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Determination of the molar mass of surface-grafted weak polyelectrolyte brushes using force spectroscopy

机译:用力谱法测定表面接枝弱聚电解质刷的摩尔质量

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摘要

The molar mass and dispersity of a polycation, poly[2-(dimethyl amino)ethyl methacrylate)] (PDMAEMA) grafted from a poly(methyl methacrylate) (PMMA) backbone, was measured by single-molecule force spectroscopy (SMFS) and shown to be consistent with results from gel permeation chromatography for the same comb polymer in aqueous solution. Comparison was then made between the comb polymer and PDMAEMA brushes that were grown from the substrate, as a function of the pH and ionic strength of the surrounding medium, and the limits of reliable characterization of the polymers are determined. A large discrepancy was observed between the responses of the comb and brush layer at low pH when the PDMAEMA molecules are extended from the supporting substrate. Here it is believed that the atomic force microscope (AFM) tip can penetrate the comb layer and selectively desorb side-chains of the comb. In the case of the well solvated PDMAEMA brushes at high pH, the tip preferentially selects larger chains, resulting in an over-estimate of the brush molar mass. The addition of salt also influenced the molar mass obtained by this technique. It is believed that salted brushes did not adhere well to the AFM tip, with subsequent desorption resulting in an underestimate of the molar mass. However, SMFS was shown to be capable of demonstrating the effect of salt on brush conformation, with greater swelling after the addition of a small amount of NaCl, but a significant decrease when 100 mM is added.
机译:通过单分子力谱法(SMFS)测量了从聚甲基丙烯酸甲酯(PMMA)骨架接枝的聚阳离子聚甲基丙烯酸2-甲基丙烯酸2-(二甲氨基乙酯)酯(PDMAEMA)的摩尔质量和分散度与水溶液中相同梳形聚合物的凝胶渗透色谱分析结果一致。然后根据周围介质的pH和离子强度,对从基材上生长的梳形聚合物刷和PDMAEMA刷进行比较,并确定了聚合物可靠表征的极限。当PDMAEMA分子从支撑基材上伸出时,在低pH值下,梳子和刷子层的响应之间存在很大差异。在此认为原子力显微镜(AFM)尖端可以穿透梳子层并选择性地解吸梳子的侧链。在高pH条件下溶剂化良好的PDMAEMA刷子的情况下,刀尖会优先选择较大的链,从而导致刷子摩尔质量的高估。盐的添加也影响通过该技术获得的摩尔质量。据认为,盐刷不能很好地粘附到AFM尖端,随后的脱附会导致摩尔质量的低估。但是,显示出SMFS能够证明盐对刷子构象的影响,添加少量NaCl后溶胀更大,但是当添加100 mM时,显着降低。

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