By means of molecular dynamics simulation, the transition of the conformations of polyvinyl chloride during a cooling process from 600 to 300 K was studied. The results show that the amorphous polyvinyl chloride chain...By means of molecular dynamics simulation, the transition of the conformations of polyvinyl chloride during a cooling process from 600 to 300 K was studied. The results show that the amorphous polyvinyl chloride chain experiences the melting state, elastic state and glass state and the conformations can be characterized by the increases of the trans-state of C--C--C--C and the near gauche-state of C--C--C--C1 with the decrease of temperature. It is found that the transition of the conformations is driven mainly by the Coulomb interaction between chain segments.展开更多
In this paper, single-walled carbon nanotubes （SWCNTs） are studied through molecular dynamics （MD） simulation. The simulations are performed at temperatures of 1 and 300K separately, with atomic interactions chara...In this paper, single-walled carbon nanotubes （SWCNTs） are studied through molecular dynamics （MD） simulation. The simulations are performed at temperatures of 1 and 300K separately, with atomic interactions characterized by the second Reactive Empirical Bond Order （REBO） potential, and temperature controlled by a certain thermostat, i.e. by separately using the velocity scaling, the Berendsen scheme, the Nose-Hoover scheme, and the generalized Langevin scheme. Results for a （5,5） SWCNT with a length of 24.5 nm show apparent distortions in nanotube configuration, which can further enter into periodic vibrations, except in simulations using the generalized Langevin thermostat, which is ascribed to periodic boundary conditions used in simulation. The periodic boundary conditions may implicitly be applied in the form of an inconsistent constraint along the axis of the nanotube. The combination of the inconsistent constraint with the cumulative errors in calculation causes the distortions of nanotubes. When the generalized Langevin thermostat is applied, inconsistently distributed errors are dispersed by the random forces, and so the distortions and vibrations disappear. This speculation is confirmed by simulation in the case without periodic boundary conditions, where no apparent distortion and vibration occur. It is also revealed that numerically induced distortions and vibrations occur only in simulation of nanotubes with a small diameter and a large length-to-diameter ratio. When MD simulation is applied to a system with a particular geometry, attention should be paid to avoiding the numerical distortion and the result infidelity.展开更多
Various types of geofluids exist in deep and ultra-deep layers in petroliferous basins.The geofluids are much more active under high-temperature and high-pressure(HTHP)conditions,but their properties are unclear.We si...Various types of geofluids exist in deep and ultra-deep layers in petroliferous basins.The geofluids are much more active under high-temperature and high-pressure(HTHP)conditions,but their properties are unclear.We simulated the mixing of different fluids in CH_(4)/C_(3)H_(8)/C_(6)H_(14)/C_(8)H_(18)-water systems and C_(6)H_(14)/C_(8)H_(18)-CO_(2)-H_(2)O systems at temperatures of 25℃ to 425℃ and pressures of 5 MPa to 105 MPa,using an in-situ micron quartz capillary tube thermal simulation system and molecular dynamics numerical simulation software.The mixing processes,patterns,and mechanisms of various fluids were analyzed at microscale under increasing temperature and pressure conditions.The results show that the miscibility of fluids in the different alkane-H_(2)O and alkane-CO_(2)-H_(2)O systems is not instantaneous,but the miscibility degree between different fluid phases increases as the temperature and pressure rise during the experiments.The physical thermal experiments(PTEs)show that the mixing process can be divided into three stages:initial miscibility,segmented dynamic miscibility,and complete miscibility.The molecular dynamics numerical simulations(MDNSs)indicate that the mixing process of fluids in the alkane-H_(2)O and alkane CO_(2)-H_(2)O systems can be divided into seven and eight stages,respectively.The carbon number affects the miscibility of alkanes and water,and the temperature and pressure required to reach the same miscibility stage with water increase with the carbon number(C_(3)H_(8),C_(6)H_(14),CH_(4),C_(8)H_(18)).CO_(2) has a critical bridge role in the miscibility of alkanes and water,and its presence significantly reduces the temperatures required to reach the initial,dynamic,and complete miscibility of alkanes and water.The results are of great significance for analyzing and understanding the miscibility of geofluids in deep and ultra-deep HTHP systems.展开更多
Atomistic detailed hydration structures of poly(vinyl methyl ether)(PVME) have been investigated by molecular dynamics simulations under 300 K at various concentrations. Both radial distribution functions and the dist...Atomistic detailed hydration structures of poly(vinyl methyl ether)(PVME) have been investigated by molecular dynamics simulations under 300 K at various concentrations. Both radial distribution functions and the distance distributions between donors and acceptors in hydrogen bonds show that the hydrogen bonds between the polymer and water are shorter by 0.005 nm than those between water molecules. The Quasi-hydrogen bonds take only 7.2% of the van der Waals interaction pairs. It was found the hydrogen bonds are not evenly distributed along the polymer chain,and there still exists a significant amount(10%) of ether oxygen atoms that are not hydrogen bonded to water at a concentration as low as 3.3%. This shows that in polymer solutions close contacts occur not only between polymer chains but also between chain segments within the polymer,which leads to inefficient contacts between ether oxygen atoms and water molecules. Variation of the quasi-hydrogen bonds with the concentration is similar to that of hydrogen bonds,but the ratio of the repeat units forming quasi-hydrogen bonds to those forming hydrogen bonds approaches 0.2. A transition was found in the demixing enthalpy at around 30% measured by dynamic testing differential scanning calorimetry(DTDSC) for aqueous solutions of a mono-dispersed low molecular weight PVME,which can be related to the transition of the fractions of hydrogen bonds and quasi-hydrogen bonds at ～27%. The transition of the fractions of hydrogen bonds and quasi-hydrogen bonds at ～27% can be used to explain the demixing enthalpy transition at 30% at a molecular scale. In addition,at the concentration of 86%,each ether oxygen atom bonded with water is assigned 1.56 water molecules on average,and 'free' water molecules emerge at the concentration of around 54%.展开更多
The transport properties of fluids in nanopores are a fundamental scientific issue in the development of tight reservoirs such as shale gas.The flow of gas in nanosized pores is affected by a size effect,therefore,the...The transport properties of fluids in nanopores are a fundamental scientific issue in the development of tight reservoirs such as shale gas.The flow of gas in nanosized pores is affected by a size effect,therefore,the conventional fluid mechanics theory cannot be applied.Based on the molecular dynamics theory,the transport process of methane in carbon nanopores was studied,including simulation of the arrangement of the wall atoms,slip and transitional flow of methane in the supercritical state and application of different driving forces.The research of this paper revealed that the configuration of the wall carbon atoms,at the microscale,has a greater influence on the density distribution and velocity distribution of methane molecules in the pores,while the change in the driving force has a greater impact on the slippage of methane at the boundary.Particularly,the theoretical model we proposed can predict the transport properties in carbon nanopores,demonstrating the sensitivity of driving force,pore configuration and the state of flow for methane gas transport,which can provide the characteristic parameters for the establishment of the seepage mathematical model.展开更多
文摘By means of molecular dynamics simulation, the transition of the conformations of polyvinyl chloride during a cooling process from 600 to 300 K was studied. The results show that the amorphous polyvinyl chloride chain experiences the melting state, elastic state and glass state and the conformations can be characterized by the increases of the trans-state of C--C--C--C and the near gauche-state of C--C--C--C1 with the decrease of temperature. It is found that the transition of the conformations is driven mainly by the Coulomb interaction between chain segments.
基金Project supported by the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No 20060003025)the State Key Program for Basic Research of China (Grant No 2003CB716201)
文摘In this paper, single-walled carbon nanotubes （SWCNTs） are studied through molecular dynamics （MD） simulation. The simulations are performed at temperatures of 1 and 300K separately, with atomic interactions characterized by the second Reactive Empirical Bond Order （REBO） potential, and temperature controlled by a certain thermostat, i.e. by separately using the velocity scaling, the Berendsen scheme, the Nose-Hoover scheme, and the generalized Langevin scheme. Results for a （5,5） SWCNT with a length of 24.5 nm show apparent distortions in nanotube configuration, which can further enter into periodic vibrations, except in simulations using the generalized Langevin thermostat, which is ascribed to periodic boundary conditions used in simulation. The periodic boundary conditions may implicitly be applied in the form of an inconsistent constraint along the axis of the nanotube. The combination of the inconsistent constraint with the cumulative errors in calculation causes the distortions of nanotubes. When the generalized Langevin thermostat is applied, inconsistently distributed errors are dispersed by the random forces, and so the distortions and vibrations disappear. This speculation is confirmed by simulation in the case without periodic boundary conditions, where no apparent distortion and vibration occur. It is also revealed that numerically induced distortions and vibrations occur only in simulation of nanotubes with a small diameter and a large length-to-diameter ratio. When MD simulation is applied to a system with a particular geometry, attention should be paid to avoiding the numerical distortion and the result infidelity.
基金supported by the National Natural Science Foundation of China(Grant Nos.42222208,41821002)the Special Fund for Taishan Scholar Project(Grant No.tsqn201909061)+1 种基金the Fundamental Research Funds for the Central Universities(Grant No.20CX06067A)Marine S&T Fund of Shandong Province for Pilot National Laboratory for Marine Science and Technology(Qingdao)(Grant No.2021QNLM020001)。
文摘Various types of geofluids exist in deep and ultra-deep layers in petroliferous basins.The geofluids are much more active under high-temperature and high-pressure(HTHP)conditions,but their properties are unclear.We simulated the mixing of different fluids in CH_(4)/C_(3)H_(8)/C_(6)H_(14)/C_(8)H_(18)-water systems and C_(6)H_(14)/C_(8)H_(18)-CO_(2)-H_(2)O systems at temperatures of 25℃ to 425℃ and pressures of 5 MPa to 105 MPa,using an in-situ micron quartz capillary tube thermal simulation system and molecular dynamics numerical simulation software.The mixing processes,patterns,and mechanisms of various fluids were analyzed at microscale under increasing temperature and pressure conditions.The results show that the miscibility of fluids in the different alkane-H_(2)O and alkane-CO_(2)-H_(2)O systems is not instantaneous,but the miscibility degree between different fluid phases increases as the temperature and pressure rise during the experiments.The physical thermal experiments(PTEs)show that the mixing process can be divided into three stages:initial miscibility,segmented dynamic miscibility,and complete miscibility.The molecular dynamics numerical simulations(MDNSs)indicate that the mixing process of fluids in the alkane-H_(2)O and alkane CO_(2)-H_(2)O systems can be divided into seven and eight stages,respectively.The carbon number affects the miscibility of alkanes and water,and the temperature and pressure required to reach the same miscibility stage with water increase with the carbon number(C_(3)H_(8),C_(6)H_(14),CH_(4),C_(8)H_(18)).CO_(2) has a critical bridge role in the miscibility of alkanes and water,and its presence significantly reduces the temperatures required to reach the initial,dynamic,and complete miscibility of alkanes and water.The results are of great significance for analyzing and understanding the miscibility of geofluids in deep and ultra-deep HTHP systems.
基金Supported by the National Natural Science Foundation of China (Grant Nos. 20474073, 20490220, 20674090 and 90612015)National Major Basic Research Project (Grant No. G1999064800)
文摘Atomistic detailed hydration structures of poly(vinyl methyl ether)(PVME) have been investigated by molecular dynamics simulations under 300 K at various concentrations. Both radial distribution functions and the distance distributions between donors and acceptors in hydrogen bonds show that the hydrogen bonds between the polymer and water are shorter by 0.005 nm than those between water molecules. The Quasi-hydrogen bonds take only 7.2% of the van der Waals interaction pairs. It was found the hydrogen bonds are not evenly distributed along the polymer chain,and there still exists a significant amount(10%) of ether oxygen atoms that are not hydrogen bonded to water at a concentration as low as 3.3%. This shows that in polymer solutions close contacts occur not only between polymer chains but also between chain segments within the polymer,which leads to inefficient contacts between ether oxygen atoms and water molecules. Variation of the quasi-hydrogen bonds with the concentration is similar to that of hydrogen bonds,but the ratio of the repeat units forming quasi-hydrogen bonds to those forming hydrogen bonds approaches 0.2. A transition was found in the demixing enthalpy at around 30% measured by dynamic testing differential scanning calorimetry(DTDSC) for aqueous solutions of a mono-dispersed low molecular weight PVME,which can be related to the transition of the fractions of hydrogen bonds and quasi-hydrogen bonds at ～27%. The transition of the fractions of hydrogen bonds and quasi-hydrogen bonds at ～27% can be used to explain the demixing enthalpy transition at 30% at a molecular scale. In addition,at the concentration of 86%,each ether oxygen atom bonded with water is assigned 1.56 water molecules on average,and 'free' water molecules emerge at the concentration of around 54%.
基金This paper was financially supported by National Science and Technology Major Project of China(Grant No.2017ZX05037001).
文摘The transport properties of fluids in nanopores are a fundamental scientific issue in the development of tight reservoirs such as shale gas.The flow of gas in nanosized pores is affected by a size effect,therefore,the conventional fluid mechanics theory cannot be applied.Based on the molecular dynamics theory,the transport process of methane in carbon nanopores was studied,including simulation of the arrangement of the wall atoms,slip and transitional flow of methane in the supercritical state and application of different driving forces.The research of this paper revealed that the configuration of the wall carbon atoms,at the microscale,has a greater influence on the density distribution and velocity distribution of methane molecules in the pores,while the change in the driving force has a greater impact on the slippage of methane at the boundary.Particularly,the theoretical model we proposed can predict the transport properties in carbon nanopores,demonstrating the sensitivity of driving force,pore configuration and the state of flow for methane gas transport,which can provide the characteristic parameters for the establishment of the seepage mathematical model.
