Abstract
The thermal properties of a series of α,ω-bis[4-(4-biphenyloxycarbonyl)phenyl]alkanedioates (BPA-n) dimeric liquid crystals containing even-number methylene spacer from n=8 to n=20 were studied. All synthesized compounds showed the same crystal form at room temperature. They were all enantiotropic nematic except BPA-20 which was monotropic. The temperature range over which the nematic phase exists becomes smaller with increasing spacer length, and finally at n=20 it no longer shows liquid crystal on heating. Thermodynamic quantities were classified as contributions from mesogens and polymethylene spacers. The overall entropies ΔSKI of transition from crystals at room temperature to the isotropic liquid were shown as a linear equation of the number, n, of methylene units as follows, ΔSKI=8.50n+95.4 J K−1 mol−1. The coefficient (8.50 J K−1 mol−1), which means the increase in ΔSKI value per methylene, corresponds well to melting entropy per methylene of n-alkanes. The entropies of transition from nematic to isotropic phase were almost constant (8.3–8.9 J K−1 mol−1), independent of the number of methylene units in the fiexible spacer.
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Ogawa, Y. Effects of Spacer Length on Thermal Properties of Dimeric Model Compounds for Main Chain Liquid Crystalline Polymers. Polym J 28, 326–329 (1996). https://doi.org/10.1295/polymj.28.326
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DOI: https://doi.org/10.1295/polymj.28.326