Effect of Chain Length Dispersity on the Mobility of Entangled Polymers

Abstract

While nearly all theoretical and computational studies of entangled polymer melts have focused on uniform samples, polymer synthesis routes always result in some dispersity, albeit narrow, of distribution of molecular weights (Đ_{M}=M_{w}/M_{n}∼1.02-1.04). Here, the effects of dispersity on chain mobility are studied for entangled, disperse melts using a coarse-grained model for polyethylene. Polymer melts with chain lengths set to follow a Schulz-Zimm distribution for the same average M_{w}=36 kg/mol with Đ_{M}=1.0 to 1.16, were studied for times of 600-800 μs using molecular dynamics simulations. This time frame is longer than the time required to reach the diffusive regime. We find that dispersity in this range does not affect the entanglement time or tube diameter. However, while there is negligible difference in the average mobility of chains for the uniform distribution Đ_{M}=1.0 and Đ_{M}=1.02, the shortest chains move significantly faster than the longest ones offering a constraint release pathway for the melts for larger Đ_{M}.

Modeling of Entangled Polymer Diffusion in Melts and Nanocomposites: A Review

“…Entangled PE melts with chain lengths of average Mw=36 kDa with PDI = 1.0–1.16, were studied for long scale times (600–800 ms) using MD simulations. It was found that polydispersity in that range did not alter the entanglement time or tube diameter [108]. There was a negligible difference in the PE dynamics for the distributions PDI =1.0 and PDI =1.02, however, the shortest chains diffused faster than the longest ones due to a constraint release mechanism [108].…”

“…It was found that polydispersity in that range did not alter the entanglement time or tube diameter [108]. There was a negligible difference in the PE dynamics for the distributions PDI =1.0 and PDI =1.02, however, the shortest chains diffused faster than the longest ones due to a constraint release mechanism [108]. It is worth to note an atomistic simulation effort (using a united atom force field) of polydisperse linear poly(ethylene) melts with PDI=1.09, showed a Rouse-scaling of D∼N−1 for chains above N=60 and a signature for the onset of a reptation regime at N=150 [109].…”

Precise modulation of molecular weight distribution for structural engineering

“…1 Minor changes in the distribution could signicantly alter the content of each component, which may result in substantial differences in the physical properties. 2 Although a few pioneering theoretical explorations [3][4][5][6][7] and experimental studies [8][9][10][11][12][13][14] have been conducted, the importance of molecular weight distribution in determining the structure and properties of macromolecules is, however, still unclear. There are mainly two obstacles: (i) precise control and quantitative modulation of molecular weight distribution is challenging; 13,15,16 and (ii) molecular weight distribution is a complex parameter consisting of multiple variables such as distribution width, symmetry, shape, etc.…”

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.…”