Dual entropic and enthalpic processes in the lower critical solution temperature phase separation of poly(vinyl methyl ether) aqueous solutions

Abstract

In the lower critical solution temperature phase separation of poly(vinyl methyl ether) aqueous solutions, the process corresponding to the weakening of the hydrogen bond interaction with increasing temperature is dominant and occurs over the entire concentration region of solutions and over a broad temperature range from 30 to 41°C, giving rise to the energetic enthalpic effect during phase separation, while the conformational change, that is, collapse of the swollen polymer coils, occurs only in the swelling polymer solution when the water concentration is above 38.3 wt %, giving rise to the entropic effect during phase separation. In addition, the entropic process corresponding to the collapse of the polymer coils occurs in a much narrow theta temperature range from 35.5 to 37°C. If the solution is held at a constant temperature for a sufficiently long time, 90% collapse of the polymer coils occurs in only the 0.5 °C temperature region between 35.5 and 36°C. Accordingly, in the enthalpic process, the most dramatic blueshift of the νC‐O bond peak occurs in the temperature range between 35 and 41°C, while this blueshift is only approximately 2 cm−1 in the temperature range from 30 to 35°C, prior to the collapse of the polymer coils due to the entropic effect. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 323–330

Quantitative determination of the spring entropy effect and its indication of the conformational change of polymer coils with varying concentration in aqueous poly(N-isopropylamide) solutions

“…Subsequently, similar entropic effects were also observed in another type III LCST behavior poly(vinyl methyl ether) (PVME) aqueous solutions. 39,42 In contrast to the complicate chemical structure in PNIPAM, the structure of PVME is very simple and there is only one polar oxygen ether atom in each repeating unit and can form hydrogen bond with a maximum of two water molecules. The DSC results showed that below the critical water concentration of 38.3 wt% (i.e.…”

“…39 Although the swelling state of polymer coils in extremely dilute solution and rather compact globule state of polymer chains in bulk state are clear for PNIPAM, it is still extremely challenging and difficult to conrm when and how the compact PNIPAM globules begin to swell with the continuous dilution of water solvent in concentrated solutions. Based on the above studies, 39,41,42 in this work, the entropic effects in the demixing enthalpy recovery behavior in different concentration regions and its implications on the conformational changes of polymer coils swelling state in aqueous PNIPAM solutions are investigated.…”