From microscopic interactions to macroscopic mechanical properties of filled elastomers / vorgelegt von Mariia Viktorova

Viktorova, Mariia

Elastomers play an important role in our everyday life. Their mechanical propertiescan be enhanced by addition of active fillers – usually carbon black or silica.The filler particles, which are initially well dispersed in the polymer matrix, inthe post-mixing stages tend to form agglomerates and filler networks in a processcalled flocculation. The mechanical properties of the resulting products depend onthe properties of the filler network, while the latter are strongly affected by thechemical composition. One of the most important effects in this context is theso-called Payne effect or the pronounced decrease in the dynamic moduli with increasingstrain amplitude in filled elastomers under cyclic loading. Up to now, itis unclear, what is the main source of the Payne effect, i.e., the polymer-filler orfiller-filler interactions. Therefore, finding a relation between the chemical compositionand the mechanical properties of filled rubbers is an important problem fromboth scientific and manufacturing points of view. To the best of our knowledge,to date, there is no theoretical model relating the chemical composition of a filledelastomer on the microscopic scale to its macroscopic mechanical properties. Thiswork presents a simulation approach combining two models. The first one is anextension of previous work, which presents a coarse-grained simulation approach tofiller flocculation based on the Metropolis Monte Carlo simulation. This algorithm,called the morphology generator, minimizes the free enthalpy of the system, whilethe interactions between different components are described via the experimentalinterface free energies. The second model, based on the assumption of local equilibrium,performs shear of the systems obtained via the morphology generator inorder to obtain storage and loss moduli as well as their ratio – the loss tangent ordamping. This simulation mimics dynamic mechanical analysis, or DMA, which iswidely used in laboratories to measure the mechanical performance of tire materials.Hence, the model provides a connection between the chemical compositioncharacterized by the experimental surface free energies and the mechanical propertiesof the material. The focus of this work is on the model parameterization andon studying the properties of filled pure polymer systems and filled polymer blendsconsisting of natural rubber, styrene-butadiene rubber and different types of fillers,such as carbon black, silica or surface modified silica. We find that the typicalrelaxation in the polymer-filler interface is much slower than the related relaxationin the bulk polymer or in the filler-filler interface. One important implication isthat this leads to a realistic form of the loss tangent in the low frequency/hightemperature regime, i.e., the regime commonly associated with rolling resistance.We propose an approach which ties the surface energies, used in the morphologygenerator, to the force constants, describing interactions in the shear simulation, by making the latter being proportional to the interface tension. We study reversible bond breaking and bond relaxation as functions of different parameters such as filler dispersion, frequency, strain amplitude and filler content. Furthermore, we consider contributions of polymer-filler and filler-filler interactions to the Payne effect in filled elastomers. In addition, we study local strain and stress distributions in the polymer matrix surrounding the filler and in the interfaces between polymer and filler, as well as between filler particles. We find that both the local stresses and the local strains, aside from being dependent on the macroscopic strain amplitude, do depend on frequency. Low excitation frequencies, allowing the local stresses and strains to relax differently, can lead to large differences between the local and the global quantities in certain parts/interfaces of the system. This phenomenon, which is also strongly influenced by filler dispersion, is likely to be of significance to fatigue and fracture in rubber composites.

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