UNIAXIAL EXTENSION OF END-LINKED POLYSTYRENE NETWORKS CONTAINING DEUTERATED FREE CHAINS STUDIED BY SMALL-ANGLE NEUTRON-SCATTERING - EFFECT OF THE NETWORK CHAINS AND THE SIZE OF THE FREE CHAINS

We have studied end-linked polystyrene networks containing labeled (de uterated) free chains subjected to an uniaxial deformation using small -angle neutron scattering(SANS). The networks are made by end-linking protonated polystyrene precursor chains of a narrow size distribution. As for statistical networks investigated earlier, the isointensity li nes on the bidimensional detector have the double-winged shape known a s ''butterfly patterns''. They appear as a figure ''8'' at small value s of the scattering vector, q, oriented along the stretching direction , corresponding to a larger scattering in the parallel than in the per pendicular direction even at q-->0. We relate the existence of butterf ly patterns to preexisting heterogeneities in the cross-linking densit y, which cause spatial fluctuations of the free chain concentration. S uch heterogeneities are therefore present even in end-linked networks. They can be attributed to imperfect chemistry or to more physical ori gins: heterogeneity in the spatial distribution of entanglement or the presence of large loops.