Theory of mechanical unfolding of homopolymer globule: all-or-none transition in force-clamp mode vs phase coexistence in position-clamp mode

Equilibrium mechanical unfolding of a globule formed by long flexible homopolymer chain collapsed in a poor solvent and subjected to an extensional force f (force-clamp mode) or extensional deformation D (position-clamp mode) is studied theoretically. Our analysis, like all previous analysis of this problem, shows that the globule behaves essentially differently in two modes of extension. In the force-clamp mode, mechanical unfolding of the globule with increasing applied force occurs without intramolecular microphase segregation, and at certain threshold value of the pulling force the globule unfolds as a whole ("all-or-none" transition). The value of the threshold force and the corresponding jump in the distance between the chain ends increase with a deterioration of the solvent quality and/or with an increase in the degree of polymerization. In the position-clamp mode, the globule unfolding occurs via intramolecular microphase coexistence of globular and extended microphases followed by an abrupt unraveling transition. Reaction force in the microphase segregation regime demonstrates an "anomalous" decrease with increasing extension. Comparison of deformation curves in force and position-clamp modes demonstrates that at weak and strong extensions the curves for two modes coincide, differences are observed in the intermediate extension range. Another unfolding scenario is typical for short globules: in both modes of extension they unfold continuously, without jumps or intramolecular microphase coexistence, by passing a sequence of uniformly elongated configurations.Comment: 19 pages, 13 figures, 1 tabl

Collapse-to-Swelling Transitions in pH- and Thermoresponsive Microgels in Aqueous Dispersions: The Thermodynamic Theory

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Can one detach a fully adsorbed flexible polymer chain by an ultra-small external force?

Full adsorption of flexible chains onto typical solid substrates occurs at a surface interaction energy of (5–10) kBT. The corresponding detachment force is in the range 10–50 pN. In contrast to “bare” solid substrates common to non-living materials, surfaces coated with brush-like polymer layers are very common in biological soft matter. We employ a simple mean-field approach to describe the effects of weak attraction between a floating long macromolecule and the brush. We show that even for a moderately thick brush a very small effective attraction is enough to produce complete binding of the long chain. The detachment force scales as $k_{B}T/W$ , where W is the brush thickness. Hence the force could be 1 to 2 orders of magnitude smaller than in the case of typical solid substrates

Unfolding of a comb-like polymer in a poor solvent : Translation of macromolecular architecture in the force-deformation spectra

A numerical self-consistent field modeling approach was employed to study the mechanical unfolding of a globule made by comb-like polymers in a poor solvent with the aim of unraveling how the macromolecular architecture affects the shape of the single-molecule force-deformation curves. We demonstrate that the dependence of the restoring force on the imposed extension of the main chain of the comb-like polymer exhibits a characteristic oscillatory shape in the intermediate deformation range. Theoretical arguments are developed that enable us to relate the shape of the patterns on the force-deformation curves to the molecular architecture (grafting density and length of the side chains) and interaction parameters. Thus, the results of our study suggest a new approach for the determination of macromolecular topology from single-molecule mechanical unfolding experiments.</p

Dendron Brushes in Polymer Medium: Interpenetration and Depletion

Structural properties of polymer brushes formed by branched tree-like macromolecules (dendrons) attached to a surface and immersed into a melt of linear polymer chains are studied by means of self-consistent field theory. The conformational swelling-to-collapse transition provoked in the brush by an increase in the degree of polymerization of mobile polymer chains is analyzed. It is demonstrated that the sharpness of this transition decreases upon branching of tethered polymers. The effect of architecture of the brush-forming macromolecules on penetration and exclusion of mobile polymers is considered. The regimes of full, partial, and peripheral penetration of mobile chains into the brush are distinguished. The depth of penetration of mobile polymers into the brush is calculated as a function of molecular masses of mobile chains and tethered dendrons, grafting density, and topological parameters of the brush-forming macromolecules. It is demonstrated that the penetration length decreases upon branching of tethered macromolecules. For sufficiently long mobile chains, the penetration length is controlled by the number of monomer units in the longest elastic path of the dendrons. The predictions of the analytical self-consistent field theory are in excellent agreement with the results of numerical modeling based on the Scheutjens-Fleer approach.</p