ABSTRACT
Analogous to hard-sphere suspensions, monodisperse thermo-responsive poly (N-isopropyl acrylamide) (PNIPAM) microgel particles beyond a volume fraction (Ï) of 0.5 freeze into face centered cubic (FCC)-hexagonal close packed (HCP) coexistence under as prepared conditions and into an FCC structure upon annealing. We report here FCC-HCP coexistence to be stable in dense PNIPAM microgel crystals (Ï > 0.74) with particles in their deswollen state (referred to as osmotically compressed microgel crystals) and the FCC structure with particles in their swollen state by performing annealing studies with different cooling rates. The structure of PNIPAM microgel crystals is characterized using static light scattering technique and UV-Visible spectroscopy and dynamics by dynamic light scattering (DLS). DLS studies reveal that the particle motion is diffusive at short times in crystals with Ï < 0.74 and sub-diffusive at short times in PNIPAM crystals with Ï > 0.74. The observed sub-diffusive behavior at short times is due to the overlap (interpenetration) of the dangling polymer chains between the shells of neighbouring PNIPAM microgel particles. Overlap is found to disappear upon heating the crystals well above their melting temperature, Tm due to reduction in the particle size. Annealing studies confirm that the overlap of dangling polymer chains between the shells of neighbouring PNIPAM spheres is responsible for the stability of FCC-HCP coexistence observed in osmotically compressed PNIPAM microgel crystals. Results are discussed in the light of recent reports of stabilizing the HCP structure in hard sphere crystals by adding interacting polymer chains.
ABSTRACT
We report here the dynamics in thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) nanogel crystals undergoing melting/freezing and verify the applicability of the dynamical criterion for melting/freezing proposed by Löwen et al. [Phys. Rev. Lett. 70, 1557 (1993)]. According to this criterion the ratio of long time diffusion coefficient (D(L)) to short time diffusion coefficients (D(S)) is ~0.1 for colloidal particles in suspension undergoing melting/freezing. Static and dynamic light scattering techniques have been employed to identify the melting/freezing transition of PNIPAM nanogel colloidal crystals of two different volume fractions φ = 0.49 and 0.79 and to measure D(L) and D(S) across the melting. In dense PNIPAM nanogel crystals undergoing melting, the ratio D(L)/D(S) is found to be less than 0.1 for the first time and this deviation is higher in the suspension with higher φ. We also show that the deviation is genuine by measuring D(L)/D(S) on shear melted charged silica colloidal liquid undergoing freezing. The mean square displacement at shorter times, close to the melting, shows subdiffusive behavior. The subdiffusive behavior, arising due to the overlap of the dangling polymer chains between shells of the neighboring particles, is argued to be the reason for the observed deviation.
ABSTRACT
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.
