Effect of material properties and processing conditions on RESS of poly(l-lactide)

Rapid-expansion experiments with CO2 + THF solutions of poly(l-lactide) (PLLA) have demonstrated that the solid-state diffusion coefficient of the solute (D) is a key variable for controlling particle size during RESS processing. PLLA polymers with melting points (Tm) of 121 and 162 °C, pre-expansion temperatures (Tpre) of 70 and 100 °C, and THF co-solvent concentrations of 10 and 20 wt.% were all investigated for their effect on the solid-state diffusion coefficient. The weight average molecular weights of the investigated PLLAs were 1340 and 6050, well below the critical molecular weight for entanglement MC, which is approximately 9000. An increase in Tm, a decrease in Tpre, and a decrease in THF concentration each resulted in a consistent decrease in both particle size and interparticle fusion. Because D ∝ exp(−Tm/T), where T is the particle temperature, these effects can be explained in terms of their impact on the solid-state diffusion coefficient. In general, RESS products from PLLA consisted predominantly of nanoparticles 30–100 nm in diameter, dispersed with either micron-sized particles or agglomerates of nano- and submicron-sized particles, with the type of dispersion depending on the value of D. These results suggest that the initially formed precipitates during RESS are nano-sized, and that the larger particles are subsequently obtained because of coalescence in the free jet, a process that is favored by higher values of D (and, thus T/Tm). In summary, then, our recent studies indicate that the best candidates for producing nanoscopic products by RESS are high-melting materials that can be processed at relatively low pre-expansion temperatures.