Chemical Structures and Phase Sequences

ClPbislOBB is not the only bent-core nematic that has a giant 63 flexo-coefficient for a few other BC compounds (whose chemical structures and phase sequences are shown in Fig. 3.11) and mixtures a similar magnitude of jesj has been obtained (see Table 3.1). 

Figure 9.12. Chemical structures and phase sequences of dimer liquid crystal com-...

Abstract Chemical structure, polymer microstructme, sequence distribution, and morphology of acid-bearing polymers are important factors in the design of polymer electrolyte membranes (PEMs) for fuel cells. The roles of ion aggregation and phase separation in vinylic- and aromatic-based polymers in proton conductivity and water transport are described. The formation, dimensions, and connectivity of ionic pathways are consistently found to play an important role in determining the physicochemical properties of PEMs. For polymers that possess low water content, phase separation and ionic channel formation significantly enhance the transport of water and protons. For membranes that contain a high... 

According to Kravelen,( l the fundamental characteristics of a polymer are the chemical structure and the molecular mass distribution pattern. The former includes the nature of the repeating units, end groups, composition of possible branches and cross-links, and defects in the structural sequence. The molecular mass distribution, which depends upon the synthesis method, provides information about the average molecular size and its irregularities. These characteristics are responsible, directly or indirectly, forthe polymer properties. They are directly responsible forthe cohesive force, packing density and potential crystallinity, and molecular mobility (with phase transitions). Indirectly, these properties control the morphology and relaxation phenomena (behavior of the polymer). 

The first report on the liquid crystalline properties of these compounds was published by Gray and Mosley [44] in 1976. The series of 4 -n-alkyl-4-cyanobiphenyls (CBn) have been widely studied by different methods due to their readily accessible nematic ranges around room temperature. The compounds have the phase sequences crystal-nematic-isotropic for CBS, CBIO, and monotropic nematic for CBS, CB4 crystal-smectic A-nematic-isotropic for CB9 crystal-smectic A-isotropic for CBll. The lower homologous CB2 is nonmesogenic. The general chemical structure of the compounds CBn is presented in Fig. 1. 

In the last few years the crystal structures of some mesogenic azine compounds have been determined [2, 133-136]. The general chemical structure of the azines is shown in Fig. 24. The crystal data and the phase sequences of the investigated mesogenic azines are presented in Table 15. 

A modified nucleotide found in RNA sequencing could either be a new nucleotide of unknown chemical structure or it could correspond to an already known modified nucleotide (up to now about 90 different modified nucleotides have been identified in RNA). Keith [124] proposed preparative purifications of major and modified ribonucleotides on cellulose plates, allowing for their further analysis by UV or mass spectrometry. Separation was realized by two-dimensional elution using the following mobile phases (1) isobutyric acid-25% ammonia-water (50 1.1 28.9,... 

An A-B diblock copolymer is a polymer consisting of a sequence of A-type monomers chemically joined to a sequence of B-type monomers. Even a small amount of incompatibility (difference in interactions) between monomers A and monomers B can induce phase transitions. However, A-homopolymer and B-homopolymer are chemically joined in a diblock therefore a system of diblocks cannot undergo a macroscopic phase separation. Instead a number of order-disorder phase transitions take place in the system between the isotropic phase and spatially ordered phases in which A-rich and B-rich domains, of the size of a diblock copolymer, are periodically arranged in lamellar, hexagonal, body-centered cubic (bcc), and the double gyroid structures. The covalent bond joining the blocks rests at the interface between A-rich and B-rich domains. 

Figure 1. Chemical structures of the monomers used in this work. Shown are a) p-phenylene diacrylate (PPDA- melting point 89° C), b) 1,6-hexanediol diacrylate (HDDA- melting point 5° C, boiling point 316° C), and c) C6M, a liquid crystalline diacrylate (phase sequence Isotropic - 116° C -> Nematic -> 86° C - Crystalline.)...

The sequence of their occurrence is determined by the rates of chemical transformations at the interfaces. It cannot yet be theoretically predicted with full confidence for any particular reaction couple A-B. Having sufficient information on the equilibrium phase diagram, thermodynamics of chemical reactions, and the structure and physical-chemical properties of the compounds, it is possible to indicate those of them, which are most likely to occur and grow first at the A-B interface. 

---