Detailed understanding of the carbon black–polymer interface in filled rubber composites

To characterize the nature of the interface between styrene-butadiene rubber (SBR) and carbon black (CB) in a filled rubber compound, we introduce an extraction protocol that uses toluene solvent in a high-pressure autoclave with sequential temperature steps of 90, 110, and 150 °C. This new approach removes unbound and physically bound (physisorbed) polymer from the CB-filled SBR compound to leave only chemically bound polymer which is 0.86% (0.0172 g polymer/g CB) and not zero as implied from extrapolating prior literature results. Rheological time sweeps on the unfilled gum SBR indicate that significant crosslinking can occur during annealing at 150 °C for this unsaturated polymer. This was the motivation for using the sequential extraction procedure versus direct extraction at 150 °C which gives a nearly six-fold higher amount of unextracted polymer due to such crosslinking. Analysis of the temperature dependence of bound rubber from 23 to 150 °C indicates that the physical interactions between the polymer and CB surface have a weaker component with a desorption energy ( E d ) of 6.8 kJ/mol and a stronger component with E d = 90.7 kJ/mol, the latter requiring an extraction temperature of 110 °C to remove the majority of it from the CB. • A new solvent extraction method quantified chemically bound rubber as 0.0172 g SBR/g CB, corresponding to an estimated 31 attached polymer chains per carbon black aggregate. • Weaker (6.8 kJ/mol) and stronger (90.7 kJ/mol) types of physical interactions between polymer chains and CB surfaces were distinguished. • The traditional process of evaluating bound rubber at room temperature gives a convoluted measure of chemical and physical interactions.