Shape-Memory Polymers Based on Carbon Nanotube Composites

Abstract

For the past two decades, researchers have been exploring the potential benefits of combining shape-memory polymers (SMP) with carbon nanotubes (CNT). By incorporating CNT as reinforcement in SMP, they have aimed to enhance the mechanical properties and improve shape fixity. However, the remarkable intrinsic properties of CNT have also opened up new paths for actuation mechanisms, including electro- and photo-thermal responses. This opens up possibilities for developing soft actuators that could lead to technological advancements in areas such as tissue engineering and soft robotics. SMP/CNT composites offer numerous advantages, including fast actuation, remote control, performance in challenging environments, complex shape deformations, and multifunctionality. This review provides an in-depth overview of the research conducted over the past few years on the production of SMP/CNT composites with both thermoset and thermoplastic matrices, with a focus on the unique contributions of CNT to the nanocomposite's response to external stimuli.

Keywords: carbon nanotubes; nanocomposites; shape-memory polymers; soft actuators.

Figure 13

(a) Photos of EVA/CNT fibers at different states: the initial state, after 1400 stretch–release cycles, and after thermal treatment; relative resistance changes of the original and thermally repaired EVA/CNT fibers with different strains at 30 mm·min⁻¹. Adapted with permission from Li et al. [107]. Copyright 2020, American Chemical Society. (b) Schematic illustration of water-sensing processes for direct and SMP-enhanced sensing. Adapted with permission from Luo et al. [225]. Copyright 2015, Elsevier. (c) Images of a stretchable spring structure i. at rest and ii. in stretched positions, and the changes in resistance of a gauge during five cycles. Reprinted with permission from Mu et al. [164]. Copyright 2017, Elsevier. (d) Relative resistance variations vs. time for a TPU/MWCNT SMP finger-like strain sensor bending and stretching. Adapted from Wu et al. [224].

Figure 1

Schematic illustration of a one-way shape-memory deformation mechanism. In a one-way type of deformation, a programming step, with an application of mechanical stress (F), temperature (T), and time (t), is required to have a temporary shape that will then be subject to an external stimulus to deform.

Figure 2

Schematic illustration of a reversible shape-memory deformation mechanism.

Figure 3

Summary of the effect of the selective localization of CNT on the properties of the corresponding polymer blends. The inset pictures represent CNT selectively located in the dispersed domains or in the continuous component of binary immiscible blends. Reprinted with permission from Qi et al. [67]. Copyright 2021, Elsevier.

Figure 4

Schematic representation of the four steps of the cyclic thermomechanical tests. Adapted with permission from Lendlein and Kelch [29]. Copyright 2002, John Wiley and Sons.

Figure 5

Spring actuator composed of CE with COOH-MWCNT/CFs: (a) shape recovery process of a spring in space and (b) spring elastic arm. Adapted with permission from Wang et al. [101]. Copyright 2022, IOP Publishing.

Figure 6

Progressing shape-memory recovery, as a function of time, of TPU, and TPU/CNT composites in a hot water bath (a,c) and a hot air oven (b,d). Adapted with permission from Vishwakarma et al. [80]. Copyright 2022, Elsevier.

Figure 7

Schematic representation of CNT electrically conductive paths: (a) network formed within a polymeric matrix; (b) added layer of bulk CNT and SMP matrix.

Figure 8

(a) Schematic representation and real images of the flower blooming process; (b) phototriggered shape change of a 3D-printed flower mimicking a flower blooming: blooming during illumination and shape recovery after turning off the light source. Reprinted with permission from Hua et al. [195]. Copyright 2018, Royal Society of Chemistry. (c) Digital and corresponding IR camera images of a gripper based on a PEO/PW composite with 0.1 wt.% MWCNT, triggered by IR light, switched on and off; (d) image of the gripper in (c) grabbing weights. Adapted with permission from Xu et al. [196]. Copyright 2022, American Chemical Society.

Figure 9

Photos of different 3D-printed models showing one-way shape-memory NIR-induced recovery: (a) mouse model, (b) small monkey model, (c) butterfly model, and (d) pentagram model of NIR-induced shape-memory performance. Reprinted with permission from Bi et al. [83]. Copyright 2020, Elsevier. (e) Localized shape recovery of a “W-shaped” stripe of a PCL-Py/SWCNT composite, recorded using a digital camera and an IR camera. Adapted with permission from Xiao et al. [111]. Copyright 2019, Elsevier.

Figure 10

Electrical activity of PVA/CNT hydrogel in $NaCl$ (a) at the initial position, and (b) after an applied voltage of 40 V. Adapted with permission from Pirahmadi et al. [115]. Copyright 2020, John Wiley and Sons.

Figure 11

(a) Deployed and folded configurations of the PCL-PCL/MWCNT bi-layer film before and after immersion in CHCl₃. shape-memory properties of MWCNT-filled cryogels. Adapted with permission from Toncheva et al. [210]. Copyright 2017, Royal Society of Chemistry. (b–d) shape-memory property of the MWCNT-filled cryogels: fast resilience and macroscopical shape-memory property (scale bar: 1 cm); (e) schematics of the application of hemostatic injectable shape-memory cryogel in a deep and irregularly shaped wound model. Adapted from Zhao et al. [211].

Figure 12

(a) Electrical self-healing of a modified PCL with MWCNT by the formation of new covalent bonds. Adapted with permission from Houbben et al. [216]. Copyright 2023, Elsevier. (b) Scheme of the healing process, and the reaction occurring during healing of an NIR-healed SBS with MWCNT. Reprinted with permission from Bai and Shi [219]. Copyright 2017, American Chemical Society. (c) Illustration of the self-healing mechanism by disulfide exchange. Adapted with permission from Miao et al. [193]. Copyright 2022, Elsevier. (d) Digital photos of the circuit constructed by a PPC/MWCNT sheet and an LED lamp at different states; (e1) the lamp is lit at the initial state; (e2) the composite sheet is cut into two parts, and the lamp is extinguished; (e3) the two parts of the sheet are connected under IR light; (e4) healed sheet by IR irradiation, which rebuilds the circuit to light the LED. Adapted with permission from Cui et al. [220]. Copyright 2020, Elsevier.