• Surface free energy
• Surface characterization
• Polylactic acid
• Precipitated calcium carbonate nanoparticles
• Nanocomposites
• Mechanical properties
• Micelle adsorption mechanism
• High-density polyethylene
• Filler toughening
• Deformation mechanism

Precipitated calcium carbonate nanoparticles (PCC), surface treated with stearin in aqueous medium, were investigated in this thesis as rigid fillers for polymer toughening. The surface characterization of a series of PCC coated with different stearin amount indicated the presence of two different types of calcium alkanoate layers on the surface. One is the chemisorbed monolayer (CSM) bonded to the active sites of PCC surface with a maximum coverage degree of around 72%. The other one is constituted by the physisorbed calcium alkanoate multilayers linked to the CSM by weak intermolecular forces. The micelle adsorption mechanism has been proposed to be the dominating process due to the limited solubility of stearin in water, compared to the coating process in solvent where a full monolayer can be achieved.
The calcium alkanoate molecules, present on the PCC surface as physisorbed multilayers, have been shown to have a complicated thermal behavior. The drying process for PCC particles resulted in molecular rearrangement from the monohydrate  phase to the anhydrous  phase. Molecular polymorphism and orientation of calcium alkanoate on the PCC surface are strongly connected with the surface free energy of the coated particles. The coated PCC particles showed a continuously decreasing surface free energy, both for the dispersion and specific components, with an increase in the amount of surface coating. Based on the investigation of the thermal transition and the determination of surface layer thickness, a molecular arrangement model has been proposed suggesting that the CSM is vertical to the PCC surface linked tail-to-tail with the physisorbed multilayers with alkyl chains oriented outwards.
This series of coated PCC nanoparticles were then applied to high-density polyethylene (HDPE) and polylactic acid (PLA) as toughening fillers. The HDPE/PCC nanocomposites achieved the expected balance between the stiffness and toughness. In fact, the yield stress showed a slightly decreasing trend while the impact strength showed a tendency of increase when the surface coating amount on the PCC surface increased. Those mechanical properties were related to the micromorphology and also to the interfacial adhesion between PCC particles and HDPE polymer matrix, which on the other hand connected to the dependence of the surface free energy from the surface coating amount of PCC fillers.
PLA/PCC nanocomposites showed a notable improvement of the elongation-at-break and toughness compared to that of pure PLA. This result can be attributed to both the weaker interfacial adhesion facilitating the debonding process and the plasticizing effect of calcium alkanoate which enhance the deformability of PLA nanocomposites. In fact, the thermal behavior study elucidated that PCC particles acted as nucleating agents for PLA, decreasing the crystallization half-time and the cold crystallization temperature of PLA composites. Also the plasticizing effect of the calcium alkanoate coated PCC nanoparticles was confirmed by the decreased glass transition temperature for PLA/PCC nanocomposites.