Tunable polylactide plasticizer design: Rigid stereoisomers
Wenxiang Xuan, Karin Odelius, Minna Hakkarainen
Abstract
Two isomeric molecules, 1,2-cyclohexanediol and isohexide (isosorbide and isomannide) demonstrated potential as rigid building blocks in PLA plasticizer design with tunable material performances. Six plasticizer candidates were synthesized via Fischer esterification in bulk of the rigid isomeric cores and green platform chemicals levulinic acid and valeric acid. The structures were confirmed by 1H NMR and ESI-MS. Based on the calculated Hansen solubility parameters, all the synthesized plasticizer candidates were expected to be miscible with PLA, which was experimentally proven by a significant decrease of glass transition temperature (Tg) and an increase in strain at break. For instance, PLA plasticized with 20 wt% cyclohexanediol levulinate (cis- and trans- mixture) portrayed the lowest Tg of 25 °C and the highest strain at break of 265%, which equals to 44 times of the initial strain at break of neat PLA. Isohexide-based plasticizers with larger rigid cores, isosorbide levulinate and isomannide levulinate enabled superior thermal stability, higher Young’s modulus and stress at break in PLA blends compared with cyclohexanediol-based plasticizers, while retaining high strain at break. Stronger influence of stereoisomerism on plasticization was also observed in PLA blends with isohexide-based plasticizers compared with cyclohexanediol-based plasticizers. Here we present a new pathway to tailor the performance of plasticizer by utilizing isomeric rigid building blocks.