Hsu Hsiao-Ping, Kremer Kurt
Max-Planck-Institut für Polymerforschung, Ackermannweg 10, Mainz 55128, Germany.
J Chem Phys. 2023 Aug 21;159(7). doi: 10.1063/5.0165902.
The glass transition temperature of confined and free-standing polymer films of varying thickness is studied by extended molecular dynamics simulations of bead-spring chains. The results are connected to the statistical properties of the polymers in the films, where the chain lengths range from short, unentangled to highly entangled. For confined films, perfect scaling of the thickness-dependent end-to-end distance and radius of gyrations normalized to their bulk values in the directions parallel and perpendicular to the surfaces is obtained. In particular, the reduced end-to-end distance in the perpendicular direction is very well described by an extended Silberberg model. For bulk polymer melts, the relation between the chain length and Tg follows the Fox-Flory equation. For films, no further confinement induced chain length effect is observed. Tg decreases and is well described by Keddie's formula, where the reduction is more pronounced for free-standing films. It is shown that Tg begins to deviate from bulk Tg at the characteristic film thickness, where the average bond orientation becomes anisotropic and the entanglement density decreases.
通过对珠簧链进行扩展分子动力学模拟，研究了不同厚度的受限和独立聚合物薄膜的玻璃化转变温度。研究结果与薄膜中聚合物的统计性质相关，其中链长范围从短的、未缠结到高度缠结。对于受限薄膜，在平行和垂直于表面的方向上，厚度依赖的端到端距离和回转半径相对于其本体值进行归一化后，得到了完美的标度关系。特别是，垂直方向上的约化端到端距离可以很好地用扩展的西尔伯格模型来描述。对于本体聚合物熔体，链长与玻璃化转变温度之间的关系遵循福克斯 - 弗洛里方程。对于薄膜，未观察到进一步的受限诱导链长效应。玻璃化转变温度降低，并且可以很好地用凯迪公式描述，其中对于独立薄膜，这种降低更为明显。结果表明，在特征薄膜厚度处，玻璃化转变温度开始偏离本体玻璃化转变温度，此时平均键取向变得各向异性，缠结密度降低。
