Dynamic rotational isomeric state model

I. Bahar and B. Erman, Macromolecules, 20, 1368 (1987). Investigation of Local Motions in Polymers by the Dynamic Rotational Isomeric State Model. 

Markov Processes and Master Equation Formalism. . . 155 2.L2 Dynamic Rotational Isomeric State Model... 

Dynamic Rotational Isomeric State Model and Assumptions... 

Dynamic Rotational Isomeric State (DRIS) Model... 

The rotational isomeric state model is a well established technique for deriving the static properties of polymers from the chemical structure. The dynamic rotational isomeric state approach is its dynamic counterpart. It is the first step for gaining insights as to the role of the intrinsic chemical structure on observed properties. It may be applied to the prediction trf the intrinsic dynamics of polymers, i.e., those occurring in the absence of external effects, and to the examination of the relative relaxation rates of different units along a givoi chain. This will pertain to both homopolymers composed of different types rrf backbone atoms, and to copolymers built of different monomeric units. 

Before embarking on a discussion of the results of these studies let us add one historical note. The difficulty with swinging the polymer tails in a conformational transition has been recognized for many years. A means of circumventing was proposed by Schatzki. Verdier and Stockmayer had earlier invoked a similar principle but used it only to produce Rouse modes. We know now that slow Rouse modes are insensitive to the details of the faster time-scale dynamics. The proposed motions are completely local, and involve going from one equilibrium rotational isomeric state to another by moving only a finite, small number of atoms. Mechanisms of this class have come to be known as crankshaft motions (a term applicable in the strictest sense only to the Schatzki proposal). Because of the limited amount of motion and the simplicity of the dynamics these models are easy to understand, analyze, and simulate. This probably contributes to the continued attention devoted to them. The crankshaft idea has helped to focus attention on the necessity to localize the motion associated with conformational transitions, but complete localization is too restrictive. There are theoretical objections that can be raised to the crankshaft mechanism, but the bottom line is that no signs of it are found in our simulations. 

Fluorescence is measured in dilute solution of model compounds for polymers of 2,6-naphthalene dicarboxylic acid and eight different glycols. The ratio of excimer to monomer emission depends on the glycol used. Studies as functions of temperature and solvent show that, in contrast with the analogous polyesters in which the naphthalene moiety is replaced with a benzene ring, there can be a substantial dynamic component to the excimer emission. Extrapolation to media of infinite viscosity shows that in the absence of rotational isomerism during the lifetime of the singlet excited state, there is an odd-even effect In the series in which the flexible spacers differ in the number of methylene units, but not in the series in which the flexible spacers differ in the number of oxyethylene units. 

FIG. 2.2 Rotation and inversion pathways of isomerization (A). Dynamic mode-coupling model of DMAAB in the excited state (B and C). 

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