Thermally-triggered free-standing shape-memory actuators

Abstract

This investigation presents a new approach to obtain free-standing thermally-triggered “two-way” shape-memory actuators by realizing multilayer structures constituted by glassy thermoset (GT) films anchored to a previously programmed liquid-crystalline network (LCN) film. The GT is obtained via dual-curing of off-stoichiometric “thiol-epoxy” mixtures, thus enabling the development of complex actuator configurations thanks to the easy processing in the intermediate stage, and a compact and resistant design due to the strong adhesion between the layers obtained upon the final curing stage of the GT. A model based on the classical multilayered beam theory to predict the maximum deflection of a “beam-like” design is proposed and its reliability is verified by experimental investigation of actuators with different configurations and LCN stretching levels. The results show the capability of these actuators to bend and unbend under various consecutive heating–cooling procedures in a controlled way. The maximum deflection can be modulated through the configuration and the LCN stretching level, showing an excellent fitting with the model predictions. The model is able to predict high actuation levels (angles of curvature ˜ 180°) and the bidirectional shape-memory behavior of the device as a function of the thickness, configuration of the layers, and the LCN stretching level. This approach enables the design of free-standing two-way actuators covering a range of bending actuation from 27 to 98% of the theoretical maximum deflection.Postprint (author's final draft

Sequential heat release: an innovative approach for the control of curing profiles during composite processing based on dual‐curing systems

“…The reaction mechanisms of nucleophile‐initiated thiol–epoxy polymerization and epoxy homopolymerization have been recently discussed in detail elsewhere . Because the kinetics of the second process are much slower than the first one, it has been possible to develop recently a new family of dual‐curable thermosetting materials that have found, so far, an application in shape‐memory devices …”

Motion control in free-standing shape-memory actuators

“…According to the theory of elasticity for multilayered beams subject to built-in stresses [30,31], with this configuration, the final actuator will bend downwards with respect to the thinner GT layer (layer 1) due to the contraction of the LCN, as illustrated in Figure 1 Once the actuator configuration is built, the 2 nd curing stage of the GT material is carried out by placing the actuator in between Teflon glass plates, applying a small constraining force to prevent the LCN shrinkage, and following a specific temperature programme. In our previous work [25], a temperature of ºC was used to limit the LCN premature shrinkage; however, GT materials with glass transition temperatures close to or equal to Tiso (120 ºC) were used in this work. Therefore, to allow completion of epoxy homopolymerization (2 nd curing stage), the following curing procedure was used: 3 hours at 90 ºC plus 1 hour at 110 ºC plus 1 hour at 135 ºC.…”

Brush‐Paintable, Temperature and Light Responsive Triple Shape‐Memory Photonic Coatings Based on Micrometer‐Sized Cholesteric Liquid Crystal Polymer Particles

“…Shape‐memory polymers (SMPs) are fascinating stimuli‐responsive materials that exhibit irreversible shape‐changes upon external stimuli such as light, temperature, or humidity. [ 1,2 ] SMPs have the ability to achieve and retain a temporal shape through the so‐called “programming” to further recover their original shape upon application of a specific stimulus. [ 3,4 ] In particular, shape‐memory photonic structures, such as photonic crystals and cholesteric liquid‐crystalline (CLC) polymers, have received attention as they exhibit simultaneous shape and structural color changes upon temperature changes.…”