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Effects of functional groups on the solubilities of polyhedraloligomeric silsesquioxanes (poss) in supercritical carbon dioxide

Kanya, Başak
In this study, the phase behavior of the binary systems containing CO2 and various hybrid polyhedral oligomeric silsesquioxanes (POSS) with different functional groups were investigated. A high-pressure view cell was used to measure the dew-points or cloud-points at temperatures between 308K and 323K, up to 30 MPa. Among the studied POSS structures, methacryl, isooctyl and octaisobutyl POSS were observed to form homogenous solutions with CO2, while octamethyl POSS was found to be insoluble in the supercritical fluid. The solubilities of both methacryl and isooctyl POSS in supercritical carbon dioxide (scCO2) decrease with increasing temperature, while isooctyl POSS exhibits nearly an order of magnitude higher solubility than methacryl POSS at both temperatures. The octaisobutyl POSS-CO2 binary system exhibits a crossover pressure, below which the solubility of the component decreases with increasing temperature. Above this pressure, as the temperature is increased, the solubility of POSS in scCO2 increases. These results show that the functional groups of POSS affect its solubility in scCO2 and the phase behavior of the POSS-CO2 binary system significantly. After the solubility experiments the obtained phase behavior data of methacryl POSS, isooctyl POSS, and octaisobutyl POSS were correlated using density based models. The models used are; Mendez-Santiago and Teja (MST) model, Chrastil model, Bartle model, del Valle and Aguilera (del Valle) model, and Kumar and Johnston (K-J) model. All of these models gave good fit to the solubility data. In the last part of this study the obtained phase equilibrium data of POSS-CO2 binary system was used to design an environmentally benign material processing application using scCO2. In this application scCO2-soluble solid POSS, which were trifluoropropyl POSS and octaisobutyl POSS, were impregnated into a CO2-philic, biodegradable and biocompatible polymer, poly (lactic acid) (PLA) with scCO2. The morphological analysis by scanning electron microscopy (SEM) analysis and thermal characterization of the impregnated PLA films and neat PLA film by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) instruments were performed. The effect of POSS addition on glass transition temperature, melting temperature, and thermal stability were investigated. It was observed that POSS could be loaded into PLA with the supercritical fluid impregnation technique. The resulted polymer composites had an increase in their Tg with respect to neat PLA showing that the intermolecular interactions within the polymer matrix were enhanced. However, sufficient loading could not be obtained with current process conditions and POSS types loaded to obtain significant improvement in the thermal stability of the films.