Energetics and Mechanisms of poly(N-isopropylacrylamide) Phase Transitions in Water–Methanol Solutions

Abstract

The phase transitions of poly(N-isopropylacrylamide) (PNIPAM) in water−methanol mixed solutions were studied in detail by high-sensitivity differential scanning calorimetry. From this study, the dependences of the transition temperature, enthalpy, heat capacity increment, and width on the methanol molar fraction (x MeOH ) were obtained. The transition temperature passed through a minimum at the methanol molar fraction x MeOH * ∼ 0.35. At x MeOH < x MeOH *, the transition enthalpy decreased quickly with the methanol content and became so small that it could not be measured, even with an increase in the polymer concentration by a hundred times (up to 150 mg mL −1 ). Furthermore, over this x MeOH range, the transition heat capacity increment being negative remained practically constant, but the transition width sharply increased. The transition thermograms were quantitatively described by the Okada−Tanaka theory, which takes into account the role of the polymer−solvent interaction cooperativity in the polymer thermoresponsivity. In terms of this approach, it is assumed that over the defined range of methanol content, PNIPAM possesses the cooperative hydro-solvation structure in the form of water−methanol complexes. The energetics of this structure smoothly decreases with the increase in the methanol content up to a complete disappearance of the structure at x MeOH > x MeOH *. In this range of the methanol content, the phase behavior of PNIPAM seems to be dictated by regularities typical of polymer solutions in organic solvents, that is, how the Flory−Huggins parameter depends on temperature.

Co-Nonsolvency Effect in Solutions of Poly(methyl methacrylate)-b-poly(N-isopropylacrylamide) Diblock Copolymers in Water/Methanol Mixtures

“…They decrease with increasing volume fraction of CD 3 OD, which is in consistency with the computational and experimental calorimetric enthalpies of PNIPAM homo­polymers in water/methanol mixtures. The endothermic enthalpy is related to the heat demanded to break the hydrogen bonds between the polymers and the solvent molecules. ,, As demonstrated in refs and , the presence of CD 3 OD as a cosolvent reduces the number and strength of the hydrogen bonds between the polymers and the solvent molecules during the phase transition, giving rise to lower values of the enthalpy. In methanol-rich solvent mixtures, T onset DSC is 26.1 ± 0.2 °C at 70 vol % of CD 3 OD.…”

“…Dalgicdir et al found that polymer hydration plays a key role in the co-nonsolvency effect of PNIPAM since methanol geometrically impedes the formation of hydrogen bonding between water and the amide groups of PNIPAM . Grinberg et al performed high-sensitivity differential scanning calorimetry (DSC) measurements and suggested that PNIPAM possesses the cooperative hydrosolvation structures formed by water/methanol complexes, leading to decreasing enthalpies with increasing methanol content . Tavagnacco et al carried out atomistic simulations on a PNIPAM chain in a water/ethanol mixture and found that not only the reduced chemical potential of water in the bulk of the solvent mixture but also the competition between water and ethanol molecules in the interactions with the polymer has a crucial impact on the transition …”

“…28 Grinberg et al performed highsensitivity differential scanning calorimetry (DSC) measurements and suggested that PNIPAM possesses the cooperative hydrosolvation structures formed by water/methanol complexes, leading to decreasing enthalpies with increasing methanol content. 29 Tavagnacco et al carried out atomistic simulations on a PNIPAM chain in a water/ethanol mixture and found that not only the reduced chemical potential of water in the bulk of the solvent mixture but also the competition between water and ethanol molecules in the interactions with the polymer has a crucial impact on the transition. 30 Due to their thermoresponsiveness, PNIPAM-based amphiphilic diblock copolymers and their self-assembled structures have been proposed for a wide variety of applications, for example, drug encapsulation and releasing systems, 31,32 microsensors, 33,34 catalysis, 35,36 and substance separation.…”

Seeking Solvation: Exploring the Role of Protein Hydration in Silk Gelation

“…Consequently, they are unlikely to be restricted to silk fibroin, but are expected to occur more generally. In this respect, flow-induced fibril formation has recently been observed in various other proteins and peptides [231], with obvious implications for various prion diseases. Indeed, similar effects may also occur with a wider range of water-soluble polymers, such as pNiPAm [232,233].…”

Photoluminescent polymer micelles with thermo-/pH-/metal responsibility and their features in selective optical sensing of Pd(ii) cations

“…1a-e Amphiphilic macromolecules used in functional nanomaterials are also called smart polymers, which can respond to heat, 2-5 pH, [6][7][8] light, [9][10][11][12][13] and solvents. [14][15][16] These smart polymers can be applied in the elds of nanosensors, bioimaging, catalysts, etc. Among them, temperatureresponsive copolymers with a lower critical solution temperature (LCST) [2][3][4][5] and upper critical solution temperature (UCST) 17,18 have been studied most widely.…”