Entropic effect implication for change in polymer coils swelling state in the demixing enthalpy recovery of aqueous poly(vinyl methyl ether) solutions

Abstract

Time-dependent demixing enthalpy recovery behavior of aqueous poly(vinyl methyl ether) (PVME) solutions exhibits distinct recovery characteristics in three concentration regions. The absence of recovery behavior below a water concentration of 38.3 wt % indicates that the PVME coil is in a globular state. The typically sigmoidal recovery behavior of demixing enthalpy above 38.3 wt % is ascribed to the reswelling of the collapsed polymer coils induced by the entropic effect. The increase in difference between the upper and lower limits indicates the continued swelling of the PVME coils. Above 65 wt %, a dominant diluting effect can be observed, and a much longer phase separation time is needed to reach the expected lower limit. In contrast, the recovery of demixing enthalpy in a wide range of water concentration (from 38.3 to 90 wt %) exhibits the same feature. The infrared spectroscopy results are in agreement with the above macroscopic differential scanning calorimetry results.

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

“…Moreover, it should be noted that in the previous investigations the determination of entropic contributions in thermodynamic energy term is usually elucidated out from a series of molecules with similar chemical or complex characters, [43][44][45][46][47] in this study and our recent work about PVME aqueous solutions 39 we showed that the entropic effect can be directly determined from a single polymer solution.…”

“…As discussed in the recent work about PVME aqueous solutions, 39 the distinctive demixing enthalpy recovery behaviors in different concentration solutions may be attributed to the entropic effect due to changes in polymer chain swelling states.…”

“…As discussed in a recent work, 39 in the demixing process of a polymer solution, the heat or enthalpy (DH total ) and free energy (DG total ) of demixing and the total demixing entropy (DS total ) can be described as:…”

“…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.…”

“…Above 65 wt%, a dominant diluting effect can be observed, and a much longer phase separation time is needed to reach the expected lower limit. 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.…”