Chain ends excite polymer cooperative motion

Among glasses, polymers stand out as the chain connectivity endows them with distinct properties in glass formation, among them the transition temperature (<i>T</i>_g) and dynamic fragility (<i>m</i>) varying with chain length. Here, we resolve the nature of the chain length-dependent behaviors, revealing the strong correlation between the number of chain ends within the cooperatively rearranging region and glassy properties including <i>T</i>_g and <i>m</i>. The correlations suggest a simple yet common mechanism of glass formation for the chain molecules, i.e., fast-relaxing chain ends alleviate the requirements of cooperativity for structural rearrangement, thus facilitating the cooperative motion that reduces <i>T</i>_g and <i>m</i> as chain length is shortened. We categorize the role of end groups between soft and rigid by proposing a physical quantifier-index of rigidity. Our results provide a unifying picture of polymer glass formation, regarding the role of chain end, length, and topology, a foundational phenomenon with implications across fields of chemistry, soft-condensed matters, and material science.