Resolving toughness-modulus conflict in carbon black reinforced natural rubber by preserving long chains

In manufacturing many natural rubber products, carbon particles and natural rubber chains are mixed by high-intensity processes, such as roll milling and internal mixing. These processes cut natural rubber chains and reduce the performance of the composite. Here we hypothesize that the performance can be enhanced by preserving long chains of natural rubber. We test this hypothesis by mixing carbon particles with natural rubber latex without cutting chains. The long chains are densely entangled and sparsely crosslinked. Above a certain volume fraction, carbon particles percolate. The percolated network of carbon particles and the crosslinked network of rubber chains interpenetrate and form strong noncovalent bonds. Preserving long chains amplifies toughness by more than an order of magnitude, from ∼3.5 kJ m^-2 to ∼63 kJ m^-2, while maintaining modulus. High toughness arises from energy dissipation across multiple length scales: along long rubber strands, across carbon particles, and in a zone of strain-induced crystallization and interfacial dissipation. Modulus is maintained through entanglements of rubber strands and percolation of carbon particles.