Heterogenous dynamics, connectivity and domain formation in polymer glasses: a fractal dimension analysis approach

Dynamical heterogeneity in glassy systems was first proposed in order to explain non-exponential relaxation patterns in single-component glassy systems, with different relaxation times contributing to produce the observed relaxation. It has been suggested that due to the influence of intramolecular interactions and chain connectivity, there is a fundamental difference between the nature of the glass transition in polymers and in standard glass-formers. Here, we study polymer collapse using fractal dimension (D_f) analysis, demonstrating that dynamical arrest upon glass transition affects the evolution of D_f in a non-trivial manner. We identify heterogenous dynamics both in bulk and near the free surface, showing characteristic domain patterns in local displacement and connectivity. We demonstrate that although covalent bonding promotes glass formation and caged dynamics, bonding sequentiality that defines a polymer chain is not critical in bulk: glassy dynamics is purely a result of the number of connections per particle, independently of how these connections are formed. However, bonding sequentiality does play an important role in the surface effects of the glass, highlighting a major difference between polymeric and colloidal glasses.