Cooperative Intramolecular Dynamics Control the Chain-Length-Dependent Glass Transition in Polymers

The glass transition is a long-standing unsolved problem in materials science. For polymers, our understanding of glass-formation particularly poor due to the added complexity chain connectivity and flexibility; structural relaxation polymers thus involves complex interplay between intra- inter-molecular cooperativity. Here we study how temperature Tg varies with molecular weight M for different polymer chemistries flexibilities. We find that Tg(M) controlled by average mass (or volume) per conformational degree freedom, `local' (involving few conformers) controls larger-scale cooperative alpha responsible Tg. propose dynamic facilitation where facilitates adjacent relaxations, leading hierarchical dynamics, can explain observations including logarithmic dependences. Our provides new relaxations which paves way predictive design based on monomer-scale metrics.