  The phase behavior of poly (N-isopropylacrylamide) nanoparticles dispersed in
aqueous medium is investigated as a function of temperature using static and
dynamic light scattering techniques. The diameter, d of the particles, as
determined by dynamic light scattering measurements on dilute dispersion showed
a decrease in size from 273 nm at 25C to 114 nm at 40C as function of
temperature with a sudden collapse of particle volume (volume phase transition)
at 32.4C. Further this nanoparticle dispersion is found to turn turbid beyond
volume phase transition. Static light scattering measurements on samples with
intermediate concentration and high concentration showed liquid-like order and
crystalline order respectively. The intensity of the Bragg peak of the
crystallized sample when monitored as a function of temperature showed crystal
to liquid transition at 26.2C and a fluid to fluid transition at 31C. The
occurrence of melting at a volume fraction of 0.85 and the absence of change in
number density across the fluid-to-fluid transition suggest that interparticle
interaction is repulsive soft-sphere below the volume phase transition. The
reported results on the phase behavior of poly(N-isopropylacrylamide) nanogel
suspensions are discussed in the light of the present results.
