Effect of structural arrest on Poisson's ratio in nanoreinforced elastomers.

We evaluate the strain amplitude (gamma) dependence of the dynamic storage and loss moduli for polybutadiene elastomers containing carbon black particles of varied size at a constant volume fraction of 0.18. Measurements of the low-strain hysteretic softening (Payne effect or unjamming transition) in both shear and uniaxial extension modes allow the gamma dependence of Poisson's ratio (nu) to be determined. For elastomers with a fully agglomerated filler network, the breakup of the jammed state with increasing strain induces a transition from nu approximately 0.35 to the limiting value for an incompressible material, nu=0.5 . This transition is more marked for samples containing smaller filler particles. The effect is similar to the change caused by temperature in the glass transition zone of amorphous polymers, reflecting the parallel between deformation in jammed systems and temperature (and the volume changes associated with it) in glasses; thus, the Payne effect resembles the glass transition in polymers and molecular liquids. We also show herein that the alleviation of structural arrest in filled rubber occurs at a constant value of strain energy, independent of the size of the reinforcing particles or the nature of the strain (shear vs tension). Contrary to the results for shear or tensile deformation, under hydrostatic pressure (volumetric strain) a value of nu=0.5 is maintained for all magnitudes of strain. Along with the absence of hysteresis in cyclic pressure-volume measurements, this demonstrates the distinct behavior of the filler network for hydrostatic compression, in which there is no relative displacement of polymer and filler.