FT-IR and 2D-IR Spectroscopic Studies on the Effect of Ions on the Phase Separation Behavior of PVME Aqueous Solution

Abstract

Aqueous solutions of some water-soluble polymers have drawn considerable interest as they exhibit phase separation above the critical temperature (LCST). [1][2][3][4] In such processes the polymer experiences great and reversible changes in its conformation and volume. Some typical examples of such polymers are poly(N-substituted acrylamide)s and poly(vinyl ether)s, which have amide or ether oxygen groups as hydrophilic moieties. In contrast to poly(ethylene oxide) (PEO) 5 and poly(propylene oxide) (PPO), 6 which usually have LCST values higher than 100˚C, poly(vinyl methyl ether) (PVME) has a lower critical separation temperature at a moderate temperature range around 35˚C which makes it suitable for industrial and medical applications such as actuators, 7 artificial muscles, 8 drug delivery systems, 9,10 and thermoresponsive surfaces. 11,12 The phase behavior of water-soluble polymer in aqueous solution reflects the interactions between polymer segments and water molecule. 13 PVME has a hydrophilic ether group, which can stabilize the aqueous solution by forming hydrogen bonds, and a hydrophobic part, which can destabilize the solution by altering bulk water. The hydrophobic interaction increases monotonously during heating, which finally breaks the balance between the hydrogen bonding and hydrophobic interaction and induces the phase separation. Heskins and Guillet 14 explained the phase separation behavior of water-soluble polymer by Flory-Huggins lattice theory in terms of thermodynamics. To describe the different types of phase separation which have been experimentally discovered, Berghmans 15-18 introduced extensive concentration dependence as a cubic polynomial function to get a better fit of the experiment data. In this respect, three types have been defined, among these, PVME belongs to type III 17 which is characterized by a bimodal demixing curve. In this case, two critical points exist in the phase transition: one is equal to zero at θ temperature, the other is almost independent of the molecular mass. This result has been confirmed by other scientists. [19][20][21][22] Maeda et al. 20 found that the phase separation of PVME proceeds in two successive steps by DSC, while Durme et al. 19 revealed that the initial stage of phase separation, above LCST, is followed by large compositional changes upon passing the temperature for the three-phase equilibrium.In order to control the phase separation temperature of PVME aqueous solution, many studies have been carried out on the systems of PVME aqueous solutions with different kinds of additives. Spevacek et al. investigated PVME/D2O system by NMR methods. [22][23][24] Schild et al. 13 have revealed that the LCST of PVME gradually increased with the increase of the concentration of methanol, while Horne et al. 25 have investigated the effects of electrolytes, urea and alcohols on the phase separation of PVME. Maeda et al. have examined the effects of inorganic 20 ions and tetraalkylammonium 26 ions on the LCST of PVME by IR and Raman spectroscopy. Durm...

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Two-dimensional correlation spectroscopy — Biannual survey 2007–2009

“…Gu and Wu [165] carried out 2D ATR-IR study of temperature-induced phase behavior of aqueous poly(vinyl methyl ether) solutions. It was found that conformational change of coilglobule transition occurs with main chain first followed by side group ether.…”

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