Design and Characterization of a Multilayered Multifield-Actuated Polymer Unimorph

Abstract

In this study, we design, fabricate and characterize a novel unimorph-like structure in which a shape memory polymer (SMP) is actuated using a magneto-active elastomer (MAE), resulting in contactless and large deformation shape change. The constituent materials are: a two-part elastomer, iron oxide microparticles, and a rapid cure epoxy. Combining the elastomer with the iron oxide particles results in an MAE actuator, such that application of an external magnetic field induces dipoles in the iron oxide particles, resulting in the actuation of the MAE. The shape memory epoxy polymers are deformed at an elevated temperature and cooled to lock-in the temporary programmed shape. Subsequent exposure to elevated temperature can then recover a permanent geometry. In a first step, the layered unimorph structure is fabricated using casting techniques in which the SMP serves as the ‘passive’ substrate, and the MAE is cast on top of this substrate or joined post-processing using adhesives. To demonstrate shape change, the unimorph is heated beyond the transition temperature of the epoxy, and a magnetic field is applied to deform the structure. Upon cooling, the shape change is locked into place. This configuration serves as proof-of-concept that the proposed magnetically-responsive polymers are capable of eliciting the shape change of an SMP through bending in response to an external magnetic stimulus. After demonstrating feasibility, the next step is to use additive manufacturing (AM) to produce both the SMP and MAE by developing and tuning MAE and SMP resin formulations for printability. Printability is assessed by characterizing the viscosities, effective yield stress, gelation times, and extent of cure of the material formulations. The bending response of the layered structure is characterized as a function of magnetic field, material composition, geometric parameters, and AM process settings. The outcome of this research is to enable AM of smart materials and devices that can monitor and adapt to their environment. The medical device industry in particular stands to benefit from customizable devices that adapt their shape for particular patients, diseases, and/or stage of healing.

Analytical modeling of a magnetoactive elastomer unimorph

“…A uniform dispersion of BHF particles inside a PDMS matrix is assumed. For pure BHF particles, the magnitude of the mass magnetization, m mH , is assumed to be constant at 40 emu g −1 , which is the average value of the measured remanent magnetization for BHF particles [43,44]. The direction of m mH is based on the poling direction, which is perpendicular to the length of the unimorph and consistent with the cast samples in the experiment.…”