Understanding Of Polymer Dynamics Research Articles

Entanglement Characteristic Time from Complex Moduli via i-Rheo GT.

Tassieri et al. have introduced a novel rheological tool called "i-Rheo GT" that allows the evaluation of the frequency-dependent materials' linear viscoelastic properties from a direct Fourier transform of the time-dependent relaxation modulus G(t), without artifacts. They adopted i-Rheo GT to exploit the information embedded in G(t) derived from molecular dynamics simulations of atomistic and quasi-atomistic models, and they estimated the polymers' entanglement characteristic time (τe) from the crossover point of the moduli at intermediate times, which had never been possible before because of the poor fitting performance, at short time scales, of the commonly used generalized Maxwell models. Here, we highlight that the values of τe reported by Tassieri et al. are significantly different (i.e., an order of magnitude smaller) from those reported in the literature, obtained from either experiments or molecular dynamics simulations of different observables. In this work, we demonstrate that consistent values of τe can be achieved if the initial values of G(t), i.e., those governed by the bond-oscillation dynamics, are discarded. These findings have been corroborated by adopting i-Rheo GT to Fourier transform the outcomes of three different molecular dynamics simulations based on the following three models: a dissipative particle dynamics model, a Kremer-Grest model, and an atomistic polyethylene model. Moreover, we have investigated the variations of τe as function of (i) the 'cadence' at which G(t) is evaluated, (ii) the spring constant of the atomic bone, and (iii) the initial value of the shear relaxation modulus G(O). The ensemble of these results confirms the effectiveness of i-Rheo GT and provide new insights into the interpretation of molecular dynamics simulations for a better understanding of polymer dynamics.

Polymers in 2D confinement: A nanoscale mechanism for thermo‐mechanical reinforcement

Acrylate‐clay nanocomposites, a 2D confined system, exhibited unusual increase of thermo‐mechanical properties. The nature of this reinforcement can be ascribed to chain dynamics modification and therefore investigated via dynamic mechanical analysis. Transmission electron microscopy and dynamic light scattering showed a strong nanoconfined regime, 2Rh ≫ d001, whereRhis the polymer's hydrodynamic radius andd001is the clay gallery spacing. The geometrical constraints to polymer dynamics led to significantenhancementof the thermo‐mechanical properties. Adding only 1 wt% nanoclay, the glass transition temperature increased significantly,ΔTg = Tg − Tg,bulk ~ 10°C, and the dynamic modulusE′ increased 10‐fold. Analysis of dynamic mechanical spectra showed an increase of relaxation timeτ, ie, polymer dynamics retardation. Furthermore, the mechanical damping tanδwas strongly attenuated evidencing the reduction of viscous dissipation. The activation energyEaof theα‐transition increased as the confined macromolecules needed to overcome higher energy barriers to achieve configurational rearrangements. The considerable increase of mechanical modulus cannot be explained by polymer composite models, rather it was associated to a “nano‐effect,” scaling with the degree of confinement asE/Ematrix ~ (2Rh/d001)n. This study paves the road for further understanding of polymer dynamics under 2D confinement and the reinforcement mechanism of thermo‐mechanical properties.

Direct Observation of Alternating Stretch‐Coil and Coil‐Stretch Transitions of Semiflexible Polymers in Microstructured Flow

Analyzing the behavior of semiflexible polymers experiencing hydrodynamic forces is an important step toward a better understanding of polymer dynamics in microfluidic applications as well as in living cells. In particular, studying conformational changes of fluorescently labeled, semiflexible actin filaments in flow fields of spatially varying flow strength will significantly contribute to this goal. The experimental situation is realized in flows through structured microchannels with alternating high‐ and low‐velocity segments. While entering the wider channel segments, the semiflexible filaments undergo a buckling transition under compression whereas they are stretched with a suppression of thermal fluctuations in the extensional regime when reentering the narrow segments. The nature of these nonequilibrium and nonstationary conformational transitions is characterized by analyzing the evolution of the end‐to‐end distances, center‐of‐mass velocities, and bending energies along the passage of the filaments through the channels. image

Mobility Gradient Induces Cross-Streamline Migration of Semiflexible Polymers.

Many aspects of modern material science and biology rely on the strategic manipulation and understanding of polymer dynamics in confining micro- and nanoflow. We directly observe and analyze nonequilibrium structural and dynamic properties of individual semiflexible actin filaments in pressure-driven microfluidic channel flow using fluorescence microscopy. Different conformational shapes, such as filaments fluctuating in an elongated manner, parabolically bent, as well as tumbling, are identified. With increasing flow velocity, a strong center-of-mass migration toward the channel walls is observed. This significant migration effect can be explained by a shear rate dependent spatial diffusivity due to a gradient in chain mobility of the semiflexible polymers.

Rheology of Ring Polymer Melts: From Linear Contaminants to Ring-Linear Blends

Ring polymers remain a challenge to our understanding of polymer dynamics. Experiments are difficult to interpret because of the uncertainty in the purity and dispersity of the sample. Using both equilibrium and nonequilibrium molecular dynamics simulations we have investigated the structure, dynamics, and rheology of perfectly controlled ring-linear polymer blends of chains of up to about 14 entanglements per chain, comparable to experimental systems. Linear contaminants increase the zero-shear viscosity of a ring polymer melt by about 10% around one-fifth of their overlap concentration. For equal concentrations of linear and ring polymers, the blend viscosity is about twice that of the pure linear melt. The diffusion coefficient of the rings decreases dramatically, while the linear polymers are mostly unaffected. Our results are supported by a primitive path analysis.

Molecular Expansion of an Individual Coiled DNA on a Graphite Surface

Direct visualization and manipulation of a single polymer on a solid surface in fluid can greatly contribute to our understanding of polymer dynamics and physics relevant to many practical applications. In this paper, the authors report a novel phenomenon that a randomly coiled DNA molecule can be gradually expanded to form a spiral-like pattern by repeatedly rotating a water droplet on a graphite surface where the DNA molecule was deposited. The process of DNA expansion was traced using atomic force microscopy relocation imaging. A simple theoretical model based on the analysis of centrifugal and resistant forces exerted on the DNA molecule during the manipulation was presented to explain the formation of the spiral-like pattern. This study should provide us insight into the movement of polymers on a solid surface under fluid flow.

Lipid Mobility Controls the Diffusion of Small Biopolymer Adsorbates

Recent studies on the diffusion of adsorbed polymers such as DNA on supported lipid bilayers have suggested that such strongly adsorbed polymers can be treated similarly to a polymer "in" a 2D fluid, but this conjecture has not been experimentally verified. To test this hypothesis and also to gain a better understanding of polymer dynamics in two dimensions, we designed an experimental protocol-the lateral transport of a short, single-stranded DNA oligonucleotide adsorbed on a supported cationic lipid bilayer. Fluorescence recovery after photobleaching (FRAP) analysis reveals that the diffusivity of the adsorbed DNA quantitatively tracks that of the underlying lipid, even though the bilayer mobility changes by 2 orders of magnitude with changes in temperature. Interestingly, our results for short, extended, adsorbed biopolymers quantitatively track those for globular proteins in lipid bilayers. We thus conclude that short macromolecules that are strongly adsorbed on lipid bilayers can be treated similarly to macromolecules in the bilayer.

The dynamics of partially extended single molecules of DNA.

The behaviour of an isolated polymer floating in a solvent forms the basis of our understanding of polymer dynamics. Classical theories describe the motion of a polymer with linear equations of motion, which yield a set of 'normal modes', analogous to the fundamental frequency and the harmonics of a vibrating violin string. But hydrodynamic interactions make polymer dynamics inherently nonlinear, and the linearizing approximations required for the normal-mode picture have therefore been questioned. Here we test the normal-mode theory by measuring the fluctuations of single molecules of DNA held in a partially extended state with optical tweezers. We find that the motion of the DNA can be described by linearly independent normal modes, and we have experimentally determined the eigenstates of the system. Furthermore, we show that the spectrum of relaxation times obeys a power law.