Influence of Polymer Topology on Glass-Formation

There is a growing interest in controlling the molecular topology of polymers to engineer the properties of materials for diverse technological applications and for the insights that topology variations can make into the nature of chain entanglement, polymer crystallization, glass-formation, etc. To gain insights into the influence of topological constraints on glass-formation, we perform molecular dynamics simulations on melts of star polymers having f equivalent arms, polymers with grafted side-groups, and knotted ring polymers having fixed minimal crossing number m_c and examine basic thermodynamic and dynamic properties of these model glass-forming liquids as a function of their fundamental topological invariants, f and other branching induces and m_c for the knotted polymers. We find that increasing topological complexity, as quantified by f and m_c, leads to a tendency for polymers to adopt a more symmetrical and compact conformational shapes, a general tendency that also exists in solution. Topologically constrained polymers exhibit increasing rigidity with increased topological complexity, i.e., topological rigidification, a primary molecular factor influencing T_g and fragility. Increasing topological complexity in these melts allows us to progressively ‘tune’ these fundamental properties of polymeric glass-forming materials, indicating that the manipulation of polymer topology offers a powerful means to tune polymer material properties in their condensed state. The applicability of the string model of glass-formation in providing a unified description of these topologically complex fluids is explored.