Ferrofluid high internal phase emulsion polymer foams for soft, magnetic materials

• Published in: Journal of magnetism and magnetic materials 2022-12, Vol.563, p.169921, Article 169921
• Main Authors: Bury, Elizabeth, Thiagarajan, Sandhiya, Lazarus, Nathan, Koh, Amanda
• Format: Article
• Language: English
• Subjects: Deformable magnetic components; Ferrofluid; High internal phase emulsion polymer foams; Soft electronics
• ISSN: 0304-8853
• DOI: 10.1016/j.jmmm.2022.169921

•Details the first successful magnetic high internal phase emulsion polymer foam (pHIPE) utilizing ferrofluid in an elastomer.•Elucidates design rules for pHIPE emulsifiers and stability that enable 60 vol% ferrofluid encapsulated in PDMS.•Final ferrofluid pHIPE had magnetic saturation/susceptibility superior to current state-of-the-art in magnetic dispersions.•Comparisons between FF/PDMS pHIPE and iron oxide/PDMS demonstrated that engineering the material interface is critical.•FF/PDMS pHIPE had a lower modulus than any high performing magnetic composite in the literature and lower than the neat PDMS. Magnetic components are an essential part of current electronics, although the materials used to make magnetic components are almost exclusively rigid and non-deformable. The high modulus and inflexible behavior of typical magnetic materials makes their integration into soft, deformable electronics challenging. Soft electronics, however, is a dynamic field pushing forward the state-of-the-art in wearable sensors, human-machine interaction, and biomimetic robots. In order to access the wide range of functionality of rigid electronics, magnetic materials must be made that are high performing and deformable. One previously unexplored route to creating such materials is the encapsulation of a high concentration of magnetic fluids, such as ferrofluids (FF), into an elastomer using high internal phase emulsification. In this work, the formulation factors required to make an effective ferrofluid high internal phase emulsion polymer foam (polyHIPE) utilizing polydimethylsiloxane (PDMS) were investigated such as the emulsifiers, polyHIPE curing kinetics, and the interaction of emulsified nanoparticles. The porous structure, magnetic properties, and mechanical properties of the final, successful FF/PDMS polyHIPE were subsequently characterized. Pores ranged between 10 and 50 μm with visible nanoparticle aggregation. FF/PDMS polyHIPE had an Ms of 18 A∙m2/kg and susceptibility of 0.69, which is reduced as compared to neat ferrofluid, as expected, but an improvement on ferrofluid polymer dispersions found in the literature. Coercivity and magnetic remanence also decreased with respect to the neat ferrofluid. Rheology demonstrated that the polyHIPE had a significantly lower modulus than the neat PDMS as well as a lower modulus than any magnetic composite material currently published. The FF/PDMS polyHIPE created in this work not only demonstrates the potential for deforma