Polymer Topology Effects on Dynamics of Comb Polymer Melts

Abstract

Controlling polymer viscosity and flow is key to their many applications through strength and processability. The topology of the polymer i.e., linear, stars, and branched, affects the macroscopic flow characteristics of melts, where introducing one branch is sufficient to increase the viscosity significantly. While a number of studies have probed the effects of polymer topology on their rheology, the molecular understanding that underlies the macroscopic behavior remains an open question. The current study uses molecular dynamics simulations to resolve the effects of topology of polymer melts on chain mobility and viscosity in the comb regime using polyethylene as a model system. A coarse-grained model where four methylene groups constitute one bead is used, and the results are transposed to the atomistic level. We find that while the number of branches only slightly affects the chain mobility and viscosity, their length strongly impacts their behavior. The results are discussed in terms of interplay between the relaxation of the branches and reptation of the backbone where the topology of the polymer affects the tube dimensions.

Comb and Bottlebrush Polymers with Superior Rheological and Mechanical Properties

“…In the current paper, we investigate the transitions from comb to bottlebrush behavior; therefore, all of these variables are used for both topologies especially in the transition from comb to bottlebrush conformations. Substantial experimental [11,[29][30][31] and coarse-grained simulation works along with scaling analysis [24,25,[32][33][34][35] have been done to classify graft polymers into comb and bottlebrush classes with distinct rheological and mechanical properties. Substantial experimental [11,[29][30][31] and coarse-grained simulation works along with scaling analysis [24,25,[32][33][34][35] have been done to classify graft polymers into comb and bottlebrush classes with distinct rheological and mechanical properties.…”

“…[24,25] Comb topologies have a backbone and side chains with an unperturbed random Gaussian conformation, while a densely grafted bottlebrush has a stretched backbone and side chains. However, both the shear and extensional viscosity increase exponentially [22,35,38,45] with the length of the entangled branches, which results in ultrahigh viscous materials with poor processing properties. Side chains were introduced on comb polymers to mainly control the rheological properties of polymers melt and solutions, especially the extensional viscosity and strain hardening behavior under extensional deformations, through variation of Z bb , Z sc , and q.…”

“…Substantial experimental [11,[29][30][31] and coarse-grained simulation works along with scaling analysis [24,25,[32][33][34][35] have been done to classify graft polymers into comb and bottlebrush classes with distinct rheological and mechanical properties. The differences observed between comb and bottlebrush structures were related to the Gaussian and stretched conformations of the backbone and side chains.…”

“…Consequently, entangled side chains induce substantially higher strain hardening than unentangled branches in comb polymers due to their higher topological constraint. However, both the shear and extensional viscosity increase exponentially [22,35,38,45] with the length of the entangled branches, which results in ultrahigh viscous materials with poor processing properties. Therefore, an optimum length of entangled branches is assumed to exist for comb polymers to improve the practically Adv.…”

Stress Relaxation of Comb Polymer Melts

“…Once the short arm relaxes, the branch point takes a random hop along the confining tube. Wijesinghe et al [22] carried out molecular dynamics simulations for a coarse grained model for entangled polyethylene comb polymers. They showed that as the branch length increases, the combination of arm retraction and backbone mobility leads to a wide spectrum of arm relaxation times and that the motion of the polymer backbone is well described by repetition over intermediate time scales.…”