We investigate the combined effects of ionizable monomer fraction f, pH, and monovalent salt concentration C_s on the swelling of weak polyelectrolyte brushes (PEBs) by using in situ ellipsometry. Our system consists of random copolymers of basic (2-(dimethylamino)ethyl acrylate, DMAEA) and neutral (2-hydroxyethyl acrylate, HEA) monomers at varying fractions of ionizable monomer. Swelling of the brushes qualitatively follows the trends predicted by scaling laws for PEBs under different charge states but quantitatively deviates at specific ionic strengths and pH values. We posit these deviations stem from the lack of excluded volume effects and assumptions of strong chain stretching in current theoretical models. Most notably, we uncover a salt-dependent, nonmonotonic hysteretic behavior as weak PEB brushes are cycled from protonated to deprotonated and back. The nonmonotonic trend of hysteresis with salt can be explained by an interplay between the protonation facilitating effects of salt in the osmotic regime and the charge screening effects in the salted regime, which make charge distribution along weak PEBs more uniform. Our results provide insight into the mechanisms that determine whether polyelectrolytes exhibit weak versus strong polyelectrolyte behavior in various environmental conditions.

中文翻译：

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弱聚电解质刷的电荷状态决定了盐依赖性溶胀和滞后行为


我们使用原位椭偏法研究了可电离单体组分 f、pH 值和单价盐浓度 C_s 对弱聚电解质刷 （PEB） 溶胀的综合影响。我们的系统由碱性（2-（二甲氨基）丙烯酸乙酯，DMAEA）和中性（2-羟乙基丙烯酸酯，HEA）单体的无规共聚物组成，这些单体具有不同的可电离单体分数。刷子的溶胀在质量上遵循不同电荷状态下 PEB 的缩放定律预测的趋势，但在特定的离子强度和 pH 值下在数量上有所不同。我们认为这些偏差源于当前理论模型中缺乏排除的体积效应和强链拉伸的假设。最值得注意的是，我们发现了一种依赖于盐的、非单调的滞后行为，因为弱 PEB 刷子从质子化循环到去质子化，然后再循环回来。盐滞后的非单调趋势可以用盐在渗透状态下的质子化促进效应和盐渍状态下的电荷屏蔽效应之间的相互作用来解释，这使得沿弱 PEB 的电荷分布更加均匀。我们的结果为确定聚电解质在各种环境条件下是否表现出弱聚电解质行为与强聚电解质行为的机制提供了见解。