The STOCKMAYER‐FIXMAN method has been applied to determine the unperturbed dimensions of a polymer chain from viscosity measurements on several fractions in a good solvent. Homopolymers yield values of the unperturbed dimensions which are independent of the nature of solvent and which hardly depend upon temperature.
Using the same method for [η] measurements in different solvents, the values obtained for random copolymers PS–PMMA are quite different from the “ideal” values calculated on the basis of additivity rules from the unperturbed dimensions of corresponding homopolymers. The discrepancy was shown to depend on the χAB parameter, which also governs the critical concentration for phase separation for a solution of the corresponding homo polymers.
Since χAB is solvent and temperature dependent, the observed variation in the unperturbed dimensions from one solvent to another can be qualitatively explained. Linear relation was found between χAB and KΘ leading to KΘ “ideal” for χAB = 0.
Moreover, the temperature dependence of unperturbed dimensions for a random copolymer, in a given solvent, is much stronger than for an homopolymer. As temperature increases χAB decreases and the unperturbed dimensions approach the “ideal” values.
These results show the influence of heterocontact interactions on chain conformation. They clearly demonstrate their importance at theta conditions by influencing the dimensions of the chain.
It has been shown recently that the application of the two parameter theory to polymer solutions is limited to the case of non polar polymers dissolved in non polar solvents.
In this work we have studied the behavior of poly‐2‐vinylpyridine in different solvents as a function of temperature.
The larger values of KΘ may be explained if it is assumed that the volume of the monomeric units is increased when the solvent (CHCl3, ethanol) is able to form H‐bonds with pyridine groups.
In other solvents (benzene, THF) there can be intramolecular hydrogen bonds which decrease the unperturbed dimensions of the molecules and intermolecular hydrogen bonds which favorize aggregations.
During this work we have also shown the existence of a transition in the unperturbed dimensions around 25°C in benzene and THF. This «order‐disorder» transition is also due to intramolecular hydrogen bonds and is very similar to what has been observed in the case of polystyrene.
It was also observed during these investigations that benzene at 15°C is a Θ‐solvent for poly‐2‐vinylpyridine.
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