Influence of temperature on the permeability of polyacrylamide hydrogels and semi-IPNs with poly(N-isopropylacrylamide)

Several polyacrylamide hydrogels and semi-interpenetrated polymer networks with poly(N-isopropylacrylamide) included were synthesised and characterised by their swelling ratio. Their permeability to Orange II was studied as a function of temperature in the range 25–45°C. In polyacrylamide hydrogels the permeability increases with temperature according to an Arrhenius relationship and the apparent activation energy for permeation decreases linearly as the swelling ratio of the hydrogel increases. In semi-interpenetrated polymer networks a more or less pronounced increase of the permeability to Orange II is observed above 32°C, the lower critical solution temperature of poly(N-isopropylacrylamide). Different values of the apparent activation energy for permeation were obtained below and above 32°C, the last one being always lower than the former one. The permeability of semi-interpenetrated polymer networks increases above 32°C though a slight shrinking of the gels is observed. This can be explained by the collapse of poly(N-isopropylacrylamide) chains resulting in an increase of the pore volume that facilitates the diffusion of the dye but simultaneously water is expelled and the swelling ratio of semi-interpenetrated polymer networks decreases.