## 3 Shape-Memory Polymer Nanocomposites (SMPNCs)
### Carbon-Based Nanocomposites
#### 3.2.2 Carbon Nanofiber (CNF) and Carbon Black (CB) Nanocomposites  
Carbon nanofibers (CNFs) and carbon blacks (CBs) are widely accessible carbon-based nanomaterials known for their distinctive mechanical properties and applications [173]. These materials form excellent electrical conductivity networks and achieve uniform thermal distribution in polymer matrices [19]. [PERSON] et al. engineered thermo-responsive SMPNCs by integrating CNFs into the matrix of PCL and polystyrene-block-polybutadiene-block-polystyrene (SBS) using a melt blending technique (Figure 18a) [174]. These nanocomposites exhibited an almost instantaneous recovery to their original configurations, with a shape-recovery ratio (R\({}_{\text{r}}\)) approaching 100%, upon heating beyond their transition temperature (T\({}_{\text{trans}}\)) to a predefined flower-like morphology. The concentration of PCL significantly influences the shape-memory performance by enhancing mechanical strength and reducing recovery time, while both PCL and CNF concentrations affect shape fixity and recovery efficiency. In another study, varying concentrations of CNFs were incorporated into a matrix of hydroxyl-terminated polybutadiene (HTPB) and polytramethylene glycol (PTMG) cross-linked polyurethane [175]. This system utilized chemically cross-linked hard-phase components for stability and elastomeric soft-phase components for reversible shape transitions, emphasizing the importance of the hard-to-soft segment ratio and CNF concentration for optimal dispersion and SMPU composite quality. Notably, composites with 1 wt.% CNFs demonstrated a recovery time of 46 s to their original shape, while those with 5 wt.% CNFs achieved recovery in only 38 s, showcasing enhanced shape-memory properties. In another study, [PERSON] et al. developed an innovative approach to enhance both the electrical conductivity and the shape-recovery efficiency of SMPU nanocomposites by integrating self-assembled CNF nanopaper layers [176]. The multilayers of self-assembled nanopapers improved the interfacial bonding and mechanical properties of the polymer composite [177; 178]. The greater the quantity of CNFs in the nanopaper, the higher the conductivity, which resulted in increased electric current amplitude and carrying capacity due to expanded conductive paths and reduced electrical resistance [179]. Consequently, SMPU nanocomposites containing four-layered CNF nanopaper demonstrated effective Joule heating-induced shape recovery upon the application of 30 V DC, achieving original shape restoration in 80 s, as confirmed by an infrared camera that captured a peak temperature of 102.5 \({}^{\circ}\)C.  
[PERSON] et al. demonstrated that SMPUs embedded with 20% carbon black (SMPU/20 CB) achieved 88.8% shape recovery within 40 s under a 4 kV electric field This performance was attributed to the enhanced mobility of hard segments facilitated by the CB additive [180]. In contrast, in situ techniques had demonstrated even higher shape-recovery rates (R\({}_{\text{r}}\)) at elevated temperatures, such as 100% recovery in 26 s under 60 V, but required a higher CB content for operation at lower voltages [181]. With increasing CB contents, the conductivity of the SMPU/CB blend was enhanced, owing to the strengthened electron tunneling effect [111]. The SMPU/20 CB filament also exhibited increase temperature with rising voltage, indicating its responsiveness to electrical stimulation for shape-memory behavior. However, it burned at multiple points when exposed to 9 kV, indicating its voltage tolerance and heating limitations. In another study, [PERSON] et al. developed a conductive composite by integrating 10 wt.% CB into a polyolefin elastomer (POE) and lauric acid (LA) blend, achieving a volume resistance of approximately 23 \(\Omega\) cm through CB percolation networks [182].