Thermotropic Nematic Polyesters Research Articles

Rheological behavior of thermotropic flexible main‐chain polyesters based on 4,4′‐dioxy‐2,2′‐dimethyl azoxybenzene mesogens. I. Oscillatory shear behavior

AbstractThe rheological properties of fractionated thermotropic nematic polyesters 4,4′‐dioxy‐2,2′‐dimethyl azoxybenzenenonanediyl (n = 7) (AZA‐9) and dodecanediyl (n = 10) (DDA‐9) magnified image have been determined under oscillatory shear in the nematic and isotropic phase. A markedly more pronounced shear thinning characterizes the polydisperse DDA‐9 polymer as compared to the fractionated polymer in the nematic state. This difference becomes less pronounced in the isotropic phase. In all cases, the elastic compliance J′ in the isotropic state is much lower than in the nematic state. The values of relaxation time λ, although lower in the isotropic state, are not very different in the nematic state. The loss tangent tan δ is significantly higher in the isotropic phase for lower molecular weights. For molecular weights of 13,000 and above, this difference becomes smaller and shifts to lower angular frequencies. For AZA‐9 and DDA‐9, η→0 the limit of the dynamic viscosity can be represented by η* = M with a value of α similar for both phases and approximately equal to 4.

Model Compounds of Main-Chain Thermotropic Nematic Polyesters

Abstract Investigation of precursors of thermotropic nematic polyesters with flexible spacers ((CH2)10 and (CH2)7) and mesogens in the main chain is reported. The mesogens are Mixtures of structural isomers of 2,2′-dimethylazoxybenzene derivatives display broad multiple melting. Several types of sequencing of mesogen and spacer were investigated. Compounds formed by the sequence mesogen-central spacer-mesogen appear to provide an adequate model of polymeric behavior. Influence of mesogen and spacer on phase transition entropies and mesogen nematic order parameter, as well as temperature dependence of alkyl chain flexibility were investigated in selected compounds. These can also serve as models for study of mesophase glasses and the semicrystalline state of nematic polymers.

Model Compounds of Main-Chain Thermotropic Nematic Polyesters

Model Compounds of Main-Chain Thermotropic Nematic Polyesters

Deuteron Magnetic Resonance Study of Order and Dynamics in Liquid Crystal Polymers

Abstract Thermotropic nematic polyesters, specifically deuterated at different positions of the polymer chain are studied by multiple pulse dynamic NMR. Analysis of the NMR experiments is achieved, employing a comprehensive model, based on the stochastic Liouville equation. Computer simulations provide the orientational distributions and conformations of the polymer chains and the correlation times of the various motions. In the anisotropic melt the correlation times for chain reorientation and trans-gauche-isomerization are in the range of 0.1–10 ns. Decreasing the temperature of the solid polymer first freezes the intermolecular motions. Thus, below the glass transition only intramolecular motions such as trans-gauche isomerization and ring flips can be detected. The chain order parameter of the nematic melt is S(ZZ) = 0.85. In addition, the chains adopt a highly extended conformation, evidenced by a trans population of n(t) = 0.8 throughout the entire spacer. This microorder is retained, when the polym...

Order in linear thermotropic nematic polyesters: The existence of two distinct molecular arrangements within polymeric nematic mesophases

Etude de poly(dioxy-4,4' dimethyl-2,2' azoxybenzene alcanedioyl): il existe un arrangement cybotactique et une phase nematique «ordinaire»

Electric Field Effects and Miscibility in Cybotactic Micellar Nematic and Ordinary Nematic Polyesters

Abstract Miscibility behavior and electro-optical instabilities are reported for two main chain thermotropic nematic polyesters formed by regularly alternating a disubstituted azoxy-benzene mesogen with a flexible spacer containing either 7 or 10 methylene units (HEPTA-9 and DDA-9, respectively). Polymer HEPTA-9 displays an ordinary nematic organization and polymer DDA-9 forms a cybotactic nematic phase. Both are totally miscible with each other, and with PAA, in the nematic phase. Williams domains and dynamic scattering modes are observed in both cases.

NMR study of some thermotropic nematic polyesters with mesogenic elements and flexible spacers in the main chain

NMR study of some thermotropic nematic polyesters with mesogenic elements and flexible spacers in the main chain

The Existence of Two Distinct Levels of Order in Thermotropic Nematic Polyesters

Abstract 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).