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 RES S processing. PLLA polymers with melting points (T-g) of 121 and 162 degrees C, pre-expansion temperatures (T-pre) of 70 and 100 degrees 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 proportional to 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. (C) 2006 Elsevier B.V. All rights reserved.