Abstract:

Shape-memory polymers are an emerging class of active polymers, having the capability of changing their shape between distinctive shapes. When exposed to an appropriate stimulus they can change their shape in a predefined way from shape (A) to a shape (B). While shape (B) is given by the initial processing step, shape (A) is determined by applying a process called programming (Scheme 1, Figure 1) [1].

Scheme 1

Within this presentation fundamental aspects of the molecular design of suitable polymer architectures, tailored programming and recovery processes of the shape-memory effect are presented.

Figure 1: Series of photographs showing the macroscopic shape-memory effect of an AB-polymer network. The Permanent shape is a rod, the temporary shape is a spiral. The pictures show the transition from temporary to permanent shape at 70°C within 20 s. Lendlein, A; Schmidt, A.M.; Langer, R.,

Proceedings of the National Academy of Sciences of the United States of America , 2001, 98(3), 842 – 847.

Initially founded on the thermally-induced dual shape-effect the concept of shape-memory polymers has been extended to other stimuli by either indirect thermal actuation or direct actuation by addressing stimuli-sensitive groups on the molecular level. By the incorporation of reversibly photoreacting molecular switches, light-induced stimulation of shape-memory polymers has been realized [2].

Remote actuation of the thermally-induced shape-memory effect in magnetic fields was enabled by the incorporation of magnetic nanoparticles of iron(III)oxide cores in a silica matrix into shape-memory thermoplasts (figure 2). Here the sample temperature is increased by inductive heating of the nanoparticles in an alternating magnetic field [3].

Figure 2: Magnetically induced shape-memory effect of a polyetherurethane with embedded magnetic nanoparticles. Within 24 sec. the temporary fixed shape (rod) recovers into the original shape (spiral) inside of a magnetic field of an inductor ring. Complex movements of shape-memory polymers had been realized by triple-shape polymers [4]. Here the incorporation of second switching segment into the molecular architecture of the shape-memory polymer network enabled three distinctive shapes (figure 3).

Figure 3: Series of photographs illustrating the triple-shape effect of a tube prepared from CL(50)EG: starting at 20 °C shape (a) which was obtained as a result of the two-step programming process having an upright diameter of 4.5 mm switches to a second programmed shape (b) with a diameter of 6.9 mm to the permanent shape (c) with a diameter of 5.8 mm. The shape change to the second programmed shape (b) and the recovery of the permanent shape (c) is triggered by subsequent heating to 40 and 60 °C.

1. Lendlein, A. and S. Kelch, Shape-memory polymers. Angewandte Chemie-International Edition, 2002. 41(12): p. 2034-2057.
2. Lendlein, A., et al., Light-induced shape-memory polymers. Nature, 2005. 434(7035): p. 879-882.
3. Mohr, R., et al., Initiation of shape-memory effect by inductive heating of magnetic nanoparticles in thermoplastic polymers. Proceedings of the National Academy of Sciences of the United States of America, 2006. 103(10): p. 3540-3545.
4. Bellin, I., et al., Polymeric triple-shape materials. Proceedings of the National Academy of Sciences of the United States of America, 2006. 103 (48): p. 18043-18047.