To clarify the mechanism involved in the appearance of antiferroelectric properties in mixtures of polymers containing achiral mesogenic side chains and their monomers, X-ray investigation was carried out in pyroelectrically active samples. The thermal stability of such systems is also reported. The X-ray investigation shows that in highly concentrated mixtures of monomer in polymer, a segregation of the excess of monomer from an optimized ratio (33% monomer/polymer) occurs.