Department of Mechanical Engineering, Technical University of Denmark, DK-2800, Kgs. Lyngby, Denmark.
Langmuir. 2013 Jun 25;29(25):7782-92. doi: 10.1021/la400827h. Epub 2013 Jun 14.
We have investigated the adsorption and lubricating properties of neutral and charged amphiphilic diblock copolymers at a hydrophobic polydimethylsiloxane (PDMS) interface in an aqueous environment. The diblock copolymers consist of a hydrophilic block of either neutral poly(ethylene glycol) (PEG) or negatively charged poly(acrylic acid) (PAA) and of a hydrophobic block of polystyrene (PS) or poly(2-methoxyethyl acrylate) (PMEA), thus generating PEG-b-X or PAA-b-X, where X block is either PS or PMEA. The molecular weight ratios were roughly 1:1 with each block ca. 5 kDa. Comparing the neutral PEG and charged PAA buoyant blocks with all other conditions identical, the former showed superior adsorption onto nonpolar, hydrophobic PDMS surfaces from a neutral aqueous solution. PEG-based copolymers showed substantial adsorption for both PS and PMEA as the anchoring block, whereas PAA-based copolymers showed effective adsorption only when PMEA was employed as the anchoring block. For PAA-b-PS, the poor adsorption properties are chiefly attributed to micellization due to the high interfacial tension between the PS core and water. The poor lubricating properties of PAA-b-PS diblock copolymer for a PDMS-PDMS sliding contact was well correlated with the poor adsorption properties. PAA-b-PMEA copolymers, despite their sizable amount of adsorbed mass, showed insignificant lubricating effects. When the charges of the PAA-b-PMEA diblock copolymers were screened by either adding NaCl to the aqueous solution or by lowering the pH, both the adsorption and lubricity improved. We ascribe the poor adsorption and inferior aqueous lubricating properties of the PAA-based diblock copolymers compared to their PEG-based counterparts mainly to the electrostatic repulsion between charged PAA blocks, hindering the facile formation of the lubricating layer under cyclic tribological stress at the sliding PDMS-PDMS interface.
我们研究了中性和带电两亲性嵌段共聚物在疏水聚二甲基硅氧烷（PDMS）界面在水相环境中的吸附和润滑性能。嵌段共聚物由亲水性嵌段组成，要么是中性的聚乙二醇（PEG），要么是带负电荷的聚丙烯酸（PAA），由疏水性嵌段组成，要么是聚苯乙烯（PS），要么是聚（2-甲氧基乙基丙烯酸酯）（PMEA），从而生成 PEG-b-X 或 PAA-b-X，其中 X 嵌段是 PS 或 PMEA。分子量比大致为 1:1，每个嵌段约为 5 kDa。将中性 PEG 和带电 PAA 浮块与所有其他条件相同进行比较，前者从中性水溶液中显示出对非极性疏水性 PDMS 表面的优越吸附性。PEG 基共聚物显示出对 PS 和 PMEA 作为锚固块的大量吸附，而 PAA 基共聚物仅在 PMEA 作为锚固块时显示出有效的吸附。对于 PAA-b-PS，由于 PS 核与水之间的界面张力高，较差的吸附性能主要归因于胶束化。PAA-b-PS 两亲性嵌段共聚物在 PDMS-PDMS 滑动接触中的较差润滑性能与较差的吸附性能密切相关。尽管 PAA-b-PMEA 共聚物吸附了大量的质量，但没有明显的润滑效果。当将 PAA-b-PMEA 两亲性嵌段共聚物的电荷通过向水溶液中添加 NaCl 或降低 pH 值进行屏蔽时，吸附和润滑性都得到了改善。我们将 PAA 基两亲性嵌段共聚物与 PEG 基嵌段共聚物相比吸附和水基润滑性能较差归因于带电荷的 PAA 块之间的静电排斥，这阻碍了在滑动 PDMS-PDMS 界面处的循环摩擦应力下形成润滑层。
