Abstract
The volume of a cylindrical polyacrylamide gel was measured when immersed in a binary mixture of isobutyric acid–water at different temperatures and weight fractions of isobutyric acid. Near the upper critical solution temperature of the binary mixture, the curve for gel volume vs. isobutyric acid weight fraction has a shoulder or a peak near the critical weight fraction. On the other hand, in a region away from the critical temperature, the gel volume decreased monotonically with increasing isobutyric acid weight fraction. The cloud point temperature of the binary mixture inside the gel was lower than that outside the gel. Thermodynamic description for the gel in the critical mixture is derived on the basis of the Ising model. By the description, the experimental results are explained consistently. The theoretical analysis shows that the shoulder and the peak appearing in the swelling behavior of the gel are respectively induced by the criticalities of the binary mixture outside and inside the gel. It also shows that the cloud point temperature lowering of the binary mixture inside the gel is attributed to the effective enhancement of the temperature of the binary mixture inside the gel induced by the presence of the gel polymer.
Highlights
Polymer gels are a three-dimensional network swollen in a solvent
We show that the anomaly of the susceptibility induces the characteristic behaviors, the presence of the shoulder and the peak in the plot of the gel volume near the critical point
The relationship between the cloud point curves outside and inside the gel shown in Figure 1 is discussed on the basis of the lattice model Hamiltonian given by Equations (2)–(4)
Summary
Polymer gels are a three-dimensional network swollen in a solvent. They shrink or swell in response to the change in solvent property due to the modification of inter- and intramolecular segment–segment interactions. In mixed solvents (binary mixtures), a synergetic solvent power should be considered to discuss the conformation of polymer chains and the gel volume. As a binary mixture approaches its critical point, the concentration fluctuations grow, and the size of the spatially inhomogeneous domains reaches the order of the pore size of the gel network. For a single polymer molecule in a critical mixture, de Gennes predicted that the polymer molecule first shrinks and swells as the mixture approaches its critical point [1,2]. In the polymer networks in binary mixtures, the microscopic changes in the size of polymer chains are integrated into macroscopic volume changes
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