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High elasticity is one of the most important characteristics of polymers. The entropy-driven elastic behavior dominates the low force regime. The entropic elasticity of single polymer chains can be...

It is generally accepted that water is deeply involved in the structures and functions of DNA and proteins. For polysaccharides, however, the role of water remains poorly understood. Amylose and natural cellulose (NC) are two polysaccharides with similar molecular structures but different linkages (α or β) between the pyranose rings. In this study, the effects of H-bonds on the single-molecule mechanics and affinity for water of amylose and NC are explored by single-molecule atomic force microscopy (AFM) and molecular dynamics (MD) simulations, respectively. The experimental results show that the single-molecule mechanics of both amylose and NC are dependent on the solvent polarity. Accordingly, the status of H-bonds of each polysaccharide can be inferred. We find that the two polysaccharides present the same status of H-bonds in a given organic solvent: the intrachain H-bonds can be formed in a nonpolar solvent (nonane), while they are completely prohibited in a highly polar solvent (dimethyl sulfoxide, ...

Since the discovery of amorphous red phosphorus (a-red P) in 1847, many possible structures have been proposed. However, the exact molecular structure has not yet been determined because of its amorphous nature. Herein several methods are used to investigate basic properties of a-red P. Data from scanning tunneling microscopy (STM) and gel permeation chromatography (GPC) confirm that a-red P is a linear inorganic polymer with a broad molecular weight distribution. The theoretical single-molecule elasticities of the possible a-red P structures are obtained by quantum mechanical (QM) calculations. The experimental single-molecule elasticity of a-red P measured by single-molecule AFM matches with the theoretical result of the zig-zag ladder structure, indicating that a-red P may adopt this structure. Although this conclusion needs further validation, this fundamental study represents progress towards solving the structure of a-red P. It is expected that the strategy utilized in this work can be applied to study other inorganic polymers.

"The Tao begets the One. One begets all things of the world." This quote from Tao Te Ching is still inspiring for scientists in chemistry and materials science: The "One" can refer to a single molecule. A macroscopic material is composed of numerous molecules. Although the relationship between the properties of the single molecule and macroscopic material is not well understood yet, it is expected that a deeper understanding of the single-chain mechanics of macromolecules will certainly facilitate the development of materials science. Atomic force microscopy-based single-molecule force spectroscopy (AFM-SMFS) has been exploited extensively as a powerful tool to study the single-chain behaviors of macromolecules. In this review, we summarize the recent advances in the emerging field of environment-dependent single-chain mechanics of synthetic polymers and biomacromolecules by means of AFM-SMFS. First, the single-chain inherent elasticities of several typical linear macromolecules are introduced, which are also confirmed by one of three polymer models with theoretical elasticities of the corresponding macromolecules obtained from quantum mechanical (QM) calculations. Then, the effects of the external environments on the single-chain mechanics of synthetic polymers and biomacromolecules are reviewed. Finally, the impacts of single-chain mechanics of macromolecules on the development of polymer science especially polymer materials are illustrated.

Ubiquitous van der Waals (vdW) forces are very important for nanostructures. Although the vdW forces between two surfaces (or two layers) have been measured for several decades, a direct detection at the single-molecule level is still difficult. Herein, we report a novel method to solve this problem in high vacuum by means of 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. A constant force plateau can be observed when a polymer chain is peeled off from a substrate in high vacuum. Accordingly, the vdW forces between one polymer repeating unit and the substrates can be obtained. The experimental results show that the vdW forces (typical range: 21–54 pN) are dependent on the species of substrates and the size of polymer repeating unit, which is in good accordance with the theoretical results. It is expected that this novel method can be applied to detect other non-covalent interactions (such as hydrogen bond and π-π stacking) at the single-molecule level in the future.

It is generally recognized that water is deeply involved in the structures and functions of DNA and proteins. For polysaccharides, however, the role of water remains unclear. Due to the force-induced conformational transition of the sugar rings, a fingerprint plateau can be observed in the single-chain force-extension (F-E) curves of amylose and some other polysaccharides in aqueous solutions. In this study, the effects of water content of the mixed solvents on the fingerprint plateau of amylose are explored by single-molecule AFM. The experimental results obtained in a series of water/alcohol mixed solvents clearly show that both the appearance and the fingerprint plateau height in the F-E curves of amylose are dependent on the water content. Since water is a good solvent for amylose but alcohols are not, the higher water content of a mixed solvent corresponds to a better solvent quality. Thus, the observed results can be associated with the solvent quality to amylose. The present study implies that water is not only a solvent but also an active constituent in the amylose solution.

Designing a main-chain visible-light-labile polymer is still a challenging task in related fields. In this work, a picolinium-caged main-chain photolabile polymer (MCPP) is firstly reported, where the C–O bonds of picolinium groups are efficiently cleaved (>95%) upon visible light irradiation (452 nm). The degradation rate of the MCPP is easily tuned by adjusting the concentration of the catalyst, irradiation intensity and time. Importantly, after photodegradation, the molecular weight of degradation products is decreased to less than 650 g mol−1. It is found that, unlike forming a helix structure of pure polyethylene glycol in water, the picolinium-caged MCPP is in a linear conformation. Moreover, nanopore-based single-molecule analysis indicates that both the MCPP and degradation products can easily pass through a biomimetic lipid membrane, and the passing speed can be tuned by varying the applied bias voltage. Cell cytotoxicity and proliferation experiments demonstrate that both the MCPP and degradation products are nontoxic, but only the MCPP is favorable for cell proliferation. On the basis of these results, it is anticipated that the MCPP is an exceptional candidate of photodegradable materials in chemistry, pharmacy, materials science and medicine.

The interactions between monovalent counterions and polyelectrolyte are important in chemical and biological systems. The condensation and screening effect of counterions complicate the polyelectrolyte solutions. By means of single-molecule AFM, the single-chain mechanics of a strong polyelectrolyte, poly(sodium styrenesulfonate) (PSSNa), in KCl aqueous solutions over almost whole concentration range have been studied. The M-FJC model has been used to describe the single-chain elasticity of PSSNa in KCl solutions with a parameter of single-chain modulus (K0). Along with the increase of the concentration of KCl from zero to almost the saturation concentration, a reentrant variation of K0 of single PSSNa chain can be observed. When [K+] is between 0.01 to 3 M, the charges on the PSSNa backbone are almost completely screened, i.e., the PSSNa chain is virtually neutral in this case. Because K0 has a positive correlation with the net charge of the polymer chain, the increased K0 at very high KCl concentrations...

Sample preparation is crucial to the studies of polymers on surfaces and interfaces. For studies using single-molecule force spectroscopy (SMFS), sample preparation is the key to obtain high-quality data. In each of the previous SMFS studies, either physisorption or chemisorption was applied in the sample preparation. However, a direct comparison on the same polymer species with both of the two strategies has not been reported yet. With two methods (physisorption, “PS”, or chemisorption, “CS”) and two surfaces (AFM tip or quartz substrate), four types of samples of poly(ethylene glycol) (PEG) can be prepared from the polymer solution. The performance of these samples with the same species of PEG are directly compared by SMFS. It is found that among these samples, two of them, i.e., tip-CS and substrate-PS, are suitable for SMFS. The advantage of substrate-PS is the simple preparation. In contrast, the advantage of tip-CS is the higher rupture force and the lower sample consumption. The former feature will be time saving if a high rupture force is needed in the analysis. The latter feature will be economic when an expensive sample is used. The other two types of samples, i.e., tip-PS and substrate-CS show lower data yield and lower rupture force. In summary, the tip-CS and substrate-PS are recommended for sample preparation in SMFS. The tip-CS is the most promising protocol, if a functionalized polymer sample is available.

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.

Non-covalent interactions are very important for nanostructures. The detection of non-covalent interactions at the molecular scale is very difficult since they are too weak to detect in most cases. The disturbance from the liquid environment can be eliminated in high vacuum, which makes the detection of non-covalent interactions possible. This review introduces a method of high vacuum single molecule force spectroscopy (SMFS) and summarizes a series of recent studies on non-covalent interactions by using SMFS. These studies include the van der Waals forces between a polymer chain and the substrate as well as the intrachain π–π interactions of a polymer chain, which cast new light on the non-covalent interactions.

Polymers with a carbon–carbon (C–C) backbone are an important class of polymers, which can be regarded as the derivatives of polyethylene (PE). To investigate the effect of side chains on the single-chain enthalpic elasticity (SCEE) of polymers with a C–C backbone, several polymers with pendants or side chains of different lengths and shapes have been studied by single-molecule AFM. We find that both length and shape of the side chains count: only the side chains that are both long and bulky (i.e., bulky dendrons of second or higher generation as side chains) affect the SCEE. Thus, only rare polymers have special SCEE. For the vast majority of polymers, the SCEE is identical to that of PE, which means that the SCEE is determined by the nature of the C–C backbone. It is expected that this conclusion can also be popularized to all polymers with various backbones. This study is an important update to the understanding of polymers at the single-chain level.

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.

The material properties of polylactic acid (PLA) are largely determined by its stereo-regularity (tacticity). To find out the origin at the molecular level, the single-chain mechanics of poly-l-lactic acid (PLLA) and poly-d,l-lactide (PDLLA) were comparatively investigated by single-molecule atomic force microscopy (AFM). At a low concentration, PLLA adopted a random-coil conformation in a good solvent. At a high concentration, however, the PLLA chain can be induced into a helix, which consumed additional energy during unfolding by further stretching. Due to the random arrangement of l- and d-repeating units in the PDLLA chain, PDLLA adopts a random-coil conformation at all concentrations. The difference in single-chain mechanics of PLLA and PDLLA at high concentrations may be the cause of their different macroscopic properties. This is the first report to reveal the stereo-regularity-dependent mechanics of a polymer at the single-molecule level, which may help to bridge the gap between understanding single-molecule and materials properties.

At crystal or larger scales, natural cellulose was generally recognized to be a rigid material. Our single-chain mechanical measurements, however, reveal that the natural cellulose is as flexible as common synthetic polymers at the single-chain level, creating new opportunities in the designing of nano materials.

Superstretchable materials have many applications in advanced technological fields but are difficult to stretch to more than 1000× their original length. A superstretchable dynamic polymer network that can be stretched to 13 000× its original length is designed. It is revealed that superstretchability of the polymer network is derived from the synergistic effect of two different types of dynamic bonds, including a small number of strong dynamic imine bonds to maintain the network integrity during stretching and a large number of weak ionic hydrogen bonds to dissipate energy. This approach provides new insights into the design of superstretchable polymers.

The single-chain mechanics of two similar thermosensitive polymers, poly(N,N-diethylacrylamide) (PDEAM) and poly(N-isopropylacrylamide) (PNIPAM), have been studied by atomic force microscopy-based single-molecule force spectroscopy (SMFS). In a typical nonpolar organic solvent, octane, both of the polymers show the same inherent elasticity, although they have different substitutional groups. However, the mechanics of the two polymers presents large differences in water. The energies needed for the rearrangement of the bound water during elongation at room temperature are estimated by the SMFS method at the single-chain level, which is ~1.13 ± 0.10 and ~5.19 ± 0.10 kJ/mol for PDEAM and PNIPAM, respectively. In addition, PNIPAM shows a temperature-dependent single-chain mechanics when the temperature is increased across the lower critical solution temperature (LCST), while PDEAM does not. These differences observed in aqueous solution originate from the different structures of the two polymers. With a hydrogen bond donor in the amide group, PNIPAM will be more hydrated when TLCST. When TLCST, PNIPAM will have larger changes in both conformation and hydration. These findings also suggest that PNIPAM is a good candidate for a thermo-driven single-molecule motor, while PDEAM is not.

Double-stranded DNA films (200 bp) were prepared on 3-aminopropyltriethoxy-silane (APS)-modified mica surfaces in the NaCl solution with concentration ranging from 0.001 to 0.1 M. By using an atomic force microscope, the force–distance curve and friction behaviour of each DNA film was studied in the NaCl solution that was used in the sample preparation. When the concentration of NaCl solution decreased from 0.1 to 0.001 M, the range of non-linear repulsion on DNA films increased from 8.6 to 29.1 nm, which suggested that lower salt concentration likely resulted in more extended DNA molecules. At the same time, the friction force on DNA films increased by 190% under a normal load of 2.5 nN, which was attributed to the conformation changes in the DNA molecules induced by the electrolyte ions. Therefore, low salt concentration is more conducive to the sequencing process, since it can not only make DNA molecules easier to reach into nanopore through extended conformation, but also reduce the passage rate by high friction. The results have important implications for the development of the third-generation sequencing technique based on nanopore.

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.

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.

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.

We utilize the extracted water-soluble wool keratin as the building block for the layer-by-layer (LbL) assembly. By adjusting the solution pH, keratin can be used as polycation or polyanion. Experimental results indicate that multilayer films of keratin together with other synthetic polyelectrolytes have been fabricated successfully. We have found that the average thickness of each bilayer of the keratin containing film is evidently larger than that of usual multilayer films previously reported. This feature is helpful to fabricate relatively thick films in less deposition cycles. The use of keratin in LbL assembly is helpful to prepare a biocompatible surface for tissue engineering.

The dominant models of polymers, i.e., the freely jointed chain (FJC) model, the wormlike chain (WLC) model and the freely rotating chain (FRC) model are modified by integrating the inherent single-molecule elasticity obtained from quantum mechanics (QM) ab-initio calculations. The QM modified models have been utilized to generate fitting curves for single-stranded DNA obtained in organic solvent. The analyses on the deviation between the fitting curve and the experimental force curve demonstrate that the QM-FRC and QM-FJC model are suitable for ssDNA, but not the QM-WLC model. We also find that one repeating unit of ssDNA is corresponding to a Kuhn segment in QM-FJC model or two rotating units in QM-FRC. Having close correlation to the inherent elasticity and real molecular structure of the polymer, QM-FJC and QM-FRC are emerging as structure relevant models.

We demonstrate the effect of concentration of extracted keratin on the rebuilding of disulfur bond during dialysis against deionized water: cross-linked networks can be formed in the aggregate when the concentration is high, whereas at low concentration, intramolecular disulfur bond is preferred. This preliminary study illustrates, for the first time, the potential of controlling the cross-linking between keratin chains by facile means without addition of other chemicals, which is helpful to construct the biomacromolecular self-assemblies.

Water, the dominant component under the physiological condition, is a complicated solvent which greatly affects the properties of solute molecules. Here, we utilize atomic force microscope-based single-molecule force spectroscopy to study the influence of water on the single-molecule elasticity of an unstructured single-stranded RNA (poly(U)). In nonpolar solvents, RNA presents its inherent elasticity, which is consistent with the theoretical single-chain elasticity calculated by quantum mechanics calculations. In aqueous buffers, however, an additional energy of 1.88 kJ/mol·base is needed for the stretching of the ssRNA chain. This energy is consumed by the bound water rearrangement (Ew) during chain elongation. Further experimental results indicate that the Ew value is uncorrelated to the salt concentrations and stretching velocity. The results obtained in an 8 M guanidine·HCl solution provide more evidence that the bound water molecules around RNA give rise to the observed deviation between aqueous and nonaqueous environments. Compared to synthetic water-soluble polymers, the value of Ew of RNA is much lower. The weak interference of water is supposed to be the precondition for the RNA secondary structure to exist in aqueous solution.

Atomic force spectroscopy (AFM)-based single-molecule force spectroscopy (SMFS) was invented in the 1990s. Since then, SMFS has been developed into a powerful tool to study the inter- and intra-molecular interactions of macromolecules. Using SMFS, a number of problems in the field of supramolecular chemistry and mechanochemistry have been studied at the single-molecule level, which are not accessible by traditional ensemble characterization methods. In this review, the principles of SMFS are introduced, followed by the discussion of several problems of contemporary interest at the interface of supramolecular chemistry and mechanochemistry of macromolecules, including single-chain elasticity of macromolecules, interactions between water and macromolecules, interactions between macromolecules and solid surface, and the interactions in supramolecular polymers.

Single molecule force spectroscopy allows the investigation of intramolecular or intermolecular interactions of polymers on a single molecule level at the solid/liquid interface, thus potentially opening a new horizon for single molecule mechanochemistry. In this article, we highlight the nascent, but rapidly developing field of single molecule force spectroscopy, and focus our discussion on the recent progress in single molecule force spectroscopy on polymers.

Single-chain manipulation of two second-generation dendronized polymers, hydrophobic and amphiphilic poly(p-phenylene), was performed by using single-molecule force spectroscopy on the basis of atomic force microscopy. In tetrahydrofuran (THF) buffer, the individual amphiphilic dendronized polymer chain exhibits larger elasticity than the corresponding hydrophobic dendronized polymer chain does. The larger elasticity of the single amphiphilic polymer chain is probably from the collapse of oligoethyleneoxy chains in THF, which is a poor solvent for them. While using CH 2 Cl 2 as a buffer, all the side groups can be expanded; the elasticity of the amphiphilic polymer chain is therefore smaller than that in THF. The individual hydrophobic polymer exhibits the same elasticity in THF and CH 2 Cl 2 because of the similar polymer chain conformation in the two buffers. Moreover, the adhesion-force measurements in a poor solvent at the interface reveal that the poor solubility of the polymer side groups as well as the hydrophobic interaction between the surface and the polymer side groups enhances the adhesion force.

AFM-based single-molecule force spectroscopy (SMFS) is a powerful tool for the investigation of the elastic properties of a single polymer. Two typical organometallic polymers bearing ferrocene (Fc) groups in the backbone, poly(ferrocenyldimethylsilane) and poly(ferrocenylmethylphenylsilane), were investigated by SMFS to reveal their single-chain mechanical properties in normal and oxidized forms. We have found that the two polymers show similar elasticity in normal forms, though bearing different side groups. However, they exhibit different enthalpic elasticity after oxidation probably because of steric effects. Moreover, all experiments confirm that the single-chain elongation of poly(ferrocenyldimethylsilane) or poly(ferrocenylmethylphenylsilane) is reversible.

We have synthesized a new kind of poly(2-acrylamido-2-methylpropanesulfonic acid) with crown ether and studied comparatively the single molecule force spectroscopy of the polymers with and without crown ether, in terms of desorption and elongation. The smooth desorption process enabled us to calculate the loading rate of the stretching process. For the two polymers, desorption forces were loading rate independent and ionic strength insensitive. Interestingly, the desorption forces of the two polymers were undistinguishable in all conditions. These findings demonstrate (1) the polymer chains adopt a trainlike (flat) conformation at the interface with a high adsorption/desorption rate, (2) the spacer, which separates the charged group from the hydrophobic backbone and combines the two properties together, should account for the retained desorption force at high salt concentration, and (3) the 20% less in linear charge density does not affect the desorption force remarkably since hydrophobic interaction domi...

Single molecule force spectroscopy (SMFS) is a new kind of technique based on atomic force microscope, which allows detecting force as low as pico-newtons directly. Herein based on our recent work, we want to demonstrate the investigation of supramolecular structures and interactions in polymer systems by SMFS, such as desorption force between polymer and substrate, identifiability of polymer micelle and its interaction with surfactant, splitting force of ion-induced helical structure in polysaccharide. It shows well that SMFS is a powerful tool in the study of supramolecular science.

We propose a simple method to isolate polymer chains individually at the quartz surface by utilizing the defects in the self-assembled monolayers of organosilane. This method allows us to measure the desorption force of a single polyelectrolyte chain from a substrate directly.

The desorption forces of poly(4-vinylpyridine) (PVP) layer assemblies in different solvents were measured directly by using single-molecule force spectroscopy (SMFS). For pure methanol was used as the solvent, sawtoothlike force profiles were obtained, indicating that the detachment of polymer loops from the substrate was achieved. On addition of acetic acid into the pure methanol, the number of sawtoothlike force profiles decreased remarkably and more force curves with single rupture peak were obtained. However, in both cases a rupture force of approximately 180 pN was found frequently, which is thought to be the desorption force of an adsorbed PVP chain from a substrate tailored with amino groups.

The influence of a basic aqueous solution on a hydrogen-bonding-directed layer-by-layer (LbL) self-assembled film, based on poly(acrylic acid) (PAA) and poly(4-vinylpyridine) (PVP), was investigated. The composition change of a multilayer film in a NaOH solution was monitored by X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and UV−vis spectroscopy. The morphology variation was observed by atomic force microscopy. A two-step variation was observed: the first step is the dissolution of PAA from the film into the basic solution; the second is the gradual reconformation of PVP polymer chains remaining on the substrate, which produces a microporous film. The evidence for the reconformation of the polymer chains was further provided by single molecule force spectroscopy. Both the release of PAA and the formation of the microporous film are attributed to the properties of the LbL film based on hydrogen bonding. This interesting and novel way to fabricate microporous films is envisag...

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.

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.