Structure–Property Relationships in PDLLA/Silica Hybrid Films: Impact of Grafting and Network Formation on Optical Behavior

Transparent PDLLA/silica hybrid films were prepared via a sol–gel process using organosilane-terminated PDLLA, and two structural motifs—graft-type and 3D-network hybrids—were systematically compared. Dynamic mechanical analysis (DMA) revealed that silica incorporation significantly restricted polymer chain mobility, increasing the onset temperature of the storage modulus from 33.9 °C for neat PDLLA to 41.5 °C and 50.3 °C for the 15 and 20 wt% graft-type hybrids, respectively. Thermogravimetric analysis (TGA) confirmed silica contents of 8.8–18.5 wt% and showed that the 10% weight-loss temperature increased by ~60 °C relative to neat PDLLA, with improvements primarily governed by silica content rather than hybrid topology. Small-angle X-ray scattering (SAXS) demonstrated uniform nanoscale dispersion with inter-domain distances of ~60–65 nm and no domain coarsening; combining these distances with the PDLLA end-to-end distance (R₀ ≈ 24–30 nm) yielded effective silica domain sizes of 30–35 nm. Porod analysis distinguished diffuse interfaces in graft-type hybrids from more correlated structures in network-type hybrids. Optically, the hybrids maintained high transparency (>90% at 400 nm) up to 18 wt% silica, while the Abbe number increased from 55 (neat PDLLA) to 73 (20 wt%). These findings provide quantitative insight into how nanoscale silica organization dictates thermomechanical, thermal, and optical behavior in PDLLA hybrids, extending the understanding established by earlier studies and supporting the continued development of PDLLA/silica hybrid materials.