Atomistic configurations of model unentangled ring polyethylene (PE) melts ranging in chain length from C 24 up to C 400 have been subjected to detailed molecular dynamics (MD) simulations in the isothermal-isobaric statistical ensemble at temperature T = 450 K and P = 1 atm. Strictly monodisperse samples were employed in all cases. We present and discuss in detail simulation results for a variety of structural, thermodynamic, conformational and dynamic properties of these systems, and their variation with chain length. Among others, these include the mean chain radius of gyration, the pair correlation function, the intrinsic molecular shape, the local dynamics, the segmental mean square displacement (msd), the chain center-of-mass self-diffusion coefficient D G , the chain terminal relaxation time τ d , the characteristic spectrum of the Rouse relaxation times τ p , and the dynamic structure factor S(q,t). In all cases, the results are compared against the corresponding data from simulations with linear PE melts of the same chain length (the linear analogues) and the predictions of the Rouse theory for polymer rings which we derive here in its entirety. The Rouse theory is found to provide a satisfactory description of the simulation findings, especially for rings with chain length between C 50 and C 170 . An important finding of our work (from the observed dependence of D G , τ p , ζ, and η 0 on chain length N) is that PE ring melts follow approximately Rouse-like dynamics even when their chain length is as long as C 400 ; this is more than twice the characteristic crossover chain length (∼C 156 ) marking the passage from Rouse to reptation dynamics for the corresponding linear PE melts. In a second step, and by mapping the simulation data onto the Rouse model, we have managed to extract the friction coefficient ζ and the zero-shear rate viscosity η 0 of the simulated ring melts. Overall, and in agreement with previous theoretical and experimental studies, our simulation results support that the structure of ring polymers in the melt is more compact than that of their linear analogues due to their nonconcatenated configurations. Additional results for the intermolecular mer-mer and center-of-mass pair correlation functions confirm that the effective correlation hole effect is more pronounced in melts of rings than in melts of linear chains.

Well-relaxed atomistic configurations of model cis-1,4-poly(butadiene) (PB) systems, ranging in molecular length from C 32 to C400, have been subjected to detailed molecular dynamics simulations in the NPT ensemble for times up to 600 ns. Results are presented for the static and (mainly) dynamic properties of these systems, such as the segmental and terminal relaxation properties, the self-diffusion coefficient, D, and the single-chain dynamic structure factor, S(q,t), at pressure P ) 1 atm and temperatures, T, between 298 and 430 K. Our simulation data demonstrate that, around C 200, D is seen to exhibit a change in its power-law dependence on molecular weight, M, deviating from a Rouse (where D ≈ M -b with b = 1) toward a reptation-like (where D ≈ M -b with b = 2.1) behavior. Following the methodology introduced by Harmandaris et al. [Macromolecules 2003[Macromolecules , 36, 1376 for linear polyethylene (PE) melts, we have further been able to successfully project atomistic cis-1,4-PB chain configurations to primitive paths, thereby mapping simulation trajectories onto the reptation model. This has allowed us to consistently calculate the friction coefficient, , per carbon atom of cis-1,4-PB melts both below and above the molecular weight for the formation of entanglements. The dependence of D on T is also presented.