Slocha Sapasakulvanit’s research while affiliated with Nanyang Technological University and other places

Bioinspired composites exhibit well-defined microstructures, where anisotropic ceramic particles are assembled and bonded by an organic matrix. However, it is difficult to fabricate these composites where both the ceramic particles and organic matrix work together to unlock toughening mechanisms, such as shear dissipation, particle rotation and interlocking, etc, that lead to stiff, strong, and tough mechanical properties. Here, we produce composites inspired by seashells, made of alumina microplatelets assembled in complex microstructures and that are physically bonded by a small amount of interpenetrated polymer network (IPN) made of polyacrylamide (PAM) and poly-N-isopropylacrylamide (PNIPAM). The fabrication employs magnetically assisted slip-casting to orient the microplatelets as desired, and in situ gelation of the IPN, followed by drying. The process was successful after carefully tuning the slip casting and gelation kinetics. Samples with horizontal, vertical, and alternating vertical and horizontal microplatelets orientations were then tested under compression. It was found that the IPN threads bonding the microplatelets acted as sacrificial bonds dissipating energy during the compression. Paired with the alternating microstructure, the IPN significantly enhanced the compressive toughness of the composites by 205% as compared to the composites with horizontal or vertical orientation only, with less than 35% reduction on the stiffness. This study demonstrates that microstructure control and design combined with a flexible and tough matrix can effectively enhance the properties of bioinspired ceramic polymer composites.

Natural strong and stiff composites exhibit complex microstructures that are responsible for their outstanding strength, stiffness, and toughness. Although horizontal nacre-like microstructures have demonstrated great potential in synthetic composites, they only demonstrate high performance along a specific loading direction. Here, we study how complex curved multilayered microstructures made of aluminum oxide microplatelets in a soft polymeric matrix can possess a combination of both stiffness and toughness. To do so, magnetically-assisted slip casting was used to direct the microplatelets into periodic orientations, as reported in previous studies, while the porous substrate used for the slip casting was designed with a raised step of different heights to control the shape of the oriented layers. The samples’ microstructures achieved in-plane modulation of the microplatelet orientation, and the mechanical properties under compression were enhanced compared to samples with layers parallel to the horizontal plane. The microstructured design proposed here could be used to make more resilient bioinspired composites.