STRAIN-INDUCED LARGE FLUCTUATIONS DURING STRESS-RELAXATION IN POLYMER MELTS OBSERVED BY SMALL-ANGLE NEUTRON-SCATTERING - LOZENGES, BUTTERFLIES, AND RELATED THEORY

We discuss the neutron scattering features of melts containing a fraction of labeled chains, and of rubbers containing a fraction of deuterated paths along several successive elementary chains. Both types of material are studied during relaxation after a fast deformation in very similar conditions. An unusual feature is the shape of the anisotropic scattering as visualized on a bidimensional detector: the isointensity lines have the shape of lozenges. A detailed review of various cases makes this appear as a general feature in SANS scattering from melts and rubbers in a state of partially relaxed deformation. We then describe results for mixtures of small labeled chains inside matrices of large entangled chains or crosslinked matrices: we find another unusual shape of the isointensity levels, called "butterfly". We propose the lozenges to be a combination of the classical elliptical scattering with a butterfly, which has a main axis orthogonal to the main axis of the ellipses. We discuss in more details the origin of the butterfly effect, by following two tracks: the first is several theories about demixing under strain, including influence of the strain on the enthalpic, elastic, and conformational terms of the free energy of mixing. The second track is the influence of heterogeneities in crosslinked materials. An explanation which would be common to the case of crosslinked and uncrossedlinked material remains to be established.