Rod-to-coil transitions

Much confusion had arisen from the polyelectrolyte-like viscosity of ionic polysoaps at low concentrations. Based on the old polyelectrolyte model of coil-to-rod transition [339-342], it was suggested that the strong increase of reduced viscosities at low concentrations would indicate a transition of intramolecularly aggregated coils to extended conformations of the polysoaps which are no more aggregated. This would mean that the polysoap characteristics would be suspended at low concentrations [126, 229, 292]. 

The interpretation of the sol-gel transition was controversial. Several groups [10,32,33,38] argued that a coil-to-rod transition was responsible for the yellow to red colour change, while other groups [30,34,49] hold the opposing view that the colour change in the polymer solutions results from the onset of aggregation. 

If the above conclusions are applied to address the abrupt thermochromism of poly(dihexylsilane) not only in the solid state but also in solution, the conclusion is that the increase in UV absorption intensity does indeed result from a coil-to-rod transition but that the red shift arises from a helix-to-helix transition of the main chain. Recent calculations have indicated that the absorption maximum gradually red shifts as the helical dihedral angle changes from 60° to... 

As early as 1932 Trommsdorf [141] and Staudinger [142] observed the viscosity of PMA in pure water to increase dramatically with decreasing polyion concentration. The same observations were made later by Kern [143]. This is in contrast to the viscosity behaviour usually found for neutral polymers. Later, Fuoss and Strauss developed their famous empirical extrapolation and explained the polyelectrolyte effect by a coil-to-rod transition because, upon dilution, the ionic strength decreases, eventually leading to a fully stretched polyion as anticipated at the time. 

The discontinuous transition observed in some cases (Figure 11) is probably due to the freezing-in of all-trans segments of the polymer backbone. This transition has been attributed to a coil-to-rod transition93, and to interaction between side-chains leading to microcrystallization94b both descriptions may be valid. 

The possibility of entropy-driven phase separation in purely hard-core fluids has been of considerable recent interest experimentally, theoretically, and via computer simulations. Systems studied include binary mixtures of spheres (or colloids) of different diameters, mixtures of large colloidal spheres and flexible polymers, mixtures of colloidal spheres and rods," and a polymer/small molecule solvent mixture under infinite dilution conditions (here an athermal conformational coil-to-globule transition can occur)." For the latter three problems, PRISM theory could be applied, but to the best of our knowledge has not. The first problem is an old one solved analytically using PY integral equation theory by Lebowitz and Rowlinson." No liquid-liquid phase separation... 

Rao, J., Luo, Z., Ge, Z. et al. (2007) Schizophrenic miceUization associated with coil-to-helix transitions based on polypeptide hybrid double hydiophihc rod-coil diblock copolymer. Biomacromolecules, 8,3871-3878. 

In a study of the transition in conformation from random coil to stiff rod by poly(acrylic acid), it was found that the point of transition depended on a number of factors, including the nature of the solvent, the temperature, the particular counterion used and the degree of dissociation (Klooster, van der Trouw Mandel, 1984). 

A theoretical treatment has recently been carried out by the author in collaboration with Matheson along the lines discussed above with appeal only to the spatial requirements of hard rods as represented in the lattice model, orientation-dependent interactions being appropriately ignored. The two transitions, one conformational and the other a cooperative intermolecular transition, are found to be mutually affected each promotes the other as expected. The coil-helix conformational transition is markedly sharpened so that it becomes virtually discrete, and hence may be represented as a transition of first-order. These deductions follow from the steric interactions of hard rods alone intermolecular attractive forces, either orientation-dependent or isotropic, are not required. 

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