Rheological Influence of Short-Chain Branching for Polymeric Materials under Shear with Variable Branch Density and Branching Architecture

Abstract

An important objective in polymer science is to manipulate the material properties of polymers by altering their molecular architecture. To this end, understanding of the fundamental role of chain branches along the polymer backbone is crucial. Although the dynamics of linear and long-branched polymers have been thoroughly investigated over the past decades, a comprehensive understanding of branching effects has not yet been obtained, particularly because of a serious lack of knowledge on the role of short-chain branches, the effects of which have mostly been neglected in favor of the standard entropic-based concepts of long polymers. Here we have comprehensively studied the general effect of short-chain branching on the rheological properties of polymeric materials using Brownian dynamics simulations for a series of SCB polymers with systematically varied branch densities and branching architectures along the chain backbone. Our results demonstrate that the short branches, via their fast random motions, give rise to a more compact and less deformed chain structure in response to the applied flow, eventually reducing the shear-thinning behavior in viscosity and the first normal stress coefficient. Most importantly, by altering the distribution of short branches along the backbone, the structural and rheological properties of the SCB system are dramatically changed. We discuss the physical origins and molecular mechanisms underlying these effects and present a detailed interpretation using a molecular-level analysis of individual chain dynamics. This information is valuable, showing us how to systematically tune the material properties by controlling the molecular architecture of branched polymers under various flow conditions.

Predicting experimental results for polyethylene by computer simulation

“…Moreover, theories based on Gaussian statistics ignore manybody effects, which can be described by long MD atomistic simulations. [177,178]; () [179][180][181][182][183][184][185][186][187][188][189][190]; () [153,191]…”

“…Baig et al have extensively worked on linear PE systems from C 10 to C 400 through NEMD simulations in both elongational [179][180][181] and shear flow [182][183][184]. They also analysed H-shaped PE [185] and more recently SCB PE [186][187][188]. Their intriguing results and conclusions have revealed: (i) that segmental orientation is the primary molecular mechanism leading to stress overshoot in transient rheological properties in linear PE atomistic models; (ii) similar shear thinning in LCB (H-shaped) PE and linear PE, but stronger tension-thickening, as experimentally observed in LCB polyolefins [172]; and (iii) a fast random Brownian kinetics inherent to SCB leading to a lesser degree of structural deformation, to reduced shear-thinning, and to less elastic stress as compared with their linear counterparts.…”

Design, Synthesis, and Self-Assembly of Polymers with Tailored Graft Distributions

“…Graft polymers display many unique properties compared to their linear analogues, such as extended chain conformations, [5][6][7][8] increased entanglement molecular weights, [9][10][11][12] and architecturedependent rheological behavior. [13][14][15][16] Recent studies have harnessed these properties in a wide variety of applications in photonics, [17][18][19] drug delivery, [20][21][22] transport, [23][24] and thermoplastics. [25][26] Continued progress in synthetic command over polymer architecture enables further studies of structure-property relationships and inspires new potential applications.…”

Molecular dynamics for linear polymer melts in bulk and confined systems under shear flow

“…In this work, we performed an in-depth analysis on the fundamental molecular mechanisms and dynamic characteristics of bulk and confined polymer melts under shear flow using atomistic nonequilibrium molecular dynamics (NEMD) simulations of unentangled (C 50 H 102 ) and weakly entangled (C 178 H 358 ) linear polyethylene (PE) melts. This work is in addition to various advanced experimental 9 – 13 and numerical 14 – 22 studies to reveal the individual chain dynamics in polymer solutions or melts under an external flow field. This molecular-level information attained by directly tracking down individual chain motions is applied to understand the rheological behaviors of representative mesoscopic and macroscopic structural and dynamical properties in response to the applied flow field in a wide range of flow strengths.…”