Colmenero J
Centro de Física de Materiales (CSIC-UPV/EHU) and Donostia International Physics Center (DIPC), Paseo Manuel de Lardizabal 5, E-20018 San Sebastián, Spain.
J Phys Condens Matter. 2015 Mar 18;27(10):103101. doi: 10.1088/0953-8984/27/10/103101. Epub 2015 Jan 30.
Traditionally, polymer melts have been considered archetypal glass-formers. This has been mainly due to the fact that these systems can easily be obtained as glasses by cooling from the melt, even at low cooling rates. However, the macromolecules, i.e. the structural units of polymer systems in general, are rather different from the standard molecules. They are long objects ('chains') made by repetition of a given chemical motif (monomer) and have intra-macromolecular barriers that limit their flexibility. The influence of these properties on, for instance, the glass-transition temperature of polymers, is a topic that has been widely studied by the polymer community almost from the early times of polymer science. However, in the framework of the glass-community, the relevant influence of intra-macromolecular barriers and chain connectivity on glass-transition phenomena of polymers has started to be recognized only recently. The aim of this review is to give an overview and to critically revise the results reported on this topic over the last years. From these results, it seems to be evident that there are two different mechanisms involved in the dynamic arrest in glass-forming polymers: (i) the intermolecular packing effects, which dominate the dynamic arrest of low molecular weight glass-forming systems; and (ii) the effect of intra-macromolecular barriers combined with chain connectivity. It has also been shown that the mode coupling theory (MCT) is a suitable theoretical framework to discuss these questions. The values found for polymers for the central MCT parameter--the so-called λ-exponent--are of the order of 0.9, clearly higher than the standard values (λ ≈ 0.7) found in systems where the dynamic arrest is mainly driven by packing effects ('standard' glass-formers). Within the MCT, this is a signature of the presence of two competing mechanisms of dynamic arrest, as it has been observed in short-ranged attractive colloids or two component mixtures with dynamic asymmetry. Moreover, recent MD-simulations of a 'bead-spring' polymer model, but including intra-macromolecular potential of different strengths, confirm that the high λ-values found in polymers are due to the effect of intra-macromolecular barriers. Although there are still open questions, these results allow to conclude that there is a fundamental difference between the nature of the glass transition in polymers and in simple (standard) glass-formers.
传统上，聚合物熔体一直被视为典型的玻璃形成体。这主要是因为即使在低冷却速率下，通过从熔体冷却，这些体系也很容易形成玻璃态。然而，大分子，即一般聚合物体系的结构单元，与标准分子有很大不同。它们是由特定化学基元（单体）重复构成的长链状物体，并且存在限制其柔性的分子内势垒。这些性质对聚合物玻璃化转变温度等的影响，几乎从聚合物科学早期起就是聚合物领域广泛研究的课题。然而，在玻璃态研究领域内，分子内势垒和链连接性对聚合物玻璃化转变现象的相关影响直到最近才开始被认识到。这篇综述的目的是给出一个概述，并批判性地审视过去几年关于这个课题报道的结果。从这些结果来看，似乎很明显在形成玻璃态的聚合物中涉及两种不同的动力学阻滞机制：（i）分子间堆积效应，它主导低分子量玻璃形成体系的动力学阻滞；以及（ii）分子内势垒与链连接性的效应。还表明模式耦合理论（MCT）是讨论这些问题的合适理论框架。聚合物的中心MCT参数——所谓的λ指数——的值约为0.9，明显高于在动力学阻滞主要由堆积效应驱动的体系（“标准”玻璃形成体）中发现的标准值（λ≈0.7）。在MCT框架内，这是存在两种竞争动力学阻滞机制的标志，正如在短程吸引胶体或具有动力学不对称性的两组分混合物中所观察到的那样。此外，最近对“珠子 - 弹簧”聚合物模型进行的分子动力学模拟，但包括不同强度的分子内势，证实了聚合物中发现的高λ值是由于分子内势垒的作用。尽管仍有一些未解决的问题，但这些结果可以得出结论，聚合物中的玻璃化转变本质与简单（标准）玻璃形成体之间存在根本差异。
