Monitoring of cluster formation and elimination in PEO solutions

It has been shown in a recent work that the formation of aggregates in dilute PEO solutions depends on the history of the samples. In the current study it is confirmed by static and quasi-elastic light scattering measurements that, as soon as a PEO of small molecular weight (M = 6500) has been dissolved in water at high temperature (89 degreesC), subsequent dissolution of this sample in methanol reveals the presence of aggregates. The dimensions and the concentration of these aggregates depend on the time the sample has stayed in water. They are very stable and cannot be destroyed later on by dissolution in water or water 0.2 N HCl at room temperature or by heating under vacuum at high temperature (160 degreesC). However, they can be separated by centrifugation of PEO solutions in methanol. The sample recovered from the top of the centrifuged part is free from aggregates, but subsequent dissolution of this sample in hot water regenerates new aggregates. These observations lead to the conclusion that the formation of aggregates in the PEO solutions is due to hydrophobic interactions.