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Growing range of correlated motion in a polymer melt on cooling towards the glass transition

机译：冷却至玻璃化转变时，聚合物熔体中相关运动的增长范围

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Many liquids cooled to low temperatures form glasses (amor-phous solids) instead of crystals. As the glass transition is approached, molecules become localized and relaxation times increase by many orders of magnitude. Many features of this 'slowing down' are reasonably well described by the mode-coupling theory of supercooled liquids. The ideal form of this theory predicts a dynamical critical temperature T_c at which the molecules become permanently trapped in the 'cage' formed by their neighbours, and vitrification occurs. Although there is no sharp transition, because molecules do eventually escape their cage, its signature can still be observed in real and simulated liquids. Unlike conventional critical phenomena (such as the behaviour at the liquid-gas critical point), the mode-coupling transition is not accompanied by a diverging static correlation length. But simulation and experiment show that liquids are dynamically heterogeneous, suggesting the possibility of a relevant 'dynamical' length scale characterizing the glass transition. Here we use computer simulations to investigate a melt of short, unentangled polymer chains over a range of temperatures for which the mode-coupling theory remains valid. We find that although density fluctuations remain short-ranged, spatial correlations between monomer displacements become long-ranged as T_c is approached on cooling. In this way, we identify a growing dynamical correlation length, and a corresponding order parameter, associated with the glass transition. This finding suggests a possible connection between well established concepts in critical phenomena and the dynamics of glass-forming liquids.
机译：许多冷却到低温的液体形成玻璃（非晶形固体）而不是晶体。随着接近玻璃化转变，分子变得局部化，弛豫时间增加了多个数量级。过冷液体的模式耦合理论可以很好地描述这种“减速”的许多特征。该理论的理想形式是预测动态临界温度T_c，在该温度下分子永久陷于由其邻居形成的“笼”中，并发生玻璃化。尽管没有急剧的转变，但是由于分子最终会逃脱它们的笼子，因此仍然可以在真实和模拟的液体中观察到其签名。与常规的临界现象（例如在液气临界点处的行为）不同，模式耦合跃迁不伴随发散的静态相关长度。但是仿真和实验表明，液体是动态非均质的，这表明可能存在相关的“动态”长度尺度来表征玻璃化转变。在这里，我们使用计算机模拟来研究在一定温度范围内模式耦合理论仍然有效的短而无缠结的聚合物链的熔体。我们发现，尽管密度波动仍然是短程的，但随着冷却时T_c的接近，单体位移之间的空间相关性变得长程。这样，我们确定了与玻璃化转变相关的不断增长的动力学相关长度以及相应的阶次参数。这一发现表明，在关键现象中公认的概念与玻璃形成液体的动力学之间可能存在联系。

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