How Topological Polymer Loops on the Nanoparticle Surface Control the Mechanical Properties of Nanocomposites

T Koga and XR Wang and ZX Huang and Y Bajaj and M Endoh and JMY Carrillo and BG Sumpter and T Masui and H Kishimoto and T Taniguchi and ZH Lin and AE Ribbe and HH Zhang and RP Li and L Wiegart and NC Osti and T Yamada and L Porcar and B Farago and J Allgaier and M Kruteva and M Monkenbusch and D Richter and M Nagao, MACROMOLECULES, 58, 9182-9198 (2025).

DOI: 10.1021/acs.macromol.5c01060

Carbon black (CB) and silica (SiO2) filled elastomers are known to be the most successful polymer nanocomposites (PNCs) in industry, where "bound rubber (BR)" (i.e., polymer chains that are physically or chemically adsorbed on the nanofiller surface) plays a critical role in their reinforcement. Here, we report a molecular-scale mechanism underlying the "BR-induced reinforcement" by integrating neutron scattering experiments and molecular dynamics simulations. Simplified non-cross-linked SiO2-filled polybutadiene (PB) and CB-filled PB reveal the critical role of topological polymer loops in the BR for the enhanced mechanical performance. The average loop size on the SiO2 surface modified with a silane coupling agent is much smaller than that on the CB surface and the loops on the SiO2 surface are densely formed, preventing interdigitation with the matrix chains. On the other hand, the larger, uncrowded loops formed on the CB surface facilitate the interdigitation with the matrix polymer chains even near the filler surface. In this way, a strong connectivity is established between a matrix and a nanofiller, resulting in an adhesive filler-polymer interface. Our findings shed light on rich and complex physics and materials design problems in PNCs, where the topological polymer structure on the nanofiller surface directly controls the macroscopic mechanical properties.

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