Temporary shape

The SMP with permanently set shape and dimensions is heated above the transition temperature the polymer is then deformed to the desired shape and dimensions. The new conhguration is allowed to cool under moderate stress to allow consolidation of the temporary shape and dimensions. To revert back to the permanent shape and dimensions, the SMP is heated again above the transition temperature and allowed to cool. 

In the second chemical category, permanent shape is set by chemical cross-links formed during processing but a temporary shape is formed when the sample is deformed above the melting temperature of the crystalline components and subsequently cooled below this temperature (Liu et al., 2(X)7). Other than thermal heating, recovery in these SMPs can be triggered by an electric current at low voltage. Liquid... 

In other invasive surgeries, bulky devices intended for implantation could be made into string-like temporary shapes to fit into laparoscopic devices for delivery and subsequent reshaping and implementation onsite. Given the high strain recovery of shape... 

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." ... 

Figure 2.5 Electroactive shape-recovery behavior of a 5 wt% MWNT-TPU composite. The sample undergoes the transition from temporary shape (linear, left) to permanent shape (helix, right) within 10 s when a constant voltage of 40 V is applied. Reprinted by permission from Wiley-VCH Verlag GmbH Co. KGaA. ...

Triple SMPs have one permanent shape and two temporary shapes, compared to the traditional double shape memory polymers (SMPs) that have only one permanent and one temporary shape. Triple SMPs can therefore provide more complex actuation than double SMPs. While double SMPs only need one reversible phase, triple SMPs generally need two reversible phases. Zhao et al. [13] first built a co-continuous architecture in immiscible polyethylene (PE)/polypropylene (PP) blends, and then prepared triple SMPs through chemical crosslinking of the blends. The co-continuous window of typical immiscible PE/PP blends is a volume fraction of PE of approximately 30-70 vol.%. This architecture can be stabilized by chemical crosslinking. Different initiators, 2,5-dimethyl-2,5-di(tert-butylperoxy)-hexane (DHBP), dicumylperoxide (DCP) coupled with divinylbenzene (DVB) (DCP-DVB), and their... 

Under specific stimulus, shape memory materials could move from a temporary shape to their original shape. The stimulus could be light, pH, or electric or magnetic field, but the most common shmulus is heat. In this case, a shape memory polymer (SMP) possesses a switch transihon temperature. When the SMP is subject to deformation, its cross-linking structure could store internal stress if it is cooled below this switch temperature. When the polymer is heated above this temperature, it returns to its original shape. Shape memory polymer blends could be achieved using irradiahon. 

Fig. 14 Access to shape-memory materials from photocross-linked metallo-supramolecular polymers. (a) Formation of shape-memory materials using light as a stimulus (a) UV light is absorbed by the metal-ligand complexes and is converted to localized heat, which disrupts the metal complexation (i>) the material can then be deformed (c) removal of the light while the material is deformed allows the metal-ligand complexes to re-form and to lock-in the temporary shape id) additional exposure to UV light allows a return to the permanent shape, (b) Images demonstrating the shape-memory behavior. Reprinted with permission from [274]. Copyright 2011 American Chemical Society...

Besides metal-complexation, hydrogen bonding can also be applied to fix a temporary shape and to create shape-memory materials [275-277]. 

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