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

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协同分子内动力学控制聚合物中依赖于链长的玻璃化转变

玻璃化转变是材料科学中长期未解决的问题。对于聚合物，由于链连接性和柔韧性增加了复杂性，我们对玻璃形成的理解特别差；因此，聚合物的结构松弛涉及分子内和分子间协同性之间的复杂相互作用。在这里，我们研究玻璃化转变温度如何${吨}_G$随分子量变化$米$适用于不同的聚合物化学成分和链的柔韧性。我们发现${吨}_G(米)$由每个构象自由度的平均质量（或体积）控制，并且“局部”分子松弛（涉及一些构象异构体）控制更大规模的合作$\alpha$放松负责${吨}_G$. 我们提出动态促进，其中局部松弛促进相邻松弛，导致分层动力学，可以解释我们的观察，包括对数${吨}_G(米)$依赖。我们的研究为聚合物中的分子弛豫和玻璃化转变提供了新的理解，这为基于单体尺度指标的聚合物预测设计铺平了道路。