Senechal Vincent, Saadaoui Hassan, Vargas-Alfredo Nelson, Rodriguez-Hernandez Juan, Drummond Carlos
CNRS, Centre de Recherche Paul Pascal (CRPP), UMR 5031, F-33600 Pessac, France.
Instituto de Ciencia y Tecnología de Polímeros, CSIC, Juan de la Cierva 3, 28006, Madrid, Spain.
Soft Matter. 2020 Aug 26;16(33):7727-7738. doi: 10.1039/d0sm00810a.
We have studied the combined influence of pH and ionic strength on the properties of brushes of a weak polyion, poly(acrylic acid), in conditions of grafting density close to the mushroom-brush crossover. By combining atomic force microscopy AFM and quartz crystal microbalance, we show that at low ionic strengths the conformational change of grafted polyions is non-monotonic with increasing pH due to the counterintuitive variation of the ionization degree. Thus, reentrant swelling of the polymer chains is observed with increasing pH. This effect is more important at low polymer grafting densities, when it is accompanied by in-plane heterogeneous distribution at intermediate pH values. In addition, we observed self-assembly on the polymer brush (formation of holes and islands) at pH values below pKa, due to the short-range attractive interaction between uncharged grafted chains. The sensitivity of the ionization of grafted chains to the physicochemical environment was also studied by measuring the interaction force between a silica tip and polymer brushes by atomic force microscopy. The dependence of the ionization of polyions on the presence of the tip points toward important charge regulation effects, in particular at pH values corresponding to partial ionization of the polyion.
我们研究了在接枝密度接近蘑菇刷-毛刷转变的条件下，pH值和离子强度对弱聚电解质聚丙烯酸刷性质的综合影响。通过结合原子力显微镜（AFM）和石英晶体微天平，我们发现，在低离子强度下，由于电离度的变化违反直觉，接枝聚离子的构象变化随pH值升高并非单调变化。因此，随着pH值升高，观察到聚合物链的折返膨胀现象。这种效应在低聚合物接枝密度时更为显著，此时在中等pH值下会伴随面内非均匀分布。此外，我们观察到，在pH值低于pKa时，聚合物刷上会发生自组装（形成孔洞和岛状结构），这是由于不带电的接枝链之间存在短程吸引相互作用。我们还通过原子力显微镜测量二氧化硅针尖与聚合物刷之间的相互作用力，研究了接枝链电离对物理化学环境的敏感性。聚离子的电离对接针尖存在的依赖性表明存在重要的电荷调节效应，特别是在对应于聚离子部分电离的pH值时。
