Temperature-dependent FTIR study on three kinds of hydrogen-bonded benzoic acid dimers in their melt states

Add time:07/19/2019    Source:sciencedirect.com

In the present paper, three kinds of benzoic acid, 4-allyloxylbenzoic acid (ABA), 4-allyloxyl-2-fluorobenzoic acid (AFBA) and 4-allyloxyl-2, 6-difluorobenzoic acid (ADFBA) have been studied by polarizing microscope (POM), differential scanning calorimeter (DSC) and temperature-dependent FTIR. POM and DSC observations show no mesomorphic properties when heating ABA and ADFBA or cooling their melts. But distinct nematic properties are recorded when cooling AFBA's melt, though its slenderness ratio is smaller than of ABA's. Temperature-dependent FTIR results indicate that the introduction of lateral fluorine atoms lowers the thermal stability of hydrogen-bonded dimers of AFBA and ADFBA, which is unfavorable for the formation of mesophase. But DSC results indicate that the introduced lateral fluorine atoms can destroy the symmetry of AFBA and ADFBA molecules, which is considered to lower their crystallizing points and is favorable for the formation of mesophase. To ABA and ADFBA, at least part of their dimers disassemble to free acids which will act as “impurity” to prevent the formation of mesophase in ABA and ADFBA melts when temperature is higher than their crystallizing point. As to AFBA, there is a temperature range from its crystallizing point (115 °C) to 120 °C in which almost all the AFBA exist in dimers. This temperature range satisfies the necessary condition for the mesophase to form in AFBA's melt. So nematic properties can be observed in AFBA's melt.

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