Optimization of polymer-solvent interaction in a multisolvent system by the UNIFAC group-contribution method

The solubility parameter scheme is an exceptionally useful tool for predicting the solubility behavior of simple polymer systems. However, this scheme is unable to handle well enough complicated systems, such as those involving donor‐acceptor interactions and those containing more than two solvents. More advanced thermodynamic treatments and computer techniques, such as UNIFAC, can be a key to solving these kinds of problems. The UNIFAC group‐contribution method utilizes a volume parameter and a surface‐area parameter for each structural group and a pair of interaction‐energy parameters for each pair of groups, which can be deduced from experimental activity data. The method was originally derived for mixtures of ordinary liquids and has been extended to polymer solutions by adding a free‐volume correction. The modified UNIFAC method can be used to estimate the activities of solvents in a polymer solution, even when no experimental data are available for the mixture. In the present study, the UNIFAC method was applied to optimize polymer‐solvent interactions in three‐solvent systems. A three‐dimensional plot, displaying polymer activity as a function of the solvent composition, was constructed for each polymer system. The minimum in polymer activity was used as the criterion for maximum polymer‐solvent interaction. Dissolution rate and solution clarity were used to test the polymer‐solvent interaction experimentally. Comparison of theoretical predictions with experimental results indicated that a better agreement could be obtained by using the UNIFAC method rather than the solubility parameter method.