Rigid rodlike molecules

Fig. 5. Surface diffusion of the rigid rodlike molecule 4-trans-2-(pyrid-4-yl-vinyl) benzoic acid on Pd(110). In (a) and (b) two consecutive STM images taken at 361 K are shown which demonstrate the 1-dim motion. Arrows indicate molecules whose position changed circles mark fractionally imaged molecules moving under the STM tip in the course of the measurement, (c) Model for the flat adsorption geometry explaining the two observed molecular orientations in the STM data. The length of the molecule is 12.5 A. (d) Arrhenius plot of single molecule hopping rates [75].

Liquid crystals consist of rigid rodlike molecules whose interaction tends to align them parallel to each other (for more details, see de Gennes, 1974 ... 

D. B. Cotts and G. C. Berry, Polymerization kinetics of rigid rodlike molecules - polycondensation of poly((benzo(l,2-D-5,4-D )bisoxazole-2,6-diyl)-l,4-phenylene). Macromolecules, 14 (1981) 930. 

U. S. Agarwal and D. V. Khakhar, Shear flow induced orientation development during homogeneous solution polymerization of rigid rodlike molecules. Macromolecules, 26 (1993) 3960-3965. 

Molecular composites as an extension of fiber reinforcement Molecular composites is designed to use rigid rodlike molecules as reinforcement for the flexible coil molecules as matrix. The patent applications on the molecular composites were made almost in the same age independently by Takayanagi in Japan in 1977 and by Helminiak in the United States in 1978. Takayanagi proposed thermoplastic nylon reinforced by poly(p-phenylene terephthalamide)(PPTA) and Helminiak proposed wet process using poly(p-phenylene benzobisthiazole)(PBT)-reinforced poly(2,5(6)benzimidazole) (ABPBI). In molecular composite (MC) [15,16,17], the fineness of reinforcement was pursued to its limit, i.e. to the molecular dimension. 

C. The only real breakthrough apart from aramids, (where chemists did the work of chain alignment by synthesizing rigid rodlike molecules) has been realised using polyethylene. 

The concentration of rigid rodlike molecules cannot be increased very much before spontaneous separation occurs into a concentrated ordered and a very dilute disordered phase, as required by thermodynamic relations, although the separation may occur at higher concentrations if there is some degree of flexibility, and may be delayed by steric hindrance so that a metastable disordered state may persist. The rotation of rigid rods at finite concentration is impeded by collisions and the rotatory relaxation time should increase rapidly with concentration. The theory has been treated by Doi and Doi and Edwards, who conclude that the relaxation time is approximately proportional to c L, where L is the molecular length. 

In atomistic simulations, molecular order can be characterised in a number of ways. For rigid rodlike molecules, orientational ordering may be defined by reference to the molecular long axis vector aji... 

Agarwal US, Mashelkar RA (1992) Diffusion of rigid rodlike molecules across interfaces implications in welding of liquid-crystalline polymers. Macromolecules 25 6703-6704 Brolly JB, Bower DI, Ward IM (1996) Diffusion and sorption of CO2 in poly(ethylene terephthal-ate) and poly(ethylene naphthalate). J Polym Sci Part B Polym Phys 34 769-780 Brooks GT (1992) Crystalline polyphthalamide composition having improved properties. US Patent 5,098,940, 24 Mar 1992... 

Liquid crystals (LCs) may be divided into two subgroups (1) lyotropic LCs, formed by mixing rigid rodlike molecules with a solvent, and (2) thermotropic LCs, formed by heating. One finds in the literature such terms as mesomorphs, mesoforms, mesomorphic states, and anisotropic liquids. The molecules in LCs have an orderly arrangement, and different orders of structures (nematic, smectic, or cholesteric structure) have been observed, as schematically shown in Figure 9.1. The kinds of molecules that form LCs generally possess certain common molecular features. The structural characteristics that determine the type of mesomorphism exhibited by various molecules have been reviewed. 

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