Micro vs nano: influence of filler size on the rheological and mechanical properties of highly-filled PLA/HAp and PCL/HAp composites

This work aims to elucidate the influence of filler particle size on the rheological behaviour, as well as the micro- and macromechanical properties, of polymer-ceramic composites. For this investigation, two widely utilised biodegradable polymers, polylactide (PLA) and polycaprolactone (PCL), were selected as the polymer matrices. Hydroxyapatite (HAp) powders with average particle sizes of 10 μm and 60 nm were used as fillers. A series of composites were prepared using twin-screw extrusion and injection moulding techniques, with micro- and nanoparticle HAp concentrations of 20 wt% and 40 wt% for PLA and 20 wt% for PCL. The produced materials were subjected to comprehensive analysis, including morphological evaluation (scanning electron microscopy), thermal characterisation (differential scanning calorimetry, thermogravimetric analysis), rheological testing and mechanical property assessment at both micro (micro indentation) and macro (tensile, impact strength) scales. Our research had three main objectives: to carry out a comparative analysis of composites with different polymer matrices, to evaluate the effect of the filler on the properties of the given polymer matrix and, finally, to investigate the effect of the filler particle size on the overall performance of the composites. In general, PLA-based materials exhibited brittleness and strong polymer-filler interactions, whereas PCL-based materials were ductile and showed weak polymer-filler interactions. The use of nanometric filler resulted in more desirable mechanical properties and improved polymer melt stability compared to composites containing micrometric filler. This research makes a significant contribution to the design and production of composite materials for biomedical applications.