No abstract

The thermally‐induced shape‐memory effect (SME) is the capability of a material to change its shape in a predefined way in response to heat. In shape‐memory polymers (SMP) this shape change is the entropy‐driven recovery of a mechanical deformation, which was obtained before by application of external stress and was temporarily fixed by formation of physical crosslinks. The high technological significance of SMP becomes apparent in many established products (e.g., packaging materials, assembling devices, textiles, and membranes) and the broad SMP development activities in the field of biomedical as well as aerospace applications (e.g., medical devices or morphing structures for aerospace vehicles). Inspired by the complex and diverse requirements of these applications fundamental research is aiming at multifunctional SMP, in which SME is combined with additional functions and is proceeding rapidly. In this review different concepts for the creation of multifunctionality are derived from the various polymer network architectures of thermally‐induced SMP. Multimaterial systems, such as nanocomposites, are described as well as one‐component polymer systems, in which independent functions are integrated. Future challenges will be to transfer the concept of multifunctionality to other emerging shape‐memory technologies like light‐sensitive SMP, reversible shape changing effects or triple‐shape polymers.

Free-standing copolymer network samples with two types of crystallizable domains are capable of a fully reversible bidirectional shape-memory effect. One set of crystallizable domains determines the shape-shifting geometry while the other provides the thermally controlled actuation capability.

Polymeric materials are clinically required for medical devices, as well as controlled drug delivery systems. Depending on the application, the polymer has to provide suitable functionalities, for example, mechanical functions or the capability to actively move, so that an implant can be inserted in a compact shape through key-hole incisions and unfold to its functional shape in the body. Shape-memory polymers, as described herein regarding their general principle, compositions and architectures, have developed to a technology platform that allows the tailored design of such multifunctionality. In this way, defined movements of implants triggered either directly or indirectly, tailored mechanical properties, capability for sterilization, biodegradability, biocompatibility and controlled drug release can be realized. This comprehensive review of the scientific and patent literature illustrates that this technology enables the development of novel medical devices that will be clinically evaluated in the near future.

Reading out the temperature-memory of polymers, which is their ability to remember the temperature where they were deformed recently, is thus far unavoidably linked to erasing this memory effect. Here temperature-memory polymer actuators (TMPAs) based on cross-linked copolymer networks exhibiting a broad melting temperature range (ΔT m ) are presented, which are capable of a long-term temperature-memory enabling more than 250 cyclic thermally controlled actuations with almost constant performance. The characteristic actuation temperatures T act s of TMPAs can be adjusted by a purely physical process, guiding a directed crystallization in a temperature range of up to 40°C by variation of the parameter T sep in a nearly linear correlation. The temperature T sep divides ΔT m into an upper T m range (T > T sep ) forming a reshapeable actuation geometry that determines the skeleton and a lower T m range (T < T sep ) that enables the temperature-controlled bidirectional actuation by crystallization-induced elongation and melting-induced contraction. The macroscopic bidirectional shape changes in TMPAs could be correlated with changes in the nanostructure of the crystallizable domains as a result of in situ Xray investigations. Potential applications of TMPAs include heat engines with adjustable rotation rate and active building facades with self-regulating sun protectors.reversible shape-memory polymer | active movement T he alignment and coupling of thermally controlled volume changes on the nanoscale has emerged as most important working principle to translate shape changes from the nanolevel to the macrolevel in polymers (1-4). In stimuli-responsive polymers capable of a free-standing shape-changing effect, this alignment is achieved during synthesis or processing by either application of external stress or the utilization of templates and fixed by covalent cross-links (5-12). Once synthesis is completed, the geometry of the shape change cannot be changed anymore (13-15) and the actuation temperature is fixed; this relies on thermal transitions with a defined temperature. Here we explored whether it is possible to implement a thermally controlled bidirectional actuation into free-standing polymers by purely physical manipulation enabling to adjust (repeatedly) the actuation temperature and (shape changing) geometry.Although programmable shape changes have been realized in shape-memory polymers (SMPs), this effect is generally a onetime, one-way effect in free-standing SMPs (16-18). In SMPs the switching domains, which can solidify by crystallization or vitrification, provide two functions: they determine the geometry of the shape change and cause the entropy elastic recovery. A reversible movement could be observed when polymers with crystallizable segments are held under an externally applied constant stress (3,8). Recently temperature-memory polymers (TMPs) enabled the programming of the switching temperature (19,20). Also this temperature-memory effect (TME) is limited to a onetime, one-way effect. The aim of th...