Hyper‐Crosslinkers Lead to Temperature‐ and pH‐Responsive Polymeric Nanogels with Unusual Volume Change

Hydrogels consisting of carboxylic acid groups and N‐isopropylacrylamide as pendants on their polymeric network usually exhibit volume expansion upon deprotonation or volume contraction when being heated. Demonstrated here is an anti‐intuitive case of a hydrogel containing multiple carboxylic acid groups at each crosslinking point in the polymeric network, which shrinks upon increasing pH from 1 to 7 at 37 °C or expands upon heating from 25 to 37 °C at pH 1. The unexpected volume change originates from the high percentage of the crosslinker in the polymers, as detected by solid‐state 13C NMR spectroscopy. In addition, the volume changes are thermally reversible. As the first example of the use of functional hyper‐crosslinkers to control the pH and thermal responses of nanogels, this work illustrates a new way to design soft materials with unusual behaviors.