Crystalline polymorphs of higher homologues of 4′-alkoxy-4-cyanobiphenyl, nOCB (n = 8, 9, 10 and 12)

Phase transition behaviour was studied for the crystalline polymorphs of 4'-alkoxy-4-cyanobiphenyl, nOCB (n = 8, 9, 10 and 12), by means of differential scanning calorimetry (DSC). The square-plate crystal form, which is composed of distinct smectic-like bilayers with infinite networks of closely arranged CN groups, appears generally for n greater than or equal to 7. It is a metastable phase for n = 7 and 8. In addition to the square-plate crystal, 8OCB has needle and parallelepiped crystal forms, which are metastable, and the most stable crystalline phase found in a commercially available powder specimen. The parallelepiped crystal shows three competitive processes depending on heating rate: (1) gradual stabilization to the most stable crystalline phase, even at room temperature, (2) transformation to the needle crystal, and (3) direct transformation to the smectic A phase. On the other hand, the square-plate crystal is stable at room temperature and transforms to another crystalline phase, which is supercooled in the commercially available powder specimen, at higher temperature for n greater than or equal to 9.