Morphology Control of Polymer Composites for Enhanced Microwave Absorption

Abstract

As the number, use, and interconnectivity of wireless devices grows, the ever-present problem of electromagnetic interference (EMI) has grown exponentially. EMI can interrupt with normal device performance and degrade the quality of functions such as taking and receiving phone calls. Ordinarily, metal coatings, tapes, or plates are used to mitigate this problem due to their ability to reflect any incoming electro-magnetic (EM) radiation. However, reflection of incoming EM allows EMI to continue at neighboring devices or circuitry. To solve this problem, as well as concerns surrounding the weight, processability, and durability of metal EMI shields, researchers have turned to polymer composites as the next generation absorbing materials for EMI mitigation. Polymer-conductive nanoparticle composite foams have shown promise as EMI absorbers due to the unique porous morphology that emerges during the foaming process. Inclusion of air in the material allows for higher degrees of EM penetration into the material where there are increased interactions with highly conductive cell walls and numerous air-filled pores. These two mechanisms lead to effective EM wave dissipation in a lightweight structure. The final morphology of the foamed material has a huge impact on the final shielding properties, but the relationship between foaming conditions and final morphology is highly variable and not easily predicted. This lack of predictive capability results in time and money intensive DOE material exploration. Taking inspiration from polymer composite foams, we have developed a model guided approach to the manufacturing of polymer composite materials with periodic porous morphologies. This approach relies on using the material’s intrinsic electromagnetic properties to perform geometric optimization in COMSOL. Simulations allow us to cut time and material costs in the development of new absorbers. Simulations also allow for the exploration of geometric parameters and configurations that are currently unachievable using modern foaming processes. Furthermore, this approach is material and processing agnostic meaning it can be applied to any material system and can be processed using the most appropriate technique for a given geometry. We have found that by optimizing simple geometric parameters such as size of pore and size of cell walls, we are able to dramatically increase the absorptive abilities of polymer composite shields. Additionally, we have successfully verified this approach for a simple poly-lactic acid/graphene nanoplatelet (PLA/xGNP) system and found that the modeling approach can successfully predict trends as well as maxima/minima of shielding phenomena for a variety of geometric arrangements. At high loadings of xGNPs, there are difficulties obtaining a homogenous dispersion of the nano-particles so we also explored the shielding properties of a polystyrene (PS)/reduced graphene oxide(rGO) system. Hot pressing of these materials leads to a segregated polymer composite structure with rGO fillers in the interfaces surrounding polystyrene domains. We studied the effect of rGO loading on the electromagnetic and EMI shielding properties of these materials. We also studied the properties of PS/rGO structures functionalized with cobalt ferrite (CF) nanoparticles. The CF allows for material interaction with the magnetic component of an incoming EM wave, leading to higher absorption.