Journal Club Theme of Jan. 15 2008: Active Nanocomposites

Actually, studying the dielectric properties of nanocomposites, and more precisely the carbon nanotubes (CNT)-polymer nanocomposites, is gaining more attention due to the extraordinary properties of the CNTs. And as you mentioned, the big part of the enhancement comes from the interface since the surface area between the matrix and the nano-fillers is really important. I came across a paper, written by Q.Z. Xue et al. that presents a model that predicts the effective electrical conductivity of CNT-composites incorporating the interface effect (KY Yan, QZ Xue, QB Zheng and LZ Hao ‘The interface effect of the effective electrical conductivity of carbon nanotube composites' Nanotechnology 18 (2007) 255705 (6pp)). In this model, based on average field theory, the interface is considered as a third phase having its own properties.

The large disparity in properties between carbon nanotubes and the polymers in which they are often imbedded leads to large expectations in terms of the effective properties the resulting nanocompiste, particularly when using 'back of the envelope' calculations such as the rule of mixtures.  However, characterization efforts have often found that the measured properties for nanocomposites are not as predicted by such methods.  This has certainly been the case in terms of the elastic properties and thermal and electrical conductivities where interface effects have been identified as potentially having a large influence on the nanocomposite properties; the elastic properties being influenced by weak van der Waals interactions, the thermal properties influenced by an interface thermal resistance, and the electrical properties influenced by electron hopping which may be treated as an interface.  In our recent efforts on modeling nanocomposites on the micromechanics level, we have certainly seen the potential for such interface effects to have a large influence on the effective properties. As most of the interface of the nanotube is along its axis, this seems to further emphasize the importance of being able to control nanotube alignment or orientation. However, regardless of nanotube orientation, the question we have often been faced with is what exactly the properties of the interface are and how can they measured?  At present, some of these properties can be assessed using lower length scale (atomistic) simulations, but this can not perfectly substitute for a direct physical measurement.  Thus it will certainly be interesting to me to see how not only the progress in modeling and characterizing the nanocomposite dielectric properties in terms of the influence of the interface, but also to see what strategies exist for probing these properties in the lab.

Another effect regarding the interface/interphase region in semi-crystalline polymer nanocomposites that I am now just starting to really appreciate is the impact that the nanoparticles can have on both the level of crystallinity as well as the crystal polymorphs present in the nanocomposite. To expand on the previous thoughts in this blog, this raises the very interesting question of how to distinguish/differentiate the effects of changes in crystallinity versus interface/interphase effects (such as an annular region ofrestricted mobility polymer due to interactions with the nanoparticle) versus the actual effects of the actual nanoinclusion itself on the effective nanocomposite properties.

For example, a paper from the Giannelis group in Advanced Materials in 2004 (see below) showed that the addition of appropriately surface-modified nanoparticles (5 wt% nanoclay) in PVDF resulted in an order-of-magnitude increase in toughness of the polymer. This large toughness enhancement was largely attributed to an increase in the amount of beta phase (at the expense of the alpha phase) of the semicrystalline PVDF present in the nanocomposite. Because the piezoelectric properties of PVDF are largely attributed to the beta crystal phase, this offers some interesting possibilities regarding using nanoparticles to control crystal morphologies and phase in these types of nanocomposites.

If you’re further interested in this work I would very highly recommend the article highlighted above and given below, and the references therein...

D Shah, P.Maiti, E. Gunn, DF Schmidt, DD Jiang, CA Batt, and EP Giannelis 'Dramatic enhancements in toughness of Polyvinylidene flouride nanocomposites via nanoclay-directed crystal structure and morphology' Advanced Materials 16, pp 1173-1177, 2004

Thanks a lot. This is a very interesting topic.  

Another ground challenge in nanocomposites is agglomeration of nano reinforcements. For instance, carbon nanotubes and nanoclays tend to agglomerate to form big particles (see papers below). This will result in a decrease in reinforcing effects as well as functionalities. In-depth discussions on this topic might lead to a true breakthrough in the practical applications of nanocomposites.   

Xiaodong Li, Hongsheng Gao, Wally A Scrivens, Dongling Fei, Xiaoyou Xu, Michael A Sutton, Anthony P Reynolds, and Michael L Myrick, "Physical 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. 

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.

Thank you Dr Zoubeida for pointing out Lewis' reference to me. As he has rightly pointed out, the interface will become increasingly prominent and its physics has to be properly accounted for especially when the size of the structure under consideration reaches nanometric dimensions.

Consider a standard capacitor (consisting of a dielectric layer sandwiched between metal electrodes) of macroscopic dimensions subject to a potential difference say 'V' across its ends. The objective is to find the electric field/potential/polarization vector profile along its length. If one goes by standard electrostatics, it can be worked out the electric field/polarization is constant in the dielectric...and both these fields drop abruptly to zero inside the metal i.e both these fields have jumps at the 'interface'. However, if one shrinks the capacitor to nanometric dimensions, the interface becomes as large as the dimensions of the dielectric layer..so if one assumes that the dielectric constant varies smoothly (what is the meaning of smooth at the atomic level?) then the electric field/polarization field wont have any jumps. Though this consideration may not be of any consequence while analysing large structures, it can have far reaching consequences for nano devices. 

I feel that characterization of interfaces is an important issue. Also i am not sure if good analytical models exist to model the interface. The following is one reference which i found which models the dielectric property at a insulator/insulator interface

F. Stern, Phys. Rev. B. 17, 5009 (1978)