Self-oscillating polymer membranes with chemically fueled pore size oscillation mediated by pH-responsive polymer

Abstract

Soft-matter materials research has considerably evolved in the last decades, mainly by promoting responsive polymer systems. Up to now, the dynamic behavior in materials was always reached by the action of an outside trigger (pH, light, etc..). This constraint has been relieved in a new class of materials that experienced self-oscillation. In this work, the self-regulating pH cycles caused by a chemical oscillator will induce autonomous pH-sensible polymer chain movements at the membrane interface, causing continuous pore-size oscillation cycles and thus a self-oscillating flux. This work involved the functionalisation of polyethersulfone commercial membranes to achieve pore size oscillations. The pH-sensitive polymer, poly(methacrylic acid) (PMAA), was obtained by deprotection of poly(tert-butyl methacrylate) (PtBuMA) synthesized by RAFT polymerization. To adapt this functionalisation to all types of commercial membranes, a thin layer of polydopamine (PDA) was deposited on the top of the membrane, which then allows a Michael-thiol-ene reaction between PDA and thiol-functionalized PMAA, obtained from prior aminolysis of PtBuMA. The functionalisation steps were characterized by XPS, SEM, water contact angle, and permeability measurements. Membranes were then placed in a filtration system containing a chemical pH oscillator to control the PMAA chain conformation through the pH cycle. Water permeation analysis showed a dependence between permeability and PMAA conformation, leading to the conclusion that there is indeed a continuous oscillation in membrane pore size.