high ϕc always suffer from various drawbacks: (i) high melt viscosities, (ii) low economic affordability, and (iii) inferior mechanical properties (esp. for ductility and toughness) [17], [18]. Therefore, decreasing ϕc efficiently has become a long-standing, major topic during the fabrication of high-performance CPC materials.
To date, forming a segregated structure in a CPC material has remained the most promising strategy for fulfilling low ϕc [19], [20], [21], [22]. In segregated CPC (s-CPC) materials, the conductive fillers are primarily located at the interfaces between the polymeric matrix particles instead of being randomly arranged throughout the entire CPC system, as shown in Fig. 2. Due to the super-high fraction and perfect mutual contact between the conductive fillers in the interfacial regions of s-CPC materials, this specific structure reduces the percolation value several times compared to conventional melt-mixed CPCs. For instance, Gupta et al. constructed a segregated CB-based conductive network in acrylonitrile–butadiene–styrene (ABS) with an ultralow ϕc (∼0.0054 vol%, which is the lowest value for CB-based CPC materials in the available literature) [23]. The mechanism for the formation of a segregated conductive network relies on a polymeric matrix with an exclusionary microstructure in which conductive fillers are allotted a constrained volume, substantially increasing the effective density of the conductive pathways at certain filler concentrations. In brief, this interesting structure provides an efficient paradigm for forming a well-established conductive network with minimal filler loading.
Three main approaches have been developed to prepare s-CPCs, as shown in Fig. 3. The first one involves compressing a mixture of polymer granules decorated with conductive fillers via dry or solution mixing to construct the segregated conductive networks (see Fig. 3a) [24], [25]. Due to the simplicity of the processing methods that include only mixing and compaction, various conductive fillers (e.g., metallic particles, CB, CNTs, and GNSs) can be distributed on the external surfaces of polymeric particles without unduly emphasizing the filler dispersion levels before being hot compressed to form bulk materials with segregated structures [26], [27], [29], [30]. However, the polymers used with this fabrication method should have relatively high melt viscosities to preserve the segregated conductive networks during hot compression molding, and the filler concentration cannot reach relatively high values (usually less than 10 wt%) due to processing difficulties. The second methodology involves dispersing conductive fillers within polymeric latex (which is called latex technology); the conductive fillers are retained within the interstitial space between the latex particles while freeze-drying the polymer emulsion (see Fig. 3b) [19], [20], [32]. Apart from the relatively complex manufacturing technology, the merits of this technique are obvious: (i) a satisfactory dispersion of conductive fillers at the surfaces of the latex particles when compared to the materials prepared through dry or solution mixing; (ii) an environmentally friendly and low cost process when only distilled water is used as the solvent; (iii) and the availability of any composition of polymer–filler systems without being limited by high melt-viscosities during melt-mixing [33], [34].
The third strategy depends on the selective distribution of conductive fillers at the interfaces of immiscible polymer blends through melt blending (see Fig. 3c) [35], [36], [37], [38]. Due to the simplicity of melt blending, this method is the first choice during the industrial production of s-CPC materials. However, forming a stable segregated conductive network at the interfaces of polymer blends is far more difficult than other technologies because this method encompasses too many influencing factors: thermodynamic coefficients (e.g., interfacial energy between the polymer matrices and conductive fillers) and kinetics parameters (e.g., mixing procedures and sequence, blending time, and shear strength) [39], [40], [41].
After Turner and co-workers initially proposed the “segregated conductive network concept” for nickel particle/high-density polyethylene (HDPE) composites in 1971 [24], [25], s-CPCs based on a numerous polymeric matrices and conductive fillers have been extensively investigated for their processing–morphology–property relationships. In addition, the related variables (e.g., grain size, polymer modulus, and processing and parameters) of s-CPCs have also been determined to optimize their performance. Nevertheless, an overview of the recent extensive progress on s-CPCs remains absent, motivating us to summarize the recent advances of s-CPC materials thoroughly in the present review article in seven sections. After the Introduction (Section 1), we will describe the ultralow percolation behaviors of s-CPCs and the major parameters influencing ϕc and σmax through the composite component, processing conditions and dispersion methods. Afterwards, the mechanical, sensing, thermoelectric and electromagnetic interference shielding (EMI) properties of the s-CPC materials are briefly described in Sections 3 Mechanical properties, 4 Sensing properties, 4.1 Temperature sensors, 4.2 Strain sensors, 4.3 Chemo sensors, 5 Thermoelectric properties, 6 EMI shielding properties. Finally, the challenges of tailoring the morphology of the segregated conductive networks and enhancing the performance of s-CPC materials are analyzed. And some future developments with s-CPCs are proposed. We hope that the above summaries are sufficiently comprehensive and representative, thereby providing a valuable reference for researchers working on high-performance CPC materials.
Section snippets
Ultralow percolation behaviors
In the last decade, significant effort toward tailoring segregated structures has been made to obtain ultralow ϕc. To clarify the highly interesting electrical percolation behaviors of s-CPCs, we summarized the ultralow percolation behaviors in terms of, for instance, the polymeric matrix, conductive filler type, and fabrication methods in Table 1. The majority of s-CPC matrices are high-melt-viscosity polymers such as ultrahigh molecular weight polyethylene (UHMWPE), polystyrene (PS) and
Mechanical properties
Although the segregated structures allow for the formation of efficient conductive networks at extremely low filler loadings, the interfacial segregation of the conductive fillers largely prevents molecular diffusion between the polymer domains. Moreover, agglomerated conductive fillers also inevitably appear, forming micro-voids along the segregated conductive pathways. These two aspects create many structural flaws that harm the mechanical behaviors of the s-CPC materials [49]. Apart from the
Sensing properties
The sensing properties of the CPCs are directly associated with the ability to disconnect the conductive channels under an external stimulus including temperature shifts, mechanical deformation, and corrosion by organic vapor or solvent [127], [128], [129]. To ensure the stability of the output signal, high filler loadings are required to build perfect conductive networks within the CPCs. Because s-CPCs exhibit low ϕc values, they have enormous potential as CPC-based sensory materials.
Thermoelectric properties
CPCs containing insulating polymer matrices and conductive fillers have been investigated as inexpensive, lightweight, and environmentally friendly alternatives to common thermoelectric devices [150], [151], [152], [153]. Nevertheless, the electrical conductivity of the conventional CPCs is relatively low (less than 100 S/m), even at high filler loading levels; the conductivity is too low to support the electron transport needed for a high-performance thermoelectric material [50], [100].
EMI shielding properties
CPCs have also been widely used for EMI shielding applications [157]. However, the high filler loading required for adequate shielding capabilities (at least above ∼20 dB) inconventional CPCs adversely affects the economic feasibility and mechanical performance [158]. Reducing the filler content without compromising the electrical conductivity and EMI SE remains a considerable challenge. Forming a segregated conductive network might resolve this problem. Nevertheless, few reports concerning
Conclusions and perspectives
The formation of a segregated conductive network throughout a polymer matrix contributes to the development of novel high-performance CPC materials. This specific distribution of conductive fillers at the interface of polymer domains results in CPCs with an efficient transport network at relatively low loading levels. In addition, the s-CPCs exhibit unique advantages (e.g., ultralow ϕc, high level-off σmax, and other multifunctionalities) compared to conventional CPCs with random networks.
These
Acknowledgments
The work was funded by the National Natural Science Foundation of China (Contract No. 50925311 and 51121001) for financial support. We also appreciated the financial support from Science and Technology Support Program of Sichuan Province (2012RZ0004) and Youth Innovative Research Team special program of Sichuan Province (2013TD0013). We are also grateful to the National Shanghai Synchrotron Radiation facility, Shanghai, and National Synchrotron Radiation Laboratory, Hefei, China, for their help
References (179)
- et al.
Progress on the morphological control of conductive network in conductive polymer composites and the use as electroactive multifunctional materials
Prog Polym Sci
(2014) - et al.
A review of vapor grown carbon nanofiber/polymer conductive composites
Carbon
(2009) - et al.
Review on the effects of injection moulding parameters on the electrical resistivity of carbon nanotube filled polymer parts
Compos Sci Technol
(2012) - et al.
Dispersion and functionalization of carbon nanotubes for polymer-based nanocomposites: a review
Composites, A
(2010) - et al.
Liquid sensing: smart polymer/CNT composites
Mater Today
(2011) - et al.
Carbon materials in composite bipolar plates for polymer electrolyte membrane fuel cells: a review of the main challenges to improve electrical performance
J Power Sources
(2011) - et al.
Fundamentals, processes and applications of high-permittivity polymer–matrix composites
Prog Mater Sci
(2012) - et al.
A review and analysis of electrical percolation in carbon nanotube polymer composites
Compos Sci Technol
(2009) - et al.
Electrical and dynamic mechanical behavior of carbon black filled polymer composites
Eur Polym J
(1996) - et al.
Electrically conductive carbon nanotube/ultrahigh molecular weight polyethylene composites with segregated and double percolated structure
Mater Lett
(2012)