In this paper, we evaluate the transferability of the coarse-grained (CG) force field (FF) of trans-1,4-polybutadiene which was built via a combined structure-based and thermodynamic quantity-based CG method at 413 K and 1 atm by systematically examining CG simulated structural and thermodynamic properties against the underlying atomistic simulation results at different temperatures. Interestingly, the derived CG force field exhibits good “state-point transferability” to some extent. For example, when applying this CG FF to the nearby state point (e.g., amorphous phase at 500 K), the resulting local conformation statistics, chain size, and local packing properties as well as density values for the CG models closely match the atomistic simulated data. When further applying this CG force field to the crystalline state at 300 K, the structural and thermodynamic properties of the crystalline phase formed for these CG and atomistic MD simulations still match within a certain level of accuracy. Furthermore, the CG torsion potential has a dual effect: for the amorphous state, the presence of an intramolecular energy barrier against rotation improves the capability of CG models to more precisely reproduce the structural properties, while in the crystalline state this CG torsion barrier suppresses the formation of the more fully stretched chain with a higher trans content. As a result, in the crystalline phase the CG model chains without torsion potentials possess a more stretched chain conformation, pack more efficiently, and have a higher crystallinity degree than its counterpart with CG torsion potentials as well as its underlying atomistic model. However, the dual effect of CG torsion potentials does not mean that we have to use different dihedral parameters to describe different state points. Both CG FFs, one with and another without torsion potentials, are able to represent the melt and the crystalline states. Overall, the phase and its structural consistency between the CG and atomistic models over other state points (e.g., crystalline phase) for which CG FFs were not explicitly parameterized very encouraging such that the combined structure-based and thermodynamic quantity-based CG method can be used to derive an optimized CG FF for multi-scale simulation of polymer systems under different thermodynamic conditions.

•Mixing gradient copolymers with gradient width disparity provides a novel route of achieving a saturated interface with few copolymers.•Increasing the polydispersity of chain lengths in bidisperse gradient copolymers requires more copolymers to saturate a given interfacial area.•For the copolymers with polydispersity of chain lengths or gradient widths, the bending modulus is smaller than the monodisperse counterparts.•Polydispersity of copolymer length or gradient width in synthesized copolymers can be utilized to effectively tune the interfacial properties.Polydispersity of copolymer lengths or gradient widths in synthesized gradient copolymers is almost inevitable and it can be utilized to tune the interfacial properties. Compared with the monodisperse counterparts, fewer amounts of gradient copolymers are required to saturate a given interfacial area, when gradient copolymers with a polydispersity of gradient widths are used. This arises from small overlap content between chain conformations of bidisperse gradient copolymers on the interfacial monolayer. In contrast, like the bidisperse diblock copolymers increasing the polydispersity of chain lengths in bidisperse gradient copolymers reduces the averaged projection area taken by each copolymer, thus more amounts of copolymers are needed to saturate a given interfacial area. Nevertheless, for the copolymers with polydispersity of chain lengths or gradient widths, the bending modulus is always smaller than the corresponding monodisperse counterparts. Thus mixing gradient copolymers with a gradient width disparity provides a novel route of achieving the saturated (tensionless) interface with fewer amounts of copolymers and of high flexibility.

•The fluorinated polyimides were synthesized from dianhydride 6FDA and aromatic diamines with bulky triptycene and pendent phenyl moieties.•The polyimide membranes, especially the GSPI-P ones revealed outstanding permeability coefficients combined with good selectivity especially for CO2/CH4 separation.•Large quantity of free volume combined with appropriate cavity size in polymeric membranes is beneficial to improve their permeability and selectivity simultaneously.The fluorinated polyimides with high fractional free volume were prepared from 6FDA and aromatic diamines with bulky triptycene and pendent phenyl moieties. These polymers showed excellent solubility, high thermal stabilities and outstanding mechanical properties. The correlation of gas separation performance with the microstructure of these polyimide membranes was investigated. The results indicated that the GSPI-P membranes based on the diamines with pendent phenyl moieties exhibited higher fractional free volumes than GSPI-T membranes derived from diamines with triptycene moieties; as a result, the former gave the much higher permeability coefficients. The gas permeability of these membranes is strongly depended on their free volume and also affected by fluorine content. The GSPI-P membranes also provided good selectivity for CO2/CH4 and CO2/N2 gas pairs because their appropriate cavity size is favorable to separate the CO2 from the other gases.

•[1] The combined structure and thermodynamics-based CG method has an advantage of constructing CG FF with good representability.•[2] In addition to the target properties, the other moduli from CG simulations agree reasonably with the atomistic results.•[3] The results from the CG model with torsions are more encouraging and agree more closely with the atomistic data.•[4] With the introduction of DPD thermostat into CGMD simulations, our CG model can mimic the dynamics of the atomistic model.•[5] The standard DPD thermostat is applicable to the steady shear flow simulation at low and moderate rates.In this paper, we combined structure-based and thermodynamic quantities-based coarse-graining (CG) approaches and adopted the bulk density, local conformational distributions and radial distribution function of the underlying atomistic model system as the target properties to derive the CG force field (FF) for the polymer system of trans-1,4-polybutadiene. The resultant CG FF exhibits good observable representability. In addition to the overall chain size, the extracted bulk modulus, isothermal compressibility, storage and loss moduli from CG model systems are of the same level of magnitude as the atomistic results, suggesting that this hybrid CG approach improves the reproduction capability of the CG model on mechanical property, pressure-dependent density fluctuations, and viscoelasticity. Upon the introduction of a standard dissipative particle dynamics (DPD) thermostat to remedy the removed degrees of freedom and reduced friction during CG, the CG model can completely mimic the dynamics of the atomistic model. Furthermore, we found that the standard DPD thermostat approach is applicable to the steady shear flow simulation at low and moderate rates and the relevant dissipative factor derived by matching diffusion coefficients can only quantitatively reproduce the shear viscosity at low shear rates. Overall, these findings demonstrate the ability of combined CG methods in optimizing CG FF to reproduce multi-properties of polymers.

The Gay-Berne (GB) model has been proved to be highly successful in the simulation of liquid crystal phases via both molecular dynamics (MD) and nonequilibrium molecular dynamics (NEMD). However, the conventional thermostats used in the simulations of GB systems, such as Nosé-Hoover and Langevin thermostats, have serious shortcomings especially in NEMD simulations. Recently, dissipative particle dynamics (DPD) has established itself as a useful thermostat for soft matter simulations, whereas the application of DPD thermostat in (NE)MD simulations is limited to the spherically isotropic potential models, such as the Lennard-Jones model. Considering the virtues of the DPD thermostat, that is, local, momentum conserved, and Galilean invariant, we extend the DPD thermostat to the non-spherical GB model. It is interesting to find that the translational DPD and rotational DPD thermostats can be used in the GB system independently and both can achieve the thermostatting effects. Also, we compared the performance of the DPD thermostat with other commonly used thermostats in NEMD simulations by investigating the streaming velocity profiles and the dynamics of phase separation in a typical but simple binary GB mixture under shear field. It is revealed that the known virtues of DPD thermostats, such as Galilean invariant, shear velocity profile-unbiased, and unscreened hydrodynamic interactions, are still intact when applying to GB systems. Finally, the appropriate parameters for the DPD thermostat in the GB system are identified for future investigations.

Benefitting from the rigid backbone and π–π stacking interactions, arylene ethynylene macrocycles (AEMs) have a high tendency toward a stable nanotube assembly, which brings about potential in transmembrane channel use. Herein, we use molecular dynamics simulations to study the transport properties of water molecules through such a macrocycle nanotube (MNT) embedded in a DPPC bilayer membrane. For comparison, we also consider a structurally less complex channel of carbon nanotube (CNT) with similar size. We find that due to the spatial distribution of the MNT interior, water density profiles exhibit more remarkable wave patterns compared to the CNT, where the water occupancy within cross-sections along the channel have a unique variation of 2–3–2–3. Water molecules inside the MNT are subject to not only shape shifting but also rotation to satisfy the steric environment, which results in an inertial loss and slows down the water flow. We further consider the effect of channel–water interaction and channel length. The water flow through MNTs and CNTs both exhibit maximum behaviors with the increase of channel–water interactions. The MNT flow becomes larger when the channel–water interaction is at low or high levels. Both the water flow decrease steeply first and then smoothly with the increase of channel length. These results indicate that the channel structure and channel–water interaction have a distinct impact on the transport properties of water molecules. As the size of MNT can be well controlled by experimental techniques, this is promising for the design of novel nanofluidic devices.

Unlike macroscale systems, symmetry breaking could lead to surprising results for nanoscale systems. Although great attention has been paid to the transport properties of water molecules through nanochannels in recent years, most of the existing studies are related to symmetric channels (e.g. cylindrical ones). Herein, we use molecular dynamics simulations to study the transport of water through a hydrophobic conical channel. Surprisingly, without any dynamic load, when the channel becomes more asymmetric (the wide radius increases) net water fluxes along the divergent direction are observed, which should be related to the thermal noise. To further explore the asymmetric properties of the conical channel, we then apply the pressure differences for the convergent and divergent directions, respectively. We find that the convergent flux is changed from smaller to larger than the divergent one with the increase of pressure. However, for the salt solution, the convergent flux is coupled to ions due to the blocking effect at low pressures; while the divergent flux is almost independent of ions. These results demonstrate some new physical insights of the ratchet effect of conical channels, and further a pressure controlled water rectification that could have deep implications for designing highly efficient nanoporous materials for sea water desalination.

In this paper, the transferability of the coarse-grained (CG) force field originally developed for the liquid crystal (LC) molecule 5CB (Zhang et al. J. Phys. Chem. B 2012, 116, 2075−2089) was investigated by its homologues 6CB and 8CB molecules. Note that, to construct the 5CB CG force field, we combined the structure-based and thermodynamic quantities-based methods and at the same time attempted to use several fragment molecular systems to derive the CG nonbonded interaction parameters. The resultant 5CB CG force field exhibits a good transferability to some extent. For example, not only the experimental densities, the local packing of atom groups, and the antiparallel arrangements of nearest neighboring molecules, but also the unique LC mesophases as well as the nematic–isotropic phase transition temperatures of 6CB and 8CB were reproduced. Meanwhile, the limitations of this 5CB CG force field were also observed, such as the phase transition from nematic to smectic was postponed to the lower temperature and the resulting smectic phase structure is single-layer-like instead of partially interdigitated bilayer-like as observed in underlying atomistic model. Apparently, more attention should be paid when applying a CG force field to the state point which is quite different from which the force field is explicitly parametrized for. The origin of the above limitations can be potentially traced back to the inherent simplifications and some approximations often adopted in the creation process of CG force field, for example, choosing symmetric CG potentials which do not explicitly include electrostatic interactions and are parametrized by reproducing the target properties of the specific nematic 5CB phase at 300 K and 1 atm, as well as using soft nonbonded potential and excluding torsion barriers. Moreover, although by construction this CG force field could inevitably incorporate both thermodynamic and local structural information on the nematic 5CB phase, the anisotropic diffusion coefficient ratios for different LC phases in both 6CB and 8CB systems are reproduced well. All these findings suggest that the multiproperty parametrization route together with fragment-based method provides a new approach to maximize the possibility to simultaneously reproduce multiple physical properties of a given molecule or related molecules with similar chemical structures at other state points.

We present a coarse-grained molecular dynamics simulation study of phase behavior of amphiphilic monolayers at the liquid crystal (LC)/water interface. The results revealed that LCs at interface can influence the lateral ordering of amphiphiles. Particularly, the amphiphile tails along with perpendicularly penetrated LCs between tails undergo a two-dimension phase transition from liquid-expanded into a liquid-condensed phase as their area density at interface reaches 0.93. While, the liquid-condensed phase of the monolayer never appears at oil/water interface with isotropic shape oil particles. These findings reveal the penetration of anisotropic LC can promote ordered lateral organization of amphiphiles. Moreover, we find the phase transition point is shifted to lower surface coverage of amphiphiles when the LCs have larger affinity to the amphiphile tails.

As a unique type of building blocks for directional self-assembly of superstructures as well as a novel class of stabilizers in polymer alloys, Janus nanoparticles if blended with immiscible homopolymers may not only open a route for fabrication of nanostructured materials with hierarchical order but also provide a means to stabilize immiscible homopolymers through formation of bicontinuous microemulsions. In this paper, by systematically investigating the phase behavior and phase dynamics of ternary systems composed of two immiscible homopolymers and Janus nanoparticles with various shapes and different dividing surface designs, we disclose the consequences and precise mechanism of varying the particle shape and the relevant dividing surface design on the ordering and compatibilizing performance of Janus nanoparticles for nanostructuring immiscible polymers. We demonstrate that a lamellar phase is only formed in systems with anisotropic “standing” Janus nanoparticles. This microphase ordering can go along with the lateral ordering of “standing” particles within lamellar layers and the exhibited in-layer arrangement is dependent on the particle shape. Moreover, the addition of Janus nanoparticles tremendously slows down the phase separation, and the compatibilizing performance is directly related to the total dividing surface area of Janus nanoparticles which is determined by the particle shapes and dividing surface designs. Particularly, the slow exponential domain growth dynamics at the late-time phase separation process follows a crossover scaling form regardless of the particle shapes and dividing surface designs, indicating that the phase separation dynamics of these systems is controlled by the same physical mechanism, that is, the interfacial aggregation of Janus nanoparticles reduces the interfacial tension to virtually zero.

Inspired by the importance of charged nanoparticles (NPs) in biomedicine and their potential applications in nanofluids, we analyze the translocation of cationic and anionic nanoparticles through a nanochannel by explicit solvent molecular dynamics simulations. We focus on the interplay of NPs and ions. The cations bind to the anionic NPs much more than the anions to the cationic NPs, which in turn affects the flux behavior. In particular, the nanochannel enhances the difference between cationic and anionic NPs during the translocation process. With the increase of salt concentration, the ion flux increases nonlinearly with external field strength, in agreement with recent experimental observations, while the NP flux decreases, which suggests a nontrivial competition between the translocation of ions and NPs. Our results revealed the important role of ions during the NP translocation and may have implications in the detection, separation, and filtration of charged NPs.

We employed dissipative particle dynamics simulations to explore the phase behavior of T-shaped ternary amphiphiles composed of rodlike cores connected by two incompatible end chains and side grafted segments. By fine-tuning the number of terminal and lateral beads, three phase diagrams for the model systems with different terminal chain lengths are constructed in terms of temperature and lateral chain length, which have some common features and mostly compare favorably with experimental studies with the exception a couple of new phases. It is worthwhile to highlight that the mixed cylindrical phase and the perforated layer phase, as the experimentally observed mesophases exclusive for facial amphiphilies, are found in simulations for the first time. Also, a novel gyroid structure is observed in series of T-shaped ternary amphiphiles for the first time. Furthermore, by evaluating the effective volume fraction of lateral chains, the phase sequence spanning from conventional smectic layer phase via perforated layer structures and polygonal cylindrical arrays to novel lamellar mesophase is established, which is not just qualitatively consistent with the related experimental findings but even the stability windows of some mesophases quantitatively correspond well to experimental results. The success of reproducing the in-plane ordering of rods in the lamellar phase as well as the generic phase diagram of such T-shaped ternary amphiphiles in great detail implies that our genetic model qualitatively captures many of the characteristics of the phase behavior of real T-shaped molecules and could serve as a satisfactory basis for further exploration of self-organization in other related soft matter systems.

Transport properties of water molecules through hydrophobic channels have been explored extensively in recent years; however, our knowledge about the transport properties of hydrophilic channels is still rather poor. Herein, we use molecular dynamics simulations to study the permeation of water molecules through a charged channel. For comparison, we first consider the pristine hydrophobic channel without charge, and we find an analytic expression that can predict the water flow through it. For uniformly charged channels, with the increase of charge density, the water flow decreases, due to the increase of roughness in the free energy profile experienced by a water molecule along the channel; while the ion flow exhibits a maximum, because of the competition between the increasing ion number and ion-channel attraction. Surprisingly, the water occupancy for positive and negative channels varies in the opposite direction, which is strongly related to the excluded volume effect of ions. Additionally, we also discuss the effect of surface charge patterns and channel sizes. These results not only enrich our understanding of the transport properties of hydrophilic channels, but also have deep implications for the design of nanometer water gates.

Polymers exhibit extended structures at high temperatures or in good solvents and collapsed configurations at low temperatures or in poor solvents. This fundamental property is crucial to the design of materials, and indeed has been extensively studied in recent years. In this paper, the collapse of polyethylene rings on an attractive surface was investigated by using molecular dynamics simulations. It is found that the collapse of ring chains on the attractive surface is of distinct difference from their free counterparts, where the collapse becomes more continuous and a one-stage instead of two-stage collapse can be identified by the specific heat. Some hairpin-like crystal structures are found at low temperatures, which are induced by the adsorption interaction of polymer-surface. For a given chain length, the results were further compared with those of the adsorbed linear chains. Due to the topological constraint of ring chains, the number of hairpin structures is clearly less than that of the linear chains. These numerical simulations may provide some new insights into the folding of ring polymers under adsorption interactions.

In this paper, the phase behavior and interfacial properties of symmetric ternary polymeric blends A/B/AB are studied by dissipative particle dynamics (DPD) simulations. By using the structure factor and nematic order parameter, we carefully characterized the diversified phases and phase transitions, and established the phase diagram of such symmetric ternary blends. It can be generally divided into four regions: disordered phase (DIS) region at high temperature, ordered lamellar phase (LAM) region, bicontinuous microemulsion (BµE) channel and phase-separated phase (2P) region at low temperature with the increase of the total volume fractions of homopolymers ΦH, which shows good accordance with that in previous experimental and theoretical reports. Furthermore, we calculated the elastic constants of 2P and LAM phase, and discussed the transition mechanisms from 2P and LAM to BμE phase, respectively. The results show a direct relevance between the phase transitions and the change of interfacial properties. Finally, we also demonstrate that the BμE channel becomes narrower in lower temperature caused by the temperature dependence of interfacial properties of ternary blends.

We use dissipative particle dynamics simulations to study the interfacial properties and their relevance to the phase transitions in ternary symmetric blends. For the blend in the lamellar (LAM) state, by analyzing the distribution of inter-bilayer distances as well as the position fluctuations and correlations of bilayers, we found that the “Discrete Harmonic” theory can describe the fluctuations of the LAM phase in ternary blends in a satisfactory way when the LAM phase is not too close to the phase transition to the bicontinuous microemulsion (BμE). In particular, as the LAM stack is swollen increasingly with the addition of homopolymers, the bilayers in the LAM phase experience a crossover from the single coherent fluctuation mode to the coexisting of coherent and incoherent fluctuation modes wherein the incoherent free membrane regime starts at larger length scales and becomes more pronounced. Meanwhile, the in-plane correlation length increases, the bending modulus and compressibility modulus decrease, indicating that as the more homopolymers are added, fluctuations between adjacent bilayers become less coherent, bilayers become more flexible, and the highly swollen LAM system becomes more susceptible to thermal fluctuations. Therefore, the phase transition from LAM to BμE is expected to occur when the bending modulus turns out to be within KBT or when the persistent length is no larger than the lamellar spacing. Also the two criteria yield the same predictions for the transition point. While for the macrophase-separated (2P) phase, we found that Helfrich model gives a good description of the fluctuation spectrum of the copolymer monolayer at long wavelengths if the 2P systems are not too close to the phase transition to BμE. With the addition of copolymers the interfacial tension reduces but the bending modulus increases. Moreover, the phase transition of 2P to BμE coincides with the saturation of the interface between homopolymer phases, i.e. attaining a vanishing interfacial tension. In a word, our results justify the notion that there exists a direct correlation between peculiar changes in interfacial properties and the phase transitions from 2P or LAM to the BμE state.

In an electric field, the orientation and self-assembly mechanisms of carbon nanotubes (CNTs) suspended in an aqueous solution are studied by molecular dynamics simulations. It is shown that the combination effect of the confined interface and the electric field drives CNTs to orient along the field direction, and the self-assembly process is determined by the competition between the CNT–CNT and CNT–water interactions. These results not only enrich our knowledge of orientation and self-assembly of nonpolar nanoparticles, but also pave the way for using the electric field as a novel tool to achieve their self-assembly. In particular, the remarkable features, such as the reversibility and fast responsiveness of orientation and self-assembly process, make CNTs attractive for many advanced applications, e.g., in creating switchable functional materials.

Dissipative particle dynamics simulations have been conducted to study the anchoring transitions of nematic liquid crystals in the presence of a rod–coil amphiphilic monolayer at the aqueous–liquid crystal interface. Instead of amphiphile interfacial coverage, the repulsion interaction parameter (aMR) between the mesogens and rod blocks of the amphiphiles is used as a tunable and quantitative parameter to control the anchoring transition. Depending on a complicated interplay between the interfacial interactions and the packing effects, we have observed a novel anchoring transition sequence of planar–tilted–homeotropic–tilted–planar when continuously decreasing the value of aMR.

Using dissipative particle dynamics simulations, we have studied the effect of Janus nanospheres on the phase separation dynamics of polymer blends. We find that Janus nanospheres significantly impede domain growth and at a later stage the average domain size approaches saturation and the growth exponent n decays to near-zero. Additionally, compared with homogeneous nanospheres, the phase separation dynamics of ternary systems containing Janus nanospheres is slower and the average size of domains is smaller at later stages of the phase separation process. This mainly arises from the inherent equatorial adsorption and low desorption probability of Janus nanospheres at interfaces, which can greatly reduce the interfacial tension and hence the driving force towards macrophase separation. In particular, such a difference in the retardation effect becomes more pronounced for the systems containing small sized nanospheres with the preferential particle-matrix interaction. Furthermore, in the later-stage of the phase separation process there exists a dynamical self-similarity in the ternary systems that undergo microphase separation and the domain growth follows a crossover scaling form. Similarly, ternary systems containing Janus nanospheres are closer to saturation and the extracted crossover scaling exponent is closer to 1 than the counterpart ternary systems containing homogeneous nanospheres. Therefore, the Janus nanospheres can be used as a more effective emulsifying or stabilizing agent than homogeneous nanospheres for immiscible polymer blends.

Using Monte Carlo simulation methods, the effects of the comonomer sequence distribution on the interfacial properties (including interfacial tension, interfacial thickness, saturated interfacial area per copolymer, and bending modulus) and interfacial structures (including chain conformations and comonomer distributions of the simulated copolymers at the interfaces) of a ternary symmetric blend containing two immiscible homopolymers and one gradient copolymer are investigated. We find that copolymers with a larger composition gradient width have a broader comonomer distribution along the interface normal, and hence more pronouncedly enlarge the interfacial thickness and reduce the interfacial tension. Furthermore, the counteraction effect, which arises from the tendency of heterogeneous segments in gradient copolymers to phase separate and enter their miscible phases to reduce the local enthalpy, decreases the stretching of copolymers along the interface normal direction. As a result, copolymers with a larger width of gradient composition can occupy a larger interfacial area and form softer monolayers at saturation and are more efficient in facilitating the formation of bicontinuous microemulsions. Additionally, chain length ratio, segregation strength, and interactions between homopolymers and copolymers can alter the interfacial character of gradient copolymers. There exists a strong coupling between the comonomer sequence distribution, chain conformation, and interfacial properties. Especially, bending modulus is mainly determined by the complicated interplay of interfacial copolymer density and interfacial chain conformation.

In this paper, with the aim to establish a rational coarse-grained (CG) model for the 4-cyano-4′-pentylbiphenyl (5CB) molecule, we construct three possible CG models (5P, 6P, and 7P) and then determine the bonded and nonbonded interaction parameters separately. For the intramolecular bonded interactions, the bond and angle distributions of the 5CB bulk phase are used as the target properties. For the nonbonded interactions between CG particles, we combine the structure-based and thermodynamic quantities-based methods for the parametrization of CG interaction potentials and attempt to use several fragment molecular systems to derive the CG nonbonded interaction parameters in order to maintain the transferability of our CG models to some extent. Finally, we fix the optimal nonbonded LJ parameters between CG bead pairs such that the results from CG simulations not only correctly reproduce the experimental density and the nematic LC state at 300 K and 1 atm but also reasonably approximate the local structural properties calculated from the underlying atomistic model. Through comparison of the resulting CG data with target properties, the 6P model is found to be the best one among the three, and then we use this model to investigate the phase behavior and dynamic properties. Our results show that the phase transition temperature from nematic to isotropic phase and the diffusion coefficients are reproduced very well, demonstrating the rationality of the 6P model. Our coarse-grained process should have implications for constructing CG models for nCB series or molecules with similar architectures.

From the perspectives of biological applications and material sciences, it is essential to understand the transport properties of water molecules through nanochannels. Although considerable effort and progress has been made in recent years, a systematic understanding of the effect of nanochannel dimension is still lacking. In this paper, we use molecular dynamics (MD) simulations to study the transport of water molecules through carbon nanotubes (CNTs) with various dimensions under pressure differences. We find an exponential decay describing the relation of the water flow and CNT lengths (L) for different pressures. The average translocation time of individual water molecules yields to a power law relation with L. We also exploit these results by comparing with the single-file transport, where some interesting relations were figured. Meanwhile, for a given CNT length, the water flow vs CNT diameters (R) can be depicted by a power law, which is found to be relevant to the water occupancy inside the nanochannel. In addition, we compare our MD results with predictions from the no-slip Hagen–Poisseuille (HP) relation. The dependence of the enhancement of the simulated water flux over the HP prediction on the CNT length and diameter supports previous MD and experimental studies. Actually, the effect of nanotube dimension is not only originated from the motion of water molecules inside the CNT but also related to thermal fluctuations in the bulk water outside the CNT. These results enrich our knowledge about the channel size effect on the water transportation, which should have deep implications for the design of nanofluidic devices.

Liquid crystals playing a crucial role in material sciences show increasing potential applications in nanotechnology and industry. Generally, thermodynamic and dynamic properties of liquid crystals strongly depend on the corresponding force fields (FF); thus, it is necessary and urgent for us to establish a reliable force field for a given liquid crystal system. In this paper, we develop a new set of FF parameters for the 5CB (4-cyano-4′-pentylbiphenyl) molecule by reoptimizing some parameters of TraPPE-UA in order to reproduce the bulk density. This strategy for the construction of 5CB FF is rather advisable as it not only provides reliable values for the Lennard-Jones parameters but also reduces the computational cost and maintains FF transferability. Indeed, our simulation results show that the phase behavior, the order parameter, conformational features, neighboring molecular pair arrangements, and diffusion properties of 5CB can be reproduced very well. We further validate the transferability of this 5CB FF by extending it to the 8CB (4-cyano-4′-octylbiphenyl) system. As a result, both the nematic and the partial bilayer smectic phases (Sm-Ad) and the nematic−isotropic and the smectic−nematic transition temperatures as well as the diffusion properties of 8CB are successfully reproduced. Therefore, this set of FF parameters originally designed for the 5CB molecule is reliable and transferable. Its effectiveness to model nCB series and molecules with similar chemical structures is expected.

The transport of water molecules through nanopores is not only crucial to biological activities but also useful for designing novel nanofluidic devices. Despite considerable effort and progress that has been made, a controllable and unidirectional water flow is still difficult to achieve and the underlying mechanism is far from being understood. In this paper, using molecular dynamics simulations, we systematically investigate the effects of an external electric field on the transport of single-file water molecules through a carbon nanotube (CNT). We find that the orientation of water molecules inside the CNT can be well-tuned by the electric field and is strongly coupled to the water flux. This orientation-induced water flux is energetically due to the asymmetrical water−water interaction along the CNT axis. The wavelike water density profiles are disturbed under strong field strengths. The frequency of flipping for the water dipoles will decrease as the field strength is increased, and the flipping events vanish completely for the relatively large field strengths. Most importantly, a critical field strength Ec related to the water flux is found. The water flux is increased as E is increased for E ≤ Ec, while it is almost unchanged for E > Ec. Thus, the electric field offers a level of governing for unidirectional water flow, which may have some biological applications and provides a route for designing efficient nanopumps.Keywords: carbon nanotube; electric field; molecular dynamics simulation; transport; water molecule

The formation of bicontinuous microemulsions (BUEs) in ternary symmetric blends including two immiscible homopolymers A and B as well as a linear gradient copolymer G is verified by Monte Carlo simulations. Four phase diagrams as a function of segregation strength and blend composition are constructed to show the effects of compositional gradient. Compared with the blends of A/B/diblock copolymers, the BUE phase occupies a broader region in the phase diagram of the A/B/G blends and is replaced by the three-phase coexistence phases of A + B + L or A + B + D at a much lower chain length ratio. Moreover, the lamellar phase is formed at a higher value of segregation strength, has a shorter periodic length and lower orientational order, and spans a narrower range of the phase diagram. A “counteraction effect” is proposed to explain those discrepancies caused by gradient copolymers.

The lipid membrane plays crucial roles in countless biologic processes, ranging from cell motility, endo- and exocytosis, and cell division to protein aggregation and trafficking. To gain a molecular insight in these biologic processes, the recently developed mesoscale simulation technique, dissipative particle dynamics (DPD) simulation, has become an invaluable tool. By providing a brief survey of existing atomistic and popular coarse-grained models used today in studying the dynamics (including vesicle formation and (protein-mediated) vesicle fusion) and phase behavior of lipid bilayers, this review illustrates how mesoscopic DPD models can be used to obtain a better understanding of these biologic processes currently inaccessible to atomistic and most coarse-grained models.

In this work, we perform a series of molecular dynamics (MD) simulations on the category of sodium alkyl sulfate (SDS-type) surfactant monolayers at the water/trichloroethylene (TCE) interface. Three separate tail-length SDS-type molecules are used. We investigate the conformation of surfactant chain (i.e., packing, orientation, and order), interfacial properties (i.e., interfacial thickness, interfacial tension, area compressibility, and bending modulus), their dependence on the chain length, and the average area per surfactant chain. We also examine the behavior of the surfactant monolayer in the metastable regime of negative surface tension with reference to collapse. The simulation has clearly shown that the very dilute monolayer is well described as a two-dimensional gas. With the increase of interfacial surfactant coverage, the monolayer is in the liquid-expanded (LE) phase. The surfactant tails at the interface become straighter, more ordered, and thicker at higher surfactant coverage. At the same time, interfacial tension of long-tail systems is always lower than that of short-tail systems. In the LE phase, the area compressibility modulus and the bending modulus increase with an increase in tail length. With a further decrease in molecular areas, the monolayer with large negative surface tension becomes unstable. Our simulations show that buckling of the monolayers is of dynamic nature as a response to mechanical instability. The further transformation pathway from buckling to bud can be controlled by the bending modulus, which depends crucially on the tail length and interfacial surfactant coverage. At a given area per molecule, the short tail chain makes the monolayer softer, and the budding process becomes more probable. For the supersaturated softer SDS monolayer, the collapse transition is initiated by the buckling of monolayers, followed primarily by budding and detachment of the nanoscale swollen micelle from the monolayer. Despite a number of extensive studies of monolayer collapse at the air/water interface, to our knowledge the conversion of surfactants from the liquid−liquid interface to swollen micellar aggregates as described here has not been reported in the literature.

A dissipative particle dynamics simulation method is used to probe the mechanism of protein-mediated membrane fusion. The coarse-grained models for proteins are designed based on the function of fusion proteins. Attractive forces have been introduced to produce a protein complex. The formation of protein complexes provides mechanical forces to bring membranes in proximity and trigger their merging. The whole fusion process is in good agreement with the scaffold hypothesis. Additionally, if self-defined interactions are also imposed on transmembrane segments (TMSs), their association will yield an unstable fusion pore, which can incorporate lipid and water to accomplish membrane fusion. It indicates the formation of a protein-lined pore will promote the stalk−pore transition and accelerate the fusion process.

A bilayer structure is an important immediate for the vesicle formation. However, the mechanism for the bilayer-vesicle transition remains unclear. In this work, a dissipative particle dynamics (DPD) simulation method was employed to study the mechanism of the bilayer-vesicle transition. A coarse-grained model was built based on a lipid molecule termed dimyristoylphosphatidylcholine (DMPC). Simulations were performed from two different initial configurations: a random dispersed solution and a tensionless bilayer. It was found that the bilayer-vesicle transition was driven by the minimization of the water-tail hydrophobic interaction energy, and was accompanied with the increase of the position entropy due to the redistribution of water molecules. The bulk pressure was reduced during the bilayer-vesicle transition, suggesting the evolved vesicle morphology was at the relatively low free energy state. The membrane in the product vesicle was a two-dimensional fluid. It can be concluded that the membrane of a vesicle is not interdigitated and most of the bonds in lipid chains are inclined to orient along the radical axis of the vesicle.

In this paper, we construct an efficient and simple coarse grained (CG) model for atactic polystyrene (PS) by using a 1:1 mapping scheme at 463 K and 1 atm pressure and derive the corresponding bonded and non-bonded potentials in the CG force field (FF) via a direct Boltzmann inversion approach and a combined structure-based and thermodynamic quantities-based CG method, respectively. For computational considerations, the non-bonded interaction between CG particles is described by Lennard-Jones (LJ) type potentials, and both the radial distribution function (RDF) and the bulk density of the atomistic simulations are taken as target properties in the parameterization of the two LJ parameters. To shed light on the choice of LJ forms of CG non-bonded potentials when designing the CG models, a series of CG models with different LJ potentials are constructed and compared in order to understand how the quality of a CG model in reproducing the structure and thermodynamic properties of chemically realistic systems is affected by the choice of non-bonded potentials. We find that with our structural and thermodynamics combined CG method to construct the CG FF at a single thermodynamic state point without any temperature dependent LJ potential correction and/or pressure optimization, the resulting CG models possess good temperature transferability in a wide range of temperatures 300–600 K, where both the target properties and several other static properties (such as thermal expansion coefficient and mean-square radius of gyration) are generally reproduced. Furthermore, the non-bonded LJ potential influences the density response of CG models to the temperature change, i.e., CG models with harder LJ potentials show better temperature transferability than the softer ones. Meanwhile, the derived Tg increases with increasing LJ repulsion strength while thermal expansion coefficients in both melt and glass states are lowered as the LJ potential hardens. With regard to the local conformation and local packing distribution functions, varying non-bonded LJ potential hardness influences only the magnitude of the peak height but does not affect the peak position, in particular the magnitude of the non-bonded potential effect on local distribution functions becomes stronger at lower temperatures. More specifically, this effect on the local chain conformation statistics at the CG level is different for the distribution of bond-lengths, bond angles and dihedrals. As a result, the size of the CG chains is fairly insensitive to the non-bonded LJ potentials within 300–600 K. In short, the CG model with the harder LJ-type non-bonded CG potential is a more realistic representation of excluded volume interactions of the underlying atomistic PS monomer and thus has the potential to generate a higher Tg to match with the atomistic systems.

In this paper, we evaluate the transferability of the coarse-grained (CG) force field (FF) of trans-1,4-polybutadiene which was built via a combined structure-based and thermodynamic quantity-based CG method at 413 K and 1 atm by systematically examining CG simulated structural and thermodynamic properties against the underlying atomistic simulation results at different temperatures. Interestingly, the derived CG force field exhibits good “state-point transferability” to some extent. For example, when applying this CG FF to the nearby state point (e.g., amorphous phase at 500 K), the resulting local conformation statistics, chain size, and local packing properties as well as density values for the CG models closely match the atomistic simulated data. When further applying this CG force field to the crystalline state at 300 K, the structural and thermodynamic properties of the crystalline phase formed for these CG and atomistic MD simulations still match within a certain level of accuracy. Furthermore, the CG torsion potential has a dual effect: for the amorphous state, the presence of an intramolecular energy barrier against rotation improves the capability of CG models to more precisely reproduce the structural properties, while in the crystalline state this CG torsion barrier suppresses the formation of the more fully stretched chain with a higher trans content. As a result, in the crystalline phase the CG model chains without torsion potentials possess a more stretched chain conformation, pack more efficiently, and have a higher crystallinity degree than its counterpart with CG torsion potentials as well as its underlying atomistic model. However, the dual effect of CG torsion potentials does not mean that we have to use different dihedral parameters to describe different state points. Both CG FFs, one with and another without torsion potentials, are able to represent the melt and the crystalline states. Overall, the phase and its structural consistency between the CG and atomistic models over other state points (e.g., crystalline phase) for which CG FFs were not explicitly parameterized very encouraging such that the combined structure-based and thermodynamic quantity-based CG method can be used to derive an optimized CG FF for multi-scale simulation of polymer systems under different thermodynamic conditions.