Static and dynamic contributions to anomalous chain dynamics in polymer blends (original) (raw)
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Entangledlike Chain Dynamics in Nonentangled Polymer Blends with Large Dynamic Asymmetry
Physical Review Letters, 2008
We discuss simulations of a simple model for polymer blends in the framework of the Rouse model. At odds with standard predictions, large dynamic asymmetry between the two components induces strong non-exponentiality of the Rouse modes for the fast component. Despite chains being much shorter than the entanglement length, it also induces dynamic features resembling a crossover to entangled-like chain dynamics. This unusual behavior is associated to strong memory effects which break down the assumption of time uncorrelation of the external forces acting on the tagged chain.
Is there a higher-order mode coupling transition in polymer blends?
The Journal of Chemical Physics, 2006
We present simulations on a binary blend of bead-spring polymer chains. The introduction of monomer size disparity yields very different relaxation times for each component of the blend. Competition between two different arrest mechanisms, namely bulk-like dynamics and confinement, leads to an anomalous relaxation scenario for the fast component, characterized by sublinear time dependence for mean squared displacements, or logarithmic decay and convex-to-concave crossover for density-density correlators. These anomalous dynamic features, which are observed over time intervals extending up to four decades, strongly resemble predictions of Mode Coupling Theory for nearby higher-order transitions. Chain connectivity extends anomalous relaxation over a wide range of blend compositions.
Chain Stiffness Intensifies the Reptation Characteristics of Polymer Dynamics in the Melt
ChemPhysChem, 2001
The reptation concept in polymer dynamics is studied for model chains with added stiffness. The main idea of a chain diffusing inside a tube can be transferred from fully flexible chains although the renormalization onto a flexible chain of fewer Kuhn segments fails. The entanglement length shrinks with increasing persistence length. If entanglement length and persistence length come to the same order of magnitude the picture of a tight tube is better suited, in which chain segments can move only along the contour, any transverse motion being much reduced. Thus, as stiffness increases, the monomers loose their freedom to perform random walks inside the tube, the "Rouse-like" part of their dynamics.
2003
A stretched exponential fits many relaxational processes ………. 14 3.2 A model for parallel relaxation (charge relaxation)……………… 17 3.3 A hierarchically constrained dynamics (spin glasses)…………… 18 3.4 Stretched exponential relaxation in a polydisperse system of noninteracting linear polymers……………………………………… 20 iv 3.5 System size dependence of relaxational processes……………… 22 3.6 The coupling model. Chaotic dynamic as a central explanation for relaxation in complex systems………………………………… 3.7 Stretched exponential and the Rouse model…………………… 4 The equilibrium relaxation of non-uniform Rouse chains 4.1 The Langevin equation for a random chain……………………… 4.2 Four types of distributions for the spring elastic constants……… 33 4.3 The eigenvalue spectra for random chains………………………. 35 4.4 The spacing between the adjacent modes………………………… 40 4.5 The dynamics of a single random chain………………………….. 4.6 The relaxation of an ensemble of non-interacting random linear polymers………………………………………….. 4.6.1 An example using the exponential ensemble…………… 4.6.2 A look for the four distributions………………………… 4.6.3 The universal nature of β for N>>1……………………… 4.6.4 The ensemble size dependence of the KWW regime……. Conclusions……………………………………………………. 5 The entropic spring constant of a polymeric chain: stress versus strain ensemble 5.1 The stress and strain ensembles for a single polymeric chain……. 5.2 The force-extension relation for a generalized freely-jointed chain in h-ensemble……………………………………………………..
Macromolecules, 2004
13 C NMR has been used to investigate the segmental dynamics of isolated polyisoprene (PI) chains in host matrices of polybutadiene (PB), poly(vinylethylene) (PVE), and polystyrene (PS). In the dilute regime, where intermolecular concentration fluctuations are minimal, the segmental dynamics of isolated PI chains do not become slaved to the segmental dynamics of the host matrix but are significantly biased toward the dynamics of pure PI. Using the framework of the Lodge-McLeish model, it is shown that self-concentration effects can account for the dilute PI segmental dynamics. The value of self-concentration was determined to be 0.41 for PI segmental dynamics in PVE blends, a value which agrees with findings at higher compositions and which is also in reasonable agreement with the Lodge-McLeish prediction (0.45 for PI). In contrast, the observed self-concentration values for dilute PI blends with PS (∼0.20) and PB (∼0.85) do not agree with the model prediction of 0.45. These results indicate the importance of self-concentration in understanding the component dynamics and the rheology of miscible polymer blends.
The molecular dynamics of polymer chains with rigid bonds. Local relaxation times
1980
Local movements of small elements (one or a pair of chain units) of the chain composed of parts joined by rigid bonds have been examined by molecular dynamics. "]7he chain was immersed in a solvent of low molecular weight. The interactions between the particles or between them and similar solvent particles are described by the Lennard-Jones potential. Chains possessing various numbers of units were examined at various temperatures and concentrations. The results of numerous experiments have been compared with the analytical results for the elastic Hearst-Harris model. The relaxation of the average eonsine of angles of rotation of the rigid chain elements is ldentmal in practme with that of the equivalent pseudo-elastic elements of the Hearst~-Harris model. Each elastic element in the model is equivalent to one rigid chain unit and the average angle between the elements in the elastxc model is similar to that between rigid chain units. The relaxation of the mean square cosine of angles of rotation is more rapid than in the elastic model. The ratio of t, the average and the mean square cosine of angles of rotation is similar to that for a separate, lagid, anisotropie particle in a viscous solvent. Pronounced anisotropy of the local relaxation properties is shown to exist; the values of the various times which can elapse in dielectrm relaxation and the depolalazation of luminescence have been established.
The Dynamics of Polymer Chains in Solution
New Trends in Nonionic (Co)Polymers and Hybrids, 2006
The physical properties and processing behavior of polymers are related to the broad range of length and times scales over which dynamical processes occur. The theory has undergone significant evolution over the last decades due to the introduction of new methods and concepts that have extended the frontier from dilute solutions, in which polymers move independently, to concentrated solutions, where many macromolecules entangle. The aim of this chapter is to review briefly the background of polymer dynamics in solution and to provide the recent achievements in this field. A good understanding of polymer dynamics in solution and the possibility to predict their behavior under flow in different media may help out to a large extent to the synthesis of high-performance polymers, with tailored properties.
Journal of Rheology, 2017
We analyze the effects of polymer concentration, C p , and physical crosslinking ratio, r c , on the linear viscoelasticity of lightly entangled, linear, associating polymers with multiple crosslinkable groups along the chain backbone. To accomplish this goal we utilize three novel tools: a robust tunable chemistry based on electrophilic methacryl-succinimidyl modified poly(N-isopropylacrylamide); a modified state-ofthe-art entanglement theory, called the discrete slip-link molecular model (DSM); and a novel experimental technique, surface fluctuation specular reflection (SFSR). This experimental technique allows extraction of the complex viscoelastic modulus covering up to six decades of frequency, from the sample's surface fluctuations at constant temperature. We demonstrate that our theoretical model with a consistent set of parameters can qualitatively reproduce the observed features of the complex viscoelastic modulus over the entire probed frequency range. Furthermore the effects of C p and r c on the viscoelastic properties agree with scaling laws predicted by the "sticky reptation model" and other published experimental data. Finally, we discuss non-additivity of the effects of entanglements and transient associations.
Polymer Motion at the Crossover from Rouse to Reptation Dynamics
Macromolecules, 1994
Employing neutron spin-echo spectroscopy, we have studied the dynamic structure factors for the relaxation of a single chain in polymer melts. We have varied the molecular weights through the transition region from unrestricted Rouse dynamics to entanglement controlled behavior. Investigating the dependence of the dynamic structure factor on the momentum transfer Q, it is possible to access the different relaxation modes separately. We found that, depending on their spatial extension in relation to the entanglement distance, larger scale relaxations are successively slowed down compared to Rouse relaxation. A comparison with macroscopic diffusion and viscosity data yields excellent internal consistency. Furthermore, we solve explicitly the generalized Rouse model by Hess12 and compare its predictions to our data. Fitting only two parameters, all the Q and molecular weight dependent structure factors can be well reproduced.