Superswelling, Supershrinking, and a Broadened Phase Transition─Amphiphilic Thermosensitivity in Sulfonate-Functionalized Azobenzene-co-NIPAAm Hydrogels and Polymers

Poly(N-isopropylacrylamide) (PNIPAAm)-based hydrogels were functionalized with a water-soluble azobenzene (WSA) comonomer bearing a sulfonate group. The WSA was thus conceptualized to be sufficiently hydrophilic to allow a high amount of incorporation of the functional monomer into the hydrogel to obtain large changes in its behavior compared to PNIPAAm. One such effect was to shift the peak value of the volume phase transition temperature (VPTT) of PNIPAAm, VPTTpeak = 34 °C, which is measured by differential scanning calorimetry (DSC). While the actual temperature for the VPTTpeak increased only slightly (by 3 °C with a WSA load of 5 mol %), the phase transition itself was substantially broadened from 5 °C for PNIPAAm to 18 °C in the functional hydrogels. In addition, while the VPTTpeak barely changed upon switching with light, the broadening of the transition also increased substantially. Macroscopically, a further unusual effect was observed: the cohydrogels showed a dual behavior, i.e., superswelling and supershrinking below and above the VPTT in comparison to PNIPAAm. The hydrophilic retention of water inside the gel could be shown to persist up to 60 °C, which is above the completed phase transition measured by DSC. This effect is likely attributed to scale, which has important implications for applications for this type of hydrogel.