Betancourt Beatriz A Pazmiño, Douglas Jack F, Starr Francis W
Department of Physics, Wesleyan University, Middletown, Connecticut 06459, USA.
Polymers Division, National Institute of Standards and Technology, Gaithersburg, Maryland, 20899, USA.
Soft Matter. 2013 Jan 7;9(1):241-254. doi: 10.1039/C2SM26800K.
Polymer-nanoparticle composites play a vital role in ongoing materials development. The behavior of the glass transition of these materials is important for their processing and applications, and also represents a problem of fundamental physical interest. Changes of the polymer glass transition temperature due to nanoparticles have been fairly well catalogued, but the breadth of the transition and how rapidly transport properties vary with temperature - termed the fragility of glass-formation - is comparatively poorly understood. In the present work, we calculate both and of a model polymer nanocomposite by molecular dynamics simulations. We systematically consider how and vary both for the material as a whole, as well as locally, for a range of nanoparticle (NP) concentrations and two polymer-NP interactions. We find large positive and negative changes in and that can be interpreted in terms of the Adam-Gibbs model of glass-formation, where the scale of the cooperative motion is identified with the scale of string-like cooperative motion. This provides a molecular perpective of fragility changes due to the addition of NPs and for glass formation more generally. We also contrast the behavior along isobaric and isochoric approaches to , since these differing paths can be important to compare experiments (isobaric) and simulations (very often isochoric). Our findings have practical implications for understanding the properties of nanocomposites and fundamental significance for understanding the properties glass-forming materials more broadly.
聚合物-纳米颗粒复合材料在当前的材料开发中起着至关重要的作用。这些材料的玻璃化转变行为对于其加工和应用很重要，并且也代表了一个具有基本物理意义的问题。由于纳米颗粒导致的聚合物玻璃化转变温度的变化已经有了相当完善的记录，但是转变的宽度以及传输性质随温度变化的速度——即玻璃形成的脆性——相对来说了解得较少。在本工作中，我们通过分子动力学模拟计算了一种模型聚合物纳米复合材料的这两个参数。我们系统地考虑了对于整个材料以及局部而言，这两个参数如何随着一系列纳米颗粒（NP）浓度和两种聚合物-纳米颗粒相互作用而变化。我们发现这两个参数有很大的正负变化，这些变化可以根据玻璃形成的亚当-吉布斯模型来解释，其中协同运动的尺度与线状协同运动的尺度相关。这为由于添加纳米颗粒以及更普遍地对于玻璃形成导致的脆性变化提供了一个分子视角。我们还对比了沿着等压和等容途径达到玻璃化转变温度时的行为，因为这些不同的途径对于比较实验（等压）和模拟（通常是等容）可能很重要。我们的发现对于理解纳米复合材料的性质具有实际意义，并且对于更广泛地理解玻璃形成材料的性质具有基本重要性。
