Viscoelasticity polymer-filler network

It is certain that the relaxation behavior of filled rubbers at large strains involves numerous complications beyond the phenomena of linear viscoelasticity in unfilled amorphous polymers. Breakdown of filler structure, strain amplification, failure of the polymer-filler bond, scission of highly extended network chains and changes in network chain configuration probably all play important roles in certain ranges of time, strain rate, and temperature. A clear understanding of the interplay of these effects is not yet at hand. 

The authors also describe the effect of temperature on the Payne effect. With increasing temperature the amplitude of the Payne effect decreases significantly (Fig. 12). Very surprisingly, enhanced Payne-like behavior was observed for rubber vulcanizates at room temperature where filler-filler and filler-polymer interaction are not observed in comparison to the typically filled vulcanizates. The authors concluded that in addition to the contribution from the filler-filler network, there are many other factors that affect the nonlinear viscoelastic behavior. Nevertheless, the Payne effect is assumed to arise from the elementary mechanism consisting of adsorption-desorption of polymer chains from the surface of the particles [50]. Besides the experimental investigation, the authors have applied the Maier... 

Three zones indicated in G, can also be detected in the graph of loss modulus (G") versus strain. In low strain, the first linear viscoelastic zone, G" of reclaimed is lower than NR due to its filler content. In higher strains, G" of NR compounds decreases due to disentanglement of polymer chains whereas, reclaimed rubber has different behavior. In medium strains, loss modulus of reclaimed rubber increases due to energy dissipation for mbber-filler and filler-filler networks breakdown and then decreases due to mbber chains disentanglement. 

From the results obtained in [344] it follows that the composites with PMF are more likely to develop a secondary network and a considerable deformation is needed to break it. As the authors of [344] note, at low frequencies the Gr(to) relationship for Specimens Nos. 4 and 5 (Table 16) has the form typical of a viscoelastic body. This kind of behavior has been attributed to the formation of the spatial skeleton of filler owing to the overlap of the thin boundary layers of polymer. The authors also note that only plastic deformations occurred in shear flow. 

Non-linear viscoelastic properties were observed for fumed silica-poly(vinyl acetate) (PVAc) composites, with varying PVAc molar mass and including a PVAc copolymer with vinyl alcohol. Dynamic mechanical moduli were measured at low strains and found to decrease with strain depending on surface treatment of the silica. The loss modulus decreased significantly with filler surface treatment and more so with lower molar mass polymer. Copolymers with vinyl alcohol presumably increased interactions with silica and decreased non-linearity. Percolation network formation or agglomeration by silica were less important than silica-polymer interactions. Silica-polymer interactions were proposed to form trapped entanglements. The reinforcement and nonlinear viscoelastic characteristics of PVAc and its vinyl alcohol copolymer were similar to observations of the Payne effect in filled elastomers, characteristic of conformations and constraints of macromolecules. ... 

Dynamic frequency tests are used to explore the microstructure and network formation of the nanocomposites in presence of multiple fillers and their chemical modifications. The storage modulus (G ) of neat PU, PU/MWCNT, PU/functionalized MWCNT/CB nanocomposites measured at 150 °C is logarithmically plotted as a function of angular frequency (m) in Fig. 13 [ 117]. Incorporation of unmodified MWCNT causes dramatic changes in viscoelasticity of polymer... 

It is also clear that activity of a filler should be related to any definite property of material. It was proposed to introduce the concept of structural, kinetic, and thermod3uiamic activity of fillers. Structural activity of a filler is its abihty to change the polymer structure on molecular and submolecular level (crystallinity degree, size and shape of submolecular domains, and their distribution, crosslink density for network pol3rmers, etc.). Kinetic activity of a filler means the ability to change molecular mobility of macromolecides in contact with a solid surface and affect in such a way the relaxation and viscoelastic properties. Finally, thermodynamic activity is a filler s ability to influence the state of thermodynamic equilibrium, phase state, and thermodynamic parameters of filled polymers — especially important for filled poljmier blends (see Chapter 7). 

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