Biobased Alkyl Chain-Driven Salicylate Plasticizers for PLA: Molecular Design Strategy for Substantially Improving Flexibility and Degradability

The application of polylactide (PLA) in many fields is challenged by its characteristically high brittleness and minimal elongation prior to breaking, despite its advantages as a compostable and biobased material; to address this, biobased salicylic acid (SA) was innovatively employed to synthesize a series of alkyl-chain-differentiated plasticizers (SAn-E, n = 4,6,8) through esterification and acylation reactions, as structurally confirmed by 1H NMR and FT-IR spectroscopy. The effects of SA and SA8-E at different loadings on the crystallization behavior, thermal properties, mechanical performance, phase morphology, and degradability of PLA, as well as the performance variations among SAn-E with different alkyl chain lengths, were systematically investigated. The results demonstrated that SA induced severe phase aggregation due to polarity mismatch, while SA8-E achieved homogeneous dispersion within PLA, drastically reducing the glass transition temperature (Tg) from 61 to 29 °C and increasing crystallinity to 17.8% at 20 wt% loading; concurrently, elongation at break surged from 3.5 to 368.6%, with impact strength enhanced 9-fold to 36.5 kJ/m2. Comparative alkyl chain analysis demonstrated SA8-E’s superiority over SA6-E and SA4-E (crystallinity: 17.8% vs 16.7% and 17.4%, respectively), attributed to its larger molecular volume improving free volume and lubricity. Durability tests further confirmed SA8-E’s low migration (1.14% in petroleum ether) and minimal volatility loss (1.08% at 70 °C/48 h), mechanistically linked to the synergistic effects of ester groups (enhancing compatibility) and long alkyl chains (improving flexibility). This study validated the success of a structure-guided strategy for synthesizing effective biobased plasticizers, offering insights into developing sustainable plasticizers and expanding the application scope of PLA.