(604b) Average and Local Distribution of Glass Transition Temperatures of Pmma-Swcnt Nanocomposites Studied Via Fluorescence

Polymer nanocomposites are the subject of increasing interest due to the unique properties that can be achieved with them. The presence of nanofillers in the polymer matrix can drastically alter the chain segmental mobility and limit the number of conformations of the polymer, changing the properties of the system. Previously [1], it was shown that addition of silica and alumina nanoparticles to the polymer matrix modifies the average Tg of the polymer. Here, both local and average values of Tg are characterized in polymer nanocomposites with single-walled carbon nanotubes (SWCNT). This is the first study to determine discrete distribution of Tg in polymer-nanofiller nanocomposites.

Single-walled carbon nanotubes functionalized with amide groups, via chemical modification of carboxyl groups introduced on the carbon nanotube surface [2], allow for a covalent bonding with poly(methyl methacrylate) (PMMA) [3] to generate PMMA/SWCNT hybrid nanoparticles that were used as nanofillers for bulk PMMA. Fluorescence was used to measure both the local Tg and the average Tg of the system. The local PMMA Tg was measured using 4-tricyanovinyl-[N-(2-hydroxyethyl)-N-ethyl]aniline (TC1)-labeled PMMA attached to the SWCNT; these labeled PMMA/SWCNT hybrid nanoparticles were embedded in a non-labeled PMMA matrix. The average Tg was obtained by including non-labeled hybrid nanoparticles in a pyrene-doped PMMA matrix. It is important to note that the labeled and non-labeled PMMA have approximately the same bulk Tg and molecular weight; therefore, the local and average measurements of Tgs can be compared.

The results for hybrid nanoparticles composed of 1 wt% of SWCNT in the polymer brushes and using a phenyl group in the amide functionalized SWCNT, show that the Tg (measured by DSC) of the PMMA brushes, labeled and non-labeled, was 412 K, representing an increase in Tg of approximately 21 K compared to neat PMMA. When 0.3 wt% of these hybrid nanoparticles are introduced in a PMMA matrix, an average Tg of 396 K is observed in the nanocomposite using fluorescence, approximately 5 K above that of neat, bulk PMMA. Also using fluorescence, the local Tg of the PMMA brushes attached to the SWCNT and influenced by the PMMA matrix is approximately 408 K, or about 17 K above the Tg of neat, bulk PMMA. This shows that the local Tg influenced by the PMMA matrix (408 K) is between the Tg of the hybrid nanoparticles (412 K) and of the average Tg of the nanocomposite (396 K). The local and the average Tg of systems with different loading levels of these PMMA/SWCNT hybrid nanoparticles are currently under study. The use of side groups of different sizes in the amide functionalized SWCNT to measure a discrete distribution of Tgs in the polymer brushes and the influence of their distance from the SWCNT surface will also be investigated.

[1] Rittigstein, P. & Torkelson, J. M. Polymer-Nanoparticle Interfacial Interactions in Polymer Nanocomposites: Confinement Effects on Glass Transition Temperature and Supression of Physical Aging. Journal of Polymer Science: Part B: Polymer Physics, in press.

[2] Ramanathan, T., Fisher, F. T., Ruoff, R. S. & Brinson, L. C. Amino-Functionalized Carbon Nanotubes for Binding to Polymers and Biological Systems. Chemistry of Materials 17, 1290-1295 (2005).

[3] Ramanathan, T., Liu, H. & Brinson, L. C. Functionalized SWNT/Polymer Nanocomposites for Dramatic Property Improvement. Journal of Polymer Science: Part B: Polymer Physics 43, 2269-2279 (2005).