Loss of affineness in gels and melts

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Abstract
We discuss measurements by mean of small angle neutron scattering of the form factor of a long chain (M > 2 x 106) in two comparably deformed materials : (i) A melt, stretched 4.6 times (leading, in a perpendicular direction, to a deformation 1/√4.6 ∼ 0.46). The long chain, deuterated, is dissolved in the matrix non-deuterated chains. (ii) A gel, crosslinked in solution, deformed by drying (deswelling of 10, leading in one direction to a deformation of 10-1/3 ∼ 0.46 too). The long chain was crosslinked with the other non-deuterated chains. We believe that a direct comparison of the two form factors is interesting for the times here involved (« rubbery plateau ») : the chain is a labelled path which is obliged, for the gel, to pass across numerous crosslinks and, for the melt, to pass through numerous entanglement points (meshes are comparable in the two cases). In addition to this direct comparison, we compare the two classical models of rubber elasticity (affine and phantom), for which we have calculated the form factor. In spite of some differences, data for the two systems agree more between them than they do with classical models : the disagreement with those suggests a loss of affineness at larger scales than the mesh