Rodlike conformation

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 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). 

Another class of soluble polysilylenes exhibits essentially no or very weak thermochromism. This class includes poly(cyclohexylmethyl- 15, 38), poly(phenylmethyl- 15, 38), (polytrimethylsilylmethyl- 15), and poly(diarylsilylenes) 46), all of which appear to be conformationally locked over a wide range of temperatures. In terms of our theoretical perspective, this behavior would arise from the steric effects of bulky substituents, which imply a large value of e and, hence, a small coupling constant Vj /e. For aryl-substituted polysilylenes, the proximity of an aromatic group to the backbone could also stabilize a highly ordered rodlike conformation via enhanced dispersion interactions. 

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... 

A complementary approach is to fix the conformation of the chain and to focus on a detailed description of the counterion distribution. Usually polyelectrolytes stretch due to the electrostatic repulsion of their charged groups. Moreover, many important polyelectrolytes have a large intrinsic stiffness (e.g., DNA, actin filaments, or microtubules). Therefore a rodlike conformation is an obvious first choice see Figure 2. The remaining problem of charged rods immersed into solution is much easier but is still far from being exactly solvable. 

Collagen molecules have a triple-helix and rodlike conformation. Their length and form depend on the type of collagen. The type I collagen molecule, tropocollagen, is approximately 300 nm long and 1.5nm in diameter. It is composed of a triplehelix atelocollagen and N-terminal and C-terminal nonhelical telopeptides (Fig. 1). 

It is likely that stiffening of chains is a result of two effects. The first effect is due to electronic delocalization and the second to counterion crowding. The second effect has been discussed in detail in Section IV. The importance of the second effect was confirmed in Ref. 43 by dissolving PANi in concentrated sulfuric acid (which is of significantly smaller size). Electronic delocalization should be the same for both acids, but the crowding effect for CSA is higher. The existence of liquid crystalline PANi(CSA) solutions indicates a rodlike conformation of the polymer chains. 

The difference between the electrostatic energy per monomer in the circular and rodlike conformations is... 

On the other hand, when the electric field is strong, DNA is no longer considered to move as a random coil polymer, but moves with extended rodlike conformation. If DNA is completely extended and moves straight to the field direction, hx ) = L , showing that the mobility in Eq. (7) is independent of DNA size ... 

The field induced orientation of poly-4BCMU in toluene in the red phase (<72 C) indicates an anisotropic polarizability tensor characteristic of a rodlike conformation. The field-induced signal from the polymer in its coil conformation (yellow phase) is extremely weak (smaller by a factor of at least 10 ) confirming the rod-coil transition. From studies of the transient response of the anisotropic light scattering from the red solution, we find a rotational relaxation time of 0.1 sec. This value is consistent with the rotational diffusion constant expected for rodlike polydiacetylene macromolecules. 

Since polydiacetylene 4BCMU has a rodlike conformation with a large axial ratio (length/diameter) in certain solvents, it is a likely candidate for a lyotropic polymer. However, it has low solubility in those solvents and no liquid crystalline phase has been observed. As described above, a rod-to-coil conformational transition can be induced in the PDA 4BCMU by changing the solvent... 

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