Reaction Regime Study of Interfacial Polyamidation

Abstract

Interfacial polyamidation is widely used for the fabrication of thin-film composite membranes. The process involves strongly coupled reaction, diffusion, and phase separation phenomena, making it difficult to identify the dominant rate-controlling mechanism during film formation. In particular, the transition between kinetic and diffusion control during film growth remains poorly understood, despite its importance in membrane fabrication. In this work, a reaction-regime framework is developed for interfacial polyamide formation by extending concepts previously proposed for polyurea synthesis via interfacial polymerization. The reaction behavior is represented using two dimensionless parameters: a dimensionless reaction rate, Ψ, and a diffusion–reaction parameter, P. The resulting log Ψ–log P relationship exhibits behavior analogous to the classical effectiveness factor–Thiele modulus correlation encountered in heterogeneous reaction systems. The analysis shows that interfacial polymerization can proceed under kinetically controlled, diffusion-controlled, or mixed-control conditions. The transition between kinetic and diffusion control occurs approximately at P = 1. For P ≫ 1, the numerical simulations closely follow the asymptotic diffusion-controlled solutions, whereas deviations from the asymptotic behavior are observed in the kinetically controlled region. The extent of these deviations depends on the characteristic diffusion and reaction times as well as the monomer ratio. Improved agreement with the asymptotic predictions is obtained when short-chain oligomers dominate the reaction zone. The influence of the reaction regime on polymer film formation and film properties is also examined. Variations in diffusivity, reaction rate, nucleation rate, and phase-volume ratio produce only modest changes in molecular weight distribution, film thickness, and polydispersity, whereas crystallinity shows greater sensitivity to the reaction regime. In contrast, the monomer ratio and limiting monomer concentration have a much stronger influence on the film properties and molecular weight development. The dimensionless framework developed in this work provides a unified description of reaction regimes in interfacial polymerization and offers a practical basis for interpreting experimental data, identifying regime transitions, and designing the operating conditions for membrane fabrication.