Influence of polymer modulus on the percolation threshold of latex-based composites
Introduction
Polymer composites containing electrically conductive filler combine the beneficial properties inherited from the polymer matrix (good toughness, flexibility, light weight) with electrical conductivity. These materials are useful for applications such as thermal resistors [1], [2], chemical sensors [3], [4], electromagnetic interference (EMI) shielding [5], [6], and electrostatic dissipation (ESD) [6], [7]. Despite their promise, a high concentration of conductive filler is often required for these composites to achieve reasonable conductivity. Greater processing viscosity and more brittle final composites accompany large filler concentration [8]. Segregated network composites, made with a polymer blend or a particulate polymer matrix, solve this problem by reducing the percolation threshold [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. The percolation threshold is the amount of filler at which the conductivity of a composite significantly increases due to the formation of an interconnected network [23]. It has been reported that the percolation threshold for randomly dispersed carbon black is around 15 vol% [24], [25], but composites with a segregated network have achieved percolation thresholds below 0.1 vol% [22].
The segregated network concept was initially formalized by Kusy in the context of lightly hot pressing dry mixtures of polymer and metal powders to create electrically conductive composites [26]. In essence, the conductive filler is given a restricted volume in which to reside that leads to network formation at low concentration. Using an immiscible polymer blend is one of the most common techniques used to form a segregated network and relies on the conductive filler being dispersed predominantly within one polymer [9], [10], [11], [12] or at the interface between the two polymers [9], [13], [14], [15]. A simpler method uses a polymer emulsion to create the segregated network by forcing the conductive particles into the interstitial space between the solid polymer particles during drying [16], [17], [18], [19], [20], [21], as illustrated in Fig. 1. The emulsion particles are relatively large (typically 100 nm–10 μm) compared to the size of conductive filler, which significantly lowers the percolation threshold of the final composite. This method is similar to using immiscible polymer blends in terms of creating excluded volume where little or no conductive particles reside. Unlike with a polymer blend, which is melt processed and has multiple phases, an emulsion has only a single polymer phase and processing can be done at room temperature [16], [18].
Electrically conductive polymer composites are typically produced by melt [27], [28] or solution-based processing [29], [30], which makes the polymer modulus negligible during processing. When using a polymer emulsion, the matrix remains solid throughout the processing steps. This is why the polymer modulus will play a key role in the final composite microstructure and ultimately influence electrical conductivity. In this work, the influence of emulsion polymer modulus on the electrical properties of carbon black-filled composites is examined. Monodisperse acrylic latexes with varying glass transition temperature were synthesized and used as the composite matrix starting material. Polymers with varying glass transition temperatures were made by changing the ratio of methyl methacrylate to butyl acrylate in the emulsion, which produced variations in room temperature polymer modulus (i.e., greater Tg polymer has greater modulus). Composites prepared with lower room temperature modulus emulsion exhibit a greater percolation threshold due to greater deformability of the polymer particles. These results reveal an additional parameter for tailoring the percolation threshold that may be useful for a variety of applications requiring flexible conductive films.
Section snippets
Materials
Methyl methacrylate (MMA), n-butyl acrylate (BA), methacrylic acid (MAA), poly(vinyl alcohol) (PVOH, Mn 85,000–124,000 g/mol), and sodium persulfate (Na2S2O8) were purchased from Sigma–Aldrich (Milwaukee, WI) and used as received. Triton X-405 (70% in water solution), from Sigma–Aldrich, and sodium dodecyl sulfate (SDS, C12H25NaO4S), from J.T. Baker (Phillipsburg, NJ), were used as surfactants. Triton X-405 is an octylphenol ethoxylate surfactant that provides steric stabilization, which
Monodispersed latex characterizations
Following emulsion polymerization, the particle size and glass transition temperature of the polymers were characterized. All samples were dried at room temperature. Particle size, BA/MMA ratio, glass transition temperature and storage moduli of these latexes are summarized in Table 2. All of the latexes have an average particle size of approximately 150 nm, with a distribution narrow enough to be called monodisperse (<1.1) [33]. The combination of an ionic and nonionic surfactant typically
Conclusion
A series of polymer emulsions with different glass transition temperatures were synthesized by varying the ratio of butyl acrylate and methyl methacrylate repeat units. The glass transition temperatures of these latexes were higher than those predicted by the Fox equation due to greater MMA reactivity that likely increased its concentration in the copolymer. Composites made using the emulsion with higher room temperature modulus (due to higher Tg) exhibited a lower percolation threshold and
Acknowledgments
Authors would like to acknowledge the Texas Engineering Experiment Station (TEES) for financial support of this research.
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