“Grafting-Through”: Growing Polymer Brushes by Supplying Monomers through the Surface

We introduce a “grafting-through” brush polymerization mechanism where monomers are supplied through the surface on which the initiators are attached rather than from solution as in the “grafting-from” technique. This is accomplished by attaching the initiator to the surface of a dialysis membrane and supplying monomers through the membrane to the growing brush. This avoids the growth of very long chains while promoting the growth of shorter chains by reversing the monomer concentration gradient found in the commonly used grafting-from technique, where monomer concentration is lowest at the substrate and highest in the surrounding solution. Reversing this monomer concentration gradient results in shorter chains experiencing a higher local monomer concentration than longer chains, thus speeding up their growth relative to the longer ones. It is shown by AFM that brush layers made by this method are thicker and have lower roughness than brushes made by a grafting-from approach. Coarse-grained molecular dynamics simulations of brush polymerizations with monomers supplied through a permeable substrate provide insight into the mechanism of the grafting-through brush growth process. Simulations show that it is possible to obtain a brush layer with a chain dispersity index approaching unity for sufficiently long chains. FTIR, contact angle measurements, SEM, and kinetic studies are used to characterize and elucidate the growth mechanism of brushes synthesized by the new grafting-through strategy.