**5. Criteria and designing protocol for a desired SMPU**

As mentioned earlier, the presence of two different "segments" (a permanent or a hard segment and a switching or a soft segment, which can be influenced by the external trigger) is essential to obtain shape memory effect in SMPs. The hard phase is responsible for retaining the original shape of the whole SMP. This hard segment can be attained through chemical crosslinking in the polymer network, incorporation of interpenetrating networks or crystalline phases (**Figure 4**). The switching segment temporarily fixes the programmed shape by a glass transition, crystallization, a transition between different liquid crystalline phases, reversible covalent or non-covalent bonds (e.g., photodimerization of coumarin, Diels-Alder reactions, and supramolecular interactions).

The polymer must deform to a desirable extent on heating at a certain temperature without any other change of its performance. This certain temperature is known as switching temperature (*T*sw), which may be a *T*<sup>g</sup> or a *T*m of switching segments of the polymers. Here, it is necessary to mention that if the *T*sw is nearer to the human body temperature, then the polymer is very useful for biomedical applications and *T*sw as *T*m is preferable over the *T*<sup>g</sup> , as the former is a sharper physical property than the latter. Upon reheating above the *T*sw, the oriented chains in case of an amorphous polymer or a crystalline polymer in the network restore the random coil conformation resulting in a macroscopic recovery of the original shape. So they have two different segments or phases in the structures: one is a net point or hard or fixed phase and the other is a soft or reversible or switching segment. Thus, the hard segment serves as a pivoting point for shape recovery and the soft segment could mainly absorb external stress applied to the polymers. They should also possess highly strain fixity rate and strain recovery rate.

The programming like applied strain or deformation rate and extent are also influenced by the shape memory effect of the polymers. The recovery ratios increased with an increasing deformation speed and with a decreasing maximum strain. Thus for good shape recovery,

**Figure 4.** General structure of SMPU containing a "permanent network," which controls the shape and a "switch phase," which can be triggered by external stimuli.

deformation speed must be fast, while overall deformation should be low. The recovery stress of SMPs increased with the decrease of stretching rate, an increase of stretching temperature, and stretch ratio. The recovery stress of SMPU largely depended on the degree of interfacial interaction. The effective SMPs can be obtained when the hard segments retain the whole shape through inter-/intra-attractions (H-bonding or dipole-dipole interaction) in SMPs chain and the soft segments freely engross external stress by extending and unfolding the molecular chains. If the interactions in the hard segments are broken at high stress, the shape memory effect will be lost and the original shape cannot be restored. Hence, the controlling of composition and the structure of hard segments and soft segments are very much required to obtain desired shape memory effect.

essential to obtain shape memory effect in SMPs. The hard phase is responsible for retaining the original shape of the whole SMP. This hard segment can be attained through chemical crosslinking in the polymer network, incorporation of interpenetrating networks or crystalline phases (**Figure 4**). The switching segment temporarily fixes the programmed shape by a glass transition, crystallization, a transition between different liquid crystalline phases, reversible covalent or non-covalent bonds (e.g., photodimerization of coumarin, Diels-Alder

The polymer must deform to a desirable extent on heating at a certain temperature without any other change of its performance. This certain temperature is known as switching temper-

sary to mention that if the *T*sw is nearer to the human body temperature, then the polymer is

a sharper physical property than the latter. Upon reheating above the *T*sw, the oriented chains in case of an amorphous polymer or a crystalline polymer in the network restore the random coil conformation resulting in a macroscopic recovery of the original shape. So they have two different segments or phases in the structures: one is a net point or hard or fixed phase and the other is a soft or reversible or switching segment. Thus, the hard segment serves as a pivoting point for shape recovery and the soft segment could mainly absorb external stress applied to the polymers. They should also possess highly strain fixity rate and strain recovery rate.

The programming like applied strain or deformation rate and extent are also influenced by the shape memory effect of the polymers. The recovery ratios increased with an increasing deformation speed and with a decreasing maximum strain. Thus for good shape recovery,

**Figure 4.** General structure of SMPU containing a "permanent network," which controls the shape and a "switch phase,"

very useful for biomedical applications and *T*sw as *T*m is preferable over the *T*<sup>g</sup>

or a *T*m of switching segments of the polymers. Here, it is neces-

, as the former is

reactions, and supramolecular interactions).

ature (*T*sw), which may be a *T*<sup>g</sup>

60 Aspects of Polyurethanes

which can be triggered by external stimuli.

SMPs are designed in such a way so that a large change in elastic modulus above and below *T*<sup>g</sup> of the switching or amorphous phase will occur. They also have micro-Brownian movement of the chain molecules at a temperature above *T*<sup>g</sup> , though rubber elasticity will present within *T*g to *T*m range due to limited molecular motion of the frozen or crystalline phase. Below *T*<sup>g</sup> , the deformation is fixed due to the frozen Brownian motion of the chains. The shape can be recovered by reheating at an elevated temperature due to the recovery of the elastic force or the strain generated during the deformation.

For crystalline SMPU, *T*m can be taken as at *T*<sup>s</sup> instead of *T*<sup>g</sup> , as a large change in elastic modulus above and below *T*m of the soft segment is possible. Further, thermoplastic SMPU with the mesogenic unit in the structure has a high value of modulus compared with conventional SMPU [25]. Crystalline state, glassy state, entanglement network, or crosslinking network can be used as a fixed structure memorizing the original shape, that is, frozen phase, the reversible phase must have a large drop-in-elastic modulus on heating at *T*<sup>s</sup> . The selection of *T*<sup>s</sup> depends on the temperature at which the elastic modulus is the highest.

From the study of shape memory effect with the molecular structure, it has been observed that high crystallinity of the soft segment region at room temperature is a necessary prerequisite for SMPU [26]. Thus, the most important thing to be required for developing SMPU is to achieve maximum crystallization and stable hard segment domains. To achieve this, various attempts have been made. These include incorporation of the mesogenic unit such as 4,4′-bis (2-hydroxy ethoxy) biphenyl [27], ionic group [28], long alkyl chain [29], and so on. Further, the studies of Kim et al. suggested that large ratio of *E*<sup>g</sup> to *E*<sup>r</sup> is essential for obtaining good shape memory effect [29]. This can be easily achieved by using crystalline soft segments, for example, polycaprolactone diols; mesogenic moiety, for example, hydroxysubstituted biphenyls; and ionomers, for example, dimethylol propionic acid as the components of SMPU.
