Communication: Role of short chain branching in polymer structure and dynamics

Abstract

A comprehensive understanding of chain-branching effects, essential for establishing general knowledge of the structure-property-phenomenon relationship in polymer science, has not yet been found, due to a critical 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 show a significant effect of short-chain branching on the structural and dynamical properties of polymeric materials, and reveal the molecular origins behind the fundamental role of short branches, via atomistic nonequilibrium molecular dynamics and mesoscopic Brownian dynamics by systematically varying the strength of the mobility of short branches. We demonstrate that the fast random Brownian kinetics inherent to short branches plays a key role in governing the overall structure and dynamics of polymers, leading to a compact molecular structure and, under external fields, to a lesser degree of structural deformation of polymer, to a reduced shear-thinning behavior, and to a smaller elastic stress, compared with their linear analogues. Their fast dynamical nature being unaffected by practical flow fields owing to their very short characteristic time scale, short branches would substantially influence (i.e., facilitate) the overall relaxation behavior of polymeric materials under various flowing 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.…”

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

“…For instance, we previously quantified the degree of slip for confined linear polyethylene (PE) melts under steady shear flow with respect to the flow strength and revealed the underlying molecular mechanisms in conjunction with the interfacial chain configurations [23]. We further examined the interfacial slip for short-chain branched (SCB) linear 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]. In addition, ring polymers, with their closed-loop molecular geometry, are known to exhibit a lower degree of chain stretching, weaker shear thinning, reduced interfacial slip, and hydrodynamic inflation toward the neutral direction under shear flow [26,[29][30][31][32].…”

Preparation, Characterization, and Rheological Behaviors of Polysiloxanes Presenting Densely Simple and Well‐Defined Short‐Branched Chains

“…In the recent years, many studies have established that physical properties of polymers are governed by the chemical structure and the molecular architecture, especially the influence of short chain branching (SCB) . By “short,” it is here referred as a branch with a molecular weight far lower than the entanglement molecular weight, M e that is usually more than 1 kg mol −1 .…”

“…By “short,” it is here referred as a branch with a molecular weight far lower than the entanglement molecular weight, M e that is usually more than 1 kg mol −1 . In a number of studies, short branched chains were introduced into various polymers to study the thermal behavior, crystallization, rheology, and other characteristics of the modified polymers . Indeed, it was shown that by varying only the chain branching of the polymer molecular architecture, one could affect the rheological behavior of the polymer .…”