Numerous materials, from biopolymers to filled rubbers, exhibit strain softening at high strain amplitudes during a strain sweep in oscillatory rheology: The modulus decreases with increasing deformation. On the other hand, if the nonlinear elastic response is analyzed within a single oscillation cycle (described by a Lissajous curve), these systems are often reported to exhibit strain hardening. We compare strain sweeps and single cycle LAOS (large amplitude oscillatory shear) analyses of stress vs strain on three very different materials. We conclude that the reported strain hardening is due to the use of a tangent modulus in the LAOS analysis, and that the overall rheology remains strain softening. To show that this conclusion is robust, we demonstrate a rescaling of the modulus that collapses the data from all the oscillatory measurements onto a single master curve that clearly exhibits the correct strain softening behavior.

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Nanocomposites consisting of polymers reinforced with filler particles are important for a wide variety of industries and processes, but although they exhibit unique viscoelastic properties and as such are widely applied in e.g. tires, the precise mechanism of their reinforcement is at best incompletely understood at present. In order to understand it at a fundamental level, and ultimately control it in practice, it is essential to determine the impact of interactions between filler particles and polymer matrix on the nanocomposite microstructure and its macroscopic dynamic mechanical properties. To this end, we performed experiments on two model systems as well as molecular dynamics simulations, aiming to determine to what extent widely used shear-distortion models of the reinforcement are applicable as well as the role played by molecular interactions on the enhancement of the mechanical properties. In both experiments and simulations a linear dependence of the reinforcement on the inverse radius of the nanoparticles was obtained. Deformation simulations of a linearly increasing strain showed an overall increase of 50% in the linear modulus when fillers were added to the polymer matrix, regardless of the use of direct interactions among the nanoparticles. Furthermore, the use of attractive nanoparticle interactions resulted in a higher matrix densification at the interfaces and to a sharp increase in the reinforcement.

Dispersing hydrophilic nanofillers in highly hydrophobic polymer matrices is widely used to tune the mechanical properties of composite material systems. The ability to control the dispersion of fillers is closely related to the mechanical tunability of such composites. In this work, we investigate the physical−chemical underpinnings of how simple end-group modification to one end of a styrene−butadiene chain modifies the dispersion of silica fillers in a polymer matrix. Using surface-sensitive spectroscopies, we directly show that polymer molecular orientation at the silica surface is strongly constrained for silanol functionalized polymers compared to nonfunctionalized polymers because of covalent interaction of silanol with silica. Silanol functionalization leads to reduced filler aggregation in composites. The results from this study demonstrate how minimal chemical modifications of polymer end groups are effective in modifying microstructural properties of composites by inducing molecular ordering of polymers at the surface of fillers.