Helix reversals

A conformationally disordered mesomorphic form is present, for instance, in the high-temperature phase I of PTFE. In this form, a long-range 3-D order is present only in the periodic pseudohexagonal placement of the chain axes [49]. In fact intramolecular helix reversals would produce the conformational disorder [50-52] and a complete intermolecular rotational disorder would be also present [49, 52,53]. 

Examples of mesomorphic forms characterized by disorder in the conformation of the chains have already been described in Section 2.6. For instance, a mesomorphic form is present in the high-temperature form I of polytetrafluoro-ethylene.106,107 In this phase the chains are in disordered conformation due to the presence of helix reversals along the chains.108-110 Moreover, intermolec-ular disorder is also present due to the random rotations of the chains around the chain axes.109 A long-range three-dimensional order is present only in the pseudohexagonal placement of the chain axes.107,109... 

It is possible that the helicity is a result of the chiral substitution itself and that the polymers with achiral substituents have, in fact, all-anti conformations. While this possibility cannot be directly ruled out, comparison of the spectroscopic data for the polymers with chiral substituents and achiral substituents, for example, 47 and 48, respectively, indicates similar main-chain dihedral angles, since the UV absorption maxima are so similar. Both polymers should therefore be latent helical, that is, contain segments of opposite screw sense separated by strong kinks (helix reversal points), with the difference being that in the case of 47 the overall numbers of P and M turns are equal, whereas for 48, one of the screw senses predominates, resulting in net helicity and optical activity. 

Recent work by this group has focused upon elastic deformation of these helices and their response to uniaxial forces.163 Under sufficient tension, a low-pitch helix reversibly separates into a straight domain with a pitch angle of 90° and a helical domain with a pitch angle of 16.5°, as shown in Figure 5.44. 

At first thought, it might be considered that the steric restrictions posed by the large fluorine atoms would cause conformational rigidity due to restricted bond rotations. Hsu, however, showed that the conformations of the TFE chains in the crystalline regions in Nafion are in fact dynamic in that they can undergo helix reversals that is, the handedness of the helix is easily reversed.These helix reversals are also seen in PTFE.2 o This disorder phenomenon causes considerable conformational entropy and is... 

Angular displacements about the helical axes were also monitored in the simulations as the helix reversals moved along the. chains. This behavior is depicted in Figure 10.6, where angular displacements have been averaged over the 20 -CF2- groups in the middle of Chains 1 and 2 during the 273 K simulation. 

The simulation results indicate that helix reversals form and migrate easily in PTFE crystal structures. Their motion in neighboring chains can be coupled. In simulations at both temperatures, the reversal bands formed mostly at chain ends where torsional motion was relatively unrestricted, but also were observed to form in the center of some of the helices. This latter point of origin, though more energetically and mechanistically difficult (because it involves coordinated motion... 

Yashima and co-workers reported the memory of macromolecular helicity of poly((4-carboxyphenyl)acetylene) (poly-98). Poly-98 itself possesses a large number of short helical units with many helix-reversal points, and is therefore achiral. However, in the presence of optically active amine 99, which can interact with the polymer s carboxyl groups, one-handed macromolecular helicity is induced in the polymer. When achiral amino alcohol 100 is added to the helical complex, chiral amine 99 bound to poly-98 is replaced by stronger base 100. Nevertheless, the newly formed complex still shows a one-handed helical conformation. Even after the removal of 99 by gel permeation chromatography, the poly-98-100 complex retains a one-handed helical conformation without a loss of helical intensity. Thus the helicity of poly-98 induced by complexation with a chiral amine was memorized after replacement by an achiral one. The half-life of the chiral memory is as long as four years at room temperature.48... 

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