Storage moduli, layered-silicate

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. 

PMMA is slightly affected by the addition of layered silicates in a wide temperature range below and above Tg. On the other hand, copolymers of methyl methacrylate (MMA) with small amounts of polar monomer show a significant increase in the storage shear modulus, G [Okamoto et al 2001]. It could be assumed that the increased polymer polarity results in better interaction with the silicate surface. This view is supported by a study performed on the effect of sulfonation of PS chains on their ability to interact with silicate. The need to provide polar functionality for PS originates in the observation that the Tg value of PS-based CPNC increases with the degree of sulfonation at a constant silicate content [Mauritz et al., 2004]. As suggested by Kawasumi et al. [1997], silicate layers, even those modified by nonpolar alkyl chains, are polar and thus immiscible with nonpolar polymers such as polyolefins (POs). 

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. 

Recently, exfoliated layered silicate-epoxy nanocomposites have been prepared from the diglycidyl ether of bisphenol-A and nadic methyl anhydride (12). The dynamic storage modulus of the nanocomposite containing 4 vol %... 

Poly(vinyl alcohol) has also been shown to provide mechanically adaptive properties to composite films comprised of a PVOH electrospun mat and a PVAc or EO-EPI matrix. Films made from such composites exhibited controlled response to hydration that resulted in a two fold decrease of the storage modulus which, in the case of the PVAc composite, was fully reversible. Expanding on this concept and in a similar motif to the aforementioned CNC composites, Korley and coworkers have recently showcased the potential use of montmorillonite in mechanically adaptive composite materials. Montmorillonite is a layered silicate with high aspect ratio and surface area and is often used in polymer composites to mechanically reinforce the resulting material, while its dispersibility and tunable surface chemistiy make its use particularly attractive. In this case, the incorporation of montmorillonite in the PVOH fibrous filler resulted in enhanced moduli when the polymer matrix consisted of EO-EPI, as opposed to the limited enhancement observed in the case of a PVAc matrix. Nonetheless, both composites showed mechanically switching properties upon hydration, with the weight fraction of montmorillonite affecting the mechanical contrast as well as the response time. ... 

Intercalated nanocomposites of PET and layered silicate were prepared with different kinds of cationic surfactants as compatibilizers by a two-step pol)unerization process a melt polymerization of bis-(2-hydroxyethyl)terephthalate followed by a solid state polymerization. The obtained nanocomposites showed a higher tensile storage modulus compared with those prepared without compatibilizers (40). 

Modified starch matrices have also been reported [151, 154]. Qiao et al. [154] prepared nanocomposites with MMT, OMMT, and thermoplastic acetylated starch (TPAS) plasticized with glycerol. They found that the TS and storage modulus of the TPAS nanocomposites were remarkably enhanced because of the interaction of layered silicates with the TPAS matrix. Moreover, the reinforcing effect of OMMT was greater than that of MMT because of the reduced hydrophilic nature of acetylated starch, which improved the dispersion of OMMT in TPAS more than unmodified MMT. These results are, therefore, in accordance with those... 

Copolymers of PBS and butylene adipate, PBSA, have been largely used to prepare bionanocomposites [338-343]. Ray and Bousmina [338] prepared PBSA/layered silicate nanocomposites by melt extrusion of PBSA and commercially available OMMT. They showed that increasing the level of interactions (miscibihty) between the organic modifier and PBSA matrix, increased the tendency of the sihcate layers to delaminate and distribute nicely within the PBSA matrix. Thermal analysis revealed that the extent of crystallinity of PBSA matrix was directly related to the extent of exfoliation of silicate layers in the nanocomposites and DMA and tensile property measurements showed concurrent improvement in mechanical properties when compared to the neat PBSA and the extent of improvement is directly related to the extent of delamination of silicate layers in the PBSA matrix. DMA also revealed remarkable increase in flexural storage modulus when compared with that of neat PBSA. Tensile properties were also improved with nanoclay addition [339], therefore in agreement with other studies reported in the literature for similar systems [344-347]. 

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... 

---