Thermal and mechanical properties of poly(lactic acid) filled with modified silicon dioxide: importance of the surface area

In this research work, the effect of the change in the surface area of silicon dioxide nanoparticles of the same size on mechanical properties of poly(lactic acid) nanocomposites (PLA) was studied, as well as the role of coupling agent amount in the compatibility of these nanomaterials. We consider a spherical silicon dioxide with a surface area of 170–200 m2/g (labeled as S–SiO2) and another considered amorphous with a surface area of 180–600 m2/g (labeled as P-SiO2). This surface areas difference plays an important role in modifying of nanoparticles polarity by incorporating a coupling agent and its integration into partially polar polymers. According to obtained results, for nanomaterials with high surface area, it was observed while increasing coupling agent amount, the elasticity of the composite was observed to increase. In contrast, in nanomaterials with spherical nanoparticles, it was observed that as the amount of coupling agent decreases, the resistance of the material increases, reaching a maximum when a 10:2 ratio is used. It was observed that behaviors for both nanoparticles were different, which gives an idea that the incorporation of nanoparticles in polymers is not an issue of coupling agent or quantity only, it is more important as it is arranged on the surface. This kind of couplings does not only affect mechanical properties, since the thermal behavior of the material was also influenced, where it was observed that particles with low surface area modify the crystallization rate when they have different percentages of coupling agent on the surface. Furthermore, it is observed that the incorporation of nanoparticles with high surface areas area does not modify the crystallization rate significantly. Besides, in both cases, it was observed that the highest crystallization rate is reached when a 10:2 ratio is used. However, the energy required to form crystals remains unchanged. Therefore, it is considered that the incorporation of nanoparticles only affects the crystal formation rate without disturbing the energy requirement for crystal formation. Finally, a maximum in the 10:2 ratio was observed for the compatibility in both particles, which was manifested in an increase in the storage module through a dynamic mechanical analysis. The rate of crystal formation as well as the number of formed crystals have a considerable effect on mechanical properties of nanocomposites when the surface area is modified.