Department of Physics , Technische Universität Darmstadt , 64289 Darmstadt , Germany.
Department of Chemistry , Technische Universität Berlin , 10623 Berlin , Germany.
Langmuir. 2018 Sep 25;34(38):11518-11525. doi: 10.1021/acs.langmuir.8b02258. Epub 2018 Sep 14.
The paper addresses the swelling of polyelectrolyte multilayers (PEMs) at varying humidity. In particular, a new model will be presented, which takes the gradual filling of voids into account. Absorption of water results in PEM swelling. This absorbed water can be distinguished into swelling and void water. Swelling water leads to an increase in thickness and a change of the optical properties of PEMs, while the void water results only in a change of the optical properties. In former studies, neutron reflectometry was used to distinguish between swelling and void water. However, as we show in this study, it is possible to resolve the two different kinds of water in PEMs by ellipsometry, a much simpler tool. The present study evaluates and interprets the refractive index of polystyrenesulfonate/polydiallyldimethylammonium chloride (PSS/PDADMAC) PEMs. Both the swelling behavior and the refractive index change as a function of relative humidity and were found to be independent of the layer number. The void model and the extended void model were used to describe the data. The void model allows fitting the experimentally determined refractive index at humidity beyond 20% RH but fails for humidity lower than 20% RH. Therefore, we modified the existing model in order to account for air-water exchange. The extended void model assumes a gradual air-water exchange at low h and describes the refractive index over the entire humidity range in a precise way. Up to 30% RH, air and water coexist. Above this threshold, the voids are completely filled with water and this threshold does not change either with layer number or with the outermost layer. Furthermore, this model allows the determination of the volume fraction of the voids (0.05 ± 0.01) and the refractive index of the pure polymer matter (1.592 ± 0.002).
本文探讨了在不同湿度下聚电解质多层膜（PEM）的溶胀。特别地，将提出一个新模型，该模型考虑了空隙的逐渐填充。水的吸收导致 PEM 溶胀。这种吸收的水可以分为溶胀水和空隙水。溶胀水会导致 PEM 厚度增加和光学性质发生变化，而空隙水只会导致光学性质发生变化。在以前的研究中，中子反射法被用于区分溶胀水和空隙水。然而，正如我们在本研究中所示，通过更简单的工具椭偏法，可以将 PEM 中的两种不同类型的水分开。本研究评估和解释了聚苯乙烯磺酸钠/聚二烯丙基二甲基氯化铵（PSS/PDADMAC）PEM 的折射率。溶胀行为和折射率变化都发现与相对湿度和层数无关。空隙模型和扩展空隙模型被用于描述数据。空隙模型允许在湿度超过 20%RH 时拟合实验确定的折射率，但在湿度低于 20%RH 时失效。因此，我们修改了现有的模型以考虑空气-水交换。扩展空隙模型假设在低 h 时空气-水交换逐渐发生，并以精确的方式描述整个湿度范围内的折射率。在 30%RH 以下，空气和水共存。超过这个阈值，空隙完全被水填充，这个阈值既不随层数变化，也不随最外层变化。此外，该模型允许确定空隙的体积分数（0.05±0.01）和纯聚合物物质的折射率（1.592±0.002）。
