Complex movements enabled by triple-shape hydrogels
AbstractStimuli-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. 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.
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.
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.