Space represents a harsh environment for all materials. This is particularly challenging for shape memory polymers (SMPs), which show significant potential for lightweight actuators for deployable space structures. Relevant conditions include UV radiation, temperature variations, and vacuum. Polymers, when exposed to such environment for prolonged period (aging), begin to break down structurally and thermodynamic properties, such as enthalpy, entropy, and specific volume, change over time. This leads to permanent modification of mechanical properties such as decreased strength and increased brittleness of the polymer. The shape recovery performance of shape memory polymer is dependent on its thermodynamic properties and energy associated with UV aging can be evaluated through a differential scanning calorimetry (DSC) test. Previous studies focus on the effects of UV exposure on chemical degradation of polymers. However, limited research has been conducted towards studying shape recovery performance of UV aged polymer through thermomechanical prestrain followed by shape recovery process. In this study, we expose SMPs in a UV environment followed by shape recovery experiments where they are prestrained and recovered at various thermomechanical conditions such as recovery temperature, strain rate and aging time. Furthermore, we use characterization techniques such as FTIR and SEM to evaluate the amount of physical degradation of SMP as a result of UV aging process. The results obtained from this study will provide insight into recovery capabilities of a SMP for space exploration.
This Conference Presentation, “Computational modeling of hot rolling process for biaxial prestraining of shape memory polymer sheets,” was recorded for the Smart Structures + Nondestructive Evaluation 2021 Digital Forum.
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.
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