Effect of polyhedral oligomeric silsesquioxane (POSS) derivative on the morphology, thermal, mechanical and surface properties of poly(lactic acid)-based nanocomposites

In this study, poly(l-lactic acid)/polyhedral oligomeric silsesquioxane derivatives nanocomposites (PLLA/POSS) were prepared using solution casting. Three different POSS molecules, two amino-POSS derivatives with different nonreactive organic substituents attached to the corner silicon atoms (aminopropylheptaisobutyl-POSS, APIBPOSS, and aminopropylheptaisooctyl-POSS, APIOPOSS) and a PLLA-g-POSS organic–inorganic hybrid (PIOPOSS-PLLA), were used in order to investigate the effect of POSS type and concentration on the morphology and properties of PLLA/POSS nanocomposites. The morphological, thermal, mechanical, and surface properties were evaluated by X-ray diffraction (XRD), transmission electron microscopy (TEM), differential scanning calorimetry (DSC), thermogravimetric analysis, tensile experiments, and water contact angle measurements. XRD data evidenced the formation of APIBPOSS crystalline aggregates at POSS contents higher than 2 wt%. TEM results showed self-assembled POSS molecules of sub-micrometer size homogeneously dispersed in the polymer matrix, the shape and size of which depended on the POSS type and concentration. The largest aggregates were formed in PLLA/APIOPOSS blends and the smallest ones in PLLA/PIOPOSS-PLLA blends, increasing the size with increasing the POSS concentration. DSC analysis showed that POSS derivatives acted as plasticizers lowering the glass transition temperature of PLLA, the role of amino-POSS derivatives as nucleating agents for PLLA and the presence of double melting peaks for the PLLA/amino-POSS nanocomposites. The thermo-oxidative stability of the nanocomposites improved, while there was no significant change in thermal stability. The presence of POSS increased the Young’s modulus and the yield point, while decreasing the elongation at break. The incorporation of APIBPOSS and PIOPOSS-PLLA increased the tensile strength. The addition of POSS nanoparticles enhanced the surface hydrophobicity of PLLA. .