Laboratory for Surface Science and Technology, Department of Materials, ETH Zurich , Vladimir-Prelog-Weg 5/10, 8093 Zurich, Switzerland.
Department of Materials Science and Technology of Polymers, MESA + Institute for Nanotechnology, University of Twente , P.O. Box 217, 7500 AE Enschede, The Netherlands.
Langmuir. 2017 May 2;33(17):4164-4171. doi: 10.1021/acs.langmuir.7b00217. Epub 2017 Apr 21.
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 °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.
亲水聚合物刷的纳米摩擦学特性可以通过横向力显微镜（LFM）方便地分析。然而，对于高度溶胀和相对较厚的聚合物刷，摩擦的测量可能会受到顺从性刷被剪切探针横向变形的趋势的强烈影响。这种现象会在记录的摩擦环中产生“倾斜”，这是由接枝的横向弯曲和拉伸引起的。在本研究中，我们强调了当应用相对较短的扫描距离时，聚合物刷的横向变形如何主要影响溶胀的 PNIPAM 刷（低于 LCST）的摩擦测量。在这些条件下，LFM 记录的能量耗散几乎完全由顺从刷在扫描方向上前后拉伸和弯曲决定，与两个滑动表面之间的动态摩擦无关。相比之下，当在 LFM 期间应用的扫描距离明显长于聚合物刷的横向变形时，探针在刷界面上的滑动变得占主导地位，可以完成对动态摩擦的正确测量。通过将温度升高到 LCST 以上，PNIPAM 刷会经历脱水并呈现出坍塌的形态，从而阻碍了扫描探针对其的横向变形。因此，在 40°C 的水中，记录的摩擦环不会出现任何倾斜，LFM 可以准确描述探针和聚合物表面之间的动态摩擦。
