Search

It is shown by means of x-ray diffraction and PMR broad line spectroscopy, that the structure of the flexible spacer drastically influences the order within the nematic phase of thermotropic polyesters based on the 4,4′ - dioxy- 2, 2′-dimethyl azoxybenzene moiety. The existence of two distinct molecular arrangements is proposed: one is characterized by an unusually high degree alignment of macromolecules, the other by a lower degree of alignment. The former is displayed by spacers with an even number of methylene units in the alkanedioic acid moiety of the spacer (odd number of bonds); the latter by a certain number of flexible spacer moieties with an odd number of methylene groups (even number of bonds), as well as in mixed spacers irregularly alternating along the macromolecular chain (copolyesters).

The homologous series of 4, 41 -dialkanoyloxydiphenyldiacetylenes with n ranging from zero to ten have been prepared. Optical microscopy, differential scanning calorimetry and X-ray diffraction have been used to study the mesomorphic behavior of these compounds. All homologs with n=1-7 are liquid crystals having nematic phases. In addition to nematic phases, these esters have extensive solid-state polymorphism. The transition enthalpies and entropies have also been calculated.

Mesomorphic polyesters were synthesized from 4, 4′-dihydroxy-α-methylstilbene and adipic acid (P-6) or (+)-3-methyl adipic acid (P-6M). P-6 forms a thermotropic nematic phase and P6-M a thermotropi...

The temperature dependence of phase structure in 5,6-dioxycarbonyl [2.2.1] bicyclohept-2-ene was investigated by Dsc, X-ray diffraction, optical microscopy and 1H NMR. The existence of an endotherm in DSC, loss of optical anisotropy, formation of diffuse diffraction patterns and drastic changes in 1H Nmr relaxations indicated the formation of a ‘plastic crystal’ phase, well below the melting point. NMR relaxation rates were measured as a function of temperature. There are sharp changes in relaxation rates at the plastic crystal transition. The transition introduces rapid molecular rotation, without translation. TIH relaxations show a change in magnitude for the rate, but no change in relaxation mechanism.