Controlled grafting of polymer brush layers from porous cellulosic membranes

Abstract

Dense polymer brushes on porous supports present powerful routes toward fouling-resistant coatings and ultra-thin, mechanically-stable selective layers. However, characterization of polymer chains grafted from nonideal, planar substrates often relies on indirect methods to determine the length, density, and location of polymeric brushes. In this study, we explore new methods to control and characterize the properties of polymer brush layers. Starting from hand-cast cellulose films and commercial cellulose membranes, we use surface-initiated atom transfer radical polymerization to graft polymeric chains. We tailor brush density and simultaneously stabilize the support by varying the proportions of initiator and crosslinker esterified to the cellulose substrate. On films with grafted polyacrylic acid (PAA) chains, we use the silver-binding method to determine brush density. We analyze brush growth behavior by directly cleaving chains from the surface of films for analysis by size exclusion chromatography, uniquely observing the divergence of two distinct populations of differing length beyond a certain molecular weight. Such behavior would be important to consider if the alignment of block copolymer layers is absolutely necessary in an application. By controlled contact of polymerization solution with the top surface, we show that brush growth can be partially directed to the top surface for asymmetric cellulose membranes with relatively low molecular weight cutoffs. In a semi-quantitative method to locate brush growth, depleted uranium was bonded to grafted PAA chains, and targeted emission x-ray spectroscopy was conducted for comparison of uranium content in various regions. Finally, we used our developed methods and findings to inform the synthesis of homopolymer and diblock copolymer brush layers, which we then used as selective layers in pressure-driven filtration and diffusion cell experiments.