title: Effect of the size of solvent molecules on the single-chain mechanics of poly(ethylene glycol): implications on a novel design of a molecular motor.
abstract: Excluded-volume (EV) interaction, also known as the EV effect, can drive the collapse of polymer chains in a polymer solution and promote the crystallization of polymer chains. Herein we report, for the first time, the effect of EV interaction on the single-chain mechanics of a polymer, poly(ethylene glycol) (PEG). By using AFM-based single-molecule force spectroscopy, the single-chain mechanics of a PEG chain has been detected in various nonpolar organic solvents with different molecule sizes. It is observed that the nonpolar solvents can be classified into two categories. In the small-sized organic solvents (e.g., tetrachloroethane and n-nonane), PEG presents its inherent elasticity, which is consistent with the theoretical single-chain elasticity from quantum mechanical calculations. However, in the middle-sized solvents (e.g., n-dodecane and n-hexadecane), the single-chain entropic elasticity of PEG is influenced by EV interactions noticeably, which indicates that the PEG chain tends to adopt a compact conformation under these conditions. To stretch a PEG chain from a free state to a fully extended state, more energy (1.54 kBT per repeating unit) is needed in small-sized organic solvents than in middle-sized organic solvents. It is expected that a partially stretched PEG chain would shrink to some extent when the solvent is changed from a middle-sized organic solvent to a small-sized one. Accordingly, a novel design of a PEG-based single-molecule motor that works with solvent stimuli is proposed.
authors.affiliation: Key Laboratory of Advanced Technologies of Materials, Ministry of Education, Southwest Jiaotong University, Chengdu 610031, China. cuishuxun@swjtu.edu.cn.
authors.forename: Zhonglong
authors.initials: Z
authors.lastname: Luo
authors.affiliation: Key Laboratory of Advanced Technologies of Materials, Ministry of Education, Southwest Jiaotong University, Chengdu 610031, China. cuishuxun@swjtu.edu.cn.
authors.forename: Bo
authors.initials: B
authors.lastname: Zhang
authors.affiliation: State Key Laboratory of Supramolecular Structure and Materials, Institute of Theoretical Chemistry, Jilin University, Changchun 130023, China.
authors.forename: Hu-Jun
authors.initials: HJ
authors.lastname: Qian
authors.affiliation: State Key Laboratory of Supramolecular Structure and Materials, Institute of Theoretical Chemistry, Jilin University, Changchun 130023, China.
authors.forename: Zhong-Yuan
authors.initials: ZY
authors.lastname: Lu
authors.affiliation: Key Laboratory of Advanced Technologies of Materials, Ministry of Education, Southwest Jiaotong University, Chengdu 610031, China. cuishuxun@swjtu.edu.cn.
authors.forename: Shuxun
authors.identifier: http://orcid.org/0000-0002-7713-7377
authors.initials: S
authors.lastname: Cui