Storage moduli, layered-silicate polymer nanocomposite

The dynamic mechanical thermal analyzer (DMTA) is an important tool for studying the structure-property relationships in polymer nanocomposites. DMTA essentially probes the relaxations in polymers, thereby providing a method to understand the mechanical behavior and the molecular structure of these materials under various conditions of stress and temperature. The dynamics of polymer chain relaxation or molecular mobility of polymer main chains and side chains is one of the factors that determine the viscoelastic properties of polymeric macromolecules. The temperature dependence of molecular mobility is characterized by different transitions in which a certain mode of chain motion occurs. A reduction of the tan 8 peak height, a shift of the peak position to higher temperatures, an extra hump or peak in the tan 8 curve above the glass transition temperature (Tg), and a relatively high value of the storage modulus often are reported in support of the dispersion process of the layered silicate. 

Hwang et al. [113] synthesized via in situ polymerization high-impact polystyrene (HlPS)/organically modifled montmorillonite (organoclay) nanocomposites. X-ray diffraction and TEM experiments revealed that intercalation of polymer chains into silicate layers was achieved, and the addition of nanoclay led to an increase in the size of the robber domain in the composites. In comparison with neat HIPS, they found that the HIPS/organoclay nanocomposites exhibited improved thermal stabiHly as well as an increase in both the complex viscosity and storage modulus, and they may have been influenced by a competition between the incorporation of clay and the decrease in the molecular weight of the polymer matrix. 

Layered silicate nanoparticles have also been used to prepare PEN-based nanocomposites through the direct intercalation of PEN polymer chains from the melt into the surface-treated clay. An internal mixer was used and exfoliated silicate layers within a PEN matrix were obtained. Mechanical and barrier properties measnred by dynamic mechanical and permeability analysis showed significant improvanents in the storage modulus and water permeabihty when compared to neat PEN (Wu and Liu, 2005). 

Lin and co-workers explained the superior mechanical properties of the PAN-Na-MMT-Si02 nanocomposites as follows The Na-MMT was exfoliated in the PAN nanocomposites (see Figure 11.7), and the enhancement of the storage modulus results from the delamination of the silicates in the PAN matrix and the strong interactions between the polymer chains and the Na-MMT layers. However, the reinforcement could be anisotropic because of the layer shape of the exfoliated Na-MMT layers. Cracks along the direction of the Na-MMT layers may not be resisted. However, in PAN-clay-silica nanocomposites, the well-dispersed Si02 particles could bridge the cracks that are not stopped by the Na-MMT layers. Therefore, the coexistence of the... 

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