Abstract

Shape-memory polymers are stimuli-responsive materials. Upon exposure to an external stimulus, they have the capability of changing their shape. A change in shape initiated by a change in temperature is called thermally induced shape-memory effect. The shape-memory effect results from the polymer's structure, ie, its morphology in combination with a certain processing and programming technology.

The polymer is processed into its permanent shape by conventional methods. Afterward, it is deformed and the desired temporary shape is fixed. The later process is called programming. Heating the programmed polymer above a temperature higher than the transition temperature, T_trans, results in activating the shape-memory effect. Consequently, the recovery of the memorized, permanent shape can be observed. The described effect is called a “one-way” shape-memory effect.

The shape-memory effect can be quantified by cyclic thermomechanical investigations. Important quantities describing the shape-memory properties of the material for a certain strain _m are the strain recovery rate, R_r and the strain fixity rate, R_f. The parameter R_r quantifies the ability of the material to memorize its permanent shape. The parameter R_f describes the ability of the material to fix the mechanical deformation _m, which has been applied during the programming process.