Electrically conducting composites are one area of interest in the Electronic Materials group at TDA Research. Conducting composites are increasingly being used in the aerospace and consumer electronics industries. The majority of these composites are made with noble metal-filled epoxies and urethanes. The metal fillers in these composites can be quite expensive and heavy (along with a number of other drawbacks). Intrinsically Conducting polymers (ICPs) could replace metal particles as the conductive element in conductive composites for use in sealants and adhesives. ICPs have advantages over metal particles, being lighter in weight and potentially more elastic or flexible. Furthermore, because sealants based on CPs form a polymer-polymer composite rather than a polymer-metal composite, the conductive and non-conductive components are more chemically compatible. Said another way, there is better adhesion or cohesion at the interface between conducting and non-conducting domains. This has been shown to lead to lower percolation thresholds (i.e. lower loading of conducting material giving higher conductivity) and improved mechanical properties in ICP-based polymer blends. The figure above right shows a number of photographs of several conducting composites that have been made at TDA.
ICPs have been studied now for well over two decades, and much has been learned in that time. Among the most successful (and more recent) examples of CPs (for example, polypyrrole, polyaniline, and polythiophene derivatives), electrical conduction arises from an extended network of overlapping p-orbitals (conjugation) over which charge is delocalized. As long as a delocalized pathway exists from one point in the material to another, electrical conduction can occur. Experimental evidence and model theories have demonstrated that this is in fact correct. Random and block copolymers of conducting monomers (such as thiophene, 3-methylthiophene, or pyrrole) and non-conducting monomers (such as methacrylate, styrene, vinyl ethers, or siloxanes) have been prepared and found to have good electronic conductivity.
So how does a conducting pathway form in composites? The figure at right shows two composites. On the left is a composite that has low loading of a conducting filler, and on the right is a composite with a higher loading of conducting filler. The composite on the right is conducting while the composite on the left is not. That is because in the more highly filled composite there is a continous pathway from particle to particle throught which electrical current can flow. There is a specific level of loading at which a composite goes from the structure on the left to the structure on the right. This level is often refered to as the percolation threshold. This percolation behavior has been explained in terms of percolation theory as well as more sophisticated models such as flocculation theory. Both theories predict that below a certain critical concentration (sometimes called the percolation threshold) of the conducting component, the composite material is an insulator. Around the critical concentration a small increase in the concentration of the conducting component increases the conductivity by several orders of magnitude. Above the critical concentration the addition of conducting material brings only a small further increase in the conductivity. The percolation threshold of conducting composites usually ranges between 10 and 30% (Vol. %), but composites of ICPs and polymers have been reported to have a much lower thresholds (1.7 to 2.6 % have been reported). Althought simple percolation cannot explain these low thresholds, flocculation theory, double percolation theory, and directed percolation have been used to predict the low thresholds seen with ICP composites.
With TDA's excellent expertise in ICP materials, we are investigating their use in new conducting composites. The photomicrographs on the right show two different blends with images taken at the same scale for each sample. The deep blue color comes from the ICP materials; the matrix here is colorless. These images clearly show that phase separation is occurring in these blends, but on a very different size scale for each of the two samples. We are discovering the relationship between these structures and how they relate to bulk properties.
The figure at right shows the bulk conductivity data obtained for a number of TDA's composite samples. These materials exhibit an exellent conductivty at very low fill fractions. We found that composites with filler content below 2.5 wt.% had unmeasureable conductivity (represented by the red dashed line). This means that the percolation threshold of these materials is somewhere around 2.5-3 wt.%. This is an impressive result for TDA's first series of composites. As we begin to explore a wider range of formulations, we expect to find materials that will perform well as conducting sealants, coatings, and adhesives.