Amorphous and crystalline states of ultrasoft colloids: A Molecular Dynamics study

In dense suspensions of multi-arm star polymers a ``\textit{reversible thermal vitrification}'' was observed experimentally under ``marginal'' solvent conditions. We have investigated the origin of this phenomenon via MD simulations at the \textit{mesoscopic scale}$^{1,2,}.$ We reported the emergence of an amorphous solid state, upon heating of the ``soft spheres''. This transient glassy state resulted from star swelling, ``free volume'' deprivation and ``dynamical arrest'' of ``soft-spheres''. We monitored the ageing of the amorphous stage towards more crystalline FCC structures. The effects of size-dispersity and arm MW on crystallization were studied qualitatively. The overall picture revealed the existence of new ``dynamically arrested'' states, all of which could be termed ``crystalline'' but differed as to the ``degree of crystallinity''. Quantitative analysis of particle trajectories supplied mean square displacement curves which at the higher temperatures are typical of ``delayed'' Fickian diffusion. Even the aged crystalline states exhibited weak diffusion in contrast with the null diffusion of the crystals resulting from a FCC initial configuration. $^{1}$ Rissanou et al., \textit{Phys. Rev. E } \textbf{71} 011402-1~:12 (2005) $^{2}$ Rissanou et al., \textit{J. Chem. Phys. }\textbf{124} 044905-1~:11 (2006)