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

Abstract

Polydispersity is inevitable in industrially produced polymers. Established theories of polymer dynamics and rheology, however, were mostly built on monodisperse linear polymers. Dynamics of polydisperse polymers is yet to be fully explored-specifically how chains of different lengths affect the dynamics of one another in a mixture. This study explored the dynamics of bidisperse polymer melts using molecular dynamics and a bead-spring chain model. Binary mixtures between a moderately entangled long-chain species and an unentangled or marginally entangled short-chain species were investigated. We found that adding short chains can significantly accelerate the dynamics of the long chains by substantially lessening their extent of entanglement. Meanwhile, although introducing long chains also hinders the motion of the short chains, it does not qualitatively alter the nature of their dynamics-unentangled short chains still follow classical Rouse dynamics even in a matrix containing entangled chains. Detailed Rouse mode analysis was used to reveal the effects of entanglement at chain segments of different scales. Stress relaxation following a step shear strain was also studied, and semi-empirical mixing rules that predict the linear viscoelasticity of polydisperse polymers based on that of monodisperse systems were evaluated with simulation results.

Effects of molecular design parameters on plasticizer performance in poly(vinyl chloride): A comprehensive molecular simulation study

“…Their di usion has to follow an entirely di erent process involving non-trivial coupling between conformational changes in both the plasticizer and polymer molecules. Di usion of longer chain molecules can be described with theories of polymer dynamics 91 , which are, however, established mainly for linear chain molecules that are chemically alike to the host matrix 92 . Theoretical description of the di usion of large and complex molecules in polymer matrices is currently not available.…”

Equilibrium and non-equilibrium molecular dynamics approaches for the linear viscoelasticity of polymer melts

“…All the results are reported in reduced LJ units and length, energy, time, and temperature values are scaled by, σ, , τ = mσ 2 / , and /k B (k B is the Boltzmann constant) respectively. The chain lengths studied range from the unentangled N = 25 and 50 to marginally entangled N = 100 and moderately entangled N = 350 cases 20 . The N = 350 case contains a total of 56000 beads in the simulation box while all other cases contain 50000 beads in each simulation box.…”

“…( 3)) and performed MD simulation in an NVT ensemble for another 500TUs during which a random velocity distribution was assigned to all the beads every 0.5TUs. Mean square internal displacement of the chains, which is a sensitive indicator of unrelaxed chain conformations 28 , was examined to ensure the convergence of the equilibration procedure -see Adeyemi et al 20 .…”

“…In practice, τ p can be obtained by fitting the TACF of the corresponding Rouse mode from EMD to eq. (20). Once τ p is known, the G(t) can be calculated by…”

“…The EMD approach again relies on the GK relation and was first reported by Sen et al 12 , followed by a number of later developments 14,15 . Many more studies reported the EMD results of G(t) but did not convert it to G (ω) and G (ω) [16][17][18][19][20] . The NEMD approach simulates the sinusoidal oscillatory shear flow (modeling the SAOS condition) and obtains G and G from the time-dependent shear stress signal.…”