Development of continuous two-dimensional thermal field-flow fractionation for polymers: Dissertation

This research work is focused on the development of instrumentation, operation, and approximate theory of a new continuous two-dimensional thermal field-flow fractionation (2D-ThFFF) technique for separation and collection of macromolecules and particles. The separation occurs in a thin disk-shaped channel, where a carrier liquid flows radially from the center towards the perimeter of the channel and a steady stream of the sample solution is introduced continuously at a second inlet close to the centre of the channel. Under influence of the thermal field, the sample components are separated in radial direction according to the analytical ThFFF principle. Simultaneously, the lower channel wall is rotating with respect to the stationary upper wall, when a shear-driven flow profile deflects the separated sample components into continuous trajectories that strike off at different angles over the 2D surface. Finally, the sample components are collected at the outer rim of the channel, and the sample concentrations in each fraction are determined using the analytical ThFFF. The samples were polystyrene polymer standards and the carrier solvents cyclohexane and cyclohexane-ethylbenzene mixture in continuous 2D-ThFFF and tetrahydrofuran in analytical ThFFF. Positive effect of the thermal field on the sample deflection was observed, although broadening of the sample zone was observed. By decreasing the channel thickness and the radial and angular flow rate of the carrier, the zone broadening was significantly narrowed. The systematic variation of the experimental parameters allowed to determine the conditions required for the continuous fractionation of polystyrene polymers according to their molar masses. As an example, almost baseline separation was achieved with two polystyrene samples of different molar masses. Meanwhile, an approximate theoretical model was developed for prediction the trajectory of the sample component zone and its angular displacement under various operating conditions. The trends in the deflection angles without and with a thermal gradient were qualitatively in agreement with predictions of the model, but significant quantitative differences between the experimental results and theoretical predictions were also found. The reasons for discrepancies between the theory and the experiment could be the following: the sample is already relaxed at the sample inlet, effect of solvent partition using binary solvent as the carrier, dispersion of the sample, the instrumental limitations, and/or geometrical imperfections. However, the theoretical model will provide quidelines for future interpretation and optimization of separations in continuous 2D-ThFFF method