L. C. Brinson, Northwestern University S.K. Kumar, L. Schadler, RPI I. Beyerlein, Antonio Redondo, Los Alamos

200 word abstract:

We propose an integrated experimental and theoretical program to characterize and model the mechanical behavior of carbon nanotube (NT) reinforced polymer composites across length and time scales spanning from the atomic to macroscopic. Our focus is to understand the deformation and failure mechanism in these materials and specifically how those are impacted by the chemistry and morphology of the NTs. We will intertwine molecular scale models, coarse graining, micromechanics and continuum mechanics to address these issues. The models will be closely coupled to experiments at several length/time scales. A hierarchical approach will allow us to assess the importance of different types of chemical interactions between NT and polymer and different types of NT morphologies on deformation from the elastic range through fracture and failure. This work will help guide the design and development of nanocomposites in general. Applications of nano-composites with designer properties are far-reaching, including aerospace structures, medical devices, and coatings. Results from this work will define new durable, lightweight materials for use in structures from the very small (MEMS and chips) to the very large (spacecraft). Even more broadly, the techniques that are developed to link length scales will be applicable across a broad range of materials modeling.

Project Report

NSF-NIRT Polymers reinforced with very small fractions of nanoscale particles such as carbon nanotubes create new kinds of materials with outstanding and useful properties. For example, light-weight plastics and coatings can be made conductive; typically brittle polymers can be made more ductile and durable; flexible electronics can be created. However, the geometrical location and shape of the nanotubes inside the polymer matrix have great influence on the resulting material properties and in order to design optimal new materials, the influence of the nanotube morphology on properties needs to be better understood. In addition to nanotube shape, the local properties of polymer near to nanotube surfaces can be altered due to both chemical and physical interaction with the nanoparticles, forming a regime of polymer called interphase polymer. Because of the large surface-volume ratio of nanoparticles, the volume fraction of interphase polymer can be quite large and dramatically affect the overall properties. Therefore, in this project, we undertook the task of examining the effect of both nanotube shape/arrangement and interphase polymer properties on overall thermomechanical and fracture properties of nanocomposites. In nanocomposites, the nanotubes are typically curved and entangled as opposed to the straight, parallel fibers in traditional composites. Therefore, we examined the effect of nanotube curvature and interface/interphase properties and their relationship to composite properties. Our work was a combined multi-scale experimental and theoretical effort to unravel the underlying mechanisms of strength and toughness properties of nanocomposites. The work included molecular dynamics modeling, micromechanics, continuum and finite element analyses as well as experimental synthesis and characterization at a range of length scales to achieve our goals. Key findings of the work include establishment of the conditions for fracture mechanics of curved and entangled nanotubes in polymers. Results show that increased waviness and longer nanotubes can improve fracture toughness, however if the nanotubes fail during crack bridging the improvements can be negated, indicating a preference for high strength nanotubes. The gradient of interphase properties from the nanotube surface into the bulk polymer and extent of the interphase zone was shown to have great impact on overall properties. The concept of "percolated" interphases was established to explain large shifts in glass transition temperature that have been observed experimentally. Clustering of nanoparticles was shown to decrease thermal property improvements and the aspect ratio of nanotubes was demonstrated to lead to changes in the nanotube network formed and thus on mechanical properties. The influence of surface treatment of the nanotubes was shown to impact the interphase properties and extent, and thus the overall material performance. Interphase properties were ascertained by a novel nanoindentation methodology. Molecular modeling supported the findings regarding interphase formation and percolation. Details of the findings and developed tools can be found in many publications resulting from this project and listed in the final report. Additionally, data sets developed are available in those papers and in doctoral theses. Overall, the results have led to development of new modeling methods ranging from molecular to micromechanics to continuum, which can be employed to predict properties of nanoparticle-polymer configurations and assist with nanocomposite design. Additionally, new experimental techniques were developed to improve nanotube structure formation in these materials and the accurately measure local and bulk properties. These new methods and tools have led to both deeper understanding of polymer-nanotube systems, development of new materials with outstanding properties, and ability to use the knowledge and techniques for design of future material systems.

Project Start
Project End
Budget Start
2004-09-01
Budget End
2011-08-31
Support Year
Fiscal Year
2004
Total Cost
$1,354,100
Indirect Cost
Name
Northwestern University at Chicago
Department
Type
DUNS #
City
Evanston
State
IL
Country
United States
Zip Code
60201