Shuxun Cui

Unexpected Temperature-dependent Single Chain Mechanics of Poly(N-isopropyl-acrylamide) in Water

Poly(N-isopropyl-acrylamide) (PNIPAM) is a paradigm thermally sensitive polymer, which has a lower critical solution temperature (LCST) of ~32 °C in water. Herein by AFM-based single molecule force spectroscopy (SMFS), we measured the single chain elasticity of PNIPAM across the LCST in water. Below LCST, the force curves obtained at different temperatures have no remarkable difference; while above LCST, an unexpected temperature dependent elasticity is observed, mainly in the middle force regime. We found that 35 °C is a turning point of the variation: from 31 to 35 °C, the middle parts of the force curves drop gradually, whereas from 35 to 40 °C, the middle parts rise gradually. A possible mechanism for the unexpected temperature dependent mechanics is proposed. The single chain contraction against external force upon heating from 35 to 40 °C may cast new light on the design of molecular devices that convert thermal energy to mechanical work.

Inherent stretching elasticity of a single polymer chain with a carbon-carbon backbone.

We study the single-chain elasticities of three kinds of neutral polymers with a carbon-carbon (C-C) backbone by atomic force microscopy-based single-molecule force spectroscopy in a nonpolar solvent (octane), aiming at measuring the inherent chain elasticity of this very important class of polymers. The finding that the single-chain elasticities of all three polymers in octane are virtually identical in the entire force region implies that the side chains of the polymers have no detectable effects on the single-chain elasticity. By utilizing the single-chain elasticity from quantum mechanics calculations, the freely rotating chain model can provide the best fitting curve when each C-C bond is set to be the rotating unit. Although there are some exceptions when the side chain is very huge, our work provides a general result for the inherent elasticity of single neutral flexible polymer chains with C-C backbones.

Fabrication of robust multilayer films by triggering the coupling reaction between phenol and primary amine groups with visible light irradiation

We prepared robust cross-linked (x-linked) multilayer films under visible light irradiation with the catalysis of a Ru(ii) complex. The x-linking is achieved by the coupling reaction between phenol group and primary amine group within the self-assembled multilayer films that were prepared beforehand. Three kinds of polymers, i.e., poly(4-vinylphenol), poly(allylamine) and poly(ethyleneimine), were selected as the model system to illustrate the concept of this strategy. Upon visible light irradiation, the chemical stability of the x-linked films towards solution etching was greatly enhanced. In previous studies, horseradish peroxidase (HRP) is often utilized to catalyze the C-C, C-O and C-N coupling structures, which is useful to prepare polymers, capsules and bulk hydrogels. We also tried to prepare the x-linked films by the catalysis of HRP. The comparison of the two methods suggests that the Ru(ii) complex method is more ideal for fabricating x-linked films. In addition, the photo-triggered chemical reaction within the films was confirmed by the solid-state (13)C NMR, XPS and FT-IR measurements. Without UV light irradiation or thermal treatment, this strategy brings many advantages. It is anticipated that this approach can be easily extended to the applications of the biological related fields in the future.

Capturing the portrait of isolated individual natural cellulose molecules

Natural cellulose molecules have a strong tendency of being aggregated into larger structures. Thus, the imaging of isolated individual cellulose molecules is hampered for a long time. In this work, we manage to observe, for the first time, the isolated individual natural cellulose chains on a sample surface by means of atomic force microscope. The advantage of the ionic liquid, in which natural cellulose can be molecularly dispersed, is considered to be the key point for the successful imaging. Moreover, we find that the surface charge can influence the morphology of the single cellulose chains upon adsorption. That is, on the positively charged surface, individual cellulose chains adopt an extended conformation; whereas on the negatively charged surface, a compact globule conformation is observed.

Single-chain polymers achieved from radical polymerization under single initiator conditions

Radical polymerization from a single initiator molecule in a microenvironment is a nearly ideal system in which bimolecular termination, solution concentration, and viscosity changes could be neglected. In this study, we provide two facile methods of preparing polymers via atom-transfer radical polymerization (ATRP) under single-initiator conditions: tether initiators on planar substrates at superlow density through mixed self-assembled monolayers (SAMs) and encapsulated single initiators in microfluidic droplets. The molecular weight (MW) of the resultant polymers characterized by atomic force microscope-based single-molecule force spectroscopy (AFM-based SMFS) showed that the single-chain ATRP had an extraordinarily faster chain propagation rate (2 unit/s) on planar substrates and gave polymers with much higher MWs (10(5)-10(6) g/mol) than those obtained from traditional ATRP (10(3)-10(5) g/mol). The former method offered a general platform for single-chain polymer synthesis and investigation, and the latter could be amplified to obtain abundant single-chain polymers with ultrahigh molecular weight (UHMW) for commercial applications.

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.

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.

The solvent quality of water for poly(N-isopropylacrylamide) in the collapsed state: Implications from single-molecule studies

Both of temperature (in water) and composition (in the water/methanol mixed solvent) can induce the coil-to-globule 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.

Real time quantification of the chemical cross-link density of a hydrogel by in situ UV-vis spectroscopy

In this paper, we prepare a hydrogel in a green method via the redox reaction between phenol groups and Eosin Y by visible light irradiation. Eosin Y shows significant absorption at 515 nm while the reduced product does not. A series of control experiments show that almost 100% of the consumed Eosin Y reacted with phenol groups to form cross-link points. On the basis of the two results, the chemical cross-link density can be easily determined quantitatively by UV-vis spectroscopy. With this method, specimens from the state of the starting solution to solid gel can be measured in situ and in real time without any pretreatment. The result obtained by this new method is proved by traditional rheological measurements.

Surface charge inversion of self-assembled monolayers by visible light irradiation: cargo loading and release by photoreactions

In this study, we find that visible light can trigger both the loading and the release of N-alkyl substituted 4-picolinium on self-assembled monolayers (SAM). The latter process will result in surface-charge inversion of the SAM, which can be used for controlled release of molecules of interest.

Revealing the formation mechanism of insoluble polydopamine by using a simplified model system

Dopamine (DA) and its polymer (PDA) have attracted broad interest in recent years. Due to the insolubility of PDA, however, it has been difficult to determine the exact structure and the polymerization mechanism. PDA is usually prepared under alkaline conditions (pH 8.5). In this case, the polymerization process may follow an equilibrium pathway to form indole-like repeating units, which lead to cross-linked structures. However, PDA can be prepared even under acidic conditions (pH 4.0) in the presence of an oxidant, ammonium persulfate (APS), which cannot be interpreted by previous mechanisms. Therefore, there should exist several pathways in parallel on the formation of PDA. Herein, a derivative of DA, 2-(4-methoxy-3-methylphenyl)ethylamine (MOE) that has fewer active sites, is used as a simplified model system to study the polymerization mechanism of DA. Experimental results from UV-Vis spectroscopy, single-molecule force spectroscopy and other measurements indicate that free radical polymerization of MOE occurs under both acidic and alkaline conditions in the presence of a polymerization initiator, APS. According to the mathematical principle of Set Theory and the fact that MOE has fewer active sites than DA, we speculate that free radical polymerization as a possible pathway should exist in parallel along with previously proposed pathways during the formation of PDA. These parallel pathways may be the main reason for the structural complexity of PDA.