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Monodisperse thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) microgel particles having a diameter of 520 nm were synthesized by free-radical precipitation polymerization and centrifuged to obtain a concentrated suspension. The centrifuged mother suspension was made to self-order into a crystalline state by repeated annealing beyond the volume phase transition (VPT) of the particles. We report here the three-dimensional (3D) real space structure, determined using a confocal laser scanning microscope, of PNIPAM microgel crystal samples prepared by two different recrystallized routes: (1) solidifying a shear melted colloidal liquid (referred as as-prepared sample) and (2) slow cooling of a colloidal liquid (referred as recrystallized sample). We have recorded images of several regions of the crystal with each region containing 15 horizontal crystal planes for determining the in-plane [two-dimensional (2D)] and 3D pair-correlation functions. The 2D pair-correlation function g(r) revealed hexagonal long-range order of particles in the layers with a lattice constant of 620 nm. The analysis of stacking sequence of layers recorded on as-prepared sample has revealed the existence of stacking disorder with an average stacking probability alpha approximately 0.42. This value of alpha together with the analysis of 3D pair-correlation function determined from particle positions revealed the structure of microgel crystals in the as-prepared sample to be random hexagonal close packing. We report the first observation of a split second peak in the 3D g(r) of the microgel crystals obtained from a shear melted liquid. Upon melting the sample above VPT and recrystallizing it the split second peak disappeared and the crystals are found to have a face centered cubic (fcc) structure with alpha approximately 0.95. From simulations, the split second peak is shown to arise from the displacement of some of the B-planes from the ideal hcp positions. The present results are discussed in light of those reported for charged and hard sphere colloidal crystals and plausible reasons for observing two different structures are also explained.

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 25 degrees C to 114 nm at 40 degrees C as function of temperature with a sudden collapse of particle volume (volume phase transition) at 32.4 degrees C. 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 26.2 degrees C and a fluid to fluid transition at 31 degrees C. 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.

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