  We consider a microscopic model of a polymer blend that is prone to phase
separation. Permanent crosslinks are introduced between randomly chosen pairs
of monomers, drawn from the Deam-Edwards distribution. Thereby, not only
density but also concentration fluctuations of the melt are quenched-in in the
gel state, which emerges upon sufficient crosslinking. We derive a Landau
expansion in terms of the order parameters for gelation and phase separation,
and analyze it on the mean-field level, including Gaussian fluctuations. The
mixed gel is characterized by thermal as well as time-persistent (glassy)
concentration fluctuations. Whereas the former are independent of the
preparation state, the latter reflect the concentration fluctuations at the
instant of crosslinking, provided the mesh size is smaller than the correlation
length of phase separation. The mixed gel becomes unstable to microphase
separation upon lowering the temperature in the gel phase. Whereas the length
scale of microphase separation is given by the mesh size, at least close to the
transition, the emergent microstructure depends on the composition and
compressibility of the melt. Hexagonal structures, as well as lamellae or
random structures with a unique wavelength, can be energetically favorable.
