Börnhorst Tobias, Scharfer Philip, Schabel Wilhelm
Thin Film Technology (TFT), Karlsruhe Institute of Technology (KIT), Kaiserstraße 12, 76131 Karlsruhe, Germany.
Langmuir. 2021 May 18;37(19):6022-6031. doi: 10.1021/acs.langmuir.1c00641. Epub 2021 May 5.
The drying behavior of two different polymers [polyvinyl pyrrolidone (PVP) and polyisobutylene (PIB)] with different glass transition temperatures are investigated and compared as a function of film thickness from micrometer (∼3 μm) to nanometer scale (∼10 nm). The focus of this study is to distinguish between solvent diffusion, polymer relaxation, and substrate confinement of polymer chain mobility toward the interface as the dominating mechanism of drying kinetics. Relaxation kinetics becomes more dominant when the film thickness is reduced, which is shown experimentally for the first time for nanometer-scale film thicknesses. Identical drying curves regardless of the film thickness of PVP/methanol indicate the limitation of solvent transport by relaxation kinetics. The viscoelastic relaxation behavior of the polymer/solvent film is modeled by a Maxwell element. The results are in accordance with the experimental drying curves and allow for the determination of the characteristic relaxation time. Relaxation limitation becomes relevant at high diffusion Deborah numbers when the relaxation time-which is a function of the deployed material and the polymer/solvent composition-is higher than the characteristic diffusion time in the film. The latter is a function of the polymer/solvent composition and the thickness of the film. Drying curves of PIB/toluene films show additional effect in a substrate-near region of about 5 nm in which polymer chain mobility is confined, resulting in decelerated solvent diffusion. Although this effect near the substrate interface is expected to be present regardless of the film thickness, it becomes more dominant when the substrate-near region represents a significant fraction of the total film thickness. The key to the derived methodology for characterization of the polymer/solvent drying process is to vary dry film thickness from micrometers to a few nanometers which allows us to determine the dominating mechanism of drying kinetics.
研究并比较了两种具有不同玻璃化转变温度的聚合物[聚乙烯吡咯烷酮（PVP）和聚异丁烯（PIB）]在从微米（约3μm）到纳米尺度（约10nm）的膜厚度范围内的干燥行为。本研究的重点是区分溶剂扩散、聚合物松弛以及聚合物链向界面移动时的基底限制，将其作为干燥动力学的主导机制。当膜厚度减小时，松弛动力学变得更加主导，这是首次在纳米级膜厚度下通过实验证明的。无论PVP/甲醇的膜厚度如何，相同的干燥曲线表明了松弛动力学对溶剂传输的限制。聚合物/溶剂膜的粘弹性松弛行为由麦克斯韦元件建模。结果与实验干燥曲线一致，并允许确定特征松弛时间。当松弛时间（它是所使用材料和聚合物/溶剂组成的函数）高于膜中的特征扩散时间时，在高扩散德博拉数下，松弛限制变得重要。后者是聚合物/溶剂组成和膜厚度的函数。PIB/甲苯膜的干燥曲线在约5nm的靠近基底区域显示出额外的效应，其中聚合物链的移动受到限制，导致溶剂扩散减速。尽管预计无论膜厚度如何，靠近基底界面的这种效应都会存在，但当靠近基底区域占总膜厚度的很大一部分时，它会变得更加主导。用于表征聚合物/溶剂干燥过程的推导方法的关键是将干膜厚度从微米变化到几纳米，这使我们能够确定干燥动力学的主导机制。
