Abstract
The occurrence and the nature of first-order transition of gels are analyzed and discussed. First, the condition for the first-order transition and the crossover between the continuous and discontinuous transitions are reviewed within the Flory-Rehner and Erman-Flory phenomenological theories. Then, experimental investigations related to this subject are reviewed and compared with the predictions of the above theories. Our attention is focused on poly(N-isopropylacrylamide) (NIPA) gel and its ionized copolymers. It is shown that the continuous as well as slightly discontinuous transitions in neutral NIPA gels can be understood semiquantitatively on the basis of the above theories. On the other hand, it is pointed out that the strongly discontinuous transitions in ionized NIPA gels are less well understood, and that the real nature of these transitions has not been clarified at all. There is a summary of the results of new observations on the discontinuous transition process in NIPA gels, which have been undertaken to shed some light on this transition. From these results, an entirely new concept of the first-order transition of gels has emerged, in which the transition between the swollen and the shrunken phases proceeds via an intermediate state of coexistence of two phases. This state is considered to be not a transient, but a stable or metastable equilibrium one and has a number of unusual features. A close similarity is pointed out between this state and the coexisting-phases state observed in some metallic alloys around their order-disorder transition.
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Abbreviations
- d:
-
diameter of cylindrical or spherical gels
- kB :
-
Boltzmann constant
- NA :
-
Avogadro's number
- T:
-
absolute temperature
- V:
-
volume of gel
- V0 :
-
volume of gel when it was formed
- v1 :
-
molar volume of solvent
- Nc :
-
number of chains
- ΔF:
-
free energy change induced when a solvent molecule is moved from the pure solvent phase into the pure polymer phase
- Δh:
-
enthalpy change induced by the same process as in ΔF
- Δs:
-
entropy change induced by the same process as in ΔF
- α:
-
linear swelling ratio
- φ:
-
volume fraction of polymer
- φ0 :
-
volume fraction of polymer when the gel was formed
- πm :
-
osmotic pressure due to mixing
- πe :
-
osmotic pressure due to network elasticity
- πi :
-
osmotic pressure due to counter ions
- πs :
-
osmotic pressure due to surface tension
- πT :
-
total osmotic pressure
- ρ:
-
radius of curvature of the surface
- γ:
-
surface tension
- χ:
-
polymer-solvent interaction parameter
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© 1993 Springer-Verlag
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Hirotsu, S. (1993). Coexistence of phases and the nature of first-order phase transition in poly-N-isopropylacrylamide gels. In: Dušek, K. (eds) Responsive Gels: Volume Transitions II. Advances in Polymer Science, vol 110. Springer, Berlin, Heidelberg. https://doi.org/10.1007/BFb0021126
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DOI: https://doi.org/10.1007/BFb0021126
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