Ehtiati Koosha, Moghaddam Saeed Z, Daugaard Anders E, Thormann Esben
Department of Chemistry, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark.
Danish Polymer Center, Department of Chemical and Biochemical Engineering, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark.
Langmuir. 2020 Mar 10;36(9):2339-2348. doi: 10.1021/acs.langmuir.9b03537. Epub 2020 Feb 27.
A weak polyelectrolyte brush is composed of a layer of polyacids or polybases grafted by one end of their chains to a substrate surface. For such brush layers immersed in an aqueous solution, the dissociation behavior of the acidic or basic groups and the structural and physical properties of the brush layer will thus be strongly dependent on the environmental conditions. For a polyacid brush layer consisting of, e.g., poly(acrylic acid), this means that the chains in the brush layer will be charged at high pH and uncharged at low pH. However, theoretical scaling laws not only foresee the structural changes occurring in response to the pH-induced dissociation behavior but also how the dissociation behavior of the brush layer depends on the ionic strength of the aqueous solution and the density of acidic groups within the brush layer. We have herein employed spectroscopic ellipsometry and a quartz crystal microbalance with dissipation monitoring (QCM-D) to experimentally evaluate the theoretically predicted dissociation and structural behavior of PAA brushes. Spectroscopic ellipsometry allows us to study the brush thickness as a function of pH and ionic strength, while QCM-D gives us an opportunity to investigate the swelling behavior of PAA brushes at various penetration depths of propagating acoustic waves. Our studies show that the dissociation degree of the carboxylic acid groups in a PAA brush increases with increasing distance from the substrate. Moreover, the ionic strength enhances carboxylic acid dissociation, such that a higher ionic strength leads to a narrower distribution and higher average dissociation degree. In conclusion, our results provide an experimental verification of the theoretically predicted gradient in the degree of dissociation of the acid groups in weak polyacid brush layers and shows that at a pH value equal to approximately the average p value of the brush, the state of the acid groups varies from being almost uncharged to almost fully dissociated depending on the ionic strength and vertical position in the brush.
弱聚电解质刷由一层通过链的一端接枝到基底表面的聚酸或聚碱组成。对于浸没在水溶液中的此类刷层，酸性或碱性基团的解离行为以及刷层的结构和物理性质将强烈依赖于环境条件。例如，对于由聚（丙烯酸）组成的聚酸刷层，这意味着刷层中的链在高pH值下带电荷，而在低pH值下不带电荷。然而，理论标度律不仅预见了响应pH诱导的解离行为而发生的结构变化，还预见了刷层的解离行为如何依赖于水溶液的离子强度和刷层内酸性基团的密度。我们在此采用椭圆偏振光谱法和带耗散监测的石英晶体微天平（QCM-D）来实验评估理论预测的聚丙烯酸（PAA）刷的解离和结构行为。椭圆偏振光谱法使我们能够研究刷厚度随pH值和离子强度的变化，而QCM-D让我们有机会研究PAA刷在传播声波的不同穿透深度下的溶胀行为。我们的研究表明，PAA刷中羧酸基团的解离度随离基底距离的增加而增加。此外，离子强度增强了羧酸的解离，使得较高的离子强度导致更窄的分布和更高的平均解离度。总之，我们的结果为理论预测的弱聚酸刷层中酸基团解离度梯度提供了实验验证，并表明在pH值约等于刷的平均p值时，酸基团的状态根据离子强度和刷中的垂直位置从几乎不带电荷变化到几乎完全解离。
