Chain extension experiments poly

It is interesting to note that junction fluctuations increase in the direction of stretching but decrease in the direction perpendicular to it. Therefore the modulus decreases in the direction of stretching, but increases in the normal direction since the state of the network probed in this direction tends to be more nearly affine. The curve of [/" ] versus 1/a is sigmoidal. The parameters k and f of poly(dimethylsiloxane) networks are determined in Figure 17 (155) the intercept of the sigmoidal curves is the phantom modulus. This Flory-Erman theory has been compared successfully with such experiments in elongation and compression (155,162,162-166). It has not yet been extended to take account of limited chain extensibility or strain-induced crystallization (167). 

Ti values may occur with such native biopolymers as ribonuclease A, deoxyribonucleic acid, and collagen, whose molecular motions are restricted, but, as yet, high values have not been observed for polysaccharides in solution, or for gels, in which these motional-restriction effects may be equivalent, or less marked. However, an extensive relaxation-study by Levy and coworkers68 on poly(n-alkyl methacrylates) may serve as a model for future experiments on polysaccharides, as this type of molecule has a main chain and side chains, albeit more mobile than those in polysaccharides. 

Fig. 1. Principle of the pulse-chase experiment. The pulse-labeled poly(ADP-ribose) chain (n residues) is shown elongated by proximal extension or distal extension. Labeled phosphate is circled. Digestion of poly(ADP-ribose) produces PR-AMP and AMP from the inner residues and Ae distal end, respectively. The expected numbers of labeled nucleotides among the total digests are written in parentheses.

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