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Title: A thermodynamic model of phase transition of poly(N-isopropylacrylamide) hydrogels in ionic solutions
S ubmerged in an aqueous solution of sodium chloride (NaCl), a poly(N-isopropylacrylamide) (PNIPAM) hydrogel c an be in one of two phases: swollen phase and collapsed phase. We measure the equilibrium volume of the h ydrogel as a function of temperature T and ionic concentration y. The hydrogel is in the swollen phase when T a nd y are low, and is in the collapsed phase when T and y are high. We develop a thermodynamic model in which t he free energy is a function of volume, temperature, and ionic concentration. The free energy also contains s everal adjustable parameters, which we best-fit to the experimental data of volume as a function of T and y. For a given pair of T and y, the free energy is a function of volume. This function has a single minimum for some pairs of (T, y), but two minima and a maximum for other pairs of (T, y). In the former, the single minimum corresponds t o either a swollen or a collapsed state. In the latter, the lower minimum corresponds to a state of equilibrium, t he higher minimum corresponds to a metastable state, and the maximum corresponds to an unstable state. When t he two minima are equal, the hydrogel undergoes phase transition. The condition of phase transition is rep - r esented as a curve on the (T, y) plane. The thermodynamic model represents the experimental data well.  more » « less
Award ID(s):
2011754
PAR ID:
10500417
Author(s) / Creator(s):
; ;
Publisher / Repository:
Elsevier
Date Published:
Journal Name:
International Journal of Solids and Structures
Volume:
257
Issue:
C
ISSN:
0020-7683
Page Range / eLocation ID:
111434
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
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