NON-AFFINE DEFORMATION AND SPATIAL FLUCTUATIONS OF THE MODULUS OBSERVED IN HETEROGENEOUS NETWORKS AND NANOCOMPOSITES

The anisotropic small-angle neutron scattering from two different mate rials is considered. One is a polymer network permeated by free, uncro ss-linked, deuteriated chains. These free chains behave as mobile spec ies. The other is made up of two networks, one being deuteriated, grow n interwoven with each other. They are made of two different polymers which are immiscible, one being in a soft, rubbery state, the other in a hard, glassy state. The scattering of the two systems displays an u nusual dependence upon the direction with respect to the stretching ax is. The scattered intensity recorded along any direction which corresp onds to an increase in the dimensions increases strongly with the elon gation ratio. The isointensity patterns, so-called butterflies, have t he shape of 8s, oriented along the stretching axis. We propose a gener al explanation: in both cases this increase is due to the separation b etween some hard, weakly deformed regions inside a softer matrix. This implies, for the network permeated by free chains, that a scattering contrast is created between hard and soft regions: the free chains mig rate into the soft regions. The increase of the correlation length, xi , along any extended direction of the sample (such as the one parallel to the stretching) reveals an intermediate regime at lower and lower values of the scattering vector, q. In this q range, the intensity, I( q), is superimposed on a limit curve, characteristic of each sample. T he perpendicular scattering is essentially unaffected. Different conti nuous-medium elasticity theories have tried to explain such an anisotr opy of the spatial fluctuations of concentration. Rather accurate meas urements allow us to detect disagreements between these theories and e xperiment. In our opinion, this confirms our more direct picture: the progressive unscreening of the structure of the spatial distribution o f the modulus.