Ze-sheng Li

The effects of Lowe–Andersen temperature controlling method on the polymer properties in mesoscopic simulations

Lowe-Andersen (LA) temperature controlling method [C. P. Lowe, Europhys. Lett. 47, 145 (1999)] is applied in a series of mesoscopic polymer simulations to test its validity and efficiency. The method is an alternative for dissipative particle dynamics simulation (DPD) technique which is also Galilean invariant. It shows excellent temperature control and gives correct radial distribution function as that from DPD simulation. The efficiency of LA method is compared with other typical DPD integration schemes and is proved to be moderately efficient. Moreover, we apply this approach to diblock copolymer microphase separation simulations. With LA method, we are able to reproduce all the results from the conventional DPD simulations. The calculated structure factors of the microphases are consistent with the experiments. We also study the microphase evolution dynamics with increasing chiN and find that the bath collision frequency Gamma does not affect the order of appearing phases. Although the thermostat does not affect the surface tension, the order-disorder transition (ODT) is somewhat sensitive to the values of Gamma, i.e., the ODT is nonmonotonic with increasing Gamma. The dynamic scaling law is also tested, showing that the relation obeys the Rouse theory with various Gamma.

Dissipative particle dynamics study on the interfaces in incompatible A∕B homopolymer blends and with their block copolymers

Dissipative particle dynamics, a simulation technique appropriate at mesoscopic scales, has been applied to investigate the interfaces in immiscible binary A/B homopolymer blends and in the ternary systems with their block copolymers. For the binary blends, the interfacial tension increases and the interface thickness decreases with increasing Flory-Huggins interaction parameter chi while the homopolymer chain length is fixed. However, when the chi parameter and one of the homopolymer chain length is fixed, increasing another homopolymer chain length will induce only a small increase on interfacial tension and slight decrease on interface thickness. For the ternary blends, adding the A-b-B block copolymer will reduce the interfacial tension. When the mole number of the block copolymer is fixed, longer block chains have higher efficiency on reducing the interfacial tension than the shorter ones. But for the block copolymers with fixed volume fraction, shorter chains will be more efficient than the longer ones on reducing the interfacial tension. Increasing the block copolymer concentration reduces interfacial tension. This effect is more prominent for shorter block copolymer chains.

Molecular dynamics simulation of the linear low-density polyethylene crystallization

By means of molecular dynamics simulation, the crystallization process of the copolymer (ethylene-co-propene) with different branch distributions is studied at the molecular level. It is shown that subglobules formed at the branch sites along the copolymer chains and subsequently coalesced into a single globule and then developed to a lamellar structure in the end. This process can be considered as crystal nucleation and growth at the early stage of the copolymer crystallization. In the nucleation the branch acts as a nucleating seed and in the crystal growth the branch is rejected from the crystal region as a defect. The driving force for the relaxation process is the attractive van der Waals interaction between the chain segments. Furthermore, it is found that the branch distribution is an important factor in determining the crystallinity of the copolymer, when the comonomer composition and the molecular weight of the copolymers are fixed, as a blocky type of copolymer will show nearly unhampered crysta...

CCNN: The last kinetically stable isomer of cyanogen

The dissociation and isomerization stability of the linear cyanogen isomer CCNN, a formally charge-transfer species comprising C2− and N2+, is theoretically investigated by means of ab initio methods, including the Hartree–Fock (HF), the Mo/ller–Plesset perturbation theory from second through fourth order (MP2, MP4SDQ, MP4SDTQ), the configuration interaction with singles and doubles (CISD), the quadratic configuration interaction with singles and doubles (QCISD) as well as with triples [QCISD(T)], and the density functional theory (DFT) including Beck’s three parameter hybrid methods with the Lee–Yang–Parr correlation functional (B3LYP) and with the Perdew–Wang 91 correlation functional (B3PW91) methods. At the QCISD(T)/6-311G(3df)//QCISD(T)/6-311G(d) level with QCISD/6-311G(d) zero-point vibrational energy (ZPVE) correction, the barriers from CCNN to the NNC three-membered ring structure with exocyclic C–C bonding and to the dissociation products C2 and N2 are predicted to be 42.1 and 51.8 kcal/mol, resp...

The crystallization of low-density polyethylene: a molecular dynamics simulation

Three models (star-shaped, H-shaped, and comb-shaped polyethylenes) are used to study the crystallization behavior of low-density polyethylene at the molecular level by means of molecular dynamics simulation. It is shown that, for the three types of polyethylene corresponding to the models, the neighboring sequences of trans bonds firstly aggregate together to form local ordered domains, and then they coalesce to a lamellar structure. In the process, the branching sites are rejected to the fold surface gradually. The driving force for the relaxation process is the attractive van der Waals interaction between the chain segments. Furthermore, it is found that the number of the branch sites and the length of the branch play an important role in determining the formation of the lamellar structure. The longer the length of the branch and the fewer the number of the branch sites, the more perfect lamellar structure can be formed.

C4N: The first CnN radical with stable cyclic isomers

The potential-energy surface of the interstellar molecule C4N is explored at the B3LYP/6-311G(d) level of theory. Thirteen isomers including the linear, three-membered ring, four-membered ring, A-like, Y-like, and cage-like structures are located as minima connected by 23 interconversion transition states. The structures of the most relevant isomers and transition states are further optimized at the QCISD/6-311G(d) level followed by single-point energy calculations at the MP4SDTQ, CCSD(T), and QCISD(T) levels with the 6-311G(2df) basis set. At the CCSD(T)/6-311G(2df)//QCISD/6-311G(d) level, the lowest-lying isomer is a linear structure CCCCN 1 followed by a CCC three-membered ring structure 4 with exocyclic CCN bonding that lies only 2.8 kcal/mol higher. The third and fourth low-lying isomers possess a CCC three-membered ring structure 5 with exocyclic CNC bonding at 21.4 kcal/mol and a linear structure CCCNC 2 at 23.4 kcal/mol, respectively. All the four isomers 1, 2, 4, and 5 and another high-lying isom...

Theoretical study on the mechanism of the reaction between CN and O2

Abstract The radical–radical reaction between CN ( 2 Σ) and O 2 ( 3 Σ g ) has been theoretically investigated at the UCCSD(T)/6-311+G(d)//UB3LYP/6-31+G(d) level. This reaction proceeds most likely through the doublet CNO 2 potential energy surface (PES) initiated by the carbon-to-oxygen attack leading to the linear NCOO ( 2 A ″) radical, followed by the direct oxygen–oxygen single-bond cleavage leading to (A) OCN ( 2 Σ)+ O ( 3 P ) , or by the sequential three-centered isomerizations and final dissociations leading to (B) CO ( 1 Σ)+ NO ( 2 Π) and (C) CO 2 ( 1 Σ g )+ N ( 2 D ) . This study may be helpful for understanding the combustion chemistry of nitrogen-containing compounds.

Theoretical study on the mechanism of the reaction: HCCCH2++C2H2→c-C3H3++C2H2

Abstract The gas phase ion–molecule reaction of propargylium ( HCCCH 2 + ) with acetylene (C2H2) to produce cyclopropenylium (c -C 3 H 3 + ) with C2H2 has been investigated theoretically at the B3LYP/6–31G(d) and single-point QCISD/6–311G(d,p) levels. The detailed mechanism for the observed isotope exchange between HCCCH 2 + and C2D2 has also discussed. Three intermediates 1 CH2CCH2CCH+, 2 H 2 C 2 · CHCCH 2 + and 3 c -C 4 H 3 –CH 2 + are shown to play important roles in the product formation and isotope exchange processes, rather than the low-lying isomers 4 c -C 3 H 2 –CH 2 + , 7 c - ( CH ) 5 + and 8 pyramidal C 5 H 5 + . Our calculated results agree well with the available experimental data and may be helpful for understanding the mechanism for combustion processes.

Theoretical studies on dynamics and thermochemistry of the reactions CF3CHCl2+Cl→CF3CCl2+HCl and CF3CHFCl+Cl→CF3CFCl+HCl

A dual-level direct dynamics study has been carried out for the two hydrogen abstraction reactions CF3CHCl2+Cl and CF3CHFCl+Cl. The geometries and frequencies of the stationary points are optimized at the BHLYP/6-311G(d,p), B3LYP/6-311G(d,p), and MP2/6-31G(d) levels, respectively, with single-point calculations for energy at the BHLYP/6-311++G(3df,2p), G3(MP2), and QCISD(T)/6-311G(d,p) levels. The enthalpies of formation for the species CF3CHCl2, CF3CHFCl, CF3CCl2, and CF3CFCl are evaluated at higher levels. With the information of the potential energy surface at BHLYP/6-311++G(3df,2p)//6-311G(d,p) level, we employ canonical variational transition-state theory with small-curvature tunneling correction to calculate the rate constants. The agreement between theoretical and experimental rate constants is good in the measured temperature range 276–382 K. The effect of fluorine substitution on reactivity of the C–H bond is discussed.

SiCNN—A New Stable Isomer with Si≡C Triple Bonding

To predict potentially stable molecules with Si≡C triple bonding, theoretical calculations at the B3LYP/6-311G(d) and CCSD(T)/6-311G(2df) (single-point) levels were employed to study the structures, energetics, and isomerization of various SiCN2 isomers. A schematic potential energy surface (PES) of SiCN2 was established to discuss the kinetic stability of the isomers. A new isomer SiCNN was found to possess a typical Si≡C triple bond, as confirmed by comparative calculations at the B3LYP, QCISD, QCISD(T), CCSD, and CCSD(T) levels on the bond lengths of SiCNN and other experimentally or theoretically known species of RSiCH (R=H, F, Cl, OH). Moreover, SiCNN resides in a very deep potential; the stabilization barrier is at least 53.2 kcal mol−1. Thus, SiCNN may be considered as the most kinetically stable isomer with Si≡C triple bonding known to date, and it may represent a very promising molecule for future experimental characterization. In addition, the stability of the other isomers, such as the four linear species SiNCN, SiNNC, NSiCN and NSiNC, a three-membered NNC ring isomer with exocyclic C−Si bonding, and a four-membered SiCNN ring isomer is discussed and compared with SiCNN.