Merlin Theodore, Ph.D. (2008)
Dissertation Topic: Studies on the Kinetics and Characterization of Epoxy Resin Reinforced with a Variety of Functionalized Multiwall Carbon Nanotubes
Major Professor: Dr. Mahesh Hosur, Professor of Materials Science & Engineering
M.S.: Mechanical Engineering, Tuskegee University
B.S.: Chemical Engineering, Tuskegee University
Employment: BMW, Moses Lake, Washington
The effect of functionalization on the multi-wall carbon nanotube (MWCNT) structure was studied as well as the effects of these functionalized MWCNTs on the curing process, decomposition process, morphological, thermal, thermo-mechanical, mechanical and electrical properties of an epoxy based nanocomposite system.
Covalent functionalization of the MWCNTs was achieved by oxidation (MWCNT-COOH), direct-fluorination (MWCNT-F), and amino-functionalization (MWCNT-NH2). Raman Spectroscopy quantified the disorder on MWCNT structure as a result of functionalization. The highest disorder was observed in the MWCNT-F at 2.00, followed by the MWCNT-NH2 at 1.78, then the MWCNT-COOH AT 1.06. FT-IR confirmed that the MWCNT surface contained the hydroxyl and ester group, C-F bond, and C-N bond as a result of oxidation, direct-fluorination, and amino functionalization, at their expected wave numbers, respectively.
Functionalized MWCNTs were used at a 1 wt % loading with 100:24 of Epon 862/Epicure-W to produce the epoxy-based nanocomposites. Functionalization not only improved the dispersion of MWCNTs in the polymer matrix but also enhanced the overall properties of the epoxy composite system. The nanocomposites showed a 33 – 59% increase in the flexural strength and 16 – 55% increase in flexural modulus in functionalized systems compared to the neat epoxy system. Dynamic mechanical analysis (DMA) results showed that only the MWCNT-F nanocomposites had an enhancement in glass transition temperature (Tg) of 12 ºC. The crosslink density of all systems increased by a factor of 2 or more with MWCNT-NH2 nanocomposite showing highest crosslink density that increased by a factor of 3 when compared to the neat. Coefficient of thermal expansion decreased for all systems which correlated well with their increase in crosslink density. The MWCNT-F and MWCNT-NH2 nanocomposite system were the only systems to form percolated networks that transferred an electric current. Electrical conductivity decreased for the MWCNT-F nanocomposites but increased for the MWCNT-NH2. SEM micrograph showed enhancements in interfacial interactions and dispersion of the MWCNTS in the matrix.
Kinetic studies indicated that the MWCNT-COOH and MWCNT-NH2 catalyzed the reaction causing an increase in reaction rate and enthalpy of reaction. MWCNT-F decreased the reaction rate, simply because the MWCNT-F has a secondary reaction with the polymer matrix. The MWCNT-COOH and MWCNT-NH2 react directly with the polymer matrix and not the curing agent in the initial stages of curing. Despite the effects on curing process, all systems achieve the same degree of conversion. Fourier Transfer-Infra Red (FT-IR) spectroscopy was also used as the second method to determine the degree of conversion of these systems and the results generated were agreeable with DSC results. The functionalized MWCNTs had no effects on the initial decomposition temperature which is observed at 350 ºC.