Bhuyan Shreyanil, Chandran Sivasurender, Pillai Dipin S
Department of Chemical Engineering, Indian Institute of Technology Kanpur, Uttar Pradesh 208016, India.
Soft and Biological Matter Laboratory, Department of Physics, Indian Institute of Technology Kanpur, Uttar Pradesh 208016, India.
Langmuir. 2024 Aug 20;40(33):17699-17709. doi: 10.1021/acs.langmuir.4c02113. Epub 2024 Aug 5.
The stability of ultrathin (<100 nm) polymer films is essential in applications like protective coatings. On the contrary, their instability may actually be desirable for the emergence of self-assembled nanoscale patterns utilized in the fabrication of functional devices. Polymer solution films exhibit two distinct kinds of instabilities, viz., dewetting (long-wave) and decomposition (short-wave). Dewetting refers to the rupture of the continuous film to form isolated domains, while decomposition leads to phase separation within the polymer solution. The focus of this work is on leveraging polar interactions between the solute and solvent molecules to tune the stability of the film. A gradient dynamics-based thin film model is developed to investigate pattern formation in a thin polar polymer solution film. The Flory-Huggins theory is suitably modified by introducing a polar interaction parameter that depends upon the concentration of the polymer and the dipole moments of monomer (μ) and solvent molecules (μ). A linear stability analysis is performed to determine the characteristic length scale and growth rate of the instabilities. It is shown that the range of concentration space for the occurrence of the decomposition mode is directly affected by the Flory interaction parameter (χ), μ, and μ, thereby serving as control parameters to tune the width of the concentration range. It is further shown that ignoring polar interactions may lead to incorrect predictions of the instability mode, including a complete loss of the decomposition mode. In addition, the long-wave dewetting length scale is found to decrease due to bulk dipolar interactions at higher polymer concentrations. Finally, numerical simulations are carried out to track the nonlinear evolution of the interface and concentration field for both the decomposition and dewetting modes of instability.
超薄（<100纳米）聚合物薄膜的稳定性在诸如防护涂层等应用中至关重要。相反，对于功能器件制造中所利用的自组装纳米级图案的出现而言，它们的不稳定性实际上可能是有利的。聚合物溶液薄膜表现出两种不同类型的不稳定性，即脱湿（长波）和分解（短波）。脱湿是指连续薄膜破裂形成孤立区域，而分解则导致聚合物溶液内的相分离。这项工作的重点是利用溶质和溶剂分子之间的极性相互作用来调节薄膜的稳定性。开发了一种基于梯度动力学的薄膜模型，以研究极性聚合物溶液薄膜中的图案形成。通过引入一个取决于聚合物浓度以及单体（μ）和溶剂分子（μ）偶极矩的极性相互作用参数，对弗洛里 - 哈金斯理论进行了适当修正。进行线性稳定性分析以确定不稳定性的特征长度尺度和增长率。结果表明，分解模式出现的浓度空间范围直接受到弗洛里相互作用参数（χ）、μ和μ的影响，从而作为调节浓度范围宽度的控制参数。进一步表明，忽略极性相互作用可能导致对不稳定性模式的错误预测，包括完全失去分解模式。此外，发现在较高聚合物浓度下，由于体偶极相互作用，长波脱湿长度尺度会减小。最后，进行了数值模拟，以跟踪分解和脱湿两种不稳定性模式下界面和浓度场的非线性演化。
