Polymer materials of reduced size and dimensionality, such as thin films, polymer nanofibres and nanotubes, exhibit exceptional mechanical properties compared with those of their macroscopic counterparts. We discuss here the abrupt increase in Young's modulus in polymer nanofibres. Using scaling estimation we show that this effect occurs when, in the amorphous (non-crystalline) part of the nanofibres, the transversal size of regions consisting of orientation-correlated macromolecules is comparable to the nanofibre diameter, thereby resulting in confinement of the supramolecular structure. We suggest that in polymer nanofibres the resulting supramolecular microstructure plays a more dominant role in the deformation process than previously thought, challenging the commonly held view that surface effects are most significant. The concept we develop also provides a way to interpret the observed--but not yet understood--temperature dependence of Young's modulus in nanofibres of different diameters.
This article reviews and discusses some open problems concerning polymer materials of reduced sizes and dimensions. Such objects exhibit exceptional physical properties when compared with their macroscopic counterparts. More specifically, abrupt increases in polymer nanofiber elastic modulus have been observed when diameters drop below a certain value. In addition, temperature dependence of elastic modulus is highly influenced by fiber diameter. Mechanical (macroscopic) analyses have failed to provide satisfactory explanations for the mechanisms ruling such features, calling for detailed microscopic examination of the systems in question. A hypothesis bridging the current knowledge gaps is presented. The key element of this hypothesis is based on confinement of the supermolecular microstructure of polymer nanofibers and its dominant role in the deformation process. This suggestion challenges the commonly held view suggesting that surface effects are the most significant parameters impacting mechanical and thermodynamic nanofiber behaviors. The review will focus on the mechanical and thermodynamic properties of electrospun polymer nanofibers, selected as representatives of nanoscale polymer objects.
Electrospun polymer nanofibers demonstrate outstanding mechanical and thermodynamic properties as compared to macroscopic-scale structures. Our previous work has demonstrated that these features are attributed to nanofiber microstructure [Nat. Nanotechnol. 2, 59 (2007)]. It is clear that this microstructure is formed during the electrospinning process, characterized by a high stretching rate and rapid evaporation. Thus, when studying microstructure formation, its fast evolution must be taken into account. This study focuses on the dynamics of a highly entangled semidilute polymer solution under extreme longitudinal acceleration. The theoretical modeling predicts substantial longitudinal stretching and transversal contraction of the polymer network caused by the jet hydrodynamic forces, transforming the network to an almost fully stretched state. This prediction was verified by x-ray phase-contrast imaging of electrospinning jets of poly(ethylene oxide) and poly(methyl methacrylate) semidilute solutions, which revealed a noticeable increase in polymer concentration at the jet center, within less than 1 mm from the jet start. Thus, the proposed mechanism is applicable to the initial stage of the microstructure formation.
We study the unzipping dynamics of individual DNA hairpins using nanopore force spectroscopy at different voltage ramp rates and temperatures. At high ramp rates the critical unzipping voltage is proportional to logV , whereV is the voltage ramp. At low ramp values we observe a crossover to another regime with a weaker dependence onV. Here we report on the dependence of these two regimes on temperature. Remarkably, the unzipping kinetics can be well described by a simple two-states model that predicts the existence of two asymptotic regimes: quasi-equilibrium unzipping at low-voltage ramps and irreversible unzipping at high ramp rates.
The postprocesses that occur in coelectrospun polymer nanofibers are investigated. The high rate of solvent evaporation during the electrospinning of fibers results in such a rapid formation of the shell of the tubular nanofibers, that the polymer molecules composing the fibers are in a nonequilibrium state. This stretched state of macromolecules is assumed to be stabilized in a solid matrix, and can account for the anomalous properties of the nanofibers. During this processing stage a considerable amount of solvent remains inside the tubular nanofibers. The evaporation of the solvent continues for several minutes, and is accompanied by further evolution of the nanofibers in both their microstates and macrostates. In this paper, we examine possible macrostate modifications of the nanofibers, such as radial buckling. The theoretical model which describes the kinetics of the solvent evaporation is found to be in good agreement with experimental observations. Thus, the physical parameters of the system in question can be estimated, and the conditions of fiber shell instability that produce buckling of the tubular nanofibers can also be predicted.
The sharp increase in elastic modulus of electrospun polymer nanofibers with decrease in their diameters is now a well‐known phenomenon. Unfortunately, up to now, the physical reasons resulting in the above size‐dependent behavior are unclear. The proposed explanation is based on the confinement concept. A manifesting mechanism of the confinement effect which provides the size‐dependent elastic modulus of electrospun polymer nanofibers is discussed. According to this model, the nanofiber polymer matrix contains anisotropic regions consisting of correlated worm‐like subchains, partially oriented along the fiber. A fiber elongation is accompanied by relative rotations of the above regions. Confinement effect is that these rotations are hindered by the fiber boundary. As a result, the elastic modulus depends on the fiber diameter. This restriction is dominant for the thin fibers, is decreasing with increase of the fiber diameter, and becomes negligible for thick fibers. Such a behavior is in good agreement with experimental observations. © 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013
The final form of tubular nanofibres produced by the co-electrospinning of two solutions (core/shell) is largely determined by the kinetics governing the buckling phenomenon. The buckling mechanism involves the evaporation of the core solution through a solidified shell resulting in a pressure difference across the fibre shell. Buckling can take place when the pressure drop across the fibre shell exceeds a critical value. In this work the physical conditions leading to fibre buckling are analysed from a kinetic point of view. A time interval, Δt, during which buckling may occur, is introduced as a single criterion determining the buckling probability. Different core/shell systems were spun by varying the surface tension, viscosity, flow rate, electric field and the diffusion coefficient of the core solvent through the fibre shell. The imaged as-spun nanofibres were analysed statistically to determine the buckling probability, and the corresponding Δt values were calculated using the values of the spinning parameters. The obtained data were fitted with an exponential distribution function which afforded determination of the characteristic time to buckling, tb. The results provide a means of predicting the buckling of tubular nanofibres. In particular, one can conclude that the dominant parameter determining the final form of the as-spun tubular nanofibres is the diffusion coefficient of the core solvent through the fibre shell.
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