Effects of soft and hard magnetic particles on the mechanical performance of ultra-soft magnetorheological elastomers

Abstract

Magnetorheological elastomers (MREs) mechanically respond to external magnetic stimuli by changing their mechanical properties and/or changing their shape. Recent studies have shown the great potential of MREs when manufactured with an extremely soft matrix and soft-magnetic particles. Under the application of an external magnetic field, such MREs present significant mechanical stiffening, and when the magnetic field is off, they show a softer response, being these alternative states fully reversible. Although soft-magnetic particles are suitable for their high magnetic susceptibility, they require the magnetic actuation to remain constant in order to achieve the magneto-mechanical stiffening. Here, we present an alternative solution based on hard-magnetic MREs to provide stiffening responses that can be sustained along time without the need of keeping the external magnetic field on. To this end, we manufacture novel extremely soft hard-magnetic MREs (stiffness in the order of 1~kPa) and characterise them under magneto-mechanical shear and confined magnetic expansion deformation modes, providing a comparison framework with the soft-magnetic counterparts. The extremely soft nature of the matrix allows for easily activating the magneto-mechanical couplings under external magnetic actuation. In this regard, we provide a novel approach by setting the magnetic actuation below the fully magnetic saturating field. In addition, free deformation tests provide hints on the microstructural transmission of torques from the hard-magnetic particles to the viscoelastic carrier matrix, resulting in macroscopic geometrical effects and complex functional morphological changes.

Polymer networks which locally rotate to accommodate stresses, torques, and deformation

“…Recent work has highlighted the importance of polymer chain orientational energies and internal torques within the network as they have the potential to be used for shape morphing [26,58], asymmetric actuation modes [27], and architected polymer networks with enhanced stimuli-responsive properties [38]. However, two of the most important and successful polymer network models, the 8-chain with the principal frame assumption (i.e.…”

“…Orientational energies often occur in stimuli-responsive polymer networks such as networks consisting of monomers with magnetic dipoles in an externally applied magnetic field [26,58,59], and networks consisting of monomers with electric dipoles in an externally applied electric field [31,48,49]. Both theory and experiments suggest that these orientational energies can lead to internal torques within the network which can be used for shape morphing [26,58,59], asymmetric actuation modes [27], and architected polymer networks with enhanced stimuli-responsive properties [38]. An important consideration then for stimuli-responsive polymers is that macro-to-micro kinematic assumption properly represents-not only the chain stretches, but also-chain orientations for a given F .…”

“…Polymer network models have been applied to a vast number of different polymer networks related to adhesives [20], biomechanics [21][22][23][24], magneto-active polymers (e.g. hard-magnetic soft materials) [25,26] and electro-active polymers [27][28][29][30][31][32]. Network models have also been used for capturing phenomena such as viscoelasticity [20,33], the curing of networks [34], excluded volume induced strain-hardening and incompressibility [35], and chain scission and fracture [36,37].…”

“…networks consisting of chains of different lengths) [36]. While it is widely accepted that the 8-chain and microsphere models (and their variants) are among the most accurate for the elasticity of rubber-like materials, the appropriate polymer network model for stimuli-responsive [25][26][27][28][29], biopolymer [22][23][24], and semi-crystalline networks [39] are still active areas of research; as is the appropriate polymer network model for fracture [36,37,40]. Recent results suggest, for instance, that the 8-chain and microsphere models are inaccurate and make nonphysical predictions for some biopolymer networks [22].…”

Curing spurious magneto‐mechanical coupling in soft non‐magnetic materials

“…16,17 However, in this subclass of MREs well-established experimental, numerical and theoretical approaches face new challenges. For pioneering experimental data we refer to Nikitin et al, 18 Stepanov et al 19,20 and rather recently Moreno-Mateos et al, 21,22 which set several opportunities for future research.…”