Relation between the Spontaneous Polarization and Alkyl Chain Length in Ferroelectric Switching of Columnar Liquid-Crystalline Ureas

Abstract

Relation between the spontaneous polarization and alkyl chain length was investigated in ferroelectric switching of N,N′-bis(3,4,5-trialkoxyphenyl)urea (1-7) possessing n-C₄H₉, n-C₈H₁₇, n-C₉H₁₉, n-C₁₀H₂₁, n-C₁₁H₂₃, n-C₁₂H₂₅, and n-C₁₆H₃₃, respectively. The phase behaviors were investigated by differential scanning calorimetry, polarized optical microscopy, and the column diameters were obtained by X-ray diffraction. Compound 2-7 exhibited rectangular and hexagonal columnar phases. All of the hexagonal columnar phases showed ferroelectric switching under a triangular wave voltage. From the plot of the spontaneous polarization against the carbon atom number of the alkyl chains, correlation between them were clarified and it was strongly suggested that the switching current peaks originated in switching of the polar columns, not in the movement of ionic impurities.

Keywords:

• Ferroelectric columnar phase
• liquid crystal
• polarity
• switching
• urea

We are grateful to Mukai Science and Technology Foundation, the Asahi Glass Foundation and Grant-in-Aid for Scientific Research (B) 19350090.

Notes

^a The transition temperatures (°C) and enthalpies (in parentheses, kcal/ mol) were determined by DSC (5°C/min) and are given above and below the arrows. Cr₁, Cr₂, and Cr indicate crystal phases, and Col_r, Col_h, and I indicate rectangular columnar, hexagonal columnar, and isotropic phases, respectively.

^b Phase behaviors reported in Reference 12.

^c The peaks could not be separated. h ₁ + h ₂ = 15.9 kcal/mol.

^a The column diameters were calculated from d(100) of the hexagonal columnar phases in XRD.