Nanobiophysics, MESA+ Institute for Nanotechnology, Faculty of Science and Technology, University of Twente, Enschede, The Netherlands.
Nanoscale. 2012 Mar 21;4(6):2072-7. doi: 10.1039/c2nr12066f. Epub 2012 Feb 13.
Recently several atomic force microscopy (AFM)-based surface property mapping techniques like pulsed force microscopy (PFM), harmonic force microscopy or Peakforce QNM® have been introduced to measure the nano- and micro-mechanical properties of materials. These modes all work at different operating frequencies. However, complex materials are known to display viscoelastic behavior, a combination of solid and fluid-like responses, depending on the frequency at which the sample is probed. In this report, we show that the frequency-dependent mechanical behavior of complex materials, such as polymer blends that are frequently used as calibration samples, is clearly measurable with AFM. Although this frequency-dependent mechanical behavior is an established observation, we demonstrate that the new high frequency mapping techniques enable AFM-based rheology with nanoscale spatial resolution over a much broader frequency range compared to previous AFM-based studies. We further highlight that it is essential to account for the frequency-dependent variation in mechanical properties when using these thin polymer samples as calibration materials for elasticity measurements by high-frequency surface property mapping techniques. These results have significant implications for the accurate interpretation of the nanomechanical properties of polymers or complex biological samples. The calibration sample is composed of a blend of soft and hard polymers, consisting of low-density polyethylene (LDPE) islands in a polystyrene (PS) surrounding, with a stiffness of 0.2 GPa and 2 GPa respectively. The spring constant of the AFM cantilever was selected to match the stiffness of LDPE. From 260 Hz to 1100 Hz the sample was imaged with the PFM method. At low frequencies (0.5-35 Hz), single-point nanoindentation was performed. In addition to the material's stiffness, the relative heights of the LDPE islands (with respect to the PS) were determined as a function of the frequency. At the lower operation frequencies for PFM, the islands exhibited lower heights than when measured with tapping mode at 120 kHz. Both spring constants and heights at the different frequencies clearly show a frequency-dependent behavior.
最近，几种基于原子力显微镜（AFM）的表面特性映射技术，如脉冲力显微镜（PFM）、谐波力显微镜或 Peakforce QNM®，已被引入用于测量材料的纳米和微机械性能。这些模式都在不同的工作频率下工作。然而，已知复杂材料表现出粘弹性行为，这是固体和流体样响应的组合，取决于样品被探测的频率。在本报告中，我们表明，复杂材料的频率相关机械行为，例如经常用作校准样品的聚合物共混物，可以通过 AFM 清楚地测量。尽管这种频率相关的机械行为是一种已建立的观察结果，但我们证明，与以前基于 AFM 的研究相比，新的高频映射技术使 AFM 能够在更宽的频率范围内以纳米级空间分辨率进行基于流变学的测量。我们进一步强调，当使用这些薄聚合物样品作为高频表面特性映射技术的弹性测量校准材料时，必须考虑机械性能的频率依赖性变化。这些结果对于准确解释聚合物或复杂生物样品的纳米力学性质具有重要意义。校准样品由软质和硬质聚合物的共混物组成，由低密度聚乙烯（LDPE）岛和聚苯乙烯（PS）周围组成，其刚度分别为 0.2 GPa 和 2 GPa。AFM 悬臂的弹性常数选择为匹配 LDPE 的刚度。使用 PFM 方法在 260 Hz 至 1100 Hz 之间对样品进行成像。在低频（0.5-35 Hz）下，进行单点纳米压痕测试。除了材料的刚度外，LDPE 岛的相对高度（相对于 PS）也作为频率的函数确定。在 PFM 的较低操作频率下，与在 120 kHz 时以敲击模式测量相比，岛显示出较低的高度。不同频率下的弹性常数和高度均明显表现出频率依赖性行为。
