A continuous RESS process to prepare PLA–PEG–PLA microparticles

A continuous RESS (rapid expansion of supercritical solution) process that combines an SEDS (solution enhanced dispersion by supercritical fluids) process and RESS process was developed to prepare the microparticles of polylactide–poly(ethylene glycol)–polylactide (PLA–PEG–PLA) triblock copolymer. The resulting microparticles exhibited a spherical shape and a narrow particle size distribution with a mean particle size of 2.2 μm. After supercritical processing, the results of Fourier transform infrared spectroscopy (FTIR) measurement indicated that the minor structural changes occurred on a molecular level; X-ray powder diffraction (XRPD) analysis revealed that the physical state of the copolymer shifted into an amorphous form; and the thermogravimetry-differential scanning calorimetry (TG-DSC) measurements demonstrated the increases in fusion temperature and decomposition temperature of the copolymer from 135.8 °C to 180.6 °C and 342.9 °C to 372.0 °C, respectively. The results indicate that the continuous RESS process could overcome the drawbacks of SEDS and RESS processes when processing the supercritical CO2 partially soluble polymers and recrystallize such polymers continuously and constantly, which would widen the application of supercritical technologies in the treatment of polymers.