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

Abstract

Rapid-expansion experiments with CO 2 + 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 ( T m) of 121 and 162 °C, pre-expansion temperatures ( T pre) 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 M C, which is approximately 9000. An increase in T m, a decrease in T pre, and a decrease in THF concentration each resulted in a consistent decrease in both particle size and interparticle fusion. Because D ∝ exp(− T m/ 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/ T m). 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.