Shrinkage behaviour of hydrophobic hydrogel during dehydration Original Research Article

A gel of poly-n-isopropylacrylamide (PNIPA) spontaneously shrinks when dehydrated at a temperature above 35 °C. The latter is the critical volume phase transition point where the system separates into a rich-polymer phase in equilibrium with pure water thus the water activity of the system at that point is practically one. This phenomenon may be attributed to hydrophobic attraction that is strong enough to push water molecules away from the vicinity of the polymer. This impacts also on the sorption isotherms of the hydrophobic gels. Compared with the hydrophilic ones, these gels have a relatively low water holding capacity. Even under very mild dehydration conditions, such as a temperature of 40 °C and water activity of 0.85, the moisture content of the gel is readily reaching a level (0.1 gwater/gsolids) where the system becomes glassy. Since dehydration processes normally take place in an environment of relatively low water activity, the surface water is readily removed to a very low level resulting in the formation of a glassy rigid skin. When strong enough, this rigid skin resists further macroscopic shrinkage.