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Constrained dynamics in interphases of model filled elastomers - effect of interface chemistry on crosslinking, local stress distribution and mechanics (DINaFil)

Subject Area Polymer Materials
Term from 2010 to 2014
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 157018565
 
Final Report Year 2016

Final Report Abstract

The goal of this project was the systematic elucidation of the most important factors that govern the unique mechanical properties of particle-filled elastomers, which find applications in many everyday commodities such as tires. A Franco-German consortium jointly funded by the ANR and the DFG sought to investigate the relations between internal structure, inhomogeneous molecular dynamics, and the mechanical properties by way of preparing and studying model nanocomposites of different crosslinked polymers filled with silica particles of known and controlled particle size, surface properties and distribution. The project combined results from advanced NMR spectroscopy, scattering techniques, mechanical spectroscopy, and computer simulations, with the aim to establish quantitative structure-property relationships. At the outset, the emphasis was on as-perfect-as-possible control and systematic variation of the dispersion state of well-defined silica spheres in an elastomer matrix, with additional control of the interaction between filler and elastomer, either by chemical bonds or by adsorption. NMR investigations on an established model system based on poly(ethyl acrylate) have revealed quantitative relationships between the temperature-dependent amount of immobilized polymer at the particle surface and the size of the transition region (i.e. the “interphase”), the total internal surface and its properties (adsorptive vs. covalently binding), the dispersion state, and the linear and non-linear mechanical properties (e.g. strain softening). Notably, only model systems with dense chemical surface grafts exhibited an additional pronounced inhomogeneity of the surrounding rubber matrix, accompanied by rather high amounts of immobilized polymer. Studies on various other silica-filled diene elastomers such as natural rubber (NR) or styrene-butadiene-rubber (SBR) demonstrated this to be a peculiarity of the model system, as in the latter systems it was found that filler addition does not lead to significant changes in the crosslinking state of the matrix or to large quantities of immoblized components. Rather low but systematically varying amounts of immobilized interphase material could reliably be identified in the SBR/silica system, where a universal relation to the surplus elasticity modulus of the filler network measured at variable temperature and agitation frequency could be established. In-depth investigations of the dynamic interphase in the model systems revealed the quantitative applicability of a glass transition temperature gradient model, by way of which NMR observables could be used for a parameter-free prediction of the thermal (calorimetric) response of the same system. More advanced so-called spin-diffusion NMR experiments served to estimate the size of the dynamic interphase, and revealed that the laterally averaged 1D gradient model is only an approximation in a sense that the systems are characterized by dynamic heterogeneities on the nm scale. Finally, NMR studies of stretched filled und unfilled rubbers enabled the quantitative assessment of chain stretching on a microscopic level. In this way, in the former case we were able to experimentally confirm theoretical predictions of the so-called matrix overstrain in filled rubbers, related to the fact that the filler particles are not deformable. In the latter case, our wealth of new NMR observables was used to assess the validity of different theoretical approaches to entangled rubber elasticity, thus contributing to solving long-standing open questions.

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