Concentration effect on aggregation and dissolution behavior of poly(N-isopropylacrylamide) in water

The concentration effect on aggregation and dissolution behavior of poly(N-isopropylacrylamide) (PNIPAM) in water was studied. Three concentration regimes with different phase behavior were identified by differential scanning calorimetry (DSC). Further optical, light-scattering, and rheological studies indicated that the appearance of different regimes arose from their corresponding solution structures below lower critical solution temperature (LCST): free chains and small clusters in regime I, large clusters in regime II, and a gel-like network in regime III. Different solution structures below LCST led to different phase-separated patterns formed above LCST: colloidal particles in regime I, large precipitate in regime II, and the sponge-like solid in regime III, which was well understood based on the overlapping parameter P. Different phase-separated patterns therefore resulted in different remixing behavior as observed by DSC. This work suggests that the swelling and collapse behavior of PNIPAM based hydrogels was controlled through the design of their phase-separated patterns, and therefore provided a way to develop high performance thermo-sensitive materials. (c) 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41669.