Nonporous membranes of self-assembled graft copolymers for chemoselective separations of small organic molecules


Chiara Vannucci


Chiara Vannucci, Ayse Asatekin

Author Affiliation: 

Tufts University


Polymeric membranes are a promising molecular separation technology, but current filtration membranes separate solutes by size, compounded in some cases by charge effects. They cannot effectively separate small organic molecules of similar size. In contrast, biological transport systems such as ion channels, porins and nuclear pore complexes exhibit exceptional separation selectivity based on specific interactions between the membrane nanochannels and a target solute. These systems combine two important structural features to achieve high selectivity: 1. Constricted pores similar in diameter to the target compound 2. Functional groups lining the pore that interact with the target during passage. We mimic this structure by designing novel amphiphilic graft copolymers that microphase separate to form ~1-nm wide nanochannels due to the immiscibility of the backbone and side-chains. Our hypothesis is that the nanoconfined interactions between the functional groups on the side chains and the solute permeating through them could be leveraged to achieve chemoselective diffusion. We synthesized a poly(vinylidene fluoride)-based graft-copolymer with aromatic-group functionalized side-chains. Solutions of this polymer are cast onto a glass plate with a casting knife and immersed in a non-solvent where the membrane is coagulated by phase inversion. Upon coagulation, microphase separation occurs between the backbone and side chains, that allows the formation of a percolated network of nanochannels, through which only the target solute of suitable size can selectively permeate and interact. Membrane selectivity is probed via diffusion tests of solute with similar size and shape but different chemical properties (e.g. aromatic and non-aromatic). Diffusion kinetics is followed with UV spectrophotometry and GCMS. We found that small organic molecules containing aromatic rings diffuse faster than non-aromatic molecules. For aromatic compounds a linear correlation between the diffusion rates and the Hansen solubility parameters was also found. We also demonstrated that the membrane nanostructure plays a crucial role in the transport process, as the selectivity of the copolymer membrane is more than twice higher than a homogeneous homopolymer membrane prepared by crosslinking the side-chain monomers. These copolymer films are promising first examples of membranes that separate small molecules based on their chemical features and nanostructure.