Polymer synthesis routes result in macromolecules with molecular weight dispersity ĐM that depends on the polymerization mechanism. The lowest dispersity polymers are those made by anionic and atom-transfer radical polymerization, which exhibit narrow distributions ĐM = Mw/Mn ∼ 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 ĐM = 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 ĐM. At longer times, the terminal relaxation time decreases with increasing ĐM. In this time range, the faster motion of the shorter chains results in constraint release for the longer chains.
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Stress Relaxation of Comb Polymer Melts
Branched polymers stress relaxation is at the center to their function as viscosity modifiers, though the fundamentals that underlie the correlation between the polymer topology and their impact on viscosity remains an open question. Here, the stress relaxation of short, branched polyethylene comb polymer melts is studied by molecular dynamics simulations. A coarse-grained model where four methylene groups constitute one bead is used, and the results are transposed to the atomistic level. For arms of length comparable to entanglement length ne of the linear polymer, we show that while increasing the number of branches with the same arm length decreases the plateau modulus, the terminal diffusive time does not change significantly. Increasing the arm length decreases the plateau modulus and increases the terminal time. As arms shorter than ne relax by the entanglement time, both the chain mobility and stress relaxation can be described by reptation of the backbone with an increased tube diameter and an increased friction coefficient; or in other words, the branches act as a solvent.
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- Award ID(s):
- 1905407
- NSF-PAR ID:
- 10350221
- Date Published:
- Journal Name:
- Tribology letters
- ISSN:
- 1573-2711
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1007/s11249-021-01432-y
