Dequidt A, Long D R, Sotta P, Sanséau O
Laboratoire Polymères et Matériaux Avancés, UMR 5268 CNRS/Rhodia, Saint-Fons, France.
Eur Phys J E Soft Matter. 2012 Jul;35(7):61. doi: 10.1140/epje/i2012-12061-6. Epub 2012 Jul 19.
Over the past twenty years experiments performed on thin polymer films deposited on substrates have shown that the glass transition temperature T(g) can either decrease or increase depending on the strength of the interactions. Over the same period, experiments have also demonstrated that the dynamics in liquids close to the glass transition temperature is strongly heterogeneous, on the scale of a few nanometers. A model for the dynamics of non-polar polymers, based on percolation of slow subunits, has been proposed and developed over the past ten years. It proposes a unified mechanism regarding these two features. By extending this model, we have developed a 3D model, solved by numerical simulations, in order to describe and calculate the mechanical properties of polymers close to the glass transition in the linear regime of deformation, with a spatial resolution corresponding to the subunit size. We focus on the case of polymers confined between two substrates with non-negligible interactions between the polymer and the substrates, a situation which may be compared to filled elastomers. We calculate the evolution of the elastic modulus as a function of temperature, for different film thicknesses and polymer-substrate interactions. In particular, this allows to calculate the corresponding increase of glass transition temperature, up to 20 K in the considered situations. Moreover, between the bulk T(g) and T(g) + 50 K the modulus of the confined layers is found to decrease very slowly in some cases, with moduli more than ten times larger than that of the pure matrix at temperatures up to T(g) + 50 K. This is consistent with what is observed in reinforced elastomers. This slow decrease of the modulus is accompanied by huge fluctuations of the stress at the scale of a few tens of nanometers that may even be negative as compared to the solicitation, in a way that may be analogous to mechanical heterogeneities observed recently in molecular dynamics simulations. As a consequence, confinement may result not only in an increase of the glass transition temperature, but in a huge broadening of the glass transition.
在过去二十年中，对沉积在基底上的聚合物薄膜进行的实验表明，玻璃化转变温度T(g)会根据相互作用强度的不同而降低或升高。在同一时期，实验还证明，在接近玻璃化转变温度的液体中，动力学在几纳米的尺度上具有很强的不均匀性。在过去十年里，基于缓慢亚基的渗流提出并发展了一种非极性聚合物动力学模型。它提出了一个关于这两个特征的统一机制。通过扩展这个模型，我们开发了一个通过数值模拟求解的三维模型，以描述和计算线性变形范围内接近玻璃化转变的聚合物的力学性能，其空间分辨率对应于亚基尺寸。我们关注聚合物被限制在两个基底之间且聚合物与基底之间存在不可忽略相互作用的情况，这种情况可与填充弹性体相比较。我们计算了不同薄膜厚度和聚合物 - 基底相互作用下弹性模量随温度的变化。特别是，这使得能够计算出玻璃化转变温度相应的升高，在所考虑的情况下可达20K。此外，在所研究的情况下，在本体T(g)和T(g)+50K之间，受限层的模量在某些情况下下降非常缓慢，在高达T(g)+50K的温度下，其模量比纯基体的模量大十多倍。这与在增强弹性体中观察到的情况一致。模量的这种缓慢下降伴随着几十纳米尺度上应力的巨大波动，与施加的应力相比甚至可能为负，其方式可能类似于最近在分子动力学模拟中观察到的力学不均匀性。因此，受限不仅可能导致玻璃化转变温度升高，还可能导致玻璃化转变的大幅展宽。
