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 (Tg) and dynamic fragility (m) 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 Tg and m. 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 Tg and m 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.