Stepped Moduli in Layered Composites

© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 7197 wileyonlinelibrary.com Circumstance-adaptive materials (CAMs) are those that can change their properties—mechanical, optical, electrical, magnetic, biological, and so on—to respond to their use or environment; [ 3–8 ] they represent a current challenge in materials science. Among the wide variety of possible CAMs, we focus on materials whose mechanical properties change in response to their circumstance. We use the word “circumstance” intentionally for its breadth: we wish to consider a range of factors that characterize the environment, including temperature, water content, strength of electric or magnetic fi elds, patterns and history of use, and others (as appropriate). There are different types of stimuli that can induce changes in mechanical properties of materials. As an example of CAMs in Nature, sea cucumbers rapidly change their stiffness in the face of danger by controlling the interactions among the collagen fi bers by secreting chemicals into their dermis. [ 9–12 ] Inspired by this mechanism, composites of cellulose fi bers or nanocrystals dispersed in polymers have been fabricated to show a reversible reduction of tensile modulus upon exposure to physiological conditions. [ 13–16 ] Coiled fi bers and braids that change porosity in response to temperature showed an increase in modulus and tensile actuation. [ 17 ] Electrical potential provides another simple way to manipulate mechanical properties. Oxidative and reductive potentials change the cross-linking capacity of ions within a gel-like matrix, [ 18 ] and electrical current induces phase transition (solid to liquid) by Joule heating. [ 19 ] Magnetorheological fl uids and elastomers exhibit tunable elastic stiffness in response to an external magnetic fi eld. [ 20,21 ]