Flexible graphene/PAM/PVA hydrogel composites with highly dispersed cobalt nanoparticles for enhanced microwave absorption.

With the increasing demand for flexible microwave-absorbing materials (MAMs), it is necessary to ensure a continuous network of MAM fillers. However, it is still a great challenge to achieve uniform loading of magnetic nanoparticles because of the uncertain reaction sites of graphene. For this research, we adopted a high-pressure homogenization strategy to realize a uniform distribution of Co nanoparticles on the graphene surface via polyethyleneimine (PEI) modification. Transferring the Co reaction sites from the randomly distributed oxygen functional groups to the uniformly distributed PEI molecules, the homogeneous graphene oxide (GO)/PEI/Co (GP/Co) dispersion ensured to form the uniform reduced graphene oxide (RGO)/PEI/Co (RGP/Co) after reduction. RGP/Co-2 achieved an effective absorption of -51.9 dB in the C band and a microwave absorption bandwidth of 5.84 GHz. In addition, RGP/Co-2 was filled into polyacrylamide/polyvinyl alcohol (PAM/PVA) hydrogel to construct a uniform and continuous network structure, and RGP/Co-2/PAM/PVA flexible MAMs were prepared. The minimum reflection loss (RLmin) is -41.5 dB under the thickness of 2.5 mm and shows excellent mechanical flexibility. In addition, RGP/Co-2/PAM/PVA exhibited excellent sensing performance. This research supplied a new perspective for the development of MAMs in flexible electronics.