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We examined the general rheological characteristics of short-chain branched (SCB) ring polyethylene (PE) melts using atomistic nonequilibrium molecular dynamics (NEMD) simulations under shear in a ...

We present the general structural and dynamical characteristics of flexible ring polymers in narrowly confined two-dimensional (2D) melt systems using atomistic molecular dynamics simulations. The results are further analyzed via direct comparison with the 2D linear analogue as well as the three-dimensional (3D) ring and linear melt systems. It is observed that dimensional restriction in 2D confined systems results in an increase in the intrinsic chain stiffness of the ring polymer. Fundamentally, this arises from an entropic penalty on polymer chains along with a reduction in the available chain configuration states in phase space and spatial choices for individual segmental walks. This feature in combination with the intermolecular interactions between neighboring ring chains leads to an overall extended interpenetrated chain configuration for the 2D ring melt. In contrast to the generally large differences in structural and dynamical properties between ring and linear polymers in 3D melt systems, relatively similar local-to-global chain structures and dynamics are observed for the 2D ring and linear melts. This is attributed to the general structural similarity (i.e., extended double-stranded chain conformations), the less effective role of the chain ends, and the absence of complex topological constraints between chains (i.e., interchain entanglement and mutual ring threading) in the 2D confined systems compared with the corresponding 3D bulk systems.

Based on the novel viewpoint that ring polymers, with their closed-loop molecular geometry, are naturally defined by an intrinsic two-dimensional topological surface, we analyzed the fundamental st...

Poly(ethylene glycol) (PEG) has attracted significant interest because of its superior antifouling properties, water solubility, and biocompatibility. However, the translation of its antifouling pr...

Rheological properties of polymer melts under weak flow reflect the orientational anisotropy of the chains, which is established experimentally as the validity of the stress-optical rule (SOR). In ...

We present a detailed analysis on the influences of short branches on the structural, conformational, and dynamical properties of short-chain branched (SCB) ring polyethylene melts using atomistic molecular dynamics simulations. Ring polymers exhibit the compact molecular structures compared to linear polymers, due to their intrinsic closed-loop geometry and an additional effective pressure via the nonconcatenation constraints between ring chains. Importantly, short branches located along the chain backbone are found to make the overall ring structure further compact via the intrinsically compact branched architecture and the fast random movement of short branches that constantly disturbs the chain conformation. Notably, the effects of short branches on the structural compactness for SCB ring polymers appear to be quantitatively very similar to those of SCB linear polymers. We also find that the structure and relaxation of the unentangled SCB ring and linear melt systems can be reasonably well characterized with the Rouse model, regardless of the short branches. Meanwhile, certain distinctive structural and dynamical features of the Rouse normal modes appear for the SCB ring systems in association with the separate effects of short branches and ring topology. Detailed analysis on several distinct torsional modes around branch points along the chain backbone reveals noticeable differences among the torsional modes with respect to the probability distributions of the gauche- and trans-states and the torsional dynamics. This is ascribed to an extra torsional stiffness imposed by the short branches. Additional analysis of the interatomic pair distribution functions for the SCB systems further confirms the fundamental role of short branches in determining the local and global chain structure.

Hierarchical and gradient structures in biological systems with special mechanical properties have inspired innovations in materials design for construction and mechanical applications. Analogous to the control of stress transfer in gradient mechanical structures, the control of electron transfer in gradient electrical structures should enable the development of high-performance electronics. This paper demonstrates a high performance electronic skin (e-skin) via the simultaneous control of tactile stress transfer to an active sensing area and the corresponding electrical current through the gradient structures. The flexible e-skin sensor has extraordinarily high piezoresistive sensitivity at low power and linearity over a broad pressure range based on the conductivity-gradient multilayer on the stiffness-gradient interlocked microdome geometry. While stiffness-gradient interlocked microdome structures allow the efficient transfer and localization of applied stress to the sensing area, the multilayered structure with gradient conductivity enables the efficient regulation of piezoresistance in response to applied pressure by gradual activation of current pathways from outer to inner layers, resulting in a pressure sensitivity of 3.8 × 105 kPa-1 with linear response over a wide range of up to 100 kPa. In addition, the sensor indicated a rapid response time of 0.016 ms, a low minimum detectable pressure level of 0.025 Pa, a low operating voltage (100 μV), and high durability during 8000 repetitive cycles of pressure application (80 kPa). The high performance of the e-skin sensor enables acoustic wave detection, differentiation of gas characterized by different densities, subtle tactile manipulation of objects, and real-time monitoring of pulse pressure waveform.

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 (ds) 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 ds and less variation in ds in the weak-to-intermediate flow regimes. Accordingly, the interfacial SCB ring system displayed the lowest ds 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.

We present a detailed study of the effects of short branches on the rheological behaviors of H-shaped long-chain branched polymers under shear and uniaxial elongational flows using (single “phantom” chain) bead-spring Brownian dynamics simulations. To clarify the fundamental role of short branches in both flow types, the short branches are distributed either along the chain backbone or along the four dangling long arms of the H-polymer. We observe that the fast random motions of the highly mobile short branches (in association with their very short characteristic relaxation time scales) constantly disturb chain conformation, generally leading to a more compact and less deformed chain structure against the applied flow. Accordingly, the structural and dynamical properties of the short-chain branched (SCB) H-polymers in response to the flow are strongly dependent on the location of the short branches along the chain. For instance, in comparison to the original H-polymer, the H-(SCB_backbone) polymer, where the short branches are allocated along the backbone, exhibits considerably less shear-thinning behavior resulting from the lesser degree of chain alignment and structural deformation of the SCB backbone. In contrast, the H-(SCB_arm) polymer, where the short branches are allocated along the four long arms, displays a higher degree of shear-thinning behavior arising from an effective tensile force (created by the tightly coiled “superbead” character of the arms via fast short-branch dynamics) that stretches out the backbone. Importantly, the fundamental role of the short branches in determining rheological characteristics of the SCB H-polymers remains unchanged, regardless of the flow type and flow strength.We present a detailed study of the effects of short branches on the rheological behaviors of H-shaped long-chain branched polymers under shear and uniaxial elongational flows using (single “phantom” chain) bead-spring Brownian dynamics simulations. To clarify the fundamental role of short branches in both flow types, the short branches are distributed either along the chain backbone or along the four dangling long arms of the H-polymer. We observe that the fast random motions of the highly mobile short branches (in association with their very short characteristic relaxation time scales) constantly disturb chain conformation, generally leading to a more compact and less deformed chain structure against the applied flow. Accordingly, the structural and dynamical properties of the short-chain branched (SCB) H-polymers in response to the flow are strongly dependent on the location of the short branches along the chain. For instance, in comparison to the original H-polymer, the H-(SCB_backbone) polymer, where ...

We present the fundamental molecular characteristics underlying the rheological behaviors of interfacial ring polymers in a wide range of flow strengths using atomistic nonequilibrium molecular dynamics simulations of confined ring polyethylene melts under shear. Ring polymer, with its intrinsic closed molecular topology, exhibits a weaker interfacial slip in the weak-to-intermediate flow regime as compared to its linear analogue because of the larger friction between the ring chains and the wall arising from a longer chain dimension in the neutral direction at the interface. In the intermediate flow regime, ring polymer displays, in addition to the standard out-of-plane wagging mechanism of local loops (similar to the linear polymer), a distinctive molecular mechanism (loop migration) wherein locally created chain loops propagate along the chain in the flow direction. This additional mechanism leads to a lesser degree of the overall decreasing tendency of interfacial slip in the intermediate flow regime....

Since carbon tetrahalides CX4 (X = Cl/Br) are adsorbed on carbon nanotubes and graphene sheets, we have studied the structures, adsorption energies, and electronic properties of CX4 adsorbed on benzene, coronene, and graphene using dispersion corrected density functional theory (DFT) with hybrid functionals. As compared with the benzene–CX4 complexes (with binding energy of ∼14/15 kJ/mol) where electrostatic energy is significant due to the halogen bonding effect, the graphene–CX4 complexes show about three times the benzene–CX4 binding energy (∼40/45 kJ/mol) where the dispersion interaction is overwhelming with insignificant electrostatic energy. Since the X atoms in CX4 are slightly positively charged and the X atom’s ends are particularly more positively charged due to the σ-hole effect, CX4 behaves as an electron acceptor. This results in electron transfer from locally negatively charged C sites of benzene/coronene to CX4. In contrast, no electron transfer occurs from graphene to CX4 because of the la...

One of the most important nonlinear rheological phenomena in flowing polymeric materials is the stress overshoot under start-up shear at sufficiently high shear rates γ, i.e., γ > τd–1, where τd is the terminal relaxation time of system. According to the well-known tube theory for entangled polymeric materials, a linear relationship between the anisotropy of the stress and that of the birefringence holds for strain rates in the τd–1 < γ < τR–1 range, where τR is the Rouse time. This implies that in such a flow range the stress overshoot behavior of entangled polymers is accurately described in terms of the chain orientation, as confirmed by previous experiments. However, there has been some recent debate on this issue, following the study of Lu et al. [ACS Macro Lett. 2014, 3, 569−573], whose coarse-grained molecular dynamics simulations produced results in apparent conflict with existing theoretical and experimental predictions, which had suggested that even for γ < τR–1, the stress overshoot is asso...

Interfacial slip plays a crucial role in a variety of fluid dynamics problems occurring in practical polymer processing, lubrication, adhesion, nanocomposites, etc. Despite many research efforts, a fundamental understanding of the underlying molecular mechanisms and dynamics is still lacking. Here, we present the intrinsic molecular characteristics of the slip phenomena by using atomistic nonequilibrium molecular dynamics simulations of polyethylene melts under shear flow. Our results identify three distinctive characteristic regimes with regard to the degree of slip and reveal the underlying molecular mechanisms for each regime: (i) the z-to-x chain rotation mechanism in the vorticity plane in the weak flow regime, which effectively diminishes the wall friction against chain movement along the flow direction, (ii) the repetitive chain detachment-attachment (out-of-plane wagging) and disentanglement mechanism in the intermediate regime, and (iii) irregular (chaotic) chain rotation and tumbling mechanisms ...

We present a detailed analysis on the effect of short-chain branches on the structure and dynamics of interfacial chains using atomistic nonequilibrium molecular dynamics simulations of confined polyethylene melts in a wide range of shear rates. The intrinsically fast random motions of the short branches constantly disturb the overall chain conformation, leading to a more compact and less deformed chain structure of the short-chain branched (SCB) polymer against the imposed flow field in comparison with the corresponding linear polymer. Moreover, such highly mobile short branches along the backbone of the SCB polymer lead to relatively weaker out-of-plane wagging dynamics of interfacial chains, with highly curvy backbone structures in the intermediate flow regime. In conjunction with the contribution of short branches (as opposed to that of the backbone) to the total interfacial friction between the chains and the wall, the SCB polymer shows a nearly constant behavior in the degree of slip (ds) with respect to shear rate in the weak-to-intermediate flow regimes. On the contrary, in the strong flow regime where irregular chain rotation and tumbling dynamics occur via intensive dynamical collisions between interfacial chains and the wall, an enhancement effect on the chain detachment from the wall, caused by short branches, leads to a steeper increase in ds for the SCB polymer than for the linear polymer. Remarkably, the SCB chains at the interface exhibit two distinct types of rolling mechanisms along the backbone, with a half-dumbbell mesoscopic structure at strong flow fields, in addition to the typical hairpin-like tumbling behavior displayed by the linear chains.

Diatomic halogen molecules X2 (X = Cl/Br) favor the edge-to-face conformation on benzene with significant electrostatic interaction via halogen bonding. In contrast, they favor the stacked conformation on graphene with negligible electrostatic interaction. As the aromatic ring expands, the inner facial side becomes almost electrostatically neutral. On coronene, the two conformations are compatible.

We present a nonequilibrium Monte Carlo (MC) methodology based on expanded nonequilibrium thermodynamic formalism to simulate entangled polymeric materials undergoing steady shear flow. Motivated by the standard kinetic theory for entangled polymers, a second-rank symmetric conformation tensor based on the entanglement segment vector was adopted in the expanded statistical-ensemble based MC method as the nonequilibrium structural variable that properly represents the deformed structure of the system by the flow. The corresponding (second-rank symmetric) conjugate thermodynamic force variable was introduced to simulate an external flow field. As a test case, we applied the GENERIC MC to C400H802 entangled linear polyethylene melts in a wide range of shear rates. Detailed analysis of the GENERIC MC results for various structural and rheological properties (such as chain size, chain orientation, mesoscale chain configuration, topological properties, and material functions) was carried out via direct comparison with the corresponding NEMD results. Overall, the GENERIC MC is shown to predict the general trends of the nonequilibrium properties of polymer systems reasonably for a wide range of flow strengths (i.e., 0.5 ≤ De ≤ 540). In conjunction with NEMD, the present MC method can thus be used to extract fundamental thermodynamic information (nonequilibrium entropy and free energy functions) of entangled polymeric systems under various flow types. Despite the general consistency between the GENERIC MC and NEMD results, some quantitative discrepancies appear in the intermediate-to-strong flow regime. This behavior stems from the inherent mean-field nature of the MC force field which is applied to the individual entanglement segments independently and uniformly along the chain, without accounting for any flow-induced structural or dynamical correlations between segments.

Biological tissues are multiresponsive and functional, and similar properties might be possible in synthetic systems by merging responsive polymers with hierarchical soft architectures. For example, mechanochromic polymers have applications in force-responsive colorimetric sensors and soft robotics, but their integration into sensitive, multifunctional devices remains challenging. Herein, a hierarchical nanoparticle-in-micropore (NP-MP) architecture in porous mechanochromic polymers, which enhances the mechanosensitivity and stretchability of mechanochromic electronic skins (e-skins), is reported. The hierarchical NP-MP structure results in stress-concentration-induced mechanochemical activation of mechanophores, significantly improving the mechanochromic sensitivity to both tensile strain and normal force (critical tensile strain: 50% and normal force: 1 N). Furthermore, the porous mechanochromic composites exhibit a reversible mechanochromism under a strain of 250%. This architecture enables a dual-mode mechanochromic e-skin for detecting static/dynamic forces via mechanochromism and triboelectricity. The hierarchical NP-MP architecture provides a general platform to develop mechanochromic composites with high sensitivity and stretchability.

Polymer behaviors at interfaces or in narrowly confined systems have important implications in a variety of practical applications, including polymer processing, polymer films, adhesion, polymer nanocomposites, lubricants, and biological membranes. However, the physics behind the distinctive structural and dynamic features created by confinement remain elusive. Experimental, theoretical, and computational studies on the basic structures of dense linear polymers at solid surfaces or in nanoconfined systems have produced contradictory results. In this study, we resolve this controversial issue by elucidating the underlying fundamental characteristics of two-dimensional monolayer melt systems composed of flexible linear chains using both atomistic and coarse-grained molecular dynamics simulations. The present findings provide basic guidelines for future theoretical and experimental analyses of two-dimensional or narrowly confined polymeric systems.

Various types of interactions between halogen (X) and π moiety (X−π interaction) including halogen bonding play important roles in forming the structures of biological, supramolecular, and nanomaterial systems containing halogens and aromatic rings. Furthermore, halogen molecules such as X2 and CX4 (X = Cl/Br) can be intercalated in graphite and bilayer graphene for doping and graphene functionalization/modification. Due to the X−π interactions, though recently highly studied, their structures are still hardly predictable. Here, using the coupled-cluster with single, double, and noniterative triple excitations (CCSD(T)), the Moller–Plesset second-order perturbation theory (MP2), and various flavors of density functional theory (DFT) methods, we study complexes of benzene (Bz) with halogen-containing molecules X2 and CX4 (X = Cl/Br) and analyze various components of the interaction energy using symmetry adapted perturbation theory (SAPT). As for the lowest energy conformers (S1), X2–Bz is found to have the...

We present detailed results for the structural and rheological properties of unknotted and unconcatenated ring polyethylene (PE) melts under shear and elongation flows via direct atomistic nonequilibrium molecular dynamics simulations. Short (C78H156) and long (C400H800) ring PE melts were subjected to planar Couette flow (PCF) and planar elongational flow (PEF) across a wide range of strain rates from linear to highly nonlinear flow regimes. The results are analyzed in detail through a direct comparison with those of the corresponding linear polymers. We found that, in comparison to their linear analogs, ring melts possess rather compact chain structures at or near the equilibrium state and exhibit a considerably lesser degree of structural deformation with respect to the applied flow strength under both PCF and PEF. The large structural resistance of ring polymers against an external flow field is attributed to the intrinsic closed-loop configuration of the ring and the topological constraint of nonconcatenation between ring chains in the melt. As a result, there appears to be a substantial discrepancy between ring and linear systems in terms of their structural and rheological properties such as chain orientation, the distribution of chain dimensions, viscosity, flow birefringence, hydrostatic pressure, the pair correlation function, and potential interaction energies. The findings and conclusions drawn in this work would be a useful guide in future exploration of the characteristic dynamical and relaxation mechanisms of ring polymers in bulk or confined systems under flowing conditions.

The birefringence on anisotropic materials under flow has been tremendously useful for understanding the physical states of nonequilibrium materials. It is, however, well-known that the linear relationship between the anisotropy of the stress and that of the birefringence breaks down above a certain flow strength, severely limiting its practical applicability. Here we present an encouraging result which helps overcome this limitation and extends our knowledge beyond the conventional the stress-optical rule regime. Through a detailed molecular-level analysis of the stress-optical behaviors of various polyethylene melts under shear via direct atomistic nonequilibrium molecular dynamics simulations, we found a universal feature in the stress-optical behaviors of polymeric materials that there exists a strictly linear relationship between the birefringence tensor and the stress tensor contributed solely by the bond-torsional interaction in the whole range of flow strength. While a limited range of chain lengt...

We present detailed results on the effect of chain branching on the topological properties of entangled polymer melts via an advanced connectivity-altering Monte Carlo (MC) algorithm. Eleven representative model linear, short-chain branched (SCB), and long-chain branched (LCB) polyethylene (PE) melts were employed, based on the total chain length and/or the longest linear chain dimension. Directly analyzing the entanglement [or the primitive path (PP)] network of the system via the Z-code, we quantified several important topological measures: (a) the PP contour length Lpp, (b) the number of entanglements Zes per chain, (c) the end-to-end length of an entanglement strand des, (d) the number of carbon atoms per entanglement strand Nes, and (e) the probability distribution for each of these quantities. The results show that the SCB polymer melts have significantly more compact overall chain conformations compared to the linear polymers, exhibiting, relative to the corresponding linear analogues, (a) ∼20% smaller values of 〈Lpp〉 (the statistical average of Lpp), (b) ∼30% smaller values of 〈Zes〉, (c) ∼20% larger values of 〈des〉, and (d) ∼50% larger values of 〈Nes〉. In contrast, despite the intrinsically smaller overall chain dimensions than those of the linear analogues, the LCB (H-shaped and A3AA3 multiarm) PE melts exhibit relatively (a) 7-8% larger values of 〈Lpp〉, (b) 6-11% larger values of 〈Zes〉 for the H-shaped melt and ∼2% smaller values of 〈Zes〉 for the A3AA3 multiarm, (c) 2-5% smaller values of 〈des〉, and (d) 7-11% smaller values of 〈Nes〉. Several interesting features were also found in the results of the probability distribution functions P for each topological measure.

While the stress relaxation has been one of the most useful protocols in polymer rheology, the underlying molecular processes have not yet been fully analyzed with direct quantification and have appeared to be somewhat ambiguous or even controversial among the existing experimental and numerical results. Here we analyzed in detail the basic molecular characteristics behind the stress relaxation phenomena of entangled linear polymer melts upon cessation of steady shear flow using atomistic nonequilibrium molecular dynamics simulations. Two separate relaxation mechanisms were identified: fast structural relaxation via the entropic chain retraction force and the induced orientational relaxation, and slow thermally-driven orientational relaxation. These processes were directly confirmed and quantified, and the physical origins were elucidated for several well-known experimental observations of stress relaxation behaviors. Mesoscopic structural analysis accounting for individual chain dynamics further reveals the conformational transition of polymer chains during relaxation. The present findings can be a useful basis for comprehending the general characteristics of the nonequilibrium relaxation phenomena exhibited by flowing polymeric materials with various molecular architectures and flow types.

Store-operated calcium entry (SOCE), an important mechanism of Ca2+ signaling in a wide range of cell types, is mediated by stromal interaction molecule (STIM), which senses the depletion of endoplasmic reticulum Ca2+ stores and binds and activates Orai channels in the plasma membrane. This inside-out mechanism of Ca2+ signaling raises an interesting question about the evolution of SOCE: How did these two proteins existing in different cellular compartments evolve to interact with each other? We investigated the gating mechanism of Caenorhabditis elegans Orai channels. Our analysis revealed a mechanism of Orai gating by STIM binding to the intracellular 2-3 loop of Orai in C. elegans that is radically different from Orai gating by STIM binding to the N and C termini of Orai in mammals. In addition, we found that the conserved hydrophobic amino acids in the 2-3 loop of Orai1 are important for the oligomerization and gating of channels and are regulated via an intramolecular interaction mechanism mediated by the N and C termini of Orai1. This study identifies a previously unknown SOCE mechanism in C. elegans and suggests that, while the STIM-Orai interaction is conserved between invertebrates and mammals, the gating mechanism for Orai channels differs considerably.

Flexible pressure sensors with a high sensitivity over a broad linear range can simplify wearable sensing systems without additional signal processing for the linear output, enabling device miniaturization and low power consumption. Here, we demonstrate a flexible ferroelectric sensor with ultrahigh pressure sensitivity and linear response over an exceptionally broad pressure range based on the material and structural design of ferroelectric composites with a multilayer interlocked microdome geometry. Due to the stress concentration between interlocked microdome arrays and increased contact area in the multilayer design, the flexible ferroelectric sensors could perceive static/dynamic pressure with high sensitivity (47.7 kPa–1, 1.3 Pa minimum detection). In addition, efficient stress distribution between stacked multilayers enables linear sensing over exceptionally broad pressure range (0.0013–353 kPa) with fast response time (20 ms) and high reliability over 5000 repetitive cycles even at an extremely hi...

The gradient stiffness between stiff epidermis and soft dermis with interlocked microridge structures in human skin induces effective stress transmission to underlying mechanoreceptors for enhanced tactile sensing. Inspired by skin structure and function, we fabricate hierarchical nanoporous and interlocked microridge structured polymers with gradient stiffness for spacer-free, ultrathin, and highly sensitive triboelectric sensors (TESs). The skin-inspired hierarchical polymers with gradient elastic modulus enhance the compressibility and contact areal differences due to effective transmission of the external stress from stiff to soft layers, resulting in highly sensitive TESs capable of detecting human vital signs and voice. In addition, the microridges in the interlocked polymers provide an effective variation of gap distance between interlocked layers without using the bulk spacer and thus facilitate the ultrathin and flexible design of TESs that could be worn on the body and detect a variety of pressing, bending, and twisting motions even in humid and underwater environments. Our TESs exhibit the highest power density (46.7 μW/cm

Despite its key importance in carbene chemistry, the amphoteric (i.e., both nucleophilic and electrophilic) behavior of the divalent carbon atom (:C) in carbenes is not well understood. The electrostatic potential (EP) around :C is often incorrectly described by simple isotropic atomic charges (particularly, as in singlet CF2); therefore, it should be described by the multipole model, which can illustrate both negative and positive EPs, favoring the positively and negatively charged species that are often present around :C. This amphotericity is much stronger in the singlet state, which has more conspicuous anisotropic charge distribution than the triplet state; this is validated by the complexation structures of carbenes interacting with Na+, Cl−, H2O, and Ag+. From the study of diverse carbenes [including CH2, CLi2/CNa2, CBe2/CMg2, CF2/CCl2, C(BH2)2/C(AlH2)2, C(CH3)2/C(SiH3)2, C(NH2)2/C(PH2)2, cyclic systems of C(CH2)2/C(CH)2, C(BHCH)2, C(CH2CH)2/C(CHCH)2, and C(NHCH)2/C(NCH)2], we elucidate the relationships between the electron configurations, electron accepting/donating strengths of atoms attached to :C, π conjugation, singlet–triplet energy gaps, anisotropic hard wall radii, anisotropic electrostatic potentials, and amphotericities of carbenes, which are vital to carbene chemistry. The (σ2, π2 or σπ) electronic configuration associated with :C on the :CA2 plane (where A is an adjacent atom) in singlet and triplet carbenes largely governs the amphoteric behaviors along the :C tip and :C face-on directions. The :C tip and :C face-on sites of σ2 singlet carbenes tend to show negative and positive EPs, favoring nucleophiles and electrophiles, respectively; meanwhile, those of π2 singlet carbenes, such as very highly π-conjugated 5-membered cyclic C(NCH)2, tend to show the opposite behavior. Open-shell σπ singlet (such as highly π-conjugated 5-membered cyclic C(CHCH)2) and triplet carbenes show less anisotropic and amphoteric behaviors.

Correction for ‘Effect of short-chain branching on interfacial polymer structure and dynamics under shear flow’ by Sohdam Jeong et al., Soft Matter, 2017, 13, 8644–8650.

In this work, we analyzed the individual chain dynamics for linear polymer melts under shear flow for bulk and confined systems using atomistic nonequilibrium molecular dynamics simulations of unentangled (C50H102) and slightly entangled (C178H358) polyethylene melts. While a certain similarity appears for the bulk and confined systems for the dynamic mechanisms of polymer chains in response to the imposed flow field, the interfacial chain dynamics near the boundary solid walls in the confined system are significantly different from the corresponding bulk chain dynamics. Detailed molecular-level analysis of the individual chain motions in a wide range of flow strengths are carried out to characterize the intrinsic molecular mechanisms of the bulk and interfacial chains in three flow regimes (weak, intermediate, and strong). These mechanisms essentially underlie various macroscopic structural and rheological properties of polymer systems, such as the mean-square chain end-to-end distance, probability distribution of the chain end-to-end distance, viscosity, and the first normal stress coefficient. Further analysis based on the mesoscopic Brightness method provides additional structural information about the polymer chains in association with their molecular mechanisms.

Various types of interactions between halogen (X) and π moiety (X-π interaction) including halogen bonding play important roles in forming the structures of biological, supramolecular, and nanomaterial systems containing halogens and aromatic rings. Furthermore, halogen molecules such as X

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.

Through the analysis of individual chain dynamics alongside the corresponding molecular structures under shear via nonequilibrium molecular dynamics simulations of C178H358 linear and short-chain branched polyethylene melts under shear flow, we observed that the conventional method based on the chain end-to-end vector (and/or the gyration tensor of chain) is susceptible to quantitatively inaccurate measurements and often misleading information in describing the rotational dynamics of polymers. Identifying the flaw as attributed to strong irregular Brownian fluctuations inherent to the chain ends associated with their large free volume and strong molecular collisions, we propose a simple, robust way based on the chain center-to-center vector connecting the two centers of mass of the bisected chain, which is shown to adequately describe polymer rotational dynamics without such shortcomings. We present further consideration that the proposed method can be useful in accurately measuring the overall chain structure and dynamics of polymeric materials with various molecular architectures, including branched and ring polymers.

To describe complex systems deeply in the nonlinear regime, advanced formulations of nonequilibrium thermodynamics such as the extended irreversible thermodynamics (EIT), the matrix model, the generalized bracket formalism, and the GENERIC (=general equation for the nonequilibrium reversible−irreversible coupling) formalism consider generalized versions of thermodynamic potentials in terms of a few, well-defined position-dependent state variables (defining the system at a coarse-grained level). Straightforward statistical mechanics considerations then imply a set of equalities for its second derivatives with respect to the corresponding state variables, typically known as Maxwell’s relations. We provide here direct numerical estimates of these relations from detailed atomistic Monte Carlo (MC) simulations of an unentangled polymeric melt coarse-grained to the level of the chain conformation tensor, under both weak and strong flows. We also report results for the nonequilbrium (i.e., relative to the quiesc...

State-of-the-art tube models for the dynamics of entangled polymer melts are usually validated on the basis of the agreement of their predictions for the linear viscoelastic properties (LVE data) of the system against experimentally measured data. We present here a more direct and fundamental test of these models based on their comparison against molecular dynamics (MD) simulation data for the dynamics of primitive paths (PPs) in the system under study. More precisely, we show how one can take advantage of a recently developed computational methodology (P. S. Stephanou, C. Baig, G. Tsolou, V. G. Mavrantzas and M. Kröger, J. Chem. Phys., 2010, 132, 124904) for calculating the most important function of all tube models, the segment survival probability ψ(s,t) and its average Ψ(t) (the overall tube survival probability), by projecting MD data of atomistically detailed samples onto the level of the primitive paths, to directly probe mechanisms proposed for chain relaxation, such as contour length fluctuation (CLF) and constraint release (CR). The simulation data for ψ(s,t) and Ψ(t) can be used next to evaluate refinements of the original Doi-Edwards reptation theory based on a modified diffusion equation for ψ(s,t) incorporating the terms proposed to account directly or indirectly for these effects (CLF and CR). The functions ψ(s,t) and Ψ(t) determined directly from the atomistic MD simulation data account automatically for all these relaxation mechanisms, as well as for any other mechanism present in the real melt. We present and discuss results from such an approach referring to model, strictly monodisperse cis- and trans-1,4- polybutadiene and polyethylene melts containing on average up to 6 entanglements per chain, simulated in full atomistic detail for times up to a few microseconds (that is, comparable to the chain disentanglement time τd). From the same simulations we also present results for two other measures of the PP dynamics in the framework of the reptation theory, the time auto-correlation function of the PP contour length L and the time auto-correlation function of the chain end-to-end vector R. Our methodology, which serves as a bridge between molecular simulations and analytical tube theories, helps quantify chain dynamics in entangled polymers and understand how it is influenced by factors like melt polydispersity and chain molecular architecture, or the presence of interfaces. It can also be straightforwardly extended to polymeric liquids under non-equilibrium conditions (e.g., subjected to a flow field) to understand the interplay between flow and entanglements. © 2011 The Royal Society of Chemistry.

We study dynamics in bidisperse melts of linear cis-1,4-polybutadiene composed of probe and matrix chains at the level of the segment survival probability function ψ(s,t) which is computed directly in the course of long atomistic molecular dynamics simulations [Stephanou et al. J. Chem. Phys. 2010, 132, 124904]. By controlling precisely the matrix chain length and composition, the effect of contour length fluctuations (CLFs) and constraint release (CR) on melt dynamics is quantified. Our study shows that (a) the values of the static topological properties of the probe chains (e.g., the average value of their primitive path (PP) contour length and its fluctuation) remain unaltered in the different matrices, but (b) their dynamical properties (including ψ(s,t) and its average over all segments s, ψ(t), the time autocorrelation function of the PP contour length, and the time autocorrelation function of the chain end-to-end vector) vary significantly from matrix to matrix. As the length of the matrix chains decreases, the functions ψ(s,t) and ψ(t) describing the reptation relaxation of the probe chains are found to decrease more rapidly. Furthermore, the relaxation of longer probe chains is seen to be delayed as the concentration of shorter matrix chains decreases. Overall, our direct computational study proves that CR is the dominant relaxation mechanism in melts of long and short cis-1,4-polybutadiene chains accounting for the majority of differences observed in their relaxation dynamics in different environments (since CLFs appear to be unaffected by compositional differences); as a result, it has a profound effect on the linear viscoelastic properties of the melt, such as the spectra of storage and loss moduli. By further analyzing the mean-square displacement of atomistic segments in the different matrices, we find that while the tube diameter is constant in the mixtures with MS ≤ Me where MS is the molecular weight of short chains and Me the entanglement molecular weight, it gradually increases in the mixtures with MS < Me. How the simulation results compare with laboratory measurements on melts of bidisperse polymers reported in the literature is also discussed. © 2010 American Chemical Society.

We present detailed results about the structural conformational rheo optical and topological properties of an entangled of C(400)H(802) linear polyethylene (PE) melt over a wide range of shear rates (covering both the linear and the highly nonlinear viscoelastic regimes) from direct nonequilibrium molecular dynamics (NEMD) simulations of a large system containing 192 chains (corresponding to 79200 interacting atomistic units). We discuss results for (i) the probability distribution of the mean square chain end to end distance and its radius of gyration (ii) the conformation tensor (iii) the material functions in steady shear (viscosity normal stress differences nonequilibrium shear compliance hydrostatic pressure) (iv) the flow birefringence (v) the orientation angle and order parameter (vi) the interaction energies and their relative importance (vii) the intermolecular pair distribution function and (viii) the intrinsic molecular shape of the chains (represented by the isosurface plots in terms of their monomer number density) all as a function of flow strength. A detailed primitive path (PP) analysis has allowed us to examine how the flow field alters the statistical properties of the underlying topological network of the melt (probability distribution functions and mean values of PP contour length of the number and size of entanglement strands etc.). Our results reveal significant distortions of all these distributions due to applied flow. One of the most important results of our work is that as the shear rate is increased the average value of the contour length goes through a maximum and the number of entanglements per chain exhibits a shear thinning behavior which bears many similarities with the corresponding behavior of the shear viscosity. Overall most of the computed theological properties of the C(400)H(802) melt change in a nonlinear way with the applied shear rate due to the simultaneous effect of (a) chain orientation and stretching (b) chain rotation and tumbling under shear and (c) chain disentanglement.

We present a powerful connectivity-altering Monte Carlo (MC) algorithm for simulating atomistically detailed models of long- and short-chain branched polymers. Based on a mix of advanced and simple moves, the algorithm allows the robust simulation of chain systems with a variety of molecular architectures: H-shaped, A3AA3 multiarm (pom-pom), and short-chain branched (SCB) ones. For the H-shaped and A3AA3 architectures (A denotes the backbone and A the arm), in particular, the recently developed intermolecular double bridging (DB) move effecting a new bridging between the chain backbones or the branches of two different chains and the intramolecular double rebridging (IDR) move effecting a new bridging between two branches of the same chain [Karayiannis et al. J. Chem. Phys. 2003, 118, 2451] are shown to provide a robust sampling of their structural and conformational characteristics at the chain level. The double concerted rotation (d-CONROT) and H-shaped branch rebridging (H-BR) moves, on the other hand,...

In this work, we employ a Hamiltonian-based procedure to derive a generalized Nose–Hoover thermostat, which generates rigorous trajectories, corresponding to the canonical ensemble, in the absence or presence of external forces. Specifically, we prove that rigorous trajectories are generated regardless of whether the total linear momentum of the system is zero, constant, or time-dependent. Through the use of peculiar and center-of-mass coordinates, we develop a thermostat that is consistent with the definition of temperature through the equipartition theorem. The generalized algorithm reduces to the conventional Nose–Hoover thermostat under the constraints that (i) the external forces are absent and (ii) the total linear momentum is zero. We show that the generalized algorithm satisfies the two criteria for rigor (Hamiltonian and non-Hamiltonian) that exist in the literature. Finally, we provide some numerical examples demonstrating the success of the generalized algorithm.

In this study, we present details of the stress–optical behavior of a linear polyethylene melt under shear using a realistic potential model. We demonstrate the existence of the critical shear stress, above which the stress–optical rule (SOR) begins to be invalid. The critical shear stress of the SOR of this melt turns out to be 5.5 MPa, which is fairly higher than 3.2 MPa at which shear thinning starts, indicating that the SOR is valid up to a point well beyond the incipient point of shear thinning. Furthermore, contrary to conventional wisdom, the breakdown of the SOR turns out not to be correlated with the saturation of chain extension and orientation: It occurs at shear rates well before maximum chain extension is obtained. In addition to the stress and birefringence tensors, we also compare two important coarse-grained second-rank tensors, the conformation and orientation tensors. The birefringence, conformation, and orientation tensors display nonlinear relationships to each other at high values of the shear stress, and the deviation from linearity begins at approximately the critical shear stress for breakdown of the SOR.

The microscopic origins of five rheological models are investigated by comparing their predictions for the conformation tensor and stress tensor with the same tensors obtained via nonequilibrium molecular dynamics simulations for n-hexadecane. Steady-state simulations were performed under both planar Couette and planar elongational flows, and the results of each are compared with rheological model predictions in the same flows, without any fitting parameters where possible. The use of the conformation tensor for comparisons between theory and experiment/simulation, rather than just the stress tensor, allows additional information to be obtained regarding the physical basis of each model examined herein. The character of the relationship between stress and conformation is examined using model predictions and simulation data.

In this work, we employ a Hamiltonian-based procedure to derive a generalized Nose barostat, which generates trajectories that rigorously satisfy the statistical mechanical isobaric–isothermal (NpT) ensemble. This generalized algorithm, unlike Nose's original NpT algorithm, maintains rigor in the presence of (i) a non-zero system momentum and (ii) non-negligible external forces. The generalized algorithm reduces to the conventional Nose NpT algorithm when neither condition is satisfied. The key element of the generalized algorithm is that the thermostat and barostat are applied only to those degrees of freedom that contribute to the temperature and pressure, which excludes, e.g. the total system momentum. We show that the generalized algorithm satisfies the two criteria for rigor (Hamiltonian and non-Hamiltonian) that exist in the literature. Finally, we provide some numerical examples demonstrating the success of the generalized algorithm.