Response of Flexible Polymers to a Sudden Elongational Flow (original) (raw)
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Macromolecules, 2003
The coil-stretch transition in the extension of polymers in two-dimensional flows was investigated near a critical boundary defined by simple shear flow. Visualization of individual molecules revealed a sharp coil-stretch transition in the steady-state length of the polymer with increasing Weissenberg number (Wi) in flows where the magnitude of the elongational component (||E||) slightly exceeded the rotational component (||Ω||). However, unlike in pure elongational flow, large fluctuations in the length of the polymer were observed near the critical point of the transition. These fluctuations result in a "softening" of the phase transition between coiled and extended states of the polymer. In flows where ||Ω|| is slightly greater than ||E||, significant transient polymer deformation was observed. However, the average length of the polymer as a function of Wi increased much more slowly than in simple shear flow.
Conformational changes of macromolecules in transient elongational flow
Polymer Engineering and Science, 1980
Calculations are reported which describe the elongation of flexible macromolecules in a pulse-like elongational How. The polymer molecules are modeled as FENE (finitely extendable nonlinear elastic) dumbbells. The results are in qualitative agreement with experimental results on dilute solutions of polyisobutylene in decalin (10) which form a two parameter family of extension versus position curves. The two parameters-in the FENE model that are used to fit the data are λH, a time constant and b, which is related to the maximum extension that can be achieved by the molecules. Quantitative comparisons are frustrated by difficulty in estimating the model parameter, b. It is suggested, based on this work, that internal self entanglements in the polymers must be considered in the determination of b.
Single-Polymer Dynamics in Steady Shear Flow
Science, 1999
The conformational dynamics of individual, flexible polymers in steady shear flow were directly observed by the use of video fluorescence microscopy. The probability distribution for the molecular extension was determined as a function of shear rate, ␥, for two different polymer relaxation times, . In contrast to the behavior in pure elongational flow, the average polymer extension in shear flow does not display a sharp coil-stretch transition. Large, aperiodic temporal fluctuations were observed, consistent with end-over-end tumbling of the molecule. The rate of these fluctuations (relative to the relaxation rate) increased as the Weissenberg number, ␥, was increased.
Chemical Engineering Science, 2009
We compare the flow-induced unraveling of a bead-spring model chain in which each stiff spring corresponds to a single Kuhn step to that of an atomistic representation of the backbone of the same polymer chain with realistic bending and torsional potentials. In our earlier work [Jain, S. and Larson, R.G. 2008. Effect of bending and torsional potentials on high-frequency viscoelasticity of dilute polymer solutions. Macromolecules 41(10), 3692-3700], we showed that in the linear viscoelastic regime, bending and torsional potentials suppress fast local diffusive modes. Now, in strong shear and extensional flows, we observe that bending and torsional potentials have only a slight effect on unraveling dynamics. However, in shear flow, we find that for relatively short coarse-grained chains having less than 20 or so Kuhn steps-representing polymers with fewer than around 150 backbone bonds-coarse-graining introduces periodic peaks in the probability distribution of steady-state stretch. We believe this occurs at high shear because of the dominance of a discrete set of stretch values each of which corresponds to an integral number of bonds that are nearly fully aligned between back-folds that are created during the chain's tumbling orbit. Even this difference between coarse-grained and realistic fine-grained chains disappears for more typical long chain lengths.
Single polymer dynamics: coil-stretch transition in a random flow
Nature
By quantitative studies of statistics of polymer stretching in a random flow and of a flow field we demonstrate that the stretching of polymer molecules in a 3D random flow occurs rather sharply via the coil-stretch transition at the value of the criterion close to theoretically predicted.
Anomalies in the coil-stretch transition of flexible polymers
The Journal of Chemical Physics, 2018
The flow-induced coil-stretch transition of high molecular weight polymers has generally been held to be of first order. But evidence of significant slowing down in the rate at which the polymers relax to equilibrium in the vicinity of the transition suggests that the thermodynamic character of the transition may be less clear-cut. The above slowing down effect is actually characteristic of a second-order transition, and it points to the existence of a broad spectrum of conformational states in the transition region, analogous to the existence of fluctuations of all length scales at a critical point. In this paper, using a path integral approach based on a free-draining finitely extensible chain model, we calculate various polymer properties as a function of elongational flow as a way of exploring different statistical mechanical details of the coil-stretch transition. These properties include the molecular weight dependence of the flow-extension curve of the polymer, the distribution of its steady-state end-to-end distances, and the characteristic relaxation time τ R of these distances. Among other findings, our calculations indicate that the coil-stretch transition is discontinuous in the N → ∞ limit, that the effective free energy of the chain is unimodal at all values of the flow, becoming broad and flat in the immediate vicinity of the transition, and that the ratio of τ R to the Rouse relaxation time increases abruptly at the transition before eventually reaching a plateau value at large flow strengths. These aspects of the coil-stretch transition place it among a larger class of unconventional nominally first-order single chain transitions that include the adsorption transition of surface-tethered polymers and the escape transition of compressed polymers.
Physics of Fluids, 2005
The dynamics of elastocapillary thinning in high molecular weight polymer solutions are reexamined using high-speed digital video microscopy. At long times, the evolution of the viscoelastic thread deviates from self-similar exponential decay and the competition of elastic, capillary, and inertial forces leads to the formation of a periodic array of beads connected by axially uniform ligaments. This configuration is itself unstable and successive instabilities propagate from the necks connecting the beads and the ligaments. This iterated process results in the development of multiple generations of beads in agreement with the predictions of Chang, Demekin, and Kalaidin ͓"Iterated stretching of viscoelastic jets," Phys. Fluids 11, 1717 ͑1999͔͒ although experiments yield a different recursion relation between successive generations. At long times, finite molecular extensibility truncates the iterated instability and axial translation of the bead arrays along the interconnecting threads leads to a progressive coalescence before the rupture of the filament.
2005
The dynamics of elastocapillary thinning in high molecular weight polymer solutions are reexamined using high-speed digital video microscopy. At long times, the evolution of the viscoelastic thread deviates from self-similar exponential decay and the competition of elastic, capillary, and inertial forces leads to the formation of a periodic array of beads connected by axially uniform ligaments. This configuration is itself unstable and successive instabilities propagate from the necks connecting the beads and the ligaments. This iterated process results in the development of multiple generations of beads in agreement with the predictions of Chang, Demekin, and Kalaidin ͓"Iterated stretching of viscoelastic jets," Phys. Fluids 11, 1717 ͑1999͔͒ although experiments yield a different recursion relation between successive generations. At long times, finite molecular extensibility truncates the iterated instability and axial translation of the bead arrays along the interconnecting threads leads to a progressive coalescence before the rupture of the filament.