Effects of functional groups on the solubilities of polyhedral oligomeric silsesquioxanes (POSS) in supercritical carbon dioxide

Abstract

, 88 pages 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 dewpoints 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. vi 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.

Synergy of supercritical CO2 and superheated H2O for enhanced processability of polyethersulfone towards open cell foams

“…In the present work, however, the relatively higher T g and the polar SO 2 groups of PES are likely causes of the favorable interaction with the scCO 2 and shH 2 O at higher soaking temperatures. Others have also reported of effect of higher solubility in scCO 2 at higher temperatures due to favorable interactions with certain functional groups . Therefore, we postulate that increasing the temperature leads to high degree of swelling by the shH 2 O and scCO 2 .…”

Supercritical processing of CO 2 -philic polyhedral oligomeric silsesquioxane (POSS)-poly( l -lactic acid) composites

“…For this, the films were processed with two different types of additives. One of them was the TFPOSS, which was shown recently to be significantly scCO2-soluble [40], and the other was OMPOSS, which was found to be insoluble in the supercritical fluid [41]. The scCO2 processing of the films were performed at 20.7 MPa and 313 K for 2 hr, after which the films were vented at a rate of 10.3…”

“…In polymer processing with scCO2, such particles can act as heterogeneous nucleating agents, which stimulate higher rate of cell nucleation by decreasing the free energy barrier for nucleation and provide new sites for cell formation [5,38,39]. We used two types of POSS to observe their effects on the formation of pores in PLLA; a CO2-philic type, trifluoropropyl polyhedral oligomeric silsesquioxane (TFPOSS) [40], and a non CO2-philic type, octamethyl polyhedral oligomeric silsesquioxane (OMPOSS) [41]. Even though in literature it was reported that for PLLA with crystallinity greater than 30%, saturation temperatures close to the melting temperature of PLLA were required to obtain a porous structure with scCO2 processing [31,42], our results showed that up to 40% porosity could be obtained in the polymer matrix when a CO2-philic POSS was added into the PLLA with crystallinity greater than 30% and the composite was processed at the scCO2 saturation temperature and pressure of 313 K and 20 MPa, respectively.…”

Density functional theory study of interactions between carbon dioxide and functionalized polyhedral oligomeric silsesquioxanes

“…However, aggregation of POSS materials and the limitation of their solubility are important hindrances of the POSS applications . Recently it was shown that POSS can be CO 2 ‐philic and solubilized in supercritical CO 2 provided that it has certain functional groups, which allows applications of POSS in environmentally benign supercritical processing of materials …”

“…This, in return, can enhance the entropy of mixing, and thus the solubility of the compound in supercritical carbon dioxide (scCO 2 ) . In previous solubility studies, trifluoropropyl POSS was found to be highly soluble in scCO 2 , while octamethyl POSS was insoluble . In a recent study trifluoropropyl POSS was used as a pore inducer in supercritical foaming of poly( L ‐lactic acid) (P L LA) with crystallinity greater than 30% .…”