Supercritical processing of CO2-philic polyhedral oligomeric silsesquioxane (POSS)-poly(L-lactic acid) composites

High molecular weight and semicrystalline poly(L-lactic acid) (P(L)LA) films were processed with supercritical carbon dioxide (scCO(2)) for environmentally friendly formation of porous films. The polymeric matrix was processed with supercritical CO2 and the system was rapidly depressurized to cause supersaturation of the polymer, leading nucleation and growth of the pores in the matrix. Even though saturation temperatures close to the melting points of the polymers are required to form porous matrices of semicrystalline polymers, we obtained porous P(L)LA with porosity up to 40% by using CO2-philic trifluoropropyl polyhedral oligomeric silsesquioxane (TFPOSS) as an additive to induce formation of pores at 313 K that is about 130 K lower than P(L)LA's melting point. This is the first time that P(L)LA with crystallinity greater than 30% foamed with the supercritical fluid at a temperature close to its critical temperature. P(L)LA without the additive did not result in a porous matrix once it was processed under the same conditions. A non-CO2-philic POSS was also used to observe the influence of CO2-philicity, which did not induce formation of pores in the matrix under the same conditions. Additive concentration (10-30 wt%), saturation temperature (313-393 K), saturation pressure (10.3-20.7 MPa), venting rate (0.2-10.3 MPa min(-1)), and saturation time (2-24 h) were among the studied process parameters. Saturation temperature and additive concentration were observed to have significant effects on the porosity and the average pore size of the polymeric matrix, while venting rate had an effect only on the average pore size. The indentation hardness values of the P(L)LA films were increased by at least 60% after the scCO(2) processing. A model antibiotic, Ceftriaxone Sodium, was loaded into the samples and about 85-95% of the loaded drug released in the first hour in in vitro conditions, which can be a preferable rate in local hard tissue antibiotic applications