PHASE BEHAVIORS AND FRACTIONATION OF POLYMER-SOLUTIONS IN SUPERCRITICAL CARBON-DIOXIDE

Supercritical carbon dioxide has been utilized to fractionate polydime thylsiloxane (PDMS), poycarbosilane (PCS) and polystyrene (PS). Throug h the observation of phase behaviors in PS-toluene-CO2 systems the exp eriment shows that only a lower critical solution temperature (LCST) h as been detected when the concentration of CO2 is 15.0 wt.% and temper atures range from 280.6 K to 373 K. We observe an upper critical solut ion temperature as well as an LCST at 16.2 wt.% of CO2 and the dramati c merge of two critical solution temperature curves by increasing CO2 concentration to 16.6 wt.%. When the concentration of CO2 was 16.6 wt. %, fractionated PS samples were obtained under different experimental conditions. Those samples were analyzed in terms of molecular weights and glass transition temperatures. For the PCS-hexane-CO2 systems we c an observe an LCST when the CO2 concentration is 19.8 wt.%, and fracti onation of PDMS also has been performed by ascending extracting pressu res from 128.6 to 207.9 bar. (C) 1998 Elsevier Science B.V.