were instrumental in obtaining oxidation resistance data. Prof. R. G. Bass generously provided information on the preparation of the hydrazines. Their contribution is gratefully acknowledged. This work has been supported, in part, by a grant from the Office of Naval Research.Registry No. I (copolymer), 115077-22-0; I (SRU), 115077-25-3;

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Polymer concentration correlations in a network swollen by a solvent are studied by small-angle neutron scattering. The network is made by statistical cross-linking of long chains in solution. At small scattering vectors, the intensity strongly increases upon swelling. Upon stretching, unusually oriented isointensity lines ("butterflies") are observed. These effects are well described by a model of fractal heterogeneities of cross-linking giving rise to nonhomogeneous swelling or deformation.PACS numbers: 64.60.Cn, 64.70.Pf, 82.70.Gg According to most current theories the properties of polymer networks are determined by their structural units alone, namely, the elementary meshes that connect two nearest-neighbor cross-links. No correlations are thought to exist on distance scales larger than the average mesh size. 1 "" 3 These approaches have not been entirely satisfactory, as their description of the elastic properties of rubbers and gels is essentially limited to semiquantitative estimates of behavior. On a microscopic scale, they cannot explain a striking anomaly which has been found recently in studies of the structure of rubbers by small-angle neutron scattering (SANS). A network containing small free labeled (deuterated) chains was subjected to a uniaxial deformation and the isointensity lines were mapped using a bidimensional neutron detector. The pattern observed was anisotropic, as expected, but unexpectedly was oriented in the wrong direction. 4,5 The observed double-winged figures, called "butterfly patterns," had their long axes aligned in the stretching direction. 4 This phenomenon has attracted attention because its understanding might bring new elements to the theoretical debate of the modeling of microstructural properties of networks. 6 " 8 In this Letter, we present the results of two experiments designed to shed some light on this situation. We look for the existence of butterfly patterns in a somewhat different situation, namely, the uniaxial elongation of statistically cross-linked gels. The appearence of such figures was predicted recently for such a case on the basis of establishment of correlations between substructures larger than the average mesh size. 9 This new model establishes a connection between the butterfly phenomenon and a strong increase of scattering intensity predicted to appear in a swelling experiment. Thus, we investigate by SANS the swelling and the stretching of networks synthesized under conditions as close as possible to the model assumptions.Semidilute solutions are good reference systems for discussing the structure of gels. They can be viewed as homogeneous networks of finite lifetime and the way they scatter light or neutrons is well described by existing theories. 3 When a contrast is established between the polymer and the solvent, their neutron-scattering intensity I(q) has an Ornstein-Zernicke form I(q)~I(0)/ (1 +q 2 £ 2 ) for qZ, < 1. £ is the correlation length scale beyond which the elementary segments of the chains can be considered as randomly distri...