摘要
Both of temperature(in water) and composition(in the water/methanol mixed solvent) can induce the coil-toglobule transition of poly(N-isopropylacrylamide)(PNIPAM). The atomic force microscope(AFM) based single molecule force spectroscopy(SMFS) has been exploited to investigate the interactions between the polymer chain and solvent at the single-molecule level. It is found that the single-chain mechanics of PNIPAM show a remarkable dependence on the two external stimuli. A confusing experimental result is that all the force-extension(F-E) curves of unfolding an individual PNIPAM globule present a feature of elastic(monotonically increasing force) stretching but not plateau(constant force) stretching predicted by theory. In this article, we clarify that the presence of the interior solvent molecules in the single-chain globule is the origin of the discrepancy between the F-E curves obtained from theory and experiment. Although both of the external stimuli do tend to lower the solvent quality for PNIPAM, water and the water/methanol mixed solvent will never be the strongly poor solvent for PNIPAM, even at the worst condition.
Both of temperature(in water) and composition(in the water/methanol mixed solvent) can induce the coil-toglobule transition of poly(N-isopropylacrylamide)(PNIPAM). The atomic force microscope(AFM) based single molecule force spectroscopy(SMFS) has been exploited to investigate the interactions between the polymer chain and solvent at the single-molecule level. It is found that the single-chain mechanics of PNIPAM show a remarkable dependence on the two external stimuli. A confusing experimental result is that all the force-extension(F-E) curves of unfolding an individual PNIPAM globule present a feature of elastic(monotonically increasing force) stretching but not plateau(constant force) stretching predicted by theory. In this article, we clarify that the presence of the interior solvent molecules in the single-chain globule is the origin of the discrepancy between the F-E curves obtained from theory and experiment. Although both of the external stimuli do tend to lower the solvent quality for PNIPAM, water and the water/methanol mixed solvent will never be the strongly poor solvent for PNIPAM, even at the worst condition.
基金
supported by the National Natural Science Foundation of China(Nos.21074102 and 21222401)