首页> 外文期刊>Langmuir: The ACS Journal of Surfaces and Colloids >Chemical force titration of plasma polymer-modified PDMS substrates by using plasma polymer-modified AFM tips
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Chemical force titration of plasma polymer-modified PDMS substrates by using plasma polymer-modified AFM tips

机译:使用等离子体聚合物改性的AFM针尖对等离子体聚合物改性的PDMS基材进行化学力滴定

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摘要

Plasma polymerization has gained increasing attention in surface functionalization. We use here chemical force titration to characterize PDMS (polydimethylsiloxane) substrates modified by maleic anhydride-pulsed plasma polymerization. The coating is hydrolyzed to promote the formation of dicarboxylic acid groups. To enhance the variation of the adhesion forces as a function of pH, we use AFM tips modified in the same way as the substrates. The pH-dependent adhesion measurements are performed at different KCl concentrations. The dicarboxylic nature of the maleic acid groups clearly emerges from the force titration curves. The surface pK(a) values (pK(a1) = 3.5 +/- 0.5 and pK(a2) = 9.5 +/- 0.5) of the dicarboxylic acids are evaluated from low electrolyte concentration solutions. The values are shifted toward higher pK(a) values when compared to maleic acid in solution. The first pK(a) appears in the titration force curve for low salt concentration as a peak. This peak changes to a sigmoidal shape at higher salt concentrations. The appearance of a peak is attributed to the formation of strong hydrogen bonds between the tip and the substrate as reported in the literature. The effect of the ionic strength on the force curves is explained by the condensation of counterions on the carboxylate groups. At high pH, the adhesion force almost vanishes. On the approach, at high pH, one first observes repulsion between the tip and the substrate, which varies exponentially with the tip/substrate distance. The decay length of this repulsion force is in good agreement with theoretical predictions of the Debye length, attesting to the electrostatic nature of the interactions. We also find that the replacement of monovalent cation K+ by the divalent cation Ca2+ leads to significant changes in the force titration curve at high pH where the dicarboxylic groups are fully ionized. We observe that the adhesion force no longer vanishes at high pH but even slightly increases with pH, an effect that is explained by Ca2+ ions bridging between two carboxylate groups.
机译:等离子体聚合在表面功能化方面已引起越来越多的关注。我们在这里使用化学力滴定法来表征由马来酸酐脉冲等离子体聚合改性的PDMS(聚二甲基硅氧烷)底物。涂层被水解以促进二羧酸基团的形成。为了增强粘附力随pH的变化,我们使用了与基材相同的AFM针尖。在不同的KCl浓度下进行pH依赖的粘附力测量。马来酸基团的二羧酸性质清楚地从力滴定曲线中显现出来。从低电解质浓度溶液评估二羧酸的表面pK(a)值(pK(a1)= 3.5 +/- 0.5和pK(a2)= 9.5 +/- 0.5)。与溶液中的马来酸相比,该值向较高的pK(a)值偏移。在低盐浓度的滴定力曲线中,第一个pK(a)出现一个峰。在较高的盐浓度下,该峰变为S形。如在文献中所报道的,峰的出现归因于尖端和基底之间的强氢键的形成。离子强度对力曲线的影响可以通过抗衡离子在羧酸酯基团上的缩合来解释。在高pH下,粘附力几乎消失。在进近过程中,在高pH值下,首先观察到尖端与基材之间的斥力,该斥力随尖端/基材的距离呈指数变化。该排斥力的衰减长度与德拜长度的理论预测非常吻合,证明了相互作用的静电性质。我们还发现,用二价阳离子Ca2 +代替一价阳离子K +会导致在高pH下力滴定曲线发生重大变化,其中二羧酸基团已被完全离子化。我们观察到,粘附力在高pH值下不再消失,而随pH值甚至略有增加,这种作用可以通过桥接在两个羧酸根之间的Ca2 +离子来解释。

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