Ferroelectric liquid crystalline polymers with strongly dipolar groups have potential to exhibit large spontaneous polarization under relatively low electric fields and are promising candidates for advanced electronic applications. In an effort to seek ferroelectric liquid crystalline phases in mesogen-free sulfonylated polyethers, we synthesized isotactic polyoxypropylenes (iPOPs) with n-alkylsulfonylpentylthioether side chains (iPOP-SC₅SO₂C_n, where n was either 12 or 8) and studied their liquid crystalline self-assembly behavior by differential scanning calorimetry, X-ray diffraction, and polarized light microscopy. The polyethers were prepared by a post-polymerization substitution reaction from isotactic poly[(R)-(-)-epichlorohydrin], which was made by methylaluminoxane-catalyzed ring-opening polymerization, and their isotacticity was confirmed by ¹³C NMR analysis. The effects of backbone tacticity, backbone-to-dipole distance, and the number of sulfonyl dipoles per side chain on the liquid crystalline self-assembly behavior were investigated by comparing iPOPs with the atactic counterparts. We found that when the sulfonyl dipole was far away from the chiral center in iPOP-SC₅SO₂C_n, the dipole-dipole interactions among the side chains were decreased, which led to more liquid crystalline phases. Smectic E (SmE) and A (SmA) phases were observed above the crystal melting point, but the chiral center in the backbone had an insignificant effect on the liquid crystalline assembly. After the thioether groups in the side chains were oxidized to sulfonyl groups, the stronger chiral center-dipole and dipole-dipole interactions in the resultant polymers, iPOP-SO₂C₅SO₂C_n, pushed the liquid crystalline phase transitions to higher temperatures. The understanding gained from this study will facilitate the design and realization of ferroelectric smectic C* phase in mesogen-free sulfonylated liquid crystalline polymers.

Ruipeng Li

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