Structural and Dynamical Characteristics of Short-Chain Branched Ring Polymer Melts at Interface under Shear Flow

Seung Heum Jeong,Tae Yong Ha,Soowon Cho,Chunggi Baig,Eun Jung Roh

doi:10.3390/polym12123068

Seung Heum Jeong, Tae Yong Ha + Show 3 more

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https://doi.org/10.3390/polym12123068

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Abstract

We present a detailed analysis of the interfacial chain structure and dynamics of confined polymer melt systems under shear over a wide range of flow strengths using atomistic nonequilibrium molecular dynamics simulations, paying particular attention to the rheological influence of the closed-loop ring geometry and short-chain branching. We analyzed the interfacial slip, characteristic molecular mechanisms, and deformed chain conformations in response to the applied flow for linear, ring, short-chain branched (SCB) linear, and SCB ring polyethylene melts. The ring topology generally enlarges the interfacial chain dimension along the neutral direction, enhancing the dynamic friction of interfacial chains moving against the wall in the flow direction. This leads to a relatively smaller degree of slip () for the ring-shaped polymers compared with their linear analogues. Furthermore, short-chain branching generally resulted in more compact and less deformed chain structures via the intrinsically fast random motions of the short branches. The short branches tend to be oriented more perpendicular (i.e., aligned in the neutral direction) than parallel to the backbone, which is mostly aligned in the flow direction, thereby enhancing the dynamic wall friction of the moving interfacial chains toward the flow direction. These features afford a relatively lower and less variation in in the weak-to-intermediate flow regimes. Accordingly, the interfacial SCB ring system displayed the lowest among the studied polymer systems throughout these regimes owing to the synergetic effects of ring geometry and short-chain branching. On the contrary, the structural disturbance exerted by the highly mobile short branches promotes the detachment of interfacial chains from the wall at strong flow fields, which results in steeper increasing behavior of the interfacial slip for the SCB polymers in the strong flow regime compared to the pure linear and ring polymers.

Highlights

Interfacial polymeric liquids in confined systems exhibit a variety of distinctive properties and phenomena compared to ordinary bulk polymeric liquids, such as solid-like behavior, high viscosity, oscillatory solvation force, and extrusion instability [1,2,3,4,5,6,7,8]
Following the previous studies [23,24,25,26], here we analyzed the degree of slip for the confined polymeric system defined as ds = 1 − γreal /γideal = Vs /Vw, where Vs is the total slip velocity occurring at the top and bottom walls and Vw is the applied velocity of the moving top wall toward the flow direction in shear flow
We evaluated the value of γreal for the ds of the confined systems as follows: the y-dimension of the simulation box (i.e., H) was divided into several bins with a constant interval, and the streaming velocity in the flow direction for each bin was calculated by averaging the x component of velocity for all atoms belonging to that bin

Summary

Introduction

Interfacial polymeric liquids in confined systems exhibit a variety of distinctive properties and phenomena compared to ordinary bulk polymeric liquids, such as solid-like behavior, high viscosity, oscillatory solvation force, and extrusion instability [1,2,3,4,5,6,7,8]. In an effort to elucidate the fundamental molecular characteristics underlying interfacial polymer slip, several nonequilibrium molecular dynamics (NEMD) simulations have been conducted for various confined polymer melt systems [23,24,25,26]. PE melts and reported the slip behavior of the SCB polymer to be quite distinct from that of the corresponding linear polymer, which was ascribed to the generally more compact and less deformed chain structures of the SCB polymer in response to the applied flow via the intrinsically fast random motions of short branches [25,27,28]. We conducted a detailed analysis of the general structural and dynamical characteristics of interfacial chains for confined SCB ring melt systems under shear flow using atomistic NEMD simulations. The results were directly compared to those for the corresponding linear, ring, and SCB linear systems

Systems Studied and Simulation Methods

Results and Discussion

Conclusions