Polyacrylic acid brushes: Surface pressure and salt-induced swelling
Polyacrylic acid brushes: Surface pressure and salt-induced swelling
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DOI：
10.1021/la991528o
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发表时间：
2000-10-31
期刊：
影响因子：
3.9
通讯作者：
Stuart, MAC
中科院分区：
文献类型：
--
作者：
Currie, EPK;Sieval, AB;Stuart, MAC
使用表面压力等温线和椭圆光度法对由聚苯乙烯-聚丙烯酸（PS-PAA）二嵌段共聚物组成的聚电解质刷进行了实验研究。测量嵌段共聚物在空气/水界面处的表面压力pi 作为不同盐浓度和pH 下接枝密度西格玛的函数。结论是，在高离子强度和低 pH 条件下，长 PAA 链的 pi>(*) 超过 bar *(sigma) 的缩放行为与分析平均场模型的预测一致。由于多元酸链吸附到空气/水界面，没有观察到退火刷在低离子强度和低 pH 下理论预测的 pi>(*) 超过 bar *(sigma) 的结垢行为。使用椭圆光度法测量疏水改性硅片上 PAA 刷的厚度，作为 pH、总离子强度 I 和 sigma 的函数。据观察，在给定的 pH 值下，刷子厚度作为 I 的函数表现出非单调性（即，它最初随着 I 的增加而增加，随后随着 I 的增加而减少）。这种非单调行为与退火电刷的理论预测一致。实验观察到与I-α相似的幂律H中的标度指数α近似于0.1，小于理论预测值（1/3）。
Polyelectrolyte brushes consisting of polystyrene-poly(acrylic acid) (PS-PAA) diblock copolymers were investigated experimentally using surface pressure isotherms and ellipsometry. The surface pressure pi of the block copolymers at the air/water interface was measured as a function of the grafting density sigma at various salt concentrations and pH. It is concluded that the scaling behavior of pi>(*) over bar *(sigma) of long PAA chains at high ionic strengths and low pH agrees with predictions of analytical mean-field models. The theoretical predicted scaling behavior of pi>(*) over bar *(sigma) for annealed brushes at low ionic strength and low pH is not observed because of adsorption of the polyacid chains to the air/water interface. The thickness of PAA brushes on hydrophobically modified Si wafers was measured with ellipsometry as a function of pH, total ionic strength I, and sigma. It is observed that at a given pH the brush thickness behaves nonmonotonically as a function of I (i.e., it initially increases and subsequently decreases with increasing I). This nonmonotonic behavior agrees with theoretical predictions for annealed brushes. The experimentally observed scaling exponent alpha in the power law H similar to I-alpha is similar to0.1, which is less than that predicted theoretically (1/3).