Rodlike polymer conformation

It is worth recalling that any of the molecular force laws given by Eqs. (13-16) are derived within the framework of the freely-jointed model which considers the polymer chain as completely limp except for the spring force which resists stretching thus f(r) is purely entropic in nature and comes from the flexibility of the joints which permits the existence of a large number of conformations. With rodlike polymers, the statistical number of conformations is reduced to one and f(r) actually vanishes when the chain is in a fully extended state. 

The distinct properties of liquid-crystalline polymer solutions arise mainly from extended conformations of the polymers. Thus it is reasonable to start theoretical considerations of liquid-crystalline polymers from those of straight rods. Long ago, Onsager [2] and Flory [3] worked out statistical thermodynamic theories for rodlike polymer solutions, which aimed at explaining the isotropic-liquid crystal phase behavior of liquid-crystalline polymer solutions. Dynamical properties of these systems have often been discussed by using the tube model theory for rodlike polymer solutions due originally to Doi and Edwards [4], This theory, the counterpart of Doi and Edward s tube model theory for flexible polymers, can intuitively explain the dynamic difference between rodlike and flexible polymers in concentrated systems [4]. 

Fig. la. Plot of Mark-Houwink equations for rodlike polymers and some conformers, numbered as in the legend given with Fig. lb. Rectangle shows the range of precision (Flo) by the selection of data which ignores the nature of the solvent and the instrumental method used in the determination of molecular weight... 

In contrast to polypeptides that have many possible conformations, poly(hexyl isocynate) is known to have a stiff rodlike helical conformation in the solid state and in a wide range of solvents, which is responsible for the formation of a nematic liquid crystalline phase.45-47 The inherent chain stiffness of this polymer is primarily determined by chemical structure rather than by intramolecular hydrogen bonding. This results in a greater stability in the stiff rodlike characteristics in the solution as compared to polypeptides. The lyotropic liquid crystalline behavior in a number of different solvents was extensively studied by Aharoni et al.48-50 In contrast to homopolymers, interesting new supramolecular structures can be expected if a flexible block is connected to the rigid polyisocyanate block (rod—coil copolymers) because the molecule imparts both microphase separation characteristics of the blocks and a tendency of rod segments to form anisotropic order. 

It is valid only for polymer conformations that do not deviate too much from the rodlike reference state. The electrostatic persistence length gives a sizable contribution to the effective persistence length only for osF > fo- This is equivalent to the condition... 

Polymer molecules in solution also display Brownian motion. Because the polymer molecule is not a simple sphere, each polymer conformation has its own diffusion characteristics. For rigid molecules, the shape of the molecule, spherical or rodlike, for instance, makes a difference. For a Unear flexible molecule, connectivity... 

Commercial PAN is normally produced as an atactic polymer with strong hydrogen-bonded intermolecular forces. Because of repulsion between cyano pendant groups and intermolecular hydrogen bonds, the molecule assumes a crystallizable rodlike conformation. The hydrogen bonds between the rodlike chains create bundles of these chains. PAN may be spun into strong fibers. It has a Tg of 104 C... 

The monomer-monomer correlation functions of flexible polyelectrolytes exhibit qualitatively the same behavior as those for rod-like molecules. The conformational changes, however, result in more pronounced and shifted peaks. From Fig. 8 we deduce a shift of the peaks of flexible chains to larger distances compared to those of rod-like chains. This is a consequence of a smaller overlap between flexible chains compared to the one between rodlike molecules. Naturally, the effect is most pronounced for densities larger than the overlap densities. The increased peak intensity corresponds to a more pronounced order in the system of flexible chains, and is a result of the more compact structure of a polymer coil. (The structural properties of flexible polyelectrolytes without medium-induced potential have been studied in [48].)... 

The fundamental questions are What is the microscopic mechanism or driving force for the transition, and what physical factors are important Two distinct possibilities have been advanced side-chain crystallization (5, 6, 17-19), which is postulated to induce polymer backbone ordering, and conformation-dependent polymer-solvent interactions that arise explicitly from electron delocalization and that stabilize an ordered rodlike conformation (20-24). Side-chain crystallization remains a qualitative suggestion that has not been developed to the point where it has predictive power and can be critically tested. However, in the solid state, the enhanced importance of packing effects makes such a mechanism more plausible (18, 19). 

Polymers assume a rodlike conformation, as opposed to the typical random coil conformation, when the chemical structure (e.g., para connected benzene rings) or molecular folding (e.g., a-helical structures) prevents internal rotation and thus local bending. Examples of such rodlike molecules in biological... 

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