- Anas Mujtaba, Melanie Keller, Sybill Ilisch, H.-J. Radusch, T. Thurn-Albrecht, K. Saalwächter, M. Beiner.
Mechanical properties and cross-link density of styrenebutadiene model composites containing fillers with bimodal particle size distribution.
Macromolecules 45, 6504-65515 (2012).
Mechanical properties and cross-link density of model
composites being solution styrene?butadiene rubbers filled with different
amounts of nanosized silica particles or mixtures of nanosized silica particles
and micrometer-sized borosilicate glass particles are studied. The cross-link
density of the rubber matrix is measured based on a double-quantum NMR
spectroscopy method. Shear data show that reinforcement and dissipation G”
in the rubber plateau range depend systematically on the total surface area of
the filler per unit composite. Different contributions to reinforcement due to
hydrodynamic effects, “filler network”, glassy polymer layer, and “occluded
rubber” are quantified based on a comparison of linear response measurements with strain sweeps performed at different
temperatures. The results show a percolation threshold at silica volume fractions of about 0.15. The load-carrying capacity of the
“filler network” decreases significantly with temperature. This may indicate the existence of a glassy polymer layer on the surface
of the filler particles which softens several ten degrees above the bulk Tg of the rubber matrix. Two regimes are found in the
dissipation above Tg which both depend systematically on the surface area of the filler system: A strongly frequency-dependent
dissipation regime with power-law behavior is observed in G”(ω) at temperatures up to 70 K above the bulk Tg, and a nearly
frequency-independent G” regime dominates at higher temperatures. The molecular nature and importance of this finding for
tire applications are discussed.