(5k) Polymer Nanocomposites: From Hierarchical Ordering to Energy Application

Multifunctional polymer nanocomposites represent the next generation of advanced materials. Two critical issues in their implementation of applications are: a) understanding and tailoring their structure and consequently properties and; b) critically evaluate the processability of such materials. We studied the structure of single walled carbon Nanotubes (SWNT) based polymer nanocomposites and related it to their mechanical properties and processability. Using neutron scattering studies (performed at NCNR, NIST) we identified the hierarchical fractal network of SWNTs dispersed in polymer matrices. With increasing SWNT concentration, the network grows self-similarly, the mesh size (nanometer length scale) decreases weakly, the floc size (micron length scale) remain constant and the number of flocs increases linearly.[1] The network growth is primarily through the formation of new flocs and inter-floc interactions mediated by the polymer dominate the elastic strength of the system.

As a direct consequence of the self-similar fractal network, the linear flow properties display ?time-temperature-composition' superposition. This superposability can be extended for non-linear deformations when the non-linear properties are scaled by the local strain experienced by the elements of the network.[2] More interestingly, under steady shear these nanocomposites show network independent behavior. The absolute stress value is a function of the nanotube loading but the characteristic time scales related to the process are independent of it. For fully grown network in viscous polymer, cluster dynamics under external shear controls the non-linear behavior of the system.[3]

Polymer Nanocomposites: Advanced polymer electrolyte materials for Li ion batteries:

Inspired by the ability of lithium to intercalate between carbon nanotubes, we prepared a well dispersed (geometrical percolation at 0.09 vol % SWNTs loading with an effective aspect ratio ~ 650) lithium dodecyl sulfate (LDS) compatibilized SWNTs-PEO nanocomposite. These nanocomposites demonstrate significant change in the melting and crystallization behavior of the PEO along with a decrease in percent crystallinity.[4] Incorporation of nanotubes acts as solid confinements to the PEO chains which not only make the chains stiffer but also provide huge steric hindrance to the chain transport. This effect coupled with high crystallization activation energy barrier leads to dramatic slowing down in PEO growth kinetics and makes the system more amorphous.[5] The observed changes in the SWNTs based nanocomposites far exceed those observed for equivalent Li+ ion concentration mixture and indicate that the SWNTs and LDS synergistically reduce PEO crystallinity. This nanocomposite with aligned SWNTs (described next) has been proposed as an alternative polymer electrolyte for Li ion batteries.

Hierarchical Polymer Nanocomposites:

The influence of well dispersed and aligned single walled carbon nanotubes to template the growth of polymer crystals from the unit-cell to lamellae is examined towards developing strategies for producing hierarchical materials. For the case where nanotubes nucleate the polymer crystals, a ?shish?kebab? structure is realized with the nanotubes and polymer crystals acting as the shish and kebab respectively and results in nanocomposites that are highly reinforced.[6] On the other hand, for the case where the nanotubes disturb the formation of polymer crystals, the oriented nanotubes, because of the short inter-tube distances even at low nanotube concentrations, cause a templating of the polymer crystals with the lamellar?normals oriented orthogonal to the nanotube axes.

The research program that I plan to develop will largely build on my postdoctoral and doctoral research and specifically, I would like to develop a research program in the following areas:

1. Dynamics of polymer chains in presence of soft and hard confinement environment, effect of size and shape

2. Conjugated polymer based advanced materials morphology for organic photovoltaics application

3. Influence of colloid addition on the structure and properties of micellar solutions

References:

1.Chatterjee, T.; Jackson, A.; Krishnamoorti, R., Hierarchical structure of carbon nanotube networks. Journal of the American Chemical Society 2008, 130, (22), 6934-+.

2.Chatterjee, T.; Krishnamoorti, R., Dynamic consequences of the fractal network of nanotube-poly(ethylene oxide) nanocomposites. Physical Review E 2007, 75, (5).

3.Chatterjee, T.; Krishnamoorti, R., Steady shear response of carbon nanotube networks dispersed in poly(ethylene oxide). Macromolecules 2008, 41, (14), 5333-5338.

4.Chatterjee, T.; Yurekli, K.; Hadjiev, V. G.; Krishnamoorti, R., Single-walled carbon nanotube dispersions in poly(ethylene oxide). Advanced Functional Materials 2005, 15, (11), 1832-1838.

5.Chatterjee, T.; Krishnamoorti, R., Isothermal crystallization behavior of poly(ethylene oxide) in presence of carbon nanotubes. (manuscript under preparaion).

6.Chatterjee, T.; Mitchell, C. A.; Hadjiev, V. G.; Krishnamoorti, R., Hierarchical polymer-nanotube composites. Advanced Materials 2007, 19, (22), 3850-3853.