Intensive studies on nacre-inspired composites with exceptional mechanical properties based on an organic/inorganic hierarchical layered structure have been conducted; however, integrating high strength, stiffness, and toughness for engineering materials still remains a challenge. We herein report the design and fabrication of polymer composites through a hydrogel-film casting method that allow for building uniformly layered organic/inorganic microstructure. Alginate (Alg) was used for an organic matrix, whose mechanical properties were controlled by Ca2+ cross-linking toward the simultaneously strong, stiff, and tough resultant composite. Alumina (Alu) microplatelets were used for horizontally aligned inorganic phase, and their alignment and interactions with the organic matrix were improved by polyvinylpyrrolidone (PVP) coating on the platelet. The composite film exhibits well-balanced elastic and plastic deformation under tensile stress, leading to high stiffness and toughness, which have not been generally achieved in microplatelet-based composite films developed in previous studies. The synergistic effect of Ca2+ cross-linking and PVP-coated Alu platelets on the mechanical properties improved polymer-platelet interfacial interactions, and platelet alignment is clearly demonstrated through mechanical tests and Fourier transform infrared and X-ray diffraction analyses. We further demonstrate that the reinforcing effect of the Alu platelet and PVP-coated platelet on the mechanical properties is dependent on humidity. Such effects are maximized at highly dry conditions, which is consistent with the model estimation. Furthermore, a thick bulk composite was produced by laminating thin films and showed high mechanical properties under flexural stress. Our design and fabrication strategies combined with the understanding of their mechanism yield an alternative approach to produce engineered composite materials.
KEYWORDS: alginate; hydrogel-film casting; ionic cross-linking; layer-by-layer structure; mechanical properties; nacre-inspired design