Aliakseyeu Aliaksei, Truong Erica, Hu Yan-Yan, Sayko Ryan, Dobrynin Andrey V, Sukhishvili Svetlana A
Department of Materials Science & Engineering, Texas A&M University, College Station, Texas 77840, United States.
Department of Chemical Engineering, Texas A&M University, College Station, Texas 77840, United States.
Macromolecules. 2024 Dec 27;58(1):240-248. doi: 10.1021/acs.macromol.4c01374. eCollection 2025 Jan 14.
This work explored solution properties of linear and star poly(methacrylic acids) with four, six, and eight arms (PMAA, 4PMAA, PMAA, and 8PMAA, respectively) of matched molecular weights in a wide range of pH, salt, and polymer concentrations. Experimental measurements of self-diffusion were performed by fluorescence correlation spectroscopy (FCS), and the results were interpreted using the scaling theory of polyelectrolyte solutions. While all PMAAs were pH sensitive and showed an increase in hydrodynamic radius ( ) with pH in the dilute regime, the of star polymers (measured at basic pH values) was significantly smaller for the star polyacids due to their more compact structure. Fully ionized star PMAAs were also found to be less sensitive to changes in salt concentration and type of the counterion compared to linear PMAA. While of fully ionized linear PMAA decreased in the series Li > Na > K > Cs in agreement with the Hofmeister series, of star PMAAs was virtually independent of type of the counterion for eight-arm PMAA. However, molecular architecture strongly affected interactions of counterions with PMAAs. In particular, Li NMR revealed that the spin-lattice relaxation time of Li ions in low-salt solutions of eight-arm PMAA was ∼2-fold smaller than that in the solution of linear PMAA, suggesting slower Li-ion dynamics within star polymers. An increase in concentration of monovalent chloride salts, , above that of the PMAA monomer unit concentration ( ) resulted in shrinking of both linear and star molecules, with the hydrodynamic size scaling as ∝ . Self-diffusion of linear and star polyelectrolytes was then studied in a wide range of polyelectrolyte concentrations (10 mol/L < < 0.5 mol/L) in low-salt (<10 mol/L of added salt) and high-salt (1 mol/L) solutions. In both the low-salt and high-salt regimes, diffusion coefficient was lower for PMAAs with a larger number of arms at a fixed . In addition, in both cases, plateaued at low polymer concentrations and decreased at higher polymer concentrations. However, while in the high-salt conditions, the concentration dependence of reflected transitions between the dilute to semidilute solution regimes as expected for neutral chains in good and theta solvents, analysis of the diffusion data in the low-salt conditions using the scaling theory revealed a different origin of the concentration dependence of . Specifically, in the low-salt solutions, both linear and star PMAAs exhibited unentangled (Rouse-like) dynamics in the entire range of polyelectrolyte concentrations.
本工作研究了具有匹配分子量的线性和星形聚（甲基丙烯酸）（分别为PMAA、四臂PMAA、六臂PMAA和八臂PMAA）在广泛的pH值、盐浓度和聚合物浓度范围内的溶液性质。通过荧光相关光谱法（FCS）进行了自扩散的实验测量，并使用聚电解质溶液的标度理论对结果进行了解释。虽然所有的PMAA都对pH敏感，并且在稀溶液状态下随着pH值的升高流体力学半径（ ）增大，但由于星形聚酸的结构更为紧凑，其在碱性pH值下测量的流体力学半径对于星形聚合物来说显著更小。还发现，与线性PMAA相比，完全电离的星形PMAA对盐浓度和抗衡离子类型的变化不太敏感。虽然完全电离的线性PMAA的流体力学半径在Li＞Na＞K＞Cs系列中减小，这与霍夫迈斯特序列一致，但对于八臂PMAA，星形PMAA的流体力学半径实际上与抗衡离子类型无关。然而，分子结构强烈影响抗衡离子与PMAA的相互作用。特别是，Li NMR显示，在八臂PMAA的低盐溶液中Li离子的自旋晶格弛豫时间 比在线性PMAA溶液中的小约2倍，这表明星形聚合物中Li离子的动力学较慢。单价氯化物盐浓度 高于PMAA单体单元浓度 时，会导致线性和星形分子收缩，流体力学尺寸 按 ∝ 标度。然后在低盐（添加盐＜10 mol/L）和高盐（1 mol/L）溶液中，在广泛的聚电解质浓度范围（10 mol/L＜ ＜0.5 mol/L）内研究了线性和星形聚电解质的自扩散。在低盐和高盐状态下，如果聚电解质的臂数固定，对于具有更多臂的PMAA，扩散系数 更低。此外，在这两种情况下， 在低聚合物浓度时达到平稳，在较高聚合物浓度时降低。然而，虽然在高盐条件下， 的浓度依赖性反映了如中性链在良溶剂和θ溶剂中所预期的从稀溶液到半稀溶液状态的转变，但使用标度理论对低盐条件下的扩散数据进行分析发现， 的浓度依赖性有不同的起源。具体来说，在低盐溶液中，线性和星形PMAA在整个聚电解质浓度范围内都表现出非缠结（类Rouse）动力学。
