Title

Well defined polymer nanoparticle hybrid materials by surface initiated polymerization from silica supports

Date of Completion

January 2004

Keywords

Chemistry, Organic|Chemistry, Polymer|Plastics Technology

Degree

Ph.D.

Abstract

Hybrid organic-inorganic nanocomposites have been synthesized consisting of a silica core surrounded by a polymer shell. Formation of uniform silica nanoparticles of controlled size and circular shape was accomplished using the method of Stöber. The resulting nanoparticles range in size from 15 to 450 nm in diameter. Reactive functionality was grafted to the particle surface using multiple types of silane coupling agents resulting in multilayer and monolayer initiator coatings. Careful selection of the nature of the surface coating allowed for the formation of nanoparticles which could be dispersed in solvents of various polarities and allowed the further attachment of different surface functionalities. The density of the surface functional groups was varied using a sequential reaction sequence in which a fraction of the particle surface was first passivated with an inert coupling agent followed by reaction of the remaining sites with the desired functional silane. Polymerization from the surface was conducted by ring opening metathesis (ROMP) and by a transition metal mediated polymerization. Characterization of the resulting nanocomposites was accomplished using nuclear magnetic resonance (NMR), thermogravimetric analysis (TGA), gel permeation chromatography (GPC), pyrolysis gas chromatography mass spectrometry (PY-GCMS), Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), and dynamic light scattering (DLS). In composites formed by ROMP, the molecular weight and polydispersity of the surface chains were controlled by careful choice of reaction time and monomer concentration, and the size of the inorganic core was varied from 15 to 100nm. The chemical structure of the nanocomposites was thoroughly investigated including the determination of the number of initiator groups, catalyst molecules, and polymer chains per particle. Variation of the number of polymer chains per unit area was also accomplished. A transition metal mediated polymerization based on a titanium trichloroalkoxide was used to form a second class of composites by polymerization of n-hexyl isocyanate from initiators tethered to the surface of silica nanoparticles. Characterization of the resulting material revealed a significant increase in size which was not attributable to a change in particle aggregation state. Additional analytical results were consistent with the conclusion that a nanocomposite had been formed in the presence of unattached polymer chains. ^