Environmentally responsive materials and structures can pave the way into an energy- and resource-efficient future concerning actuating devices in the field of engineering and construction. To evaluate the application potential of this responsiveness onto environmental stimuli, like humidity or solar radiation, in technical applications, it is necessary to precisely characterize the magnitude and range of stimuli that trigger certain reactions of a material and the resulting kinetics with suitable testing equipment and techniques. The overall aim is to correlate actuation potential and mechanical properties in terms of civil engineering applications.
Sorption behavior of Cottonid, which is an efficient adaptive material for humidity-driven actuators, was characterized in cyclic actuation tests. The tests were performed in an alternating climate chamber using a customized specimen holder, instrumented with digital image correlation (DIC) and passive thermography. DIC was used for precise actuation strain measurements to derive sorption hystereses and to calculate possible energy losses of different Cottonid variants during swelling and shrinking. Taking into account effects of another environmental stimuli besides humidity, Cottonid specimens were further exposed to UV radiation in long-term experiments. Effects of photodegradation were analyzed using scanning electron microscopy, roughness measurements and tensile tests.
Sorption hystereses analysis has proven to be a useful tool for the assessment of actuation potential of different Cottonid variants. Therefore, the optimal process window for stimuli-responsive element production could be defined. Possible application areas could be correlated with the material thickness, since it has a strong influence onto resulting actuation ranges. The thermoelastic response of Cottonid when strained elastically was used to characterize the shift of the onset of yielding caused by a variation of material specific parameters or environmental conditions. On the basis of this results, tailor-made functional materials shall be generated in future where stimuli-responsiveness can be adjusted through the manufacturing process.