Studies on the theory and applications of polymer separation by thermal field-flow fractionation

Thermal field-flow fractionation is a member of the field-flow fractionation (FFF) family of high-resolution separation techniques proposed by J. C. Giddings. The FFF techniques utilize an external field that is applied perpendicular to the axis of a Poiseuille flow to drive polymeric or colloidal species to the different velocity laminae to achieve differential separation. Thermal FFF offers a solution to the many problems that exist in the field of separation and characterization of complex polymeric materials whose molecular weights extend into the ultrahigh range.; A thermal FFF laboratory system was constructed to examine the applicability of thermal FFF to the separation of polysaccharides. Model polysaccharides widely used in biological research and clinical work were successfully fractionated in dimethyl sulfoxide according to differences in their molecular weights. Calibration plots were developed and used to obtain the molecular weight distributions for these materials. The determined calibration constants are physicochemical properties of the polymer-solvent systems and are expected to apply universally to other laboratories.; More complex industrial polysaccharides such as starch and cellulose were also investigated. Starch samples were successfully separated into amylose and amylopectin fractions. It was discovered that the retention of a cationic starch sample was significantly enhanced when an electrolyte was added to the solvent. An on-line multiangle laser light scattering detector was coupled with the thermal FFF to directly determine the molecular weight distributions for starch and cellulose.; In order to understand the behavior observed for the cationic starch in an ionic strength modified carrier in a thermal FFF system, a more comprehensive study was carried out using model polyvinylpyridines. Dependence of retention and sample recovery on the ionic strength of the solvent was determined experimentally. A hypothesis based on the polyelectrolyte screening effect and the electrostatic repulsions in the wall region was proposed to interpret the experimental results.; High-speed polymer separation was achieved by operating thermal FFF at high flow rates in a lift-modulated hyperlayer mode. Measurements were made using linear polystyrene standards to determine the elastic/entropic force at different flow rates and temperature gradients. A hypothesis is proposed to predict the molecular weight dependence and flow rate dependence for the entropic force. The results should prove useful in the understanding of the rheological behavior of polymers under shear flow.