How Can You Measure the Statistic Parameters?

The determination of the end-to-end distance (or its variables, as s and a factors) and the radius of gyration are fundamental for the conformational analysis of macromolecules in solution.

Working under the q state is the simplest method to perform this study, since long range interactions do not take place, and the polymer remains in its unperturbed dimension, being only influenced by factors of structural nature. However, q state is not always easily found. For this reason semi-empirical methods must be applied, also taking into account the non-s state, and separating the short-range interactions from the long-range interactions.

To this end, there are some methods based both on light scattering or the viscosity, that only are applicable to dilute solutions. Both methods allow knowing the r2o and s2o values, under q state, or r2 under non-q state. Nevertheless, the measurements by light scattering suffer from several errors when low molecular weight polymers are analyzed. On the other hand, determinations made by viscometry, a method more intensely used, has serious disadvantages when trying to find suitable q solvents. These difficulties have concentrated the efforts on the determination of the unperturbed dimensions under non-q state.

From intrinsic viscosity measurements, that is the parameter that increases according to the molecular weight and represents the capacity of a polymer to increase the viscosity of the solution, it has also been possible to evaluate the linear expansion factor a. The obtained results, considering a polymer in a given solvent and temperature, showed that as the chain molecular weight increases, the factor a also increases (Flory, 1971).

The determination of the conformation factor s, that allows knowing the degree of flexibility shown by macromolecules, can also be obtained from measurements of intrinsic viscosity, once the value of r2 has been established. This data is extremely useful when trying to correlate macromolecules of diverse chemical structure.

It has been found that bulky groups, as well as their specific interactions, cause a remarkable increase in the value of coefficient s (Abuin S. et al, 1987). On the contrary, the introduction of -CH2 groups in the chain confers certain flexibility to it, causing that s factor is smaller to those homologous without the presence of these groups.

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