Microstructural design for enhanced shape memory behavior of 4D printed composites based on carbon nanotube/polylactic acid filament

Four-dimensional (4D) printing technology, combining 3D printing with stimuli-responsive materials, has attracted extensive attention for smart additive manufacturing. This work focuses on microstructural design, shape recovery behavior and recovery force characterization of 4D printed angle-ply laminated and rectangular braided preforms and their silicone elastomer matrix composites. The angle-ply laminated and rectangular braided preforms were printed using polylactic acid (PLA) and carbon nanotube reinforced PLA (CNT/PLA) based shape memory polymer (SMP) filaments. The X-ray micro-computed tomography, a high resolution and nondestructive imaging technique, was employed to characterize the microstructures of 4D printed specimens and their post-bending damage morphology. The effects of microstructure, CNT filler and silicone elastomer matrix on shape recovery behavior, recovery force and flexural property were studied. The infilling of CNT enabled earlier initiation of specimen shape recovery process, improved the recovery force up to 144% and enhanced the flexural load and modulus. The infusion of silicone matrix increased the final shape recovery ratio to 97.3%, improved the recovery force and enhanced flexural property. The knowledge gained in this research will benefit future design optimization for smarter, faster and stronger actuators.