Infrared spectroscopic study on the crystalline polymorphism and phase transition behavior of mesogenic 4-heptyloxy-4′-cyanobiphenyl

Add time:09/04/2019    Source:sciencedirect.com

Infrared (IR) spectral changes during heating at the rate of 0.2°C min−1 were measured for three kinds of solid state 4-heptyloxy-4′-cyanobiphenyl (7OCB), i.e., square-plate and needle crystals, and commercially available powder. Frequencies and intensities of CN stretching vibrations of the plate and needle single-crystal samples show discrete changes, indicating that the transition to a nematic phase, which is completed at 54.6°C, proceeds through the following sequences; for the plate single crystal, plate crystal → (approx. 26°C) state P → (approx. 36 ∼ 42°C) state A → (approx. 51 ∼ 54.6°C) nematic phase; for the needle single crystal, needle crystal → (approx. 49°C) state A → (approx. 51 ∼ 54.6°C) nematic phase; for the commercial powder, state A′ → (approx. 51 ∼ 54.6°C) nematic phase. The states P and A are intermediate states, and state A′ is thermodynamically the most stable state. The plate crystal sample in a KBr disk takes on state A′ at room temperature, being converted to the nematic phase over the temperature range of approx. 51 ∼ 54.6°C; on the other hand, the needle crystal sample in the KBr disk exists in the needle crystal state, which is converted directly to the nematic phase over the temperature range of approx. 51 ∼ 54.6°C. Thus, the transition behavior of the KBr disks is appreciably different from that of the single crystal samples.

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