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Nanoparticle amount, and not size, determines chain alignment and nonlinear hardening in polymer nanocomposites

Significance When straining materials (e.g., pulling a rubber band), they initially deform with a certain stiffness; if one pulls harder, some materials strengthen. This phenomenon, known as nonlinear strain hardening, is a critical feature of composite polymer materials—polymers with reinforcing filler particles—used in, e.g., car tires. Engineering properties such as modulus, toughness, and strength of nanocomposites have been traditionally optimized through trial and error by changing the size and amount of fillers. Our work elucidates the molecular origin of strain hardening in polymer nanocomposites, showing that filler amount, but not size, sets the strain-hardening properties based on interfiller chain elongation. The insensitivity to filler size provides a facile concept to independently tune linear and nonlinear mechanics in composites. Polymer nanocomposites—materials in which a polymer matrix is blended with nanoparticles (or fillers)—strengthen under sufficiently large strains. Such strain hardening is critical to their function, especially for materials that bear large cyclic loads such as car tires or bearing sealants. Although the reinforcement (i.e., the increase in the linear elasticity) by the addition of filler particles is phenomenologically understood, considerably less is known about strain hardening (the nonlinear elasticity). Here, we elucidate the molecular origin of strain hardening using uniaxial tensile loading, microspectroscopy of polymer chain alignment, and theory. The strain-hardening behavior and chain alignment are found to depend on the volume fraction, but not on the size of nanofillers. This contrasts with reinforcement, which depends on both volume fraction and size of nanofillers, potentially allowing linear and nonlinear elasticity of nanocomposites to be tuned independently.