Detailed Molecular-Dynamics Study On Structural and Dynamical Properties of Unentangled Ring Polyethylene Melts: Comprehensive Analysis of the Rouse Theory and Simulation

Motivated by recently reported experimental data [1] on the peculiar rheological behaviour of polymer rings, we have undertaken a systematic and detailed investigation of the structural and mainly the dynamical and rheological properties of model ring polyethylene (PE) melts, ranging in molecular length from N =24 up to N=400 carbon atoms per molecule [C24, C400] with the help of atomistic Molecular Dynamics (MD) simulations at temperature T=450 K [2]. We present results concerning the size of the ring molecules and their spatial packing which show that ring systems are characterized by a more compact and denser structure than their linear analogues; this is consistent with previous theoretical [3] and experimental studies [4]. Additionally, we investigate the dynamical and rheological characteristics of these systems obtained either directly from the atomistic MD trajectories (e.g., the chain self-diffusion coefficient D, the terminal relaxation time τd, and the dynamic structure factor S(q,t)) or indirectly (e.g., the friction coefficient ζ, the spectrum of relaxation times τp, and the zero-shear rate viscosity η0) by mapping the MD results onto the predictions of the Rouse theory for cyclic systems [2]. In agreement with previous experimental studies [5], our simulations support that ring systems exhibit a faster dynamics and a lower viscosity than linear PE melts of the same molecular weight. Furthermore, the dependence of D, τp, ζ and η0 on molecular weight reveals that all ring PE systems examined here follow a Rouse-like dynamical behavior even for chain lengths as long as N=400 which is above the crossover regime from Rouse to reptation for linear PE melts [6].

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