Phase behavior of biodegradable poly(L-lactic acid) in supercritical solvents and cosolvents

Poly(L-lactic acid) (PLLA), a biodegradable polymer from plant sources, is widely employed in biomedical applications for its biocompatibility and versatile mechanical properties. However, comprehending its phase behavior in various solvents poses a challenge for broader applications. In this study, the phase behavior of the polymer was investigated in various solvent and cosolvent media and combinations at a wide range of temperatures and pressures to gain insight into the polymer properties. PLLA of varying intrinsic viscosity (IV): [0.15 dl/g, with an estimated weight average molecular weight (M-w) range of approximately: 1600 to 2400 g/mol; 1.0 dl/g (M-w): 100,000 g/mol); and 1.8 dl/g (M-w): 80,000 to 100,000 g/mol] was employed for this study. The phase behavior of the polymer across different compositions of PLLA and solvent (CH2F2, CHF3, CO2, DME, F-22) in a wide range of temperatures from 333 to 425 K and high pressure 1.55 to 172 MPa was investigated. Miscibility of PLLA in DME and F-22 is predicted to be higher with increasing in concentration than the other cosolvents. Furthermore, in the case of the supercritical solvent CHF3 and DME the phase transition pressure curve of PLLA was observed to exhibit a lower critical solution temperature (LCST) type with a positive gradient with increasing temperature. However, lower phase transition pressure was noticed for lower molecular weight PLLA in supercritical solvent CHF3 and DME, i.e., of the order IV 0.15 dl/g<IV 1.0 dl/g<IV 1.8 dl/g. The phase transition pressure of PLLA (IV 1.8 dl/g) in solvent CO2 with increasing the concentration of co-solvent (F-22, DME) from 0 to similar to 97 wt% shifted from UCST to LCST type. The LCST curve of PLLA (IV 0.15) in DME intersects with the vapor(V)-liquid(L) curve at T: 353 K and P below 5 MPa, indicating the coexistence of V+L phases.