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

L. Roy Xu's picture

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,

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

f2After 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:

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.

>Af3 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:

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:

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.

Zhigang Suo's picture

Dear Roy:  Thank you very much for this introduction.  I have not followed up on the literature on polymer-matrix composites.  You have descibed problems (or weaknesses) of these materials.  Do they have any unusually good aspects of mechanical behavior?

L. Roy Xu's picture

Dear Zhigang,

Thanks for your suggestion,
I added a few advantages for nanocomposites in my introduction part: "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."

Zhigang Suo's picture

Dear Roy:  Thank you.  Further questions:

  1. Do polymer-matrix nanocomposites have unusual mechnaical bahavior, such as better fatigue endurance?
  2. Do you have any referencecs on how people use polymer-matrix nanocomposites?  
L. Roy Xu's picture

Dear Zhigang,

Since nanocomposite is a very broad area, my knowledge is limited.My answers to your two questions:

1) Polymer-matrix nanocomposites do have some unusual mechanical behaviors, such as high hardness, improved thermal expansion
coefficients (compared to traditional composites). Obviously, these properties
are not key parameters for structural materials.

2) I don't know any application of nanocomposites yet. I hear nanocomposite coating
is close to ship applications (no reference). 

Hope iMechanica  readers and users will find more....

MichelleLOyen's picture

I should point out these materials are directly analagous to many biological composite materials, such as bone and nacre, in having composite phases with a large modulus mismatch and features at the nano-scale.  There are many biomimetics applications possible in this context.  This is also a place where the discussion of local variability and its effects as a potential toughening mechanism have been discussed, see, for example,

Tai K, Dao M, Suresh S, Palazoglu A, Ortiz C, 2007. Nanoscale heterogeneity promotes energy dissipation in bone . Nature Materials 6, 454-462. 

Such arguments have suggested that local inhomogeneity is a good thing--agglomeration of phases and other traditionally "non-ideal" mixing behavior might actually be useful in the right circumstances.  This conclusion may rely, however, on materials with a very high volume fraction of the "filler" phase compared with the polymer "matrix". 

Konstantin Volokh's picture

Dear Roy,

The scattering of the fracture toughness may be affected by the notch sharpness –see:  



L. Roy Xu's picture

Dear Kosta,

I read these two interesting papers and totally agree with your conclusions. For any
fracture toughness measurement, a "mathematically sharp" initial crack and a plane-strain
condition are two key issues. For polymer specimens (also for polymer-matrix
nanocomposites), we usually use a razor blade to get a "sharp crack" (indeed, a
sharp notch) with a tip radius of the order of tens of microns. But we cannot
guarantee that each specimen has the same notch radius. Therefore, the fracture
toughness scattering of polymers should be larger than that of metals (using an
initial crack from fatigue-a "natural" crack).

However, the fracture toughness scattering for polymer-matrix nanocomposites is much
larger than that of polymers. I believe the major reason to cause mechanical property
uncertainty is due to the special material configurations of nanocomposites,
such as large interface areas to induce initial defects during material processing.  Notch sharpness might be the second factor.

Arun K. Subramaniyan's picture

The relationship between toughness and the relevant microstructure length scale pointed out in the paper is interesting. We reported a similar behavior in nanoclay reinforced composites and a quantitative method for finding the validity of stress intensity factor as a measure of fracture toughness in one of our papers: Composites Part A: Applied Science and Manufacturing Volume 38, Issue 1,
January 2007,
Pages 34-43, doi:10.1016/j.compositesa.2006.01.021

L. Roy Xu's picture

Dear Teng,

 Yes. I noticed this discussion and we're including this topic
in this J-club discussion.

Xiaodong Li's picture

Thanks Roy for posting this theme in JClub, very timely. In a lot of cases, nanoreinforcements like nanotubes and nanowires form bundles (agglomeration). Only the outer-layer nanotubes in the bundle are really carry the load from the matrix. For nanoclay reinforced composites, blocks of nanoclays were found. In each block, there were about ten nanoclays stacking together. This may be why the mechanical properties of nanocomposites are sometimes lower than the theorectically predicted values. Team work (experiments and modeling) are greatly needed.

Xiaodong Li, Hongsheng Gao, Wally A. Scrivens, Dongling Fei, Xiaoyou Xu, Michael A. Sutton, Anthony P. Reynolds and Michael L. Myrick, "Nanomechanical Characterization of Single-Walled Carbon Nanotube-Reinforced Epoxy Composites," Nanotechnology,15 (2004) 1416-1423. 

Xiaodong Li, Hongsheng Gao, Wally A Scrivens, Dongling Fei, Xiaoyou Xu, Michael A Sutton, Anthony P Reynolds, and Michael L Myrick, "Reinforcing Mechanisms of Single-walled Carbon Nanotube- Reinforced Polymer Composites, " Journal of Nanoscience and Nanotechnology, 7 (2007) 2301-2308.

Xiaodong Li, Hongsheng Gao, Wally A. Scrivens, Dongling Fei , Michael A. Sutton, Anthony P. Reynolds and Michael L. Myrick, "Structural and Mechanical Characterization of Nanoclay-Reinforced Agarose Nanocomposites," Nanotechnology, 16 (2005) 2020-2029.

L. Roy Xu's picture

Dear Xiaodong,

Agglomeration is another factor to reduce mechanical properties of nanocomposites. I didn't mention this issue in my introduction part since it was discussed in Zoubeida Ounaies's Jan. 15
2008 J-Club theme: Active
 (Try to minimiz  overlap among different J-Club themes)

Dear Roy,Thank you very much for posting this nice theme. Could you please give me some information about recent modeling techniques based on continuum mechanics approach for predicting the overall mechanical properties of such nanocomposites (especially CNT/polymer composites)? In addition, please tell me your comments about the validity of such techniques in nano scale. Kind regards,

L. Roy Xu's picture

Dear Kazem, 

You may check Prof. Yong Huang's website on recent modeling on nanocomposites (I also attached his papers in this discussion).

Using today's experimental techniques, we are not able to measure the nanoscale displacement or force inside
nanocomposites. So modeling and simulations are very important.

Liying Jiang's picture

Dear Kazem,We have some work related to the continuum modeling of the mechanical properties of nanocomposites. Since the nanocomposites surface aspect ratio is drastic high (i.e., high interface area per unit volume of the composite), the behavior of the CNT/matrix interfaces may significantly influence the macroscopic behavior of composites.  However, modeling of CNT/polymer interfaces has always been a challenge because it is difficult to account for the van der Waals force in continuum models. We have developed a cohesive law for CNT/polymer matrix interfaces based on the van der Waals force. This atomic-based continuum model accurately accounts for the vdW forces in the continuum model and avoids any assumed phenomenological cohesive law.  We applied this simple, analytical cohesive law to study the mechanical behavior of carbon nanotube reinforced polymer composites. The results clearly demonstrated how the interfaces and CNT sizes would affect the macroscopic behavior of the composites. Our work provided a direct link between the overall mechanical properties and the nanoscale behavior of the interfaces. The related papers are the follows, hope they are helpful:

·         Jiang, L.Y., Huang, Y. G., Jiang, H., Ravichandran, G., Gao, H., Hwang, K.C. and Liu, B., 2006, A cohesive law for carbon nanotube/polymer interfaces based on the van der Waals force. Journal of the Mechanics and Physics of Solids, 54: 2436-2452.

·     H. Tan, L. Y. Jiang, Y. Huang, B. Liu and K. C. Hwang, The effect of van der Waals-based interface cohesive law on carbon nanotube-reinforced composite materials, Compos. Sci. Technol, 67: 2941-2946.

For the development of applications for nanostructured materials and composites the key is in understanding the structure/size dependence of the properties of nanomaterials on the bulk properties.  For example, the structure influences both the electrical and mechanical behavior.

E. T. Thostenson and T-W. Chou, "Processing-Structure-Multi-Functional Property Relationship in Carbon Nanotube/Epoxy Composites," Carbon, 44(14) 2869-3148 (2006).

Basic understanding of the interrelationship between the micro/nano structure (constituent materials, interface/interphase, dispersion, etc.) and the coupling of mechanical and electrical behavior can lead to the development of novel multifunctional composites.  An example of a unique multifunctional composite is the development of nanotube-based techniques for in situ health monitoring and detection of defects in fiber composites.  Here the nanoscale reinforcement is a small volume fraction but imparts new mechanical/electrical functionality.

E. T. Thostenson and T-W. Chou, "Carbon Nanotube Networks: Sensing of Distributed Strain and Damage for Life Prediction and Self-Healing," Advanced Materials, 18(22) 2837-2841 (2006).

E. T. Thostenson and T-W. Chou, "Real-Time in situ Sensing of Damage Evolution in Advanced Fiber Composites using Carbon Nanotube Networks," Nanotechnology, 19(21) 215713 (2008).

Frank Fisher's picture

Roy- Thanks for starting a very interesting discussion on polymer nanocomposites! I enjoyed going through the comments and the references included within. As a few others have mentioned in this post, a significant challenge in this area is understanding how how the significant surface area present within the nanocomposite can alter the properties of the polymer matrix (and how this influences the overall behavior of the nanocomposite). For example, it is well known that polymer behavior can change dramatically when interacting with a surface (as shown in the extensive thin film polymer and polymer brush literature, respectively). We've previously shown that through careful processing of the polymer nanocomposite we can facilitate an adhered layer of polymer on the nanotube surface, which can drastically enhance the overall response.

A. Eitan, F.T. Fisher, R. Andrews, L.C. Brinson, and L.S. Schadler (2006). "Reinforcement mechanisms in MWCNT-filled polycarbonate”, Composites Science and Technology, 66, 1159-1170

More recently, we've become interested in semi-crystalline polymer nanocomposites, where the added complication of changes in the crystallinity of the polymer matrix now also come into play. This adds the question of how to differentiate reinforcement provided by the nanoparticle with respect to property changes due to changes in crystallinity. For example, often is it reported that property enhancements are realized with small loadings of nanoparticles, and then these enhancements 'disappear' for higher nanoparticles loadings... changes in crystallinity follow a similiar behavior (i.e. large changes in crystallinity for small nanoparticles loadings, with further changes in crystallinity plateauing after a certain nanoparticle loading). Even further complicating the issue (ugh!), crystallinity changes can be very sensitive to the thermo-mechanical history experienced by the nanocomposite during processing.

G. Mago, D.M. Kalyon, and F.T. Fisher (2008). “Deformation induced crystallization and associated morphology development of carbon nanotube - PVDF nanocomposites, Journal of Nanoscience and Nanotechnology, in press. (hopefully to appear soon)

Considered from a different perspective, however, this offers the possibility of adding nanoparticles in such a way as to control crystallinity in polymer nanocomposites. For example, we've shown that adding nanotubes (under certain processing conditions) can provide some control of the crystal morphology present within the nanocomposite; for example, facilitating the desirable beta-phase in the piezoelectric polymer PVDF.

G. Mago, D.M. Kalyon, and F.T. Fisher (2008). “Membranes of Polyvinylidene fluoride (PVDF) and PVDF nanocomposites with carbon nanotubes via immersion precipitation”, Journal of Nanomaterials (special issue on Nanomechanics and Nanostructured Multifunctional Materials), Article ID 759825. doi:10.1155/2008/759825

I agree with Erik's comment that multifunctionality is a key factor when discussing the potential of polymer nanocomposites.

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