Long alkyl chain dimethylammonioalkoxydicyanoethenolates as new zwitterionic thermotropic liquid crystals

The liquid crystalline thermotropic behaviour of a series of long alkyl cha in dimethylammonioalkoxydicyanoethenolates of the type CH3-(CH2)(n-1)-N+ (C H3)(2)-(CH2)(p)-O-CO-C- (CN)(2) (n = 10-18, even values only, with p = 2, 3 ; p = 3 for n = 16) was analysed through differential scanning calorimetry, X-ray diffraction and polarizing optical microscopy. Smectic A, columnar a nd cubic mesophases were observed in addition to a crystalline phase. The c rystalline structure corresponds to a single layered lamellar arrangement o f molecules characterized by a tilt angle of the paraffin chains of about 5 0 degrees and 56 degrees for p = 2 and 3, respectively. Above the melting p oint (70-100 degrees C), all amphiphiles n-p, except 10-2 and 12-3 which be came immediately isotropic, display one or more mesophases: smectic A for 1 4-2, 16-2, 16-3, 18-2 and 18-3 (zwitterionic bilayers with a thickness of a bout 12 Angstrom), a hexagonal columnar phase for 12-2, 14-3 and 16-4 (mono tropic for 12-2, zwitterionic cylinders of radius of about 12 Angstrom), an d an inverted bicontinuous cubic phase (Ia3d space group) for 14-2, 16-3 an d 18-3 (rod-like elements of radius of about 10 Angstrom). For 14-2 and 18- 3, the cubic mesophase tends to form slowly from the smectic A phases both on heating and cooling. The amphiphiles displaying the columnar mesophase p ossess the lowest and an identical ratio of alkyl tail length to zwitterion length. The temperature range of mesophase stability is a strongly increas ing function of n and a decreasing function of p. For the longest alkyl tai l, mesophases extend over more than 130 degrees C up to the degradation tem peratures (T greater than or equal to 210 degrees C). Strong dipolar intera ctions between the zwitterionic groups in their stretched conformation (mu > 25 D) within the liquid apolar and incompatible paraffin matrix are the d riving force for the thermotropic behaviour of these new amphiphiles.