Excellent UV Resistance of Polylactide by Interfacial Stereocomplexation with Double-Shell-Structured TiO₂ Nanohybrids

The durable application of polylactide (PLA) under atmospheric conditions is restricted by its poor ultraviolet (UV) stability. To improve the UV stability of polymers, titanium dioxide (TiO2) is often used as a UV light capture agent. However, TiO2 is also a photocatalytic agent, with detrimental effects on the polymer properties. To overcome these two conflicting issues, we used the following approach. TiO2 nanoparticles were first coated with silicon dioxide (SiO2) (with a SiO2 shell content of 5.3 wt %). Subsequently, poly(d-lactide) (PDLA) was grafted onto TiO2@SiO2 nanoparticles, approximately 20 wt %, via a ring-opening polymerization of d-lactide to obtain well-designed double-shell TiO2@SiO2-g-PDLA nanohybrids. These double-shell nanoparticles could be well dispersed in a poly(l-lactide) (PLLA) matrix making use of the stereocomplexation between the two enantiomers. In our concept, the inner SiO2 shell on the TiO2 nanoparticles prevents the direct contact between TiO2 and the PLLA matrix and hence considerably restricts the detrimental photocatalytic effect of TiO2 on PLLA degradation. Additionally, the outer PDLA shell facilitates an improved dispersion of these nanohybrid particles by interfacial stereocomplexation with its enantiomer PLLA. As a consequence, the PLLA/TiO2@SiO2-g-PDLA nanocomposites simultaneously possess excellent UV-shielding property, high(er) tensile strength (>60 MPa), and superior UV resistance, for example, the mechanical properties remain at a level of >90% after 72 h of UV irradiation. In our view, this work provides a novel strategy to make advanced PLA nanocomposites with improved mechanical properties and excellent UV resistance, which enables potential application of PLA in more critical areas such as in durable packaging and fiber/textile applications.