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Complex movements enabled by triple-shape hydrogels

Abstract

Stimuli-sensitive hydrogels provide the ability to store large amounts of water, exhibit soft tissuelike mechanical properties, and are capable to respond to a stimulus like temperature, changes in pH, or a variation of the concentration of ions.[1] The strategy to incorporate crystallizable switching segments as side chains in a hydrophilic polymer network resulted in hydrogels with on demand directed movements when heat was applied, whereby constant degrees of swelling during shape switches were obtained.[2] Here we report about hydrogels providing complex movements of soft materials with two almost independent shape changes, which were triggered by a thermally-induced triple-shape effect.[3] Triple-shape hydrogels (TSHGs) with two different semi-crystalline switching segments integrated as side chains were designed, whereby interferences of volume changes could be avoided as the degrees of swelling were almost independent of different shapes and temperatures. Two distinct shapes were implemented by a two-step programming procedure resulting in shape fixity ratios of generally > 50%. While shorter side chains orient perpendicular to the hydrophilic main chain, side chains with a higher molecular weight gain lower orientation after deformation as detected by means of X-ray scattering. Furthermore, it was observed that the orientation of the switching domains is not a key requirement for adequate shape fixity and recovery ratios of TSHGs. Therefore, these soft materials, which provide complex directed movements, are potential material candidates for two-step self-unfolding devices or soft temperature-sensitive actuators, e.g. smart valves for flow rate control in an aqueous media.
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