Journal Club Theme of 1 May 2008: Mechanical Behaviors of Polymer-matrix Nanocomposites

1. Definition of nanocomposites Nanocomposites are a novel class of composite materials whose reinforcements have dimensions in the range of 1-100 nm. Although nanoscale reinforcements (or nanofillers) of nanocomposites have different kinds of fillers such as nanofibers, nanowires, nanotubes and nanoparticles etc, their mechanical behaviors have some common features. Figure 1 shows a potential use of nanocomposites as multifunctional materials. Since many important chemical and physical interactions are governed by surfaces and surface properties, and nanoscale reinforcements have a large surface area for a given volume, nanocomposites are ideal multifunctional materials. For nanotube-reinforced materials, coupled mechanical and electric properties of nanotubes can be used for very small-scale health monitoring.

In terms of matrices, polymer-matrix is commonly used for nanocomposites.  Since this J-club theme is focused on mechanical behaviors and analysis, our discussions are still helpful to understand the mechanical behaviors of nanocomposite materials with other matrices such as ceramics and metals.  For reviews of general nanocomposites, I would refer you to,

• Thostenson, E. T., C. Li, T.-W. Chou, 2005, Nanocomposites in context, Composites Science and Technology, 65, 491-516
• Hussain, F., Hojjati, M., Okamoto, M, Gorga, R.E., 2006, Polymer-matrix nanocomposites, Processing, Manufacturing, and Application: An Overview, Journal of Composite Materials, 40, 1511-1575.

2. Connections with other J-club themes

This J-club theme is closely related to two previous J-club themes: Xiaodong Li's May 2007: Experimental Mechanics of Nanobuilding Blocks, and Zoubeida Ounaies's Jan. 15 2008: Active Nanocomposites. The previous two discussion leaders provided excellent insight into the material aspects of nanocomposites, while the present discussion tries to analyze some special mechanical behaviors from the solid mechanics viewpoint. Any overlap is minimized and the readers may read the two previous themes to obtain a complete understanding of mechanical behaviors of nanocomposites.

3. Special mechanics phenomena of nanocomposites

Traditional composite materials used as structural materials are continuous fiber-reinforced composites (carbon fiber/epoxy etc), which are different from nanocomposites in terms of reinforcements. Here, only unique mechanical behaviors of nanocomposites resulting from their special discontinuous reinforcements and interfaces will be discussed. 

a) Stiffness improvement and nanofiber/tube waviness effects

After very stiff nanotubes or nanofibers (Young's modulus E~1000GPa) are added into soft polymer matrices such as epoxy (E~4GPa), the stiffness of the nanocomposite should be increased.  However, the composite stiffness is often below classical micromechanics predictions  because the nanofibers/nanotubes are often curved inside the matrix due to their very high aspect ratio (Figure 2 shows a TEM image of uniformly distributed nanofibers inside an epoxy matrix from our previous work). New micromechanics model was proposed to analyze this special phenomenon:

• Luo, J. J. I. M. Daniel, 2003, Characterization and Modeling of Mechanical Behavior of Polymer/Clay Nanocomposites, Composites Science and Technology, 63, 1607-1616.
• Fisher, F. T., Bradshaw, R. D., Brinson, L. C.  2003. Fiber Waviness in Nanotube-Reinforced Polymer Composites: I. Modulus predictions using effective nanotube properties, Composites Science and Technology, 63, 1689-1703.
• Shi, D. L., X. Q. Feng, Y. Huang, K. C. Hwang, and H. Gao. 2004, The Effect o f Nanotube Waviness and Agglomeration on the Elastic Property of Carbon Nanotube-reinforced Composites, Journal of Engineering Materials and Technology, 126, 250-257,

b) Strength and failure mechanics

Two key parameters for engineering  materials--tensile strength and mode-I fracture toughness of the nanocomposite are not as high as we would expect. The fracture toughness of the nanocomposites is slightly higher than that of the baseline epoxy matrix, but sometimes it is even less than that of the pure epoxy!  As seen in Figure 3, for tensile experiments on nanofiber/PEEK composites, with the increase in nanoscale reinforcements, the Young's modulus of the nanocomposites will increase (slope of the initial elastic region). Therefore, the final tensile strength is controlled by the failure strain. However, the failure strain of the nanocomposite significantly decreases with the increase of the weight percent of the nanofibers (from 5% to 15%).  So the tensile strength increase is limited.

>A major reason is that very strong nanotube/nanofibers inside nanocomposite materials are not fully loaded due to low efficiency of interfacial shear load transferring. Since the interfacial shear stress is related to the shear modulus of the matrix, a soft polymeric matrix only offers very limited load transferring from the matrix to the strong nanotube/nanofiber (should be better for ceramic and metal matrices due to their high stiffness properties).  Therefore, future nanocomposite materials for structural applications would require nanoscale reinforcements to carry load directly (continuous nanotubes or nanofibers, and aligned discontinuous nanotubes are not enough). These papers are very helpful:

• Sandler, J., Werner, P., Shaffer, M.S.P., Demchuk, V., Altstadt, V. and Windle, A.H., 2002. Carbon-nanofibre-reinforced poly(ether ether ketone) composites, Composites-- Part A: Appl. Sci. & Manufac., 33, 1033-39.
• Li, X.D., Gao, H.S., Scrivens, W.A., Fei, D.L., Xu, X.Y., Sutton, M.A., Reynolds, A.P., and Myrick, M.L., 2004, Nanomechanical characterization of single-walled carbon nanotube-reinforced epoxy composites.  Nanotechnology, 15, 1416-1423.
• Tsai, J., and Sun, C.T., 2004, Effect of platelet dispersion on the load transfer efficiency in nanoclay composites. Journal of Composite Materials, 38, 567-579.

c) Interface mechanics issue

It should be noticed that strong interfacial bonding (such as covalent binding) is a necessary condition, not a sufficient condition in order to increase the failure strength of nanocomposite materials due to the interfacial shear stress transferring mechanism for discontinuous nanofiber/nanotubes.  Indeed, the high mismatch in the elastic properties of the matrix and the nanoscale reinforcement (4GPa vs. 1000 GPa) will lead to interfacial debonding at the matrix and the nanotube/nanofiber end, when compared to traditional composites with much less stiffness mismatch (the stress singularity order is around -0.1 in our nanofiber/epoxy composites, compared to -0.5 for a traditional crack case).  In terms of mechanics modeling, since the smallest dimension of any nanoscale reinforcement is greater than 1 nm, continuum mechanics model is widely employed to analyze mechanical behaviors of nanocomposites, except for the interface mechanics case, when nanomechanics model is necessary:

•  Odegard, G.M.  R.B. Pipes and P. Hubert, 2004. Comparison of two models of SWCN polymer composites, Compos Sci Technol 64, 1011–1020
• Tan H, Jiang LY, Huang Y, Liu B, and Hwang KC, 2007.  The effect of van der Waals-based interface cohesive law on carbon nanotube-reinforced composite materials, Composites Science and Technology, 67, 2941-2946

d) Uncertain mechanical properties

There is always a large scatter in the strength and fracture toughness data of nanocomposites. This phenomenon might result from very large interfacial bonding area of nanocomposites compared to the same traditional composite materials with the same fiber/particle volume percents.  As a result, initial interfacial defects are easily induced in nanocomposites than traditional composites, and lead to a large scatter in nanocomposite failure strengths.

• Liu, W., Hoa, S.V., and Pugh, M., 2005, Fracture toughness and water uptake of high-performance epoxy/nanoclay nanocomposites. Composites Science and Technology, 65: 2364-73.

Here I briefly summarize major mechanical behaviors of nanocomposites (other properties such as impact and fatigue are not addressed). Indeed, I try to propose more problems for you to solve in the future. Hope more insight would be explored through discussion with iMechanica users. Some papers are uploaded as attachments if any user cannot access on-line papers.