聚乙二醇在高真空条件下的单链弹性

Single-chain Elasticity of Poly(ethylene glycol) in High Vacuum

在过去二十年间,高分子的单链弹性已经得到了广泛的研究.然而由于环境和高分子之间往往有着复杂的相互作用,实验中很难得到高分子在严格无扰状态下的单链弹性(即本征弹性).为此,利用单分子力谱技术研究了高真空条件下聚乙二醇(PEG)的单链弹性.结果表明,由于高真空条件下溶剂分子的干扰被消除,PEG在这一准无扰状态下呈现其本征弹性.在非极性有机溶剂中,由于溶剂分子和PEG之间只有微弱的范德华力作用,PEG表现出和高真空中基本一致的弹性.然而,在不同环境中,力曲线的低力区(F<100pN)存在着细微的差异.这一现象可归因于不同条件下基底与PEG链之间的吸附力不同.采用的高真空力谱可用于研究其他高分子单链在准无扰状态下的本征弹性.

The elasticity of a single polymer chain has been widely investigated in last decades. However, the direct measurement of the single polymer elasticity in an unperturbed state (i.e. inherent elasticity) remains a challenge. The main obstacle in this regard is that most force measurements are carried out in a liquid environment. The single polymer elasticity may be strongly affected by the complex interactions between solvent molecules and polymer such as van der Waals (vdW) forces, hydrogen bonds and/or thermal motions. For instance, the single-chain elasticity of poly(ethylene glycol)(PEG) in water is different from that in nonpolar organic solvents, since hydrogen bonds can be formed between PEG and water molecules. In this study, the single-chain elasticity of PEG is investigated in high vacuum by means of atomic force microscopy (AFM)-based single-molecule force spectroscopy (SMFS). Solvent molecules and surface adsorbed water are removed thoroughly under high vacuum so that the situation is greatly simplified. PEG is dissolved in DI water to a concentration of 50 μg/mL, which is used for the polymer physisorption on a quartz substrate. Then, the sample is rinsed with abundant DI water to remove the loosely adsorbed polymer and dried by air flow. After that, the AFM chamber is pumped down to ca. 7.0×10^-4 Pa to achieve high vacuum, where almost all adsorbed water molecules can be removed from the environment. The results show that PEG maintains its inherent elasticity in high vacuum, which can be well described by an elastic model of a single polymer chain (QM-FRC model) when F>100 pN. In a nonpolar organic solvent (nonane), since there are only vdW forces between solvent molecules and PEG, PEG presents an elasticity virtually identical to that in high vacuum. However, a slight difference can be observed in the low force region (F<100 pN) in different environments. The long plateau (ca. 45 pN) observed in high vacuum can be attributed to the adsorption/desorption force (mainly vdW forces) of PEG on the substrate. It is greatly anticipated that the method used in the current study can be applied to investigate the inherent elasticity of other polymers in the future.