Influence of polydispersity of poly(lactic acid) on particle formation by rapid expansion of supercritical CO2 solutions

Poly(lactic acid) (PLA) particles were generated by rapid expansion of supercritical PLA/CO₂ solutions (RESS). Two different PLA samples, one with high (PDI = 2.4) and the other one with low (PDI = 1.4) polydispersity but similar number average molecular weight, were compared. After micronization, the polymers were analysed by rapid-scan infrared spectroscopy, scanning electron microscopy, size-exclusion chromatography, differential scanning calorimetry, and NMR spectroscopy. Our investigation reveals that the polydispersity of the polymers strongly affects the size but not the shape of the particles. We found larger particles (∼730 nm) for the PLA with high polydispersity than for the PLA with low polydispersity (∼270 nm). In both cases, spherical particles were formed. Moreover, our results clearly show that PLA with high polydispersity is less suitable for RESS processing because the low-molecular weight chains are depleted over time and process conditions are thus not constant.