Functionalization of hydrogen-terminated silicon with polybetaine brushes via surface-initiated reversible addition-fragmentation chain-transfer (RAFT) polymerization

Controlled grafting of polybetaine brushes onto hydrogen-terminated Si(100) substrates (Si-H substrates) was carried out via surface-initiated reversible addition-fragmentation chain-transfer (RAFT) polymerization. The azo initiator was immobilized on the Si-H surface through a three-step process: (i) coupling of an omega-unsaturated alkyl ester to the Si-H surface under UV irradiation, (ii) reduction of the alkyl ester into a hydroxyl group by LiAlH4, and (iii) esterification of the hydroxyl group with the initiator 4,4'-azo-bis(4-cyanopentanoic acid) after acid chlorination. In the presence of the chain-transfer agent, RAFT-mediated polymerization of the sulfobetaine monomer N,N'-dimethyl(methylmethacryloyl ethyl) ammonium propane sulfonate (DMAPS) was initiated from the surface-immobilized azo initiator to produce DMAPS polymer (PDMAPS) brushes on the silicon substrate (Si-g-PDMAPS). X-ray photoelectron spectroscopy (XPS) analysis indicated that the PDMAPS brushes had been successfully grafted onto the silicon surface. Atomic force microscopy (AFM) images revealed changes in the surface topography of the silicon substrates and the presence of polymer overlayers. Ellipsometry results indicated that the thickness of the polybetaine film increased linearly with the polymerization time. The living characteristics of the end functionality of the polybetaine brushes from the RAFT-mediated process was ascertained by block copolymerization with the anionic monomer sodium 4-styrene sulfonate (SS) to form the diblock polymer brushes (a Si-g-PDMAPS-b-PSS surface).