Surface Modification of Silica Nanoparticles
Surface modification of nanosized silica particles by polymer grafting is gaining attention. This can be attributed to the fact that it provides a unique opportunity to engineer the interfacial properties of these modified particles; at the same time the mechanical and thermal properties of the polymers can be improved. Controlled free radical polymerization is a versatile technique which affords control over molecular weight, molecular weight distribution, architecture and functionalities of the resulting polymer. Three commonly used controlled free radical polymerizations include nitroxide- mediated polymerization (NMP), atom transfer radical polymerization (ATRP) and reversible addition fragmentation transfer (RAFT) polymerization. ATRP and RAFT polymerization were explored in order to modify the silica surface with well-defined polymer brushes.
A novel click-functionalized RAFT chain transfer agent (RAFT CTA) was synthesized which opened up the possibility of using RAFT polymerization and click chemistry together in surface modification. Using this RAFT CTA, the surface of silica nanoparticles was modified with polystyrene and polyacrylamide brushes via the “grafting to” approach. Both tethered polystyrene and polyacrylamide chains were found in the brush regime. The combination of ATRP and click chemistry was also explored for surface modification.
A combination of RAFT polymerization and click chemistry was also studied to modify the surface via the “grafting from” approach. Our strategy included the (1) “grafting from” approach for brush formation (2) facile click reaction to immobilize the RAFT agent (3) synthesis of R-supported chain transfer agent and (4) use of the more active trithiocarbonate RAFT agent. Grafting density obtained by this method was significantly higher than reported values in the literature.
Polystyrene (PS) grafted silica nanoparticles were also prepared by a tandem process that simultaneously employs reversible addition fragmentation transfer (RAFT) polymerization and click chemistry. The click reaction doesn’t interfere with RAFT polymerization. With a suitable choice of a Cu(I) catalyst, it is possible to perform both RAFT polymerization and click chemistry together. In a single pot procedure, azide-modified silica, an alkyne-functionalized RAFT agent and styrene were combined to produce the desired product. As deduced by thermal gravimetric and elemental analysis, the grafting density of PS on the silica in the tandem process was intermediate between the “grafting to” and “grafting from” techniques. Relative rates of RAFT polymerization and click reaction were altered to control grafting density.
ATRP was also used to modify the surface of silica nanoparticles via the “grafting from” approach. The surfaces of silica with homopolymers and diblock copolymers brushes were modified using surface initiated ATRP. The polymer grafted silica particles were characterized by FT-IR, TGA, XPS and elemental analysis.
School:The University of Akron
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:silica nanoparticle polymer brush surface modification grafting density hybrid nanopartcles
Date of Publication:01/01/2008