(334g) Phase Behavior of PS/Tetramethyl Orthosilicate/CO2 Ternary System

Phase Behavior of PS/Tetramethyl orthosilicate/CO₂ Ternary System

              In recent years, for the effective energy usage, development of high performance heat insulator is the subject of research for engineers. Polymer foam that inside of the foam filled with silica aerogel is regarded as one of the promising candidate for high performance heat insulator. This composite material will be commercially produced with a supercritical carbon dioxide (CO₂) assisted extruding of the mixture of the polymer and tetraalkoxysilane. As the foaming pressure depend on the phase separation, phase behavior of the systems is very important. In the previous work, we examined Tetramethyl orthosilicate (TMOS)/CO₂ binary systems and poly(methyl methacrylate) (PMMA)/TMOS/CO₂ ternary systems.

              In this work, phase diagrams of poly(styrene) (PS)/ TMOS/CO₂ ternary systems were measured. PS/TMOS system is mutual solubility system at atmospheric pressure. Phase equilibrium measurements were performed by visual observation of the Vapor-Liquid (VL) separation, Liquid-Liquid (LL) separation and Vapor-Liquid-Liquid (VLL) separation with a synthetic method at wide range of temperatures and Pressures. Experimental conditions for the ternary systems were at PS weight fraction from 10 wt % to 40 wt % and PS molecular weight from 35,000 to 250,000. The results of the measurements were compared with the results of the TMOS/CO₂ binary systems and PMMA/TMOS/CO₂ ternary systems.

              The PS/TMOS/CO₂ ternary system showed two types of different phase behavior with the amount of the CO₂ mass fraction. When CO₂ mass fraction was small, it showed the behavior of the Vapor-Liquid (VL) separation, and the VL equilibrium line appeared. On the other hand, when CO₂ mass fraction was large, as CO₂ acts as a poor solvent for PS/TMOS, Liquid-Liquid (LL) phase separation occurred. Consequently the VLL and LL equilibrium line appeared. As PS molecular weight was increased, the LL equilibrium line was shifted to the lower CO₂ mass fraction. It seems that because the affinity of the PS and CO₂ is reduced due to the increase in molecular weight, it became easier to LL phase separation. Effect of molecular weight was not observed in the case of the VL and VLL equilibrium line.

              Further, with the increase in temperature, the VL equilibrium line shifted to the high pressure. In contrast, the LL lines of two temperatures (40 ^oC and 80 ^oC) crossed each other. It seems that this phenomenon is the same effect as the retrograde crystallization. The VL and VLL equilibrium lines of the ternary systems and the VL equilibrium lines of the TMOS/CO₂ binary systems were almost identical. Thus, the polymer is not involved in to the separation of carbon dioxide from the mixture.

              In addition, this work was compared with PMMA/TMOS/CO₂ ternary system. The impact of differences in the polymer was not found in the VL and VLL equilibrium line, but in the LL equilibrium line, the equilibrium line with PS showed a rise in the region of lower CO₂ mass fraction than that with PMMA. This reason is considered that the affinity with TMOS is good towards PS than PMMA.