Reentrant phase transition in pH-responsive microgel suspensions

We report here experimental evidence of a reentrant [fluid (liquid-like) to crystal to fluid (liquid-like)] phase transition in pH-responsive poly(N-isopropyl acrylamide)-co-poly(acrylic acid) [PNIPAM-co-PAac] microgel suspensions. Upon increasing the pH (NaOH), a liquid-like ordered PNIPAM-co-PAac microgel suspension changes to crystalline order and reenters into liquid-like order with a further increase in pH (NaOH). The observed reentrant phase behavior is explained in terms of an increase in volume fraction caused by osmotic swelling of microgels as well as an increase in Coulombic repulsion between microgels due to dissociation of acrylic acid groups at the initial pH (NaOH) and then a decrease in the volume fraction caused by osmotic deswelling of microgels along with screening of the Coulombic interactions between microgels by Na+ ions at higher pH (NaOH). Furthermore, as a consequence of reentrant structural transition, PNIPAM-co-PAac microgel suspensions exhibit reentrant viscoelastic properties, i.e., a suspension in a viscoelastic liquid-like state changes to a viscoelastic solid-like state upon increasing pH (NaOH) and reenters into a viscoelastic liquid-like state with further increase of pH (NaOH). Present results are discussed in line with the earlier reported experimental and simulation work on microgels and other charged colloidal suspensions.