STRAIN-INDUCED FLUCTUATIONS IN POLYMER NETWORKS, MELTS AND GELS (BUTTERFLY PATTERNS)

We present several examples of an unexpected behaviour, occurring in d ifferent types of polymer networks (permanent or temporary) containing either small or large proportions of mobile species. The spatial fluc tuations of local concentration of the mobile probes appear (through n eutron scattering experiments) to be anisotropically enhanced (essenti ally parallel to the elongation axis) when the samples are uniaxially stretched. This behaviour may arise either from an anisotropic enhance ment of the thermal fluctuations under strain or from the revealing of ''quenched'' large scale heterogeneities. It has not been possible ye t to decide which of these rather different explanations is the more s atisfactory.