L. Tschersich’s research while affiliated with Hochschule Merseburg and other places

For the first time, a quantitative theoretical analysis (liquid/liquid phase equilibria treated by means of the continuous thermodynamics) of the operating characteristics of continuous polymer fractionation (CPF) was performed. The results of these calculations were compared with data published for CPF of polyethylene. It turned out that the efficiency of the conventional CPF corresponds to approximately two theoretical plates only. For this reason, several improvements, suggested by theoretical considerations, were realized experimentally, for which purpose the system dichloromethane/diethylene glycol/bisphenol-A polycarbonate was chosen. The pulsating sieve-bottom column was replaced by a nonpulsating column filled with glass beads. In this manner, the number of theoretical plates could be raised considerably. A further improvement of the fractionation efficiency results from the reflux of part of the polymer contained in the sol phase. In praxi, this situation was realized by putting a condensor on top of the column and introducing the feed somewhere near its upper third. After predictive calculations and orienting experiments, 125 g of a polycarbonate with Mw = 29 kg/mol and a nonuniformity U = 1.3 were fractionated in four consecutive CPF runs (where the gels were directly used as feed for the next step) into five fractions of approximately equal weight. Except for the lowest-molecular-weight fraction, one obtains nonuniformities on the order of 0.1.

In an earlier paper, continuous thermodynamics was applied to calculate polymer fractionation procedures leading to lucid equations favorable for computer simulations. The present paper refines that treatment by applying more sophisticated Gibbs free energy relations. The simulation results agree closely with the experimental data, assuming the Flory-Huggins interaction parameter to be a function of the concentration.

Based on continuous thermodynamics and its application to the theory of successive polymer fractionation procedures, a theory of column fractionation is developed. In continuous thermodynamics the polydispersity of polymers is accounted for by the direct use of the continuous distribution function in the thermodynamic equations. In this way equations which are favorable for computer simulations are obtained. As an example, Baker-Williams fractionation is chosen for presenting theory and computer simulation. The generalization to other column fractionations based on solubility differences is easily possible.

Continuous thermodynamics was developed in recent years and applied successfully to the liquid-liquid equilibrium of poly disperse polymer solutions. Continuous thermodynamics is based on the direct use of continuous distribution functions in the thermodynamic equations. There is no need for a reduction to pseudocomponents. This paper presents the application of continuous thermodynamics to successive polymer fractionation procedures based on solubility differences. In this way, simple equations for the distribution function of the different polymer fractions are obtained. Furthermore, the other equations describing the fractionation possess a lucid structure favorable for computer simulations of the fractionation procedures.

Leuna, Merseburg, Techn. Hochsch., Diss., 1991.