Fluorescence study on the mechanism of rapid shrinking of grafted poly(N-isopropylacrylamide) gels and semi-IPN gels

In order to clarify in nanometer scale the mechanism of rapid shrinking of grafted poly(N-isopropylacrylamide) (PNIPAM) hydrogels during temperature jump from room temperature to 40 °C, temperature dependences of dansyl fluorescence were studied for the rapid-shrinking grafted PNIPAM gels in which dansyl groups are attached to graft chains or network main chains. Gradual decrease in dansyl fluorescence peak wavelength, λf, with the increase in temperature between 17 and 32 °C for grafted chains in the grafted PNIPAM gel from 540 to 500 nm showed the occurrence of coil-globule transition of graft chains during this temperature range. However, λf of network main chains showed abrupt decrease at 30–34 °C from 540 to 510 nm reproducing the macroscopic volume phase transition of PNIPAM gels. The coil-globule transition and further aggregation of graft chains enhance the formation of heterogeneous structure during the temperature jump, providing water-expelling channels at the surface skin layer of the collapsed PNIPAM gel. The possible importance of large-scale heterogeneity with a much larger size than the mesh size of network on the rapid-shrinking is suggested, because this type of large-scale heterogeneity can be introduced by the network formation in the presence of linear macromonomer. A post-crosslinked semi-IPN gel also showed rapid shrinking and gradual decrease in λf for the linear polymer chains inside.