Quantifying the Shape-Memory Effect of Polymers by Cyclic Thermomechanical Tests

Shape-memory polymers (SMPs) can be deformed and fixed in defined temporary shapes, which significantly differ from their original shape and remain unchanged until exposed to heat or other stimuli. The shape-memory effect (SME) of polymers is generally quantified in cyclic thermomechanical experiments, which allow to simultaneously control stress or strain and temperature during programming and recovery. Characteristic shape-memory quantities such as the shape-fixity ratio R-f and shape-recovery ratio R-r as well as the specific response temperatures can be determined from the obtained stress-temperature-strain curves. This review reports on the most common cyclic testing methods utilizing tensile, bending, and compression experiments for examination of dual-shape polymers. Furthermore, recently developed testing protocols for triple- and multi-shape polymers as well as materials that exhibit a temperature-memory effect or can show a two-way SME under constant stress are discussed.