A ferroelectric instability is predicted to occur in an experimentally accessible temperature range in a main-chain (semiflexible) liquid crystalline polymer with directed polar mesogenic segments. The long-range part of the dipole-dipole interaction is separated and its effect is completely accounted for in the average electric field. The remaining (local) free energy density is derived in the form of an expansion in powers of the spontaneous polarization. We derive simple expressions for the instability temperature and the polar mean-field coupling in a polar nematic polymer in terms of the molecular model parameters. Using simple estimates we show that ferroelectric ordering is more likely to occur in nematic polymers with directed polar segments than in low molecular weight nematic liquid crystals composed of polar molecules.
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