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

Abstract

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.nanocomposites | nonlinear elasticity | strain stiffening | polymer bridging | polymer chain alignment M any synthetic and natural materials around us increase their elastic modulus upon large deformation after initial softening-a phenomenon that is known as work or strain hardening, which is critical to their function. In ductile polymer materials, the strain-hardening behavior is essential for their functional lifetime, resilience, and toughness-all key parameters of their practical uses-because these materials repetitively bear large loads (1, 2). Many industrial and consumer polymeric materials are composites, in which (hard) nanoscale inorganic particles, or fillers, are blended with polymer matrices to tailor their mechanical properties. In preparing such nanocomposites, filler−filler and filler−matrix interaction, filler dispersion, and polymer properties all affect the linear (low strain) and nonlinear (high strain) mechanical response in nontrivial ways (3). Although a massive volume of work has attempted to clarify the mechanism of reinforcement (increased linear elasticity) at low strain and of nonlinear strain softening (the Payne and Mullins effects) at medium strain, a comparatively much smaller body of work exists that focuses on the mechanism of strain hardening in polymer composite materials.In analogy to rubber elasticity at large deformations, strain hardening in polymer composites is typically attributed to the increasing resistance to deformation of extended and oriented polymer chains (4-7). However, it has been shown that polymer chain alignment during strain hardening is strongly affected by dispersing fillers within the host polymer matrix (8-10). To account for these observations, one needs to establish the relation between the macroscopically observed strain hardening and the microscopic chain alignment that is affected by the presence of fillers.The connection between chain alignment and strain hardening in glassy polymer composites is purported to occur because the fillers act as "entanglement attractors." In this...

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Linear and Nonlinear Viscoelastic Modulus of Rubber

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Bioinspired Wear‐Resistant and Ultradurable Functional Gradient Coatings

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