## 3 Shape-Memory Polymer Nanocomposites (SMPNCs)
### Carbon-Based Nanocomposites
#### 3.2.1 Carbon Nanotube (CNT) Nanocomposites
[PERSON] and colleagues developed SMPCs based on cardanol-based benzoxazine and CNTs, aiming to leverage their high conductivity for intelligent applications [156]. The SMP/CNT combination displays excellent sensitivity to changes in pressure and bending, along with commendable stability and recoverability. In another study, [PERSON] and coworkers explored a reversible SME using a segregated CNT/poly(ethylene-co-vinyl acetate) (EVA) composite, achieving actuation at a remarkably low voltage of 15 V [157]. This segregated CNT/EVA composite was distinguished by its superior reversible actuation, evidenced by its capability to lift and lower a target object under low voltage application. Further advancing the application of shape-memory composites, [PERSON] et al. utilized a CNT/EVA nanocomposite for the reversible actuation of 3D structures through a synergy of chemical and physical programming, facilitated by NIR irradiation [158]. Figure 17a depicted a locally reversible actuation, as demonstrated by the wings of a model responding to NIR irradiation with an upward swinging motion, which reverted to their original position upon cessation of the irradiation. The addition of CNTs to polymer matrices significantly influences the crystallization behavior of the resulting composites, as documented in various studies [55]. This impact varies across studies depending on factors such as the nature of the polymer matrix, dispersion quality, CNT-polymer interactions, thermal treatment, and other variables [159; 160; 161; 162]. Generally, MWCNTs have been identified as potent nucleation agents that expedite the crystallization process in polymers, affecting key parameters such as crystallization temperature, degree of crystallinity, and crystallite dimensions [163; 164]. However, an increase in crystallinity can impede the elastomer phase from returning to its original shape, enhancing temporary shape retention and diminishing the shape-recovery ratio with increased MWCNT content [165; 166; 167]. This phenomenon is attributed to the displacement of MWCNTs during deformation and their subsequent failure to return to initial positions, potentially causing permanent deformation in the composite material [167; 13]. In a related study, [PERSON] et al. developed SMPCs that respond to heat and electricity, consisting of poly(ethylene-1-octene) (PEO) and poly(styrene-b-isoprene-b-styrene) (SIS), with MWCNTs serving as fillers [168]. The resulting composite exhibited a noticeable rise in stiffness and brittleness, while tensile strength and elongation at break decreased with increasing MWCNT content, leading to restricted molecular mobility and premature fractures [169; 170]. Although nanofillers can enhance polymer matrix stiffness, this often results in higher recovery stress levels but not necessarily improved shape recovery [171]. When assessing heat-induced actuators, \"C-shaped\" samples demonstrated that composites with increased MWCNT content exhibited longer recovery times due to hindered elastomer molecule mobility [169]. The 10 CNT composite took 600 s to recover, twice as long as the blend, attributed to MWCNT-induced inhibition. In contrast, electrical voltage rapidly triggered shape recovery in the 5 CNT and 10 CNT composites, with the 5 CNT outperforming due to a lower concentration of MWCNT aggregates, showcasing the efficiency of electrical stimulus compared to heat-induced recovery (Figure 17b). [PERSON] et al. developed continuous fiber-reinforced electro-induced shape memory auxetic composites (CFRSMCs) using a 3D-printed PLA/TPU/CNT blend [172]. Adding carbon fiber reduced the thermal recovery ratio (R\({}_{\text{r}}\)) from 96.7% to 89.5% due to fiber-matrix slippage, despite enhancing shape recovery speed. Electrically, CFRSMCs achieved rapid recovery (R\({}_{\text{r}}\) = 94% in 25 s at 10 V), demonstrating the efficiency of their conductive network.