Polymer foaming with supercritical carbon dioxide

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2015
Novendra, Novendra
High molecular weight semi crystalline poly(L-lactic acid) (PLLA) was processed with environmentally benign and non-toxic supercritical carbon dioxide with and without the addition of two different additives to form porous polymeric thin films for biomedical applications such as drug delivery. One of the additives was a CO2-philic additive, trifluoropropyl polyhedral oligomeric silsesquioxane (TFPOSS), and the other was a non CO2-philic additive, octamethyl polyhedral oligomeric silsesquioxane (OMPOSS). Even though PLLA has good mechanical properties, there are only a few studies in literature on porous PLLA preparation with supercritical CO2 since the processability of this polymer with the supercritical fluid at moderate temperatures and pressures is quite poor. In this study, the chosen processing parameters were temperature, saturation pressure, venting rate, saturation time and additive concentration in the ranges of 313-393 K, 10.3-20.7 MPa, 0.2-10.3 MPa.min-1, 2-24 h, 10-30 wt%, respectively. The results showed that while PLLA did not form a porous structure when it was processed with supercritical CO2 at a temperature as low as 313 K and a moderate pressure of 20.7 MPa, addition of 10 wt% TFPOSS to PLLA improved its foamability when it was processed with supercritical carbon dioxide (scCO2) at the same conditions. About 27% porosity was obtained in the PLLA film with the addition of 10% TFPOSS at these conditions in 2 h processing with scCO2. At the same processing conditions, with an increase of TFPOSS to 30 wt% in the polymer matrix, the extent of porosity was improved further to value of 40%. At the same processing conditions, when 30% non CO2-philic additive (OMPOSS) was used, i.e. does not form a homogenous mixture with the CO2 at any temperature and pressure, no porous structure was obtained. This shows that the porosity improvement was obtained due to the CO2-philicity of the used additive. Conditions of processing with scCO2 can change many aspects of the polymeric films as well. In the supercritical foaming of PLLA, it was observed that increase in pressure at constant temperature resulted in increase in the pore density and decreased the average pore size. Processing at a higher temperature at constant pressure, increase the average pore size. The average pore size decreased, by decreasing the venting rate while keeping all the other conditions constant. The indentation hardness of the processed films was also analyzed and it was observed that this property was directly proportional with the processing time and POSS concentration in the PLLA. The maximum indentation hardness was obtained as 3541 MPa, with the use of 30% POSS, at the supercritical processing conditions of 313 K and 20.7 MPa, 24 h. Similarly, crystallinity of the films also increases after supercritical processing. This shows that CO2 had a plasticizing effect on the solid polymer, so that polymer chains gain some mobility and reorganize themselves due to scCO2 processing which is leading to higher crystallinity. However, none of the parameters and the additives had a significant effect on the crystallinity of the obtained films. Finally when the obtained films with the enhanced porosity were tested for drug delivery, which is the targeted biomedical application, the films exhibited a burst release behavior, which can be suitable for local antibiotic applications. It was observed that at 313 K and 20.7 MPa, complete extraction of TFPOSS from the PLLA films was obtained during the processing, while at higher temperatures, complete extraction was not observed. In order to estimate the solubility of TFPOSS in scCO2 at the processing conditions, the solubility data of TFPOSS obtained from literature was modeled. Six different semi-empirical density-based models, including Chrastil, Chrastil modified by Wang, Del Valle and Aguilera, Kumar and Johnston, Mendez-Santiago and Teja, and Bartle models were used. The best solubility predicting performance was obtained with Chrastil modified by Wang model, which also exhibited a linear self-consistency test.

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Citation Formats
N. Novendra, “Polymer foaming with supercritical carbon dioxide,” M.S. - Master of Science, Middle East Technical University, 2015.