Surface tension of the polymer network of a gel

The surface tension of poly(N-sopropylacrylamide) gel is measured as a function of temperature. It is found that the surface tension of the gel suddenly decreases as the gel undergoes the discontinuous volume phase transition. The surface tensions of the polymer network of the gel are evaluated on the basis of a simple theory. The results yield that the surface tension of the poly(N-isopropylacrylamide) gel is mainly determined by the area occupied by the polymer chain on the surface of the gel in most of the temperature regions. In addition, it is also found that the surface tension of the polymer network of the gel shows a singular behavior in the immediate vicinity of the volume phase transition temperature of the gel.