An experimental study of polyurea membrane formation by interfacial polycondensation

Interfacial polycondensation has been studied for many applications, and most importantly for such niche applications such as microencapsulation and membrane synthesis. The fast kinetics and the complexity of the process involving the interplay of several rate and equilibrium processes make it difficult to study the reaction and explore the effect of kinetics on the properties of the polymer film that forms. We show in this study that a dispersed phase configuration such as used in microencapsulation, with a fast on-line pH measurement, is a convenient way to study these reactions. Apart from the ease of obtaining kinetic information, the technique results in self-supporting films which can be recovered and characterized for structure by a variety of techniques. In this study, the intrinsic variables which influence observed reaction velocities, such as reactant concentrations and film thickness, have been varied through experimental parameters that can be more conveniently set. Although the intrinsic kinetics is fast, it is still possible that intrinsic chemical kinetics play a significant role in the overall mechanism, since the films formed are very thin. Data obtained under such conditions, with an excess of the organic-side monomer (hexamethylene-1,6-diisocyanate, HMDI), show a first order dependence of monomer consumption rate on the aqueous-side monomer (hexamethylene-1,6-diamine, HMDA). The effect of solvent on the observed rates shows some interesting characteristics, counterintuitive if a correlation is sought with the partition coefficient of HMDA into the solvent. It is shown that rates correlate better with solvent polarity. The polymer formed is of low molecular weight in general. The molecular weight shows a dependence on the mole ratio of monomers, with a ratio (HMDI/HMDA) slightly in excess of 1 being the most conducive to molecular weight development. The polymer is of semicrystalline structure. with the crystallinity determined by the conditions of reaction. These findings are of interest to applications such as controlled release and membrane separations, in which permeation rate through the membrane is of importance. (C) 2008 Elsevier B.V. All rights reserved.