摘要/Abstract
脯氨酸聚酯(PPEs)是一类分子量较高且分子量分布较窄的新型可降解脂肪族聚酯, 有巨大的应用潜力. 然而, 还没有关于PPEs力学性质的报道. 本工作使用基于原子力显微镜的单分子力谱(AFM-based SMFS)实验技术和熵焓弹性耦合模型研究了PNC12PE (一种PPEs)在极性有机溶剂中的力学性质. 熵焓弹性耦合模型对PNC12PE的实验力曲线的拟合结果很好, 并且可以将PNC12PE重复单元视作库恩链段, 说明得到了PNC12PE的本征弹性. PNC12PE的典型实验力曲线和C—C骨架的理论力曲线在整个力区间(100~2000 pN)内基本重合, 说明两者单分子链的柔性接近. 此外, PNC12PE主链上的四氢吡咯环在拉伸力(F<2200 pN)作用下不发生异构化, 表明PNC12PE在单分子层面上具有较高的力学稳定性. 本研究加深了人们对PPEs单分子链力学性质的认识, 有助于PPEs的深入研究和应用.
关键词: 脯氨酸聚酯, 单分子力谱, 单链弹性, 力学稳定性, 分子柔性
Proline-based polyesters (PPEs) are novel degradable aliphatic polyesters with high molecular weight and complete recyclability under mild and economic conditions. Because of that, PPEs can be used as a candidate to solve the environmental problems caused by the petroleum-based synthetic polymers. However, the study of the mechanical property of PPEs, which is important for the applications, has not been reported yet. Herein, we utilized atomic force microscopy (AFM)-based single-molecule force spectroscopy (SMFS) and quantum mechanics (QM) calculations to study the mechanical property of PNC12PE (a kind of PPEs) in polar organic solvent at the single-molecule level. PNC12PE was dissolved in methyl isobutyl ketone (MIBK) to a concentration of 10 μg/mL. To prepare the sample for single-molecule AFM, a few drops of the PNC12PE solution were deposited onto the clean glass substrate for 15 min. Then, the sample was rinsed with abundant ethanol to remove the loosely adsorbed polymer. After being dried by air flow, the sample was immediately used in the experiments. The spring constant of each AFM cantilever, which was around 45 pN/nm, was obtained by the thermo- excitation method. No remarkable differences can be found between different force-extension (F-E) curves with various stretching velocities, suggesting that the single-molecule experiments were carried out under quasi-thermodynamic equilibrium conditions. It is found that the experimental single-chain elasticity of PNC12PE consists with the result of the QM calculations and the Kuhn length is structural relevant, indicating that the inherent elasticity of PNC12PE is obtained. The normalized F-E curves of PNC12PE and the QM-freely rotating chain (FRC) model fitting curve of C—C backbone (polyethylene) have little difference, meaning that the main chain of PNC12PE is as soft as the common polymers with C—C backbone. In addition, the pyrrolidine rings in the main chain of PNC12PE are mechanically stable (F<2200 pN) during SMFS experiment, indicating that PNC12PE is suitable for the applications requiring high mechanical stability.
Key words: proline-based polyester, single-molecule force spectroscopy, single-chain elasticity, mechanical stability, molecular flexibility
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