Self-folding origami of prestrained shape memory polymer by resistive Joule-heating

摘要

Shape memory polymers (SMPs) are studied extensively for self-folding origami due to their low cost, large strain recovery and low activation energy. SMPs utilize viscoelastic strain recovery to induce shape change, wherein an external stimulus, e.g. light or electricity, heats the material above the glass transition temperature to accelerate the recovery. Application of electric current to a conductive SMP composite produces Joule-heating, which provides higher energy density and a shorter self-folding time compared to other stimuli. Previous research has focused on Joule-heat induced shape recovery using SMP samples containing uniformly dispersed conductive fillers. Application of an electric field to these samples causes them to heat and change shape uniformly, thus limiting the ability to fold locally. In contrast, the present study focuses on shape recovery of a prestrained SMP sheets using localized resistive Joule-heating via a nichrome wire. The localized heat input applied to the SMP enables self-folding in specific regions of the sample. A previously prestrained polymer sample, experiences a differential shrinking between its top and bottom surface when subjected to the local Joule-heat. The differential shrinking causes the polymer to have a strain gradient along the thickness, which results in self-folding of the sample. This paper studies the thermal and mechanical response of Joule-heat induced self-folding of polymer sheets subjected to varying applied current and electrical resistance. Furthermore, an in-house polymer prestraining sequence is also reported.