A modelling study of interfacial polyamidation

Abstract

Interfacial polymerization (IP) is a fast polymerization technique that does not have stringent requirements on operating conditions or extreme monomer purity. The fast reaction kinetics, and the involvement of a complex array of several transport, reaction, and equilibrium processes, make its mathematical modelling challenging, and very few attempts have been made to model IP reaction, especially to predict the film properties. In particular, no detailed models exist for the commercially important polyamide system, in which the liberation of HCl brings in some unique complications and features.This work extends the earlier work on polyurea (Dhumal and Suresh, 2009) to the interfacial reaction of a diamine with a diacid chloride and accounts for different physicochemical processes associated with the reaction, such as ionic equilibria in the aqueous phase, external mass transfer, monomer diffusion through the film, solution thermodynamics, phase separation kinetics, and film formation. Flory–Huggins theory has been used to calculate the solubility limits of the oligomers in different solvents. A competition between nucleation and spinodal decomposition-based phase separation has been postulated to predict the film’s crystallinity.Since only the unprotonated fraction of the amine monomer takes part in the reaction, HCl evolved in of the reaction, and restricts monomer availability in the reaction zone. The present model explains the experimental observation that the reaction ceases in spite of both monomers being still available, as being due to the amine monomer being almost entirely protonated by about 33% conversion. The consequences of this on the reaction kinetics and film properties are explored.