  The phase behavior of a cross-linked polymer blend made of two incompatible
species, $A$ and $B$, of different chemical nature is analyzed. Besides a
homogeneous phase, this system also exhibits two microphases and a phase of
total segregation. The transition between the homogeneous and the microphase is
continuous along a $\lambda$-line; a first-order phase boundaries separate the
microphase and the disordered phase from the phase of complete segregation. The
critical line meets the first-order phase boundaries at an end point. Scaling
arguments indicate that, close to any end point, the equations for the
first-order phase boundaries exhibit nonanaliticities associated with the
singularities present at the thermodynamic functions near the critical line.
Explicit expressions for the phase boundaries near the end point for a
cross-linked polymer mixture are obtained and checked for singularities.
