Effects of Lateral Deformation by Thermoresponsive Polymer Brushes on the Measured Friction Forces
Effects of Lateral Deformation by Thermoresponsive Polymer Brushes on the Measured Friction Forces
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DOI：
10.1021/acs.langmuir.7b00217
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发表时间：
2017-05-02
期刊：
影响因子：
3.9
通讯作者：
Benetti, Edmondo M.
中科院分区：
文献类型：
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作者：
Ramakrishna, Shivaprakash N.;Cirelli, Marco;Benetti, Edmondo M.
利用侧向力显微镜（LFM）方便地分析了亲水聚合物刷的纳米摩擦学性能。然而，对于高度膨胀和相对较厚的聚合物刷，摩擦的测量可能会受到剪切探针的侧向变形趋势的强烈影响。这种现象在记录的摩擦环中引起“倾斜”，这是由移植物的侧向弯曲和拉伸产生的。在本研究中，我们强调了当相对较短的扫描距离应用时，刷侧变形如何主要影响肿胀的PNIPAM刷（低于LCST）的摩擦测量。在此条件下，LFM记录的能量耗散几乎完全由柔性电刷沿扫描方向的前后拉伸和弯曲决定，而与两个滑动表面之间的动摩擦无关。相比之下，当LFM过程中施加的扫描距离相对大于电刷横向变形时，探针在电刷界面上的滑动占主导地位，可以实现正确的动态摩擦测量。通过将温度提高到lst以上，PNIPAM刷体脱水并呈现塌陷形态，从而阻碍了扫描探针的横向变形。因此，在40摄氏度的水中，记录的摩擦回路没有显示任何倾斜，LFM准确地描述了探针和聚合物表面之间的动态摩擦。
The nanotribological properties of hydrophilic polymer brushes are conveniently analyzed by lateral force microscopy (LFM). However, the measurement of friction for highly swollen and relatively thick polymer brushes can be strongly affected by the tendency of the compliant brush to be laterally deformed by the shearing probe. This phenomenon induces a "tilting" in the recorded friction loops, which is generated by the lateral bending and stretching of the grafts. In this study we highlight how the brush lateral deformation mainly affects the friction measurements of swollen PNIPAM brushes (below LCST) when relatively short scanning distances are applied. Under these conditions, the energy dissipation recorded by LFM is almost uniquely determined by stretching and bending of the compliant brush back and forth along the scanning direction, and it is not correlated to dynamic friction between two sliding surfaces. In contrast, when the scanning distance applied during LFM is relevantly longer than the brush lateral deformation, sliding of the probe on the brush interface becomes dominant, and a correct measurement of dynamic friction can be accomplished. By increasing the temperature above the LCST, the PNIPAM brushes undergo dehydration and assume a collapsed morphology, thereby hindering their lateral deformation by scanning probe. Hence, at 40 degrees C in water the recorded friction loops do not show any tilting and LFM accurately describes the dynamic friction between the probe and the polymer surface.