Researchers from the Higher School of Economics and the University of Leipzig have created a model which enables the timely and effective prediction of polymer behavior in mixed solvents. This is the first scientific work to explain, using statistical mechanics, the effect of suppression of co-nonsolvency at high pressures. The findings have been published in the journal Soft Matter: http://pubs.rsc.org/en/content/articlelanding/2017/sm/c7sm01637a#!divAbstract
A good solvent is one in which a polymer will dissolve well. Let's take water and methanol: both are good solvents, if used separately, for a polymer such as poly(N-isopropylacrylamide) (PNIPA). However, should these two solvents be mixed, the polymer will no longer dissolve. Such is the paradoxical nature of what is known as 'co-nonsolvency'. And, despite the fact that this phenomenon has long been known to scientists, the physical mechanisms responsible have never been systematically studied - until now.
'Water tends to form hydrogen bonds with the macromolecule PNIPA, as does methanol. However, the hydrogen bonds between PNIPA and the alcohol functional group on a methanol molecule are stronger than the hydrogen bonds formed when it's in water. This 'competition' means that, at a certain concentration of alcohol, the polymer chains fold more easily, forming compact globules which collect the alcohol and displace the water. It was thought that this phenomenon could only occur in systems where hydrogen bonds can form. However, co-nonsolvency was experimentally observed as early as 1978 in the polystyrene-cyclohexane-dimethylformamide ternary system - a system in which there are no hydrogen bonds. It was consequently deemed by researchers working under the guidance of Professor Kremer to be a common physical effect,' explains Yury Budkov, Associate Professor at HSE Tikhonov Moscow Institute of Electronics and Mathematics.
The researchers formulated a model of a flexible polymer chain in mixed solvents and showed that, within a certain earlier predicted range of parameters of intermolecular interactions and co-solvent concentrations, the polymer chain collapses. That is, co-nonsolvency is observed. Also, it had been previously demonstrated that at high pressures, co-nonsolvency can be suppressed, however this mechanism had never been explained theoretically. Researchers were able to confirm this conclusion using general physical principles and explained the mechanism of suppression of co-nonsolvency on the microscale in detail.
The mathematical model created by scientists enables the timely prediction of polymer behavior for a wide range of parameters. Not only are the findings of academic interest, they are also an integral contributor to developing methods for the express analysis of real polymer systems in real mixed solvents. This significantly saves time during the experimental phase. The research findings will be applied in fields such as medicine, pharmacology (targeted delivery of drug compounds and the synthesis of long-acting drugs) and the food industry (production of suspensions such as mayonnaise, ketchup and dairy products).
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Journal
Soft Matter