  Melts of 3-dimensional dendritic beads-springs, namely coarse-grained
soft-clusters, are studied by molecular dynamics simulations. The goal is to
elucidate the unique dynamics of giant molecules, or generally speaking,
3-dimensional architectured polymers. When constituted by more than the
critical number around 200 beads, soft-clusters cannot diffuse or relax far
above their glass transition temperature, although relaxation can happen on the
level of beads. Each soft-cluster can only rotate in the cage formed by
neighboring soft-clusters. Such a non-diffusing state would transform to the
liquid state at exceptionally high temperature, e.g. 10 times the glass
transition temperature. Agreeing with experiments, 3D hierarchies lead to
unique dynamics, especially their divergent relaxation times with the number of
beads. These unique dynamics are in sharp contrast with 1-dimensional
chain-like polymers. We name such a special state as 'cooperative glass',
because of the 'cooperation' of the 3D-connected beads. The design of
soft-clusters may also resemble cooperative rearranging regions where
cooperativeness is contributed by low temperature, thus offer further insights
into the glass problem.
