Nanoparticles reorganizations in polymer nanocomposites under large deformation

The structural evolution of nanoparticles (NPs) dispersion under uniaxial stretching and extension/ retraction deformation cycles above T-g was investigated in model silica/polymethylmethacrylate (PMMA) nanocomposites (PNCs) by a combination of Small Angle X-ray Scattering (SAXS) and Transmission Electronic Microscopy (TEM). The different structure displacements and reorganizations can be quantitatively characterized as a function of elongation ratio, silica volume fraction and NP size. At low NP volume fraction, a rotation/orientation of non-connected aggregates is observed. along the stretching direction, while the reinforcement is low and might be limited by the large-scale aggregates. At high volume fraction, the stress strain curves exhibit three regimes. (i) At low stretching ratio, in the linear deformation regime, reinforcement is driven by the primary network filler structure. (ii) Above a few percent of deformation, a yield is observed and can be associated to the network breakdown as revealed by cyclic extension/retraction experiments. (iii) As a result of this yield, at larger deformation, the stress curve appears as shifted upward with respect to the one of pure polymer. A persistence of this vertical shift (constant value up to large deformation) might be related with SAXS measurements to a non-affine deformation of the NPs network due to new structural arrangements, while in a second case, the decrease of stress to the pure polymer value with increasing deformation is related with observation of affine deformation after the yield. Finally, affinity and non-affinity after yielding are discussed for all the systems according to the strength of the NP-NP interaction. (C) 2014 Elsevier Ltd. All rights reserved.