Single-chain Elasticity Research Articles

Interrogation of Single Synthetic Polymer Chains and Polysaccharides by AFM‐Based Force Spectroscopy

This contribution reviews selected mechanical experiments on individual flexible macromolecules using single-molecule force spectroscopy (SMFS) based on atomic force microscopy. Focus is placed on the analysis of elasticity and conformational changes in single polymer chains upon variation of the external environment, as well as on conformational changes induced by the mechanical stress applied to individual macromolecular chains. Various experimental strategies regarding single-molecule manipulation and SMFS testing are discussed, as is theoretical analysis through single-chain elasticity models derived from statistical mechanics. Moreover, a complete record, reported to date, of the parameters obtained when applying the models to fit experimental results on synthetic polymers and polysaccharides is presented.

Weakly Bound Water Molecules Shorten Single-Stranded DNA

In this paper, we measure the single chain elasticity of an oligomer single-stranded DNA (ssDNA) in both aqueous and nonaqueous, apolar liquid environments by AFM-based single molecule force spectroscopy. We find a marked deviation between the force-extension relations recorded for the two conditions. This difference is attributed to the additional energy required to break the H-bond-directed water bridges around the ssDNA chain in aqueous solutions, which are nonexistent in organic solvents. The results obtained in 8 M guanidine-HCl solution provide more evidence that water bridges around ssDNA originate the observed deviation. On the basis of the results obtained by an ab initio quantum mechanics calculation, a parameter-free freely rotating chain model is proposed. We find that this model is in perfect agreement with the experimental force-extension curve obtained in organic solvents, which further corroborates our assumption. On the basis of the experimental results, it is suggested that the weak H-bonding between ssDNA and water molecules may be a precondition for stable double-stranded DNA to exist in water.

Toward understanding the effect of substitutes and solvents on entropic and enthalpic elasticity of single dendronized copolymers

Using single molecule force spectroscopy (SMFS), we have investigated the single chain elasticity of a series of dendronized copolymers, whose original backbone is styryl dendron and maleic anhydride. The amilysis reaction of maleic anhydride introduces amide and carboxyl side groups. Hence, the copolymer bears three type side groups, Fréchet type dendrons, amides and carboxyls. The single chain elasticity of the polymers in tetrahydrofuran (THF) and chloroform (CHCl3) has been obtained, respectively. There is no hysteresis between the stretching and relaxing force curves of individual polymer chain, indicating that the elongation is carried out at the equilibrium condition. The force curves can be fitted well by the modified freely jointed chain model, which suggests that the elongation of the polymer chain is controlled by the entropic elasticity in the low-force region but dominated by enthalpic elasticity in the high-force region. The polymer with first-generation dendrons shows different elasticities in the two solvents, which should result from the different hydrogen bonds of the dendronized copolymer in two solvents. When the dendron groups are changed to be second and third generations, the single chain elasticity of the polymers is the same in two solvents. Hence, the elasticity of the polymer chain can be different depending on the properties of side groups and solvents.

Grafting of single, stimuli-responsive poly(ferrocenylsilane) polymer chains to gold surfaces.

Redox-responsive poly(ferrocenylsilane) (PFS) polymer molecules were attached individually to gold surfaces for force spectroscopy experiments on the single molecule level. By grafting ethylenesulfide-functionalized PFS into the defects of preformed self-assembled monolayers (SAMs) of different omega-mercaptoalkanols on Au(111), the surface coverage of PFS macromolecules could be conveniently controlled. Atomic force microscopy (AFM), contact angle, as well as cyclic and differential pulse voltammetry measurements were carried out to characterize the morphology, wettability, and surface coverage of the grafted layers. The values of the PFS surface coverage were found to depend on the chain length of the omega-mercaptoalkanol molecules and on the concentration of the PFS solution but not on the insertion time or on the molar mass of PFS. The equilibrium surface coverages were successfully described by Langmuir adsorption isotherms. For low-surface coverage values (< 6.2 x 10(-4) chain/nm2), achieved by PFS insertion from very dilute solutions (8 x 10(-6) M) into long-chain SAMs, AFM and differential pulse voltammetry showed that surfaces exposing isolated individual polymer chains were obtained. The isolated PFS macromolecules were subjected to in situ AFM-based single molecule force spectroscopy (SMFS) measurements. The single chain elasticity of PFS in isopropanol (and ethanol) was fitted with the modified freely jointed chain (m-FJC) model. This procedure yielded a Kuhn segment length of 0.33 +/- 0.05 nm and a segment elasticity of 32 +/- 5 nN/nm.

Single chain elasticity and thermoelasticity of polyethylene

Single-chain elasticity of polyethylene at θ point up to 90% of stretching with respect to its contour length is computed by Monte Carlo simulation of an atomistic model in continuous space. The elasticity law together with the free-energy and the internal energy variations with stretching are found to be very well represented by the wormlike chain model up to 65% of the chain elongation, provided the persistence length is treated as a temperature-dependent parameter. Beyond this value of elongation simple ideal chain models are not able to describe the Monte Carlo data in a thermodynamic consistent way. This study reinforces the use of the wormlike chain model to interpret experimental data on the elasticity of synthetic polymers in the finite extensibility regime, provided the chain is not yet in its fully stretched regime. Specific solvent effects on the elasticity law and the partition between energetic and entropic contributions to single chain elasticity are investigated.

Force Spectroscopy Study on Poly(acrylamide) Derivatives: Effects of Substitutes and Buffers on Single-Chain Elasticity

Single-chain elasticity of two important poly(acrylamide) derivatives, poly(N,N‘-dimethylacrylamide) (PDMA) and poly(N,N‘-diethylacrylamide) (PDEA), was investigated by an atomic force microscopy (AFM)-based single molecule force spectroscopy (SMFS). SMFS revealed that the single chain of PDEA was stiffer than that of PDMA due to different bulky substitutes. It was also found from the SMFS study that the elasticity of PDMA and PDEA was enhanced in urea buffer solutions, probably due to hydrogen-bond interactions between urea molecules and the side groups of poly(acrylamide) derivatives.

Polyelectrolyte gels in poor solvent: Elastic moduli

We study theoretically using scaling arguments the behavior of polyelectrolyte gels in poor solvents. Following the classical picture of Katchalsky, our approach is based on single-chain elasticity but it accounts for the recently proposed pearl necklace structure of polyelectrolytes in poor solvents. The elasticity both of gels at swelling equilibrium and of partially swollen gels is studied when parameters such as the ionic strength or the fraction of charged monomers are varied. Our theory could be useful to interpret recent experiments performed in Strasbourg that show that if identical gel samples are swollen to the same extent at different pH the sample with the highest charge has the lowest shear modulus.

Polyelectrolyte gel elasticity in poor solvent

AbstractPolyelectrolytes in poor solvents are expected to show a necklace structure where collapsed polymer pearls are linked by stretched polymer strands. This prediction by Kantor and Kardar worked out by Dobrynin Rubinstein and Obukhov Macromol. 1996,29,2974 has mainly be confirmed by computer simulations of Dobrynin; Darinskii; Holm and Kremer. We study the unwinding of pearls under an external force. Polyelectrolyte gels in poor solvents are then discussed, mainly on the basis of single chain elasticity following Katchalsky.