Viscoelastic Response of Dispersed Entangled Polymer Melts

Abstract

Polymer synthesis routes result in macromolecules with molecular weight dispersity Đ M that depends on the polymerization mechanism. The lowest dispersity polymers are those made by anionic and atom-transfer radical polymerization, which exhibit narrow distributions Đ M = M w /M n ∼ 1.02−1.04. Even for small dispersity, the chain length can vary by a factor of two from the average. The impact of chain length dispersity on the viscoelastic response remains an open question. Here, the effects of dispersity on stress relaxation and shear viscosity of entangled polyethylene melts are studied using molecular dynamics simulations. Melts with chain length dispersity, which follow a Schulz−Zimm (SZ) distribution with Đ M = 1.0−1.16, are studied for times up to 800 μs, longer than the terminal time. These systems are compared to those with binary and ternary distributions. The stress relaxation functions are extracted from the Green−Kubo relation and from stress relaxation following a uniaxial extension. At short and intermediate time scales, both the mean squared displacement and the stress relaxation function G(t) are independent of Đ M . At longer times, the terminal relaxation time decreases with increasing Đ M . In this time range, the faster motion of the shorter chains results in constraint release for the longer chains.

Stress Relaxation of Comb Polymer Melts

“…An alternative method is to follow the experimental procedure and apply a small strain and measure stress relaxation. Although computationally more efficient, the challenge is to apply a strain large enough to reliably measure the stress yet remain in the linear response regime [36,37]. As seen in Fig.…”

“…Results for G(t) presented here are an average of the three components. We also measured the normal stress decay after deforming polymer chains in a melt by a small step strain [36,37]. This was done by applying a uniaxial elongation to deform the simulation cell in the x-direction L x = L while shrinking the simulation cell in the other two directions L y = L Z = L∕…”

Dynamics and stress relaxation of bidisperse polymer melts with unentangled and moderately entangled chains

“…However, most contributions focused on monodisperse polymer melts [17][18][19][20] and only a very small number of studies investigated the effects of polydispersity. 10,11,[21][22][23][24] A bidisperse system provides the simplest case where interactions between different chain lengths can be investigated. Barsky 10 studied chain diffusion dynamics in bidisperse mixtures of Kremer-Grest (KG) model chains and observed that mixing with longer chains reduces the mobility of the shorter chains, while mixing with shorter chains accelerates the motion of longer chains.…”

Dynamics and Stress Relaxation of Bidisperse Polymer Melts with Unentangled and Moderately Entangled Chains

“…MD has been widely used in the study of polymer dynamics, for which the bead-spring chain model pioneered by Kremer and Grest 14 has been particularly instrumental. However, most contributions focused on monodisperse polymer melts [14][15][16][17] and only a very small number of studies investigated the effects of polydispersity 10,11,[18][19][20][21] . A bidisperse system provides the simplest case where interactions between different chain lengths can be investigated.…”

“…For example, Kopf et al 11 simulated mixtures in which monomers in one species are heavier than the other. Polydisperse polymers with realistic, albeit very narrow, molecular weight distributions were simulated more recently using a coarse-grained model for polyethylene 20,21 . It was found that keeping the weightaverage molecular weight constant, chain dynamics accelerates with polydispersity.…”