Ahlawat Vikhyaat, Deopa Surya Pratap S, Patil Shivprasad
Department of Physics, Indian Institute of Science Education and Research (IISER) Pune, Pashan Road, Pune 411008, India.
Nanomaterials (Basel). 2022 Feb 3;12(3):526. doi: 10.3390/nano12030526.
We estimate the elasticity of single polymer chains using atomic force microscope (AFM)-based oscillatory experiments. An accurate estimate of elasticity using AFM is limited by assumptions in describing the dynamics of an oscillating cantilever. Here, we use a home-built fiber-interferometry-based detection system that allows a simple and universal point-mass description of cantilever oscillations. By oscillating the cantilever base and detecting changes in cantilever oscillations with an interferometer, we extracted stiffness versus extension profiles for polymers. For polyethylene glycol (PEG) in a good solvent, stiffness-extension data showed significant deviation from conventional force-extension curves (FECs) measured in constant velocity pulling experiments. Furthermore, modeling stiffness data with an entropic worm-like chain (WLC) model yielded a persistence length of (0.5 ± 0.2 nm) compared to anomaly low value (0.12 nm ± 0.01) in conventional pulling experiments. This value also matched well with equilibrium measurements performed using magnetic tweezers. In contrast, polystyrene (PS) in a poor solvent, like water, showed no deviation between the two experiments. However, the stiffness profile for PS in good solvent (8M Urea) showed significant deviation from conventional force-extension curves. We obtained a persistence length of (0.8 ± 0.2 nm) compared to (0.22 nm ± 0.01) in pulling experiments. Our unambiguous measurements using interferometer yield physically acceptable values of persistence length. It validates the WLC model in good solvents but suggests caution for its use in poor solvents.
我们使用基于原子力显微镜（AFM）的振荡实验来估计单个聚合物链的弹性。使用AFM准确估计弹性受到描述振荡悬臂梁动力学时所作假设的限制。在这里，我们使用了一个基于光纤干涉测量的自制检测系统，该系统允许对悬臂梁振荡进行简单且通用的质点描述。通过振荡悬臂梁基部并用干涉仪检测悬臂梁振荡的变化，我们提取了聚合物的刚度与伸长曲线。对于处于良溶剂中的聚乙二醇（PEG），刚度-伸长数据与在恒速拉伸实验中测得的传统力-伸长曲线（FEC）有显著偏差。此外，用熵弹性蠕虫状链（WLC）模型对刚度数据进行建模，得到的持久长度为（0.5±0.2纳米），相比之下，传统拉伸实验中的异常低值为（0.12纳米±0.01）。这个值也与使用磁镊进行的平衡测量结果非常吻合。相比之下，处于不良溶剂（如水）中的聚苯乙烯（PS）在这两种实验之间没有显示出偏差。然而，处于良溶剂（8M尿素）中的PS的刚度曲线与传统力-伸长曲线有显著偏差。我们得到的持久长度为（0.8±0.2纳米），而在拉伸实验中为（0.22纳米±0.01）。我们使用干涉仪进行的明确测量得出了物理上可接受的持久长度值。这验证了WLC模型在良溶剂中的适用性，但在不良溶剂中使用时需谨慎。
