Expectations vs. reality in nacre-like composites: dominating role of particle packing and polymer confinement in mechanical performance

After decades of research, mimicking the intricate structure of nacre shells with flawlessly packed blocks remains a laborious task in composite material design. For practical reasons, less ideal alternatives with reduced packing densities below 70 vol.% are often being explored. However, the extent to which the features of the nacre structure can be exploited remains unclear. This paper investigates whether mimicking nacre design in non-densely packed composites can still deliver exceptional mechanical performance. A wide range of ceramic particles (80–100 µm, including spheres and platelets) and methacrylate-based polymers was studied. All the composites exhibited little variation in strength (100–150 MPa) and E-modulus regardless of hierarchical structure, particle size, shape, or interfacial bonding, highlighting the greater importance of particle packing over these factors for ceramic loadings below 65 vol.%. In particular, the benefits of micron-sized anisotropic particles were diminished by the fundamental challenges in aligning such blocks: although these assemblies significantly enhanced fracture resistance, the elastic modulus was still lower than expected (25 GPa). A polydisperse mixture of irregularly shaped micron-sized particles surprisingly achieved a high elastic modulus of 20 GPa, suggesting that an optimized size distribution can provide benefits comparable to those of particle anisotropy. Composites loaded with small particles (< 500 nm) exhibited two key effects: the solvation shells contributed to the total organic content significantly, limiting the maximum ceramic loading, and the polymer confined within small interparticle voids exhibited increased stiffness, leading to more brittle fracture despite the abundance of organic phase. Both phenomena should be accounted for in theoretical simulations and the practical design of composite materials.