The role of connectivity in polymer glasses

The glass transition of supercooled liquids is an unsolved problem in condensed matter physics: agreement on the cause of the slowing down of structural relaxation in glass-forming liquids is lacking. Glasses are amorphous solids, which do not possess the long range order of crystalline solids, yet display arrested dynamics and the shear elastic modulus characteristic of equilibrium elasticity. It has been postulated that due to the influence of chain connectivity, there is a difference between the glass transition in polymers and standard glass-formers. In this work we focus on the role of connectivity in polymer glasses, demonstrating that although covalent bonding promotes glass formation and caged dynamics, bonding sequentiality in a polymer chain is not critical: glassy dynamics in polymers is a result of the number of connections per particle, independently of how connections are formed. Our findings agree with the Phillips-Thorpe topological constraint theory, where glass-forming ability is determined by comparing the number of atomic degrees of freedom with the number of interatomic force field constraints. We also study the heterogenous dynamics of model coarse-grained polymer chains, demonstrating characteristic domain patterns in local displacement and connectivity.