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We develop a coarse-graining procedure,using bead–rod chainswith varying numbers of rods per Kuhn length (Nr,K), to model the conformational dynamics of polymer chainswith bending stiffness and self-attraction. We find the conditionsunder which increased chain resolution leads to converged resultsfor the effect of bending stiffness, attractive interaction strength(ε), and shearing flow on the conformational dynamics. As chainstiffness increases, we find a collapse transition that becomes steeperas attractive strength increases. The collapsed state depends notonly on the net dimensionless attractive strength ε* = εNr,Kbetween each Kuhn step of the chain butalso on the ratio of bead diameter to Kuhn length (σ*). At highε* (>0.8/σ*), collapsed globules are produced at moderatechain diameter σ* = 1/4, while for fat chains with σ*= 1, helices are formed, and for thin ones with σ* = 1/16, tori,folded bundles, and finally globules are formed as ε* increases. [ABSTRACT FROM AUTHOR]

Using multiscale modeling, includingmolecular dynamics simulations, with both united-atom and coarse-grainedforce fields, as well as Brownian dynamics simulations with stillhigher levels of coarse-graining, we explain the long-mysterious absenceof high frequency modes in the viscoelastic spectrum of isolated polymerchains in good solvents, reported years ago by Schrag and co-workers.The relaxation spectrum we obtain for a chain of 30 monomers at atomisticresolution is, remarkably, a single exponential, while that of a coarse-grainedchain of 100 monomers is well fit by only two modes. These resultsare very surprising in view of the many degrees of freedom possessedby these chains, and in view of the many relaxation modes presentin melts of such chains. However, the result agrees perfectly withexperimental observations of Schrag and co-workers. We also performed Browniandynamics (BD) simulations in which the explicit solvent moleculesare replaced by a viscous continuum, using chain models of varyingdegrees of resolution, both in the presence and absence of hydrodynamicinteractions (HI). Although the local dynamics is suppressed by theaddition of bending, torsion, side groups and excluded volume interactions(as suggested in Jain and Larson(15)), none of the BD simulations predicta single exponential relaxation for a short polymer chain. The comparisonof the relaxation of the bond vectors from different models indicatesan additional slow-down in the presence of explicit solvent molecules,which is critical to obtain a single exponential relaxation for shortchains. Our results indicate that the chain dynamics at small lengthscales (down to a few Kuhn steps) are significantly different fromthe predictions of models based on a continuum solvent, and finallyhelp explain the experimental results of Schrag and co-workers. [ABSTRACT FROM AUTHOR]

Using Brownian dynamics simulations of polymer chainsover a widerange of resolution, we find universal scaling laws for polymer coildimensions and tumbling time. At high shear rates γ̇,the coil thickness in the gradient direction becomes independent ofchain length, scaling as NK0γ̇–1/4,where NKdenotes the numbers of Kuhn steps,and the tumbling time scales as NKγ̇–3/4, correcting scaling laws presented in prior studies.We find this to be a consequence of the formation of loops whose lengthis limited by the time required to stretch them and derive scalinglaws from a balance of convection and diffusion of monomers. We findthat, in the absence of hydrodynamic interaction (HI) and excludedvolume (EV), for wormlike chains, the shrinkage in chain stretch observedat ultrahigh shear rates is pushed out to arbitrarily high shear ratesif the chain is resolved increasingly finely below the persistencelength. Finally, scaling laws in the presence of excluded volume andhydrodynamic interactions are derived that are expected to be validfor long chains at high shear rates. [ABSTRACT FROM AUTHOR]