Layered fillers storage modulus

As already reported by several authors, the addition of carbon nanotubes did not affect the storage modulus in the glassy region, nevertheless a strong increase with the filler content is observed in the rubbery region. In conventional composites, this increase is mainly attributed to interfacial interactions leading to introduction of additional cross-links into the network by the filler. These interfacial interactions contribute to the formation of an adsorption layer whose thickness has been estimated around 2 or 3 nm and where... 

Fig. 17. Double logarithmic plot of the small strain storage modulus vs filler volume fraction for E-SBR/BR(m)-microgel composites with varying size of the BR-microgels as indicated (open symbols). The solid line represents a master curve with slope 3.5 estimated from Eq. (72) with layer thickness A=2 nm (solid symbols). Experimental data are taken from [94,95]...

Schaefer et al. (19) studied the interphase microstructure of ternary polymer composites consisting of polypropylene, ethylene-propylene-diene-terpolymer (EPDM), and different types of inorganic fillers (e.g., kaolin clay and barium sulfate). They used extraction and dynamic mechanical methods to relate the thickness of absorbed polymer coatings on filler particles to mechanical properties. The extraction of composite samples with xylene solvent for prolonged periods of time indicated that the bound polymer around filler particles increased from 3 to 12 nm thick between kaolin to barium sulfate filler types. Solid-state Nuclear Magnetic Resonance (NMR) analyses of the bound polymer layers indicated that EPDM was the main constituent adsorbed to the filler particles. Without doubt, the existence of an interphase microstructure was shown to exist and have a rather sizable thickness. They proceeded to use this interphase model to fit a modified van der Poel equation to compute the storage modulus G (T) and loss modulus G"(T) properties. 

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. ... 

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