Shape recovery force

Most recently, Zhu et al. (2009) and Chen et al. (2009) reported another new type of supramolecular polyurethane network that contains a relatively high fraction of UPy groups for shape memory materials (SMMs). However, to date there has been no systematic account of the SMEs of pyridine-containing polyurethanes (PUPys), and previous studies have not explained the relationship between SMEs and non-covalent bonds. This chapter aims to fill that gap with a detailed study of the thermally-induced SMEs of BIN-SMPUs based on an investigation of their morphology and properties. The influence of the BINA content and pyridine ring fractions on shape fixity, shape recovery, shape stability, thermal-dependent shape recovery and shape recovery force is also systematically examined. 

Shape recovery force improves as the BINA content drops in the PUPy-BDO series, and as the MDI-BDO content increases in the PUPy-MDI series the addition of MDI-BDO canthtrs improve the shape recovery force. 

Finally, it should be noted that although shape recovery can be achieved either by moisture or by water, the final shape recovery of PUPys triggered by water and moisture is still less than that triggered by heat. This is due to the fact that water interrupts the hydrogen bonds both in the reversible phase and plysical net points, resulting in significant reduction in the shape recovery force. Water-driven or moisture-induced switches, as well as stable net points or domains, are the two necessary conditions for SMPs that display water-influenced or moisture-induced SMEs... 

Thermoplastic polyurethane-carbon nanofiber, TPU-CNF, conposites were prepared in situ in a chaotic mixer. Two types of CNF with different levels of surface oxidation were mixed with a shape memory TPU. Electrical condnctivity, thermal behavior, and mechanical properties were investigated. Electrical and thermal-induced shape recovery behaviors were evaluated. TPU with treated CNF showed better dispersion, crystallinity, tensile properties, and better shape memory properties snch as higher shape recovery force than their counterparts with rmtreated CNF. 

In rubber testing the elastic structure is not damaged and the original shape is restored, since the deforming forces are lower than the recovery forces which are exerted by the elastic nature of rubber. In this respect rubber test methods differ from those applied to metals, bitumens, waxes, greases and ceramics where measurements are made of permanent deformation. 

Shape-memory cycle of a thermo-responsive SMP. The typical shape-memory cycle of a thermo-responsive SMP consists of the following steps 1) start with a SMP in its original shape (permanent shape) 2) heat the SMP above its thermal transition temperature (Ttrans) and deform the SMP by applying an external force, cool well below 3 rans and remove the constraint to obtain the temporary shape with energy stored and 3) heat the pre-deformed SMP above Ttrans at which point the SMP releases the stored energy and recovers the permanent shape (shape recovery). Reprinted and adapted by permission from Cambridge University Press." ... 

Obviously, the stress recovery ratio is much lower than the strain recovery ratio. The difference is even bigger for cold-drawing programming. In other words, the SMPF has a good memory of strain or shape, but a poor memory of stress or load. Actually, this is trae not only for SMPFs but also for almost all shape memory polymers. On the one hand, this shows the limitations of SMPs in certain applications, such as those that need a large recovery force. On the other hand, this also provides a unique opportunity for researchers to develop SMPs with a higher recovery stress and a higher stress recovery ratio. 

Figure 7.29 Cyclic tension and thermal-induced shape recovery of the SMPFs are repeatable. The shape recovery of the SMPFs closes macrocracks in the composite and exerts a sustained compressive force on the crack interfaces. Source [33] Reproduced with permission from the Royal Society...

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