Nanocomposites clay aspect ratio

Comparison of experimental observations with barrier model predictions clearly indicated that increase of clay content has an impact on effective clay aspect ratio and alignment, which in turn affect the nanocomposite barrier properties. At the same time, the barrier models could serve as indirect tools for quantification of the degree of exfo-liation/intercalation/aggregation in the production nanocomposite prodncts. 

Figure 4.2 Relative permeability of polymer-clay nanocomposites as a function of the clay aspect ratio.

Average Aspect Ratio of Clay Layers Present in Different Nanocomposites... 

Dispersion The degree of dispersion of the nanoplatelets is determined by the degree of delamination of the clay. The fully delaminated (exfoliated) nanocomposite presents much higher values for the tortuosity factor and the aspect ratio in comparison with the partially delaminated (intercalated) nanocomposite. This means that the clay particles that grow as aggregates or books of sheets must be broken up or exfoliated into individual sheets that have a thickness of the order of 1 nm, with lengths and widths of the order of 500 nm. 

Maiti and Bhowmick [205] have established a good correlation between the aspect ratio of clay in fluoroelastomer/clay nanocomposites and the transport properties of solvent. For fluoroelastomer/clay nanocomposites, permeability decreases significantly with the addition of only 4 phr of the unmodified mont-morillonite clay (0.14 x 10 s cm2 s ) compared with that of neat polymer (2.29 x 10 8 cm2 s-1)-... 

Another advantage of PCNs is the substantial decrease in permeability. The drastic decrease in permeability is attributed to the large aspect ratio of the clay layers, which increase the tortuous path of the gas as it diffuses into the nanocomposites. 

Since nanoparticles in PNC are orders of magnitude smaller than conventional reinforcements, the models developed for composites are not applicable to nanocomposites. However, development of a universal model for PNC is challenging since the shape, size, and dispersion of the nanoparticles vary widely from one system to another. On the one hand, exfoliated clay provides vast surface areas of solid particles (ca. 800 m /g) with a large aspect ratio that adsorb and solidify a substantial amount of the matrix polymer, but on the other hand, the mesoscale intercalated clay stacks have a much smaller specific surface area and small aspect ratio. However, in both these cases the particle-particle and particle-matrix interactions are much more important than in conventional composites, affecting the rheological and mechanical behavior. Thus, the PNC models must include the thermodynamic interactions, often neglected for standard composites. 

Fis Fischer, B., Ziadeh, M., Pfaff, A., Breu, J., Altstadt, V. Impact of large aspect ratio, shear-stiff, mica-like clay on mechanical behaviour of PMMA/clay nanocomposites. Polymer 53 (2012) 3230-3237. 

However, the tensile strength of in-situ polymerized nanocomposites is higher than other systems. The improvement in tensile property of nanocomposites occurs at the expense of strain to failure. The clay platelets dispersion and interaction with polyamide matrix are important factors that influence the mechanical properties. The high aspect ratio of the clay platelets and exfoliated clay platelets in nanocomposites are responsible for the improvement in mechanical properties. 

The water absorption behavior of the clay-epoxy nanocomposites in terms of maximum water uptake and diffusion coefficient are given in Table 9.18 [47]. The presence of nanoparticle as reinforcement reduces the water absorption of the composite system. The maximum water uptake of epoxy decreases gradually with the increase in clay content. The maximum water absorption decreases by 14.1,17.9 and 24.8% after the incorporation of 1, 3 and 5 wt% nano-clay, respectively, compared with neat epoxy. The presence of high aspect ratio nanofillers can create a tortuous pathway for water molecules to diffuse and enhances the resistance to water absorption. The diffusivity also decreases in the same manner and a significant reduction in diffusivity is obtained for the composites containing 5 wt% clay [112]. 

Durmus, A., Woo, M., Kasgoz, A., and Macosko, C. W. 2008. Mechanical properties of linear low density polyethylene (LLDPE)/clay nanocomposites Estimation of aspect ratio and interfacial strength by composite models. Journal of Macromolecular Science, Part B Physics 47 608-619. 

Lu, C. and Mai, Y. W. 2005. Influence of aspect ratio on barrier properties of polymer-clay nanocomposites. Physical Review Letters 95 088303 1-4. 

Following from the same considerations, it can be demonstrated that a fiber-like shape, instead of a platelet-like, can minimize the sharp increase of viscosity due to the addition of nanoclays to polymer melts, and therefore improve the processability of the nanocomposite. Different factors contribute to the viscosity of a nanocomposite (a) the polymer-polymer network, (b) the clay-clay network, and (c) the polymer-clay interaction. Supposing that the chemistry of the system is fixed, the first contribution is invariant and the second depends only on the particle aspect ratio, according to percolation theory [7] the higher the aspect ratio, the higher the viscosity of the composite. Comparing the effect of two nanoclay particles with the same aspect ratio, one with a rod-like shape and the other with... 

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