Department of Chemistry and Biochemistry, The Florida State University, Tallahassee, Florida 32306-4390, USA.
Soft Matter. 2018 Feb 28;14(9):1699-1708. doi: 10.1039/c7sm02461d.
Ultrathin films of complexed polycation poly(diallyldimethylammonium), PDADMA, and polyanion poly(styrenesulfonate), PSS, were prepared on silicon wafers using the layer-by-layer adsorption technique. When terminated with PDADMA, all films had excess PDADMA, which was balanced by counterions. Neutron reflectivity of these as-made multilayers was compared with measurements on multilayers which had been further processed to ensure 1 : 1 stoichiometry of PDADMA and PSS. The compositions of all films, including polymers and counterions, were determined experimentally rather than by fitting, reducing the number of fit parameters required to model the reflectivity. For each sample, acetate, either protiated, CHCOO, or deuterated, CDCOO, served as the counterion. All films were maintained dry under vacuum. Scattering length density profiles were constrained to fit reflectivity data from samples having either counterion. The best fits were obtained with uniform counterion concentrations, even for stoichiometric samples that had been exposed to PDADMA for ca. 5 minutes, showing that surprisingly fast and complete transport of excess cationic charge occurs throughout the multilayer during its construction.
使用层层吸附技术，在硅片上制备了复合聚阳离子聚二甲基丙烯酰胺（PDADMA）和聚阴离子聚苯乙烯磺酸盐（PSS）的超薄薄膜。当用 PDADMA 终止时，所有薄膜都有过量的 PDADMA，由抗衡离子平衡。这些未经进一步处理的多层膜的中子反射率与经过进一步处理以确保 PDADMA 和 PSS 的 1:1 化学计量的多层膜的测量结果进行了比较。所有薄膜的组成，包括聚合物和抗衡离子，都是通过实验确定的，而不是通过拟合确定的，这减少了建模反射率所需的拟合参数的数量。对于每个样品，乙酸盐，无论是质子化的 CHCOO，还是氘代的 CDCOO，都用作抗衡离子。所有薄膜都在真空下保持干燥。散射长度密度分布被约束为拟合具有任何抗衡离子的样品的反射率数据。即使对于暴露于 PDADMA 约 5 分钟的化学计量样品，也可以获得具有均匀抗衡离子浓度的最佳拟合，这表明在多层膜的构建过程中，过量的阳离子电荷会出乎意料地快速和完全地传输到整个多层膜中。
