Structural study on polymorphism of long-chain dicarboxylic acids using oblique transmission method for micro FT-IR spectrometers.

Polymorphism and higher-order structures of even-number dicarboxylic acids from hexadecanedioic acid to eicosanedioic acid have been studied by micro-FTIR spectroscopy and microscopic observation. In order to obtain the three-dimensional structural information, the oblique transmission method was incorporated into a micro-FTIR spectrometer equipped with a couple of Cassegrain lenses. The IR spectra showed that the solid states of dicarboxylic acid bore a marked structural similarity to those of n-saturated fatty acids. It was found that a solution-grown single crystal of dicarboxylic acids was an aggregate of lamellae whose thickness exceeded by far their molecular length. The surfaces of lamellae were covered with hydrocarbon segments taking a hairpin structure, and the linkage of hydrogen-bonded dicarboxylic acids formed a folded chain structure. The result of oblique transmission measurements showed that the stacking mode of lamellae varied depending on crystallization conditions.