**3. Properties and characterization of shape memory polyurethane**

Transition temperature (*T*trans), shape fixity (*R*<sup>f</sup> ), shape recovery ratio (*R*<sup>r</sup> ), maximum recoverable strain (*ε*max), and maximum recovery stress (*σ*max) are the important parameters that are used to describe shape memory effects of a polymeric material [22].

*Shape fixity:* Shape fixity is the extent of a temporary shape being fixed for an SMP. It is also known as strain fixity or shape retention. The shape fixity is thus equal to the percentage of the ratio of fixed deformation to total deformation (Eq. (1))

$$R\_{\gamma} = \frac{\varepsilon}{\mathcal{E}\_{\text{load}}} \times 100\% \tag{1}$$

*Shape recovery:* Shape recovery is defined as the ability of a polymeric material to memorize the original shape from a temporary deformed shape. Therefore, the shape recovery is the percentage of the ratio of deformation recovered by the specimen to the deformation taken place to the specimen (Eq. (2))

$$R\_r = \frac{\varepsilon - \varepsilon\_{\text{noonyary}}}{\varepsilon} \times 100\,\%\tag{2}$$

*Recovery rate:* This parameter describes the speed, that is, the rate of recovery from a programmed shape to its original shape upon triggering of a proper stimulus. It can also be said as the speed of recovery process or shape recovery speed.

*T*trans is usually equal to *T*<sup>g</sup> for an amorphous SMPU or *T*m for a crystalline SMPU. This is generally measured by standard thermal analysis techniques such as differential scanning calorimetry (DSC) or dynamic mechanical analysis (DMA). Conveniently, a shape memory cycle (SMC) as mentioned in **Figure 1** is performed in order to determine *R*f and *R*<sup>r</sup> , the changes of sample dimension are manually measured, and *R*f and *R*r are calculated with the measured data using Eqs. (1) and (2), respectively. However, a mechanical testing equipment with a climate chamber (possess heating and cooling facilities) is the best choice to evaluate *ε*max by elongating the testing sample to its failure at *T*trans. In addition to that, the SMC using such thermo-mechanical analyzer can precisely evaluate different shape memory parameters such as *R*<sup>f</sup> , *R*<sup>r</sup> , *σ*max, and shape recovery rates. This method can accurately record the time progress of temperature, stress, and

**Figure 1.** Thermomechanical cycle of SMPs.

strain. An example of a typical SMC is shown in **Figure 2a**. Instead, the SMC may also be demonstrated in a three-dimensional (3D) diagram as shown in **Figure 2b**. In this 3D diagram, the three axes are temperature, strain, and stress. Especially, time information is absent there. This absence does not impede the determination of *R*<sup>f</sup> and *R*<sup>r</sup> . Basically, the use of such a 3D diagram is very well known in the literature. The absence of time information may be moderately

*Shape fixity:* Shape fixity is the extent of a temporary shape being fixed for an SMP. It is also known as strain fixity or shape retention. The shape fixity is thus equal to the percentage of

*Shape recovery:* Shape recovery is defined as the ability of a polymeric material to memorize the original shape from a temporary deformed shape. Therefore, the shape recovery is the percentage of the ratio of deformation recovered by the specimen to the deformation taken

*Recovery rate:* This parameter describes the speed, that is, the rate of recovery from a programmed shape to its original shape upon triggering of a proper stimulus. It can also be said

ally measured by standard thermal analysis techniques such as differential scanning calorimetry (DSC) or dynamic mechanical analysis (DMA). Conveniently, a shape memory cycle (SMC)

dimension are manually measured, and *R*f and *R*r are calculated with the measured data using Eqs. (1) and (2), respectively. However, a mechanical testing equipment with a climate chamber (possess heating and cooling facilities) is the best choice to evaluate *ε*max by elongating the testing sample to its failure at *T*trans. In addition to that, the SMC using such thermo-mechanical ana-

recovery rates. This method can accurately record the time progress of temperature, stress, and

*<sup>ε</sup>*load × <sup>100</sup>% (1)

*ε* × 100% (2)

, the changes of sample

, *σ*max, and shape

, *R*<sup>r</sup>

for an amorphous SMPU or *T*m for a crystalline SMPU. This is gener-

the ratio of fixed deformation to total deformation (Eq. (1))

*Rf* <sup>=</sup> \_\_\_\_ *<sup>ε</sup>*

*Rr* <sup>=</sup> *<sup>ε</sup>* <sup>−</sup> *<sup>ε</sup>* \_\_\_\_\_\_\_ recovery

as the speed of recovery process or shape recovery speed.

as mentioned in **Figure 1** is performed in order to determine *R*f and *R*<sup>r</sup>

lyzer can precisely evaluate different shape memory parameters such as *R*<sup>f</sup>

place to the specimen (Eq. (2))

56 Aspects of Polyurethanes

*T*trans is usually equal to *T*<sup>g</sup>

**Figure 1.** Thermomechanical cycle of SMPs.

**Figure 2.** Shape memory cycle test: (a) 2D diagram (reproduced with permission from Ref. [2]) and (b) 3D diagram (reproduced with permission from Ref. [3].

unfavorable for a more sophisticated experiment of SMC. For example, if the testing sample is annealed under a constant stress at any stage of the experiment, the information about the annealing time and the strain reaches equilibrium or not during the annealing process would not be known. The rapid strain recovery rate *V*<sup>r</sup> can also be calculated from the strain curve in the recovery portion of the SMC (**Figure 2a**), the time derivative of the strain as defined in Eq. (3)

$$V\_r = \begin{array}{c} \frac{\delta \varepsilon}{\delta t} \times 100\% \end{array} \tag{3}$$
