Thermal field-flow fractionation separates poJymers with high selectivity according to their Soret coefficient, ST, hence, according to their molar mass, and therefore consitutes an efficient physicochemical tool for the determination of the Soret coefficient of a given polymer in the carrier liquid from its retention time. However, the polymer concentration in the sample influences the retention time and, hence, the value of ST derived from it. An experimental study of the influence of sample concentration on retention, ST, and peak shape was performed for the polystyrene-decahn system over a relatively large temperature domain and for various molar masses. It is found that the retention time and the value of ST increase with increasing sample concentration, the more so as the cold wall temperature is lower. This appears to be in contradiction with the general non-equilibrium thermodynamic expression derived for polymer-solvent systems with positive second virial coefficients, such as the present system over the temperature range investigated. There seems to be a temperature for which the dependence of S T on sample concentration vanishes. This temperature is about 375 K for the polystyrene-decalin system. As the sample concentration increases, the peak barycentre and the standard deviation increases. As the peaks are fronting, the skewness is negative and becomes more negative as the sample concentration increases. The peak skewness appears to be a good indicator of the onset of sample concentration effects. The threshold concentration, forwhich these effects begin to become significant, decreases with increasing molar mass.
Experimental study of the Concentration dependence of the Soret Coefficient by Thermal Field Flow Fractionation: the case of polystyrene in decalin
CONTADO, Catia;DONDI, Francesco;
2002
Abstract
Thermal field-flow fractionation separates poJymers with high selectivity according to their Soret coefficient, ST, hence, according to their molar mass, and therefore consitutes an efficient physicochemical tool for the determination of the Soret coefficient of a given polymer in the carrier liquid from its retention time. However, the polymer concentration in the sample influences the retention time and, hence, the value of ST derived from it. An experimental study of the influence of sample concentration on retention, ST, and peak shape was performed for the polystyrene-decahn system over a relatively large temperature domain and for various molar masses. It is found that the retention time and the value of ST increase with increasing sample concentration, the more so as the cold wall temperature is lower. This appears to be in contradiction with the general non-equilibrium thermodynamic expression derived for polymer-solvent systems with positive second virial coefficients, such as the present system over the temperature range investigated. There seems to be a temperature for which the dependence of S T on sample concentration vanishes. This temperature is about 375 K for the polystyrene-decalin system. As the sample concentration increases, the peak barycentre and the standard deviation increases. As the peaks are fronting, the skewness is negative and becomes more negative as the sample concentration increases. The peak skewness appears to be a good indicator of the onset of sample concentration effects. The threshold concentration, forwhich these effects begin to become significant, decreases with increasing molar mass.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
