Evidence on Chain Dimensions from Chemical Properties

Evidence on Chain Dimensions from Chemical Properties 

The total number of cross-links per gram is directly proportional to radiation dose. Either RSMW or JRg(Mw)2/3 should be independent of initial molecular weight, depending on whether all repeating units or only those near the surface of the coils have equal probabilities for intermolecular cross-link formation. Complete coil collapse is clearly ruled out by the results. Moreover, detailed calculations (46) show that a trend in the product RgMw due to systematic differences in intramolecular cross-linking, should be observed even for only partially collapsed random coils. If the coil radii in bulk polystyrene were reduced by even as much 

More quantitative chemical evidence for random coil configuration comes from cyclization equilibria in chain molecules (49). According to the random coil model there must be a very definite relationship among the concentrations of x-mer rings in an equilibrated system, since the cyclization equilibrium constant Kx should depend on configurational entropy and therefore on equilibrium chain and ring dimensions. Values of r2 /Af deduced from experimental values on Kx for polydimethylsiloxane, both in bulk and in concentrated solution, agree very well with unperturbed dimensions deduced from dilute solution measurements(49). 

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