Title

Synthesis and characterization of multi-phase materials at the nanoscale

Date of Completion

January 2006

Keywords

Chemistry, Polymer

Degree

Ph.D.

Abstract

We have prepared 2D polymer nanocomposites from synthetic silica nanowafers via surface-initiated polymerization (SIP). The silica nanowafers were norbornene-functionalized, inorganic-organic materials of magadiite-like structure synthesized using a hydrothermal-silylation process. The organic moieties were covalently bound to the inorganic substrate, in contrast to conventional organoclay-nanocomposites involving primarily ionic interactions. The functionalized nanowafers were modified with Grubbs catalyst. Polymer was then grafted from the nanowafer via ring-opening metathesis polymerization (ROMP). This is the first report, that we are aware of, of a nanocomposite prepared using SIP (the "grafting from approach") from initiators that are covalently bound to a planar nanostructure. Chemical and structural characterizations of the nanowafers and nanocomposites were done using X-ray diffraction (WAXD), infrared and H1 NMR spectroscopy, thermogravimetric analysis, atomic force microscopy and transmission electron microscopy. X-ray diffraction data showed that the nanowafers were not fully exfoliated. Infrared and H1 NMR spectroscopy verified the chemical structures of the nanowafers and polymer nanocomposites. Thermogravimetric analysis confirmed that polymer chains grow with increasing reaction time and that they are covalently bound to the inorganic substrates. Transmission electron microscopy provided the morphology of the substrate and product. Polymer appeared to be chemically bound to the surface of the nanoplatelets and not just physically adsorbed. Variation of the surface coverage using an alkane spacer was also possible. Polymer nanocomposites were prepared from these multi-functionalized substrates. Polymer growth was monitored using FTIR and TGA. Grafted polymer was successfully isolated and further characterized for molecular weight, polydispersity and grafting density. The calculated grafting densities were relatively high and fall within the polymer brush regime. AFM was used to evaluate the mechanical properties of the nanocomposites. The calculated moduli of the polymer were significantly lower than that of the silicate substrate. The nanocomposites and nanowafers differed in surface elastic properties. The measured Tg of nanocomposites with lower grafting density was lower than that of the neat polymer. This was attributed to the larger free volume for polymer chain motions since the polymer chains are farther apart from each other.^