Energetics and Mechanisms of poly(N-isopropylacrylamide) Phase Transitions in Water–Methanol Solutionsстатья
Статья опубликована в высокорейтинговом журнале
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Дата последнего поиска статьи во внешних источниках: 10 февраля 2021 г.
Аннотация:The phase transitions of poly(N-isopropylacrylamide)(PNIPAM) in water−methanol mixed solutions were studied in detail byhigh-sensitivity differential scanning calorimetry. From this study, thedependences of the transition temperature, enthalpy, heat capacityincrement, and width on the methanol molar fraction (xMeOH) wereobtained. The transition temperature passed through a minimum at themethanol molar fraction xMeOH* ∼ 0.35. At xMeOH < xMeOH*, thetransition enthalpy decreased quickly with the methanol content andbecame so small that it could not be measured, even with an increase inthe polymer concentration by a hundred times (up to 150 mg mL−1).Furthermore, over this xMeOH range, the transition heat capacity increment being negative remained practically constant, but thetransition width sharply increased. The transition thermograms were quantitatively described by the Okada−Tanaka theory, whichtakes into account the role of the polymer−solvent interaction cooperativity in the polymer thermoresponsivity. In terms of thisapproach, it is assumed that over the defined range of methanol content, PNIPAM possesses the cooperative hydro-solvationstructure in the form of water−methanol complexes. The energetics of this structure smoothly decreases with the increase in themethanol content up to a complete disappearance of the structure at xMeOH > xMeOH*. In this range of the methanol content, thephase behavior of PNIPAM seems to be dictated by regularities typical of polymer solutions in organic solvents, that is, how theFlory−Huggins parameter depends on temperature.