Influence of humidity on the rheology of thermoresponsive shape memory polymers

Abstract Shape Memory Polymers (SMPs) have the inherent ability to maintain a reversible temporary shape and restore a permanent shape under an external trigger. The class of materials has great potential to contribute to smart applications in soft robotics, aerospace, actuation and biomedicine. In these potential application domains, materials are often exposed to large fluctuations due to humidity influences. Therefore, a novel approach is developed to characterize the stronlgy coupled thermal, humidity and time-dependent behavior of polyurethane-based SMP. Weight gain measurements with disk samples of dimension 35$$\,\times $$ 35 $$\times $$ 1.5 $$\text {mm}^\text {3}$$ and linear expansion tests with rectangular samples of dimension 10 $$\times $$ 40 $$\times $$ 1.0 $$\text {mm}^\text {3}$$ at different relative humidity are carried out to perform the isothermal and isohumid dynamic measurements in thermodynamic equilibrium. The time-temperature superposition is used to characterize and compare the viscoelastic properties at different relative humidity. Concerning effective material properties, a major finding of this investigation is the horizontal shift of the material parameter in the temperature space due to the presence of humidity. Thus, the humidity-dependent material behavior is fully described by a humidity-dependent glass transition temperature. The measured experiments provide a full description of the thermal, humidity and mechanical behavior of SMPs. Graphical abstract