We present large scale Monte Carlo simulations of polymer blends, cross-linked in the absence of interactions and quenched into the two-phase region. Our results show, in agreement with the experiments, that the characteristic length of the microphases is much larger than the mean radius of gyration of the network strands. We calculate the structure function at different times and for different interaction parameters and compare the results to various theoretical models. We find that the individual strands do not deform much; such deformations cannot explain the scale of microseparation. On the other hand, we find large cooperative rearrangements of the network under quenching; this stresses the role played by the topological disorder in the formation of microphases.