Spring-bead chain

FIG. 6 Illustration of the bond fluctuation Monte Carlo model and the standard bead-spring chain (see, e.g. [4]). 

The behavior of a bead-spring chain immersed in a flowing solvent could be envisioned as the following under the influence of hydrodynamic drag forces (fH), each bead tends to move differently and to distort the equilibrium distance. It is pulled back, however, by the entropic need of the molecule to retain its coiled shape, represented by the restoring forces (fs) and materialized by the spring in the model. The random bombardment of the solvent molecules on the polymer beads is taken into account by time smoothed Brownian forces (fB). Finally inertial forces (f1) are introduced into the forces balance equation by the bead mass (m) times the acceleration ( ) of one bead relative to the others ... 

From the standpoint of thermodynamics, the essential quantity which governs the course of a chemical reaction is the chemical potential of the system or, in the case of interest, the free energy storage within the molecular coil. This quantity, unfortunately, is difficult to evaluate in non-steady flow. At modest extension ratios (X < 4), the free energy storage of a freely-jointed bead-spring chain is... 

The system used in the simulations usually consists of solid walls and lubricant molecules, but the specific arrangement of the system depends on the problem under investigation. In early studies, hard spherical molecules, interacting with each other through the Lennard-Jones (L-J) potential, were adopted to model the lubricant [27], but recently we tend to take more realistic models for describing the lubricant molecules. The alkane molecules with flexible linear chains [28,29] and bead-spring chains [7,30] are the examples for the most commonly used molecular architectures. The inter- and intra-molecular potentials, as well as the interactions between the lubricant molecule and solid wall, have to be properly defined in order to get reliable results. Readers who intend to learn more about the specific techniques of the simulations are referred to Refs. [27-29]. 

Fig. 6.2. Illustration of the mapping procedure for a 2 1 mapping where the repeat unit of a BPA-PC chain is replaced by two monomers of a generalized bead spring chain. The geometrical centers of the carbonate group and the geometrical center of the isopropyli-dene group, respectively, are mapped onto the centers of the new spherical beads. From [43]...

Fig. 1 a Model bead-spring chain interacting through bond potential Dj, bond angle potential Uq, and van der Waals potential C7v(jw> and b the form of the bond angle potential Ug... 

To say nothing about the different equivalent forms of the theory of the Brownian motion that has been discussed by many authors (Chandrasekhar 1943 Gardiner 1983), there exist different approaches (Rouse 1953 Zimm 1956 Cerf 1958 Peterlin 1967) to the dynamics of a bead-spring chain in the flow of viscous liquid.1 In this chapter, we shall try to formulate the theory in a unified way, embracing all the above-mentioned approaches simultaneously. Some parameters are used to characterise the motion of the particles and interaction inside the coil. This phenomenological (or, better to say, mesoscopic) approach permits the formulation of overall results regardless to the extent to which the mechanism of a particular effect is understood. 

Our model of a polyelectrolyte solution consists of Np flexible bead-spring-chains which are located in a simulation box of length L with periodic boundary conditions. For each chain, a fraction / of the N monomers is monovalently charged (v=l), and fN oppositely charged monovalent counterions are added to obtain an electrically neutral system. In some cases Ns pairs of salt ions were added. The density is given in form of the charged... 

In the Rouse model, a chain of N monomers is mapped onto a bead spring chain of N beads connected by springs. 

Massah, H. Hanratty, T.J. Added stresses because of the presence of FENE-P bead-spring chains in a random velocity field. J. Fluid Mech. 1997, 337, 67-101. 

The starting point of a molecular constitutive theory is a simple mechanical model for the molecule that captures its most salient traits. Thus, flexible polymer molecules have been represented by elastic dumbbells and bead-spring chains, and rigid polymers by rigid dumbbells and rigid rods. For its simplicity, the evolution of the model molecule is easily described by a convection-diffusion equation. Then a Fokker-Planck equation is written for the probability distribution function of an ensemble of these molecules. Finally, the macroscopic stress tensor is derived in terms of the distribution function. This kinetic theory framework was pioneered by Kirkwood (see, for example, Ref. ). 

Bird, R. B., Dotson, P. J., and Johnson, N. L., Polymer solution rheology based on a finitely extensible bead-spring chain model, J. Non-Newtonian Fluid Mech., 7, 213-235 (1980). 

Agarwal and Mashelkar first analyzed contradictory reports from Kim [190], Gryte [191], and Singh [192, 193], and proposed a simple mechanistic model [209]. In stark contrast to the concept of preferential scission of side chains, their model reveals decreased shear stability by grafting side chains. They extended Odell and Keller s bead-rod model [27]. The backbone was modeled as a fully extended rod with Ni = 2m+ beads (Fig. 25). p grafted bead-spring chains having g beads with... 

In 1944 Kramers [1] published a phase-space kinetic theory for the steady-state potential flow of monodisperse dilute polymer systems in which the polymer molecule is modeled as a freely jointed bead-rod chain. Subsequent scholars developed kinetic theories for shearing flows of monodisperse dilute polymer solutions Kirkwood [2] for freely rotating bead-rod chains with equilibnum-averaged hydrodynamic interaction. Rouse [3] and Zimm [4] for freely jointed bead-spring chains, and others. These theories were all formulated m the configuration space of a single polymer chain. 

Once the singlet distribution function has been found, we are in a position to evaluate the various contributions to the fluxes that depend on (see Table 1). In this section we discuss the contnbutions to the stress tensor, and in the next two sections the contnbutions to the mass and heat flux vectors. In these sections, for illustrative purposes, we restrict ourselves to the Rouse bead-spring chain and the Hookean dumbbell models, for which we can use the singlet distribution functions , given in Eqs. (13.5) and (13.8). 

The most studied relaxation processes from the point of view of molecular theories are those governing relaxation function, G,(t), in equation [7.2.4]. According to the Rouse theory, a macromolecule is modeled by a bead-spring chain. The beads are the centers of hydrodynamic interaction of a molecule with a solvent while the springs model elastic linkage between the beads. The polymer macromolecule is subdivided into a number of equal segments (submolecules or subchains) within which the equilibrium is supposed to be achieved thus the model does not permit to describe small-scale motions that are smaller in size than the statistical segment. Maximal relaxation time in a spectrum is expressed in terms of macroscopic parameters of the system, which can be easily measured ... 

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