iMechanica - failure analysis - Comments

Matrix changes within polymer nanocomposites

Frank Fisher — Sun, 18 May 2008 15:14:51 -0400

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.

Multifunctional Nanocomposites

Erik T. Thostenson — Thu, 15 May 2008 15:06:54 -0400

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).

Paper link

Liying Jiang — Wed, 14 May 2008 12:18:03 -0400

Sorry, the link to the first paper does not work. Here is the link again.

· 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.

Dear Kazem,We have some work

Liying Jiang — Wed, 14 May 2008 12:06:28 -0400

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:

· 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.

Toughness and Crack tip sharpness in nanocomposites

Arun K. Subramaniyan — Sun, 11 May 2008 01:17:18 -0400

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

Reply to Kazem on modeling and validity

Luoyu Roy Xu — Tue, 06 May 2008 13:14:31 -0400

Dear Kazem,

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

http://www.civil.northwestern.edu/people/huang.html

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.

Reply to Xiaodong on nanocomposite agglomeration

Luoyu Roy Xu — Tue, 06 May 2008 12:22:00 -0400

Dear Xiaodong,

Yes.
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
Nanocomposites. (Try to minimiz overlap among different J-Club themes)

Reply to Teng

Luoyu Roy Xu — Tue, 06 May 2008 11:40:13 -0400

Dear Teng,

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

Polymer nanocomposite applications

MichelleLOyen — Tue, 06 May 2008 10:54:47 -0400

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".

Nice Topic

Kazem Yazdchi — Tue, 06 May 2008 07:01:46 -0400

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,

Agglomeration - experiments and modeling in nanocomposites

Xiaodong Li — Mon, 05 May 2008 21:32:00 -0400

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.

Another recent discussion on polymer-matrix nanocomposites

Teng Li — Mon, 05 May 2008 16:39:02 -0400

FYI

http://imechanica.org/node/3134

Reply to Zhigang on applications of nanocomposites

Luoyu Roy Xu — Mon, 05 May 2008 13:59:41 -0400

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....

Reply to Kosta on toughness scattering

Luoyu Roy Xu — Fri, 02 May 2008 08:17:48 -0400

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.