Carbon-based fillers are good thermal conductive fillers. The incorporation of these fillers enhance thermal conductivity and improve the heat distribution between the shape memory device and heat source
[16][17][18][19][20]. Carbon nanotubes have a high aspect ratio, which results in high mechanical strength. Fonseca et al. reported that the reduction of the particle size of the filler improves the thermomechanical properties of the material. They have improved the CNT dispersion in thermoplastic polyurethane by functionalization. The carboxylation of CNT established the linkage between CNT and the matrix and improved the thermal diffusivity of the nanocomposite. Reinforcement of CNT into the natural rubber matrix made the composite susceptible to near-infrared laser irradiation, which actsas a trigger to the shape memory process
[21]. Lai et al. melted blended natural rubber/paraffin wax/CNT composite and studied the two-way shape memory effect, which involves melt-induced contraction and cooling-induced elongation behaviour. The measurements were conducted using a dynamic mechanic analyser
[16].They heated the rubber/paraffin wax/CNT composite to the deformation temperature (Td) of 90 °C at a heating rate of 5 °C/min, then elongated it to the elongation value of 120 kPa, fixed the sample shape, and then cooled it to 10 °C. Then the load was removed, recovery was noted, and the procedure was repeated.
Figure 2A shows the one-way shape memory cycle and
Figure 2B shows the two-way memory cycle, in which the sample is again heated to a Td of 90 °C, elongated up to 450 kPa, cooled up to 10 °C, and repeated.
Figure 2C(a–e) shows the near infrared laser-induced shape memory effects of the NR/paraffin wax/CNT sample and
Figure 2D(a–e) shows the images of the sunlight-induced shape memory effects of the NR blend composites. However, applied external stress is needed for the vapour-triggered shape memory process; they overcame this issue by replacing the paraffin wax with beeswax. By adjusting the beeswax composition they have attained the solvent vapour-triggered process
[22][23].
Figure 2. (
A) One-way shape memory cycle for the natural rubber/paraffin wax 6:4 blend. (
B) Two-way shape memory cycle for the NR. (
C). Near-infrared laser-induced shape memory effects of NR/PW 6:4–0.5 CNT [(
a) original sample, (
b) irradiated for 15 s, (
c) 30 s, (
d) 90 s, and (
e) 120 s]. (
D) Sunlight-induced shape memory effects of NR/PW 6:4–0.5 CNT [(
a) original sample, (
b) irradiated for 30 s, (
c) 60 s, (
d) 90 s, and (
e) for 120 s] (Reproduced with permission from
[16], Elsevier. 2019).
Graphene oxide is layered filler that shows high mechanical and thermal properties; when we reduce the surface oxygen group present in the graphene oxide, it becomes electrically conductive, reduced graphene oxide. Sarmadet et al. used the graphene oxide, reduced the graphene oxide (rGO), and functionalized the reduced graphene, as a filler in the polyurethane matrix. The shape memory effect was studied and a 99.1% of shape fixity value and 96.7% shape recovery value for 5 wt% TPU composite, reinforced with GO: rGO hybrid filler, was obtained
[24][25].
Figure 3A illustrates the morphology of the GO-based shape memory material [(a) GO platelets, (b) rGO platelets, and (c) GO:rGO hybrid filler] and
Figure 3B shows the shape fixity, recovery, and the molecular mechanism of the shape memory behaviour [(a) Shape memory thermo-mechanical cycle, (b) The molecular mechanism of Shape memory behavior (Blue lines: molecular chains with low mobility below Tg; red lines: molecular chains with high mobility above Tg), (c) shape fixity and (d) shape recovery on neat TPU, TPU/GO, TPU/rGO and hybrid TPU/GO:rGO composites with 1, 2 and 5 wt% filler content]
Figure 3. (
A) Illustrated morphology of the GO-based shape memory material (TPU/GO: rGO) [(
a) GO platelets, (
b) rGO platelets, and (
c) GO:rGO hybrid filler] and the (
B) molecular mechanism of the shape memory behaviour, shape fixity, and shape recovery [(
a) Shape memory thermo-mechanical cycle, (
b) The molecular mechanism of Shape memory behavior (Blue lines: molecular chains with low mobility below Tg; red lines: molecular chains with high mobility above Tg), (
c) shape fixity and (
d) shape recovery on neat TPU, TPU/GO, TPU/rGO and hybrid TPU/GO:rGO composites with 1, 2 and 5 wt% filler content] (Reproduced with permission from
[24], Elsevier, 2019).
The incorporation of hybrid fillers, such as CNT/GO, CNT/nano clay, and CNT/carbon black, was also explored by different researchers; there are plenty of hybrid filler combinations that need to be studied
[26][27][28]. Liu et al. synthesized the graphene oxide/Trans-1,4-polyisoprene (GO/TPI) nanocomposite and improved the mechanical and thermal properties of the composites at 0.9 phr GO composition; they have also studied the effect of temperature on the rate of shape recovery. They find that the rate of shape recovery increases with temperature
[29].