Thermally-Induced Shape-Memory Effect

FIGURE 19.4 Molecular mechanism and macroscopic effect of a shape-memory polymer, (a) Schematic representation of the thermally induced shape-memory effect of a polymer network with (b) Shape recovery of a stent with T = 52° in water at 37°C. The stent gradually changed from its... 

Heuchel, M., Sauter, T., Kratz, K., and Lendlein, A. (2013) Thermally induced shape-memory effects in polymers quantification and related modeling approaches. Journal of Polymer Science Part B Polymer Physics, 51, 621-637. 

Conventional shape memory polymers are segmented polyfur-ethane) s and have hard segments that include aromatic moieties (70,71). But a series of other thermoplastic block copolymers with thermally induced shape memory effects have been summarized (69). 

Indirect Actuation of Thermally-Induced Shape-Memory Effect... 

Fig. 3. Schematic demonstration of the molecular mechanism of the thermally induced shape-memory effect for a multiblock copolymer, Ttrans = Tm. If the rise in temperature is higher than Ttrans of the switching segments, these segments are flexible (marked red, here) and the polymer can be deformed elastically. The temporary shape is fixed by cooling down below Ttrans (marked blue, here). If the poljrmer is heated up again the permanent shape is recovered.

Table 1. Possible Combinations of Hard-Segment- and Switching-Segment-Determining Blocks in Linear, Thermoplastic Block Copolymers with Thermally Induced Shape-Memory Effect ...

Abstract This chapter presents the thermaUy-indnced shape memory properties of supramolecular shape memory polyurethane (SMPU) containing pyriine moieties, and compares them with those of other types of SMPU. llie influence of the BINA content and the MDI-BDO content on the thermally-induced shape memory effects (SMEs) of these SMPUs are discussed, including shape fixity, shape recovery and shape recovery stress, as well as the temperature-dependent strain recovery process. A description and analysis of the mechanism of thermaUy-induced SMEs is also provided, covering the effect of temperature and pyridine content on the hydrogen bonds present in SMPUs. 

Figure 1.2 Schematics of thermally induced shape memory polymer effect.

Abstract This chapter introduces the moisture-induced shape memory effect (SME) observed in supramolecular shape memory polymers, particularly shape memory polyurethane (SMPU), containing pyridine moieties. The moisture absorption of polyurethane networks containing pyridine moeties (PUPy) is discussed followed by an investigation into the effect of relative humidity (RH), temperature, BINA content and MDI-BDO content. The induence of moisture absorption on both the thermal properties and the dynamic mechanical properties of SMPUs is also described, along with the moisture-induced SME mechanism. 

The presence of oxygen-containing fimctional groups on the surface of the carbon nanofibers improved their dispersion in the TPU matrix. Electrical conductivity in TPU-CNF and TPU-CNFOX materials was a synergistic effect produced by the chaotic mixing and the morphology of these materials. Young s modulus at room temperature was controlled by PCL crystals, while at 60 °C it was eontrolled by the presence of carbon nanofibers. CNFOX composites provided better performance in thermally-induced shape memory, while CNF is reconunended for use with electrical shape memory actuation. 

It is noted that while the majority of constitutive modeling focuses on thermally induced dual-shape memory behavior, triple-shape and multishape SMPs have been developed recently and they call for constimtive modeling [1]. In addition, the effect of programming temperature and strain rate on the constimtive behavior also needs modeling [2]. Furthermore, some recent smdies have found that while the shape recovery ratio can be 100%, other mechanical properties such as recovery stress or modulus become smaller and smaller as the thermomechanical cycles increase, which has been explained by the shape memory effect in the microscopic scale [24]. Obviously, these new findings also call for constitutive modeling. 

When the material is heated above the melting point of the soft segment, but below glass transition temperature of the hard segment, the stresses and strains are relieved and the material returns to its original shape. The recovery of the original shape, which is induced by an increase in temperature, is called the thermal shape memory effect. Properties that describe the shape memory capabU-... 

Pig. 1. Schematic demonstration of the thermally induced one-way shape-memory effect. By the programming process the permanent shape is transferred to the temporary shape. Heating up of the sample to a temperature above the switching transition Ttrans initiates the recovery of the permanent shape. From Ref 2. 

Shape memory materials are able to memorize a second, permanent shape besides their actual, temporary shape. After application of an external stimulus, e.g. an increase in temperature, such a material can be transferred into its memorized, permanent shape. The process of programming and restoring a shape can be repeated several times. This behaviour is called the thermally induced one-way shape memory effect. 

Shape memory effect in polymers is normally achieved by relaxation of the polymeric chains due to changes in temperature.When the programming and recovery stages in the shape memory effect are produced by changes in temperature, it is said that the effect is thermally induced. 

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