Fast-responsive semi-interpenetrating hydrogel networks imaged with confocal fluorescence microscopy

The composition of semi-interpenetrating polymer networks (semi-IPNs) based on responsive N-isopropylacrylamide (NIPAAm) hydrogels has been shown to affect the kinetics of the volume phase transition. Several N-alkyl-substituted acrylamides were used as the linear polymers in a crosslinked NIPAAm network, and the kinetics was observed as a function of crosslinking density and linear polymer concentration. The time required for collapse of the network could be reduced by as much as 90%, with little change to the corresponding swelling ratio and volume phase transition temperature. However, the underlying changes in network morphology are not known, and here we present kinetics data in combination with imaging of the resulting hydrogel networks. The crosslinked networks and the linear polymers were fluorescently labeled, and the resulting morphology was imaged with confocal fluorescence microscopy and two-photon laser scanning microscopy. The most hydrophilic of the linear polymers was acrylamide, which was shown to phase separate during polymerization. The hydrophilic domains become more interconnected at higher concentrations of the crosslinker and the linear polymers. This correlates well with the kinetics of the volume phase transition for the corresponding networks. The semi-IPNs containing more hydrophobic linear polymers had very similar morphology, but some domains were present, ranging from 500 nm to 2 μm and increasing in size with increased linear polymer concentration. The time scale of collapse was an order of magnitude faster than expected, based on size of the hydrophobic N-alkyl group, when the linear polymer had the same lower critical solution temperature as the hydrogel network. This is an indication that the simultaneous collapse of the linear polymer and the crosslinked network contributes to the fast response of these semi-IPNs.