Re-thinking polyamide thin film formation: How does interfacial destabilization dictate film morphology?

Unravelling synthesis-morphology-performance relations of thin-film polyamide (PA) selective layers prepared via interfacial polymerization (IP) is essential for tailor-made membrane design. Here, an approach is outlined, which links synthesis and morphology in terms of the interfacial stability of a system undergoing IP, with the notion that a transition from a stable to an unstable state corresponds to a morphological shift of the resultant film, e.g., from smooth to crumpled. Within the proposed framework, physical parameters related to a variety of synthesis conditions are identified and classified based on their effect on two defined criteria: the polymer formation rate and/or the system susceptibility to instability. A conceptual phase diagram maps the morphological regimes with respect to the relevant parameter space. Our analysis, based on available literature data, illustrates how PA morphology, as observed in published SEM images, transitions from smooth to crumpled, exhibits a remarkable correlation with the defined parameter space, and hereby the stability of the system. Re-thinking experimental results in this manner highlights not only the underlying physics, but also identifies potential pitfalls when variations of synthesis conditions result in conflicting effects. Our motivation is to encourage experimental design based on the developed framework supplemented by theoretical quantification, which will strengthen the fundamental understanding of IP for a given chemistry, with the goal of providing a clear physical toolbox for IP-based membrane design.