Shape-Memory Polymers

The ability of polymers to respond to external stimuli is of high scientific and technological significance. In the last few years, research activities have been intensified substantially, exploring whether stimuli-sensitive polymers can be designed that move actively. In this review actively-moving materials were classified according to the underlying mechanisms enabling the shape changes: shape-memory polymers and shape-changing polymers/shape-changing gels were identified. The application spectra of these materials as well as the current developments were elucidated and general molecular design principles presented. When applicable, a further distinction according to the applied stimulus was made.

The development of shape-memory polymer composites (SMPCs) enables high recovery stress levels as well as novel functions such as electrical conductivity, magnetism, and biofunctionality. In this review chapter the substantial enhancement in mechanical properties of shape-memory polymers (SMPs) by incorporating small amounts of stiff fillers will be highlighted exemplarily for clay and polyhedral oligomeric silsesquioxanes (POSS). Three different functions resulting from adding functional fillers to SMP-matrices will be introduced and discussed: magnetic SMPCs with different types of magnetic nanoparticles, conductive SMPCs based on carbon nanotubes (CNTs), carbon black (CB), short carbon fiber (SCF), and biofunctional SMPCs containing hydroxyapatite (HA). Indirect induction of the shape-memory effect (SME) was realized for magnetic and conductive SMPCs either by exposure to an alternating magnetic field or by application of electrical current. Major challenges in design and fundamental understanding of polymer composites are the complexity of the composite structure, and the relationship between structural parameters and properties/functions, which is essential for tailoring SMPCs for specific applications. Therefore the novel functions and enhanced properties of SMPCs will be described considering the micro-/nanostructural parameters, such as dimension, shape, distribution, volume fraction, and alignment of fillers as well as interfacial interaction between the polymer matrix and dispersed fillers. Finally, an outlook is given describing the future challenges of this exciting research field as well as potential applications including automotive, aerospace, sensors, and biomedical applications.

Shape-memory polymers (SMPs) are able to fix a temporary deformed shape and recover their original permanent shape upon application of an external stimulus such as heat or light. A shape-memory functionalization can be realized for polymer based materials with an appropriate morphology by application of a specific shape-memory creation procedure (SMCP). Specific characterization methods have been tailored to explore the structure-function relations of SMPs in respective applications. This paper reviews characterization methods on different length scales from the molecular to the macroscopic level.

On the molecular morphological level SMPs are comprised of netpoints determining the permanent shape and reversible crosslinks fixing the temporary shape. For polymers with covalent permanent netpoints the crosslinking density plays an important role, which can be quantified by means of swelling experiments or nuclear magnetic resonance (NMR) methods. In contrast, thermoplastic SMPs are typically phase-segregated polymers, where each domain is related to a different thermal transition, which can be explored by differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA). Further suitable techniques for investigations of the SMP morphology on different levels of hierarchy are polarized light microscopy (POM), scanning or transmission electron microscopy (SEM, TEM) and atomic force microscopy (AFM) as well as wide and small X-ray scattering (WAXS, SAXS).

On the macroscopic level the extent to which a temporary deformation can be fixed and the recovery of the permanent shape or the recovery stress are the most important characteristics of the shape-memory effect (SME), which can be quantified in cyclic, thermomechanical tensile tests or bending tests. Such cyclic tests consist of a SMCP module that can be performed either under stress or strain control followed by a recovery module under stress-free or constant strain conditions. The obtained shape-memory properties are strongly influenced by temperature dependent test parameters like deformation and fixation temperature or applied heating and cooling rate. In addition cyclic, photomechanical testing of light-induced dual-shape polymers, where the temporary shape is fixed by photoreversible chemical crosslinks and the testing of magnetically-induced shape-memory composites are described. In contrast multi-phase polymer networks, which exhibit a triple-shape effect, are explored in cyclic, thermomechanical experiments utilizing a specific two-step SMCP. Furthermore a selection of application-oriented tests for characterization of SME is presented.

Finally, as part of a comprehensive characterization, modeling approaches for simulating the thermomechanical behavior of SMPs are presented. At the beginning linear viscoelastic models were applied consisting of coupled spring, dashpot and frictional elements. More recent approaches consider in detail the specific molecular transition underlying the SME, e.g. glass or melting transition. Currently models that incorporate the strain rate dependence and time dependent behavior are under development.

Shape-memory polymers (SMPs) are a class of mechanically functional “smart” materials that have generated substantial interest for biomedical applications. SMPs offer the ability to promote minimally invasive surgery, provide structural support, exert stabilizing forces, elute therapeutic agents, and biodegrade. This review focuses on several areas of biomedicine including vascular, orthopedic, and neuronal applications with respect to the progress and potential for SMPs to improve the standard of treatment in these areas. Fundamental studies on proposed biomedical SMP systems are discussed with regards to biodegradability, tailorability, sterilization, and biocompatibility. Lastly, a proposed research and development pathway for SMP-based biomedical devices is proposed based on trends in the recent literature.

Biodegradable shape-memory polymers (SMPs) have attracted significant interest for biomedical applications. Modern concepts for biofunctional implants often comprise the controlled release of bioactive compounds to gain specific biofunctionalities. Therefore, a general strategy has been suggested for polymer systems combining degradability and shape-memory capability with controlled release of drugs. This chapter provides a detailed description of the molecular basis for such multifunctional SMPs including the selection of building blocks, the polymer morphology, and the three dimensional architecture. Moreover, drug loading and release, drug effects on thermomechanical properties of SMPs, and drug release patterns in a physiological environment are described and potential applications in minimally-invasive surgery are discussed.