Dynamics in thermo-responsive nanogel crystals undergoing melting.

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.