Structural evolution of a-Si:H/SiO2 multilayers upon step by step thermal annealing

a-Si:H/SiO_2 multilayers were prepared by alternatively changing plasma enhanced chemical vapour deposition of a-Si:H layers and in situ plasma oxidation process. Subsequently, as-grown samples were annealed at temperatures from 350℃ to 1100℃ in N_2 ambient with an increment of 100℃. The evolution of bonding configurations and structures with annealing treatments was systematically investigated by Fourier-transform infrared spectroscopy. The peak position of Si－O stretching vibration of SiO_2 layers shift to 1087cm^{-1} after annealing at 1100℃, which demonstrates that the SiO_2 films fabricated by plasma oxidation after high temperature annealing can have similar properties to the thermal grown ones. A Si－O vibration from interfacial SiO_x was identified: the value x was found to increase as increasing the annealing temperature, which is ascribed to the cooperation of hydrogen effusion and reordering of the oxygen bond in SiO_x networks. The H-related bonds were observed in the form of H－Si－O_3 and H－Si－Si_{3-n}O_n (n=1－2) configurations, which are supposed to be present in SiO_2 and interfacial SiO_x layers, respectively. The H atoms bonded in different bonding configurations effuse at different temperatures due to their different desorption energies.     

Expected and unexpected etherification of 2-per(poly)fluoroethyl 4-substitutedphenol with β-haloethanol

Under different reaction conditions, the expected and unexpected reaction products (of which X-ray crystallographic structure was presented) of etherification of 2-per(poly)fluoroethyl 4-substituted phenol with β-haloethanol can be obtained respectively.     

活化/去活化过程对高密度表面引发聚合反应动力学的影响

利用粗粒化分子动力学方法提出了一个包含活化/去活化可逆过程的表面引发聚合反应模型,用以研究高引发密度表面引发聚合反应过程中不同聚合速率和不同转化几率(活化/去活化)对产物聚合物刷性质的影响.可通过调节聚合反应几率和活化/去活化过程发生的比率,在相对短的时间内制备高引发密度下低分散性的聚合物刷.     

Ab Initio Study of Structural and Electronic Properties of Hexagonal BC2N

We investigate hexagonal BC2N in graphite unit cells using the first-principles method and calculate the total energies, lattice parameters, and electronic band structures after full relaxation. It is shown that stable hexagonal BC2N should be stacked sequentially with one graphite layer and one h-BN layer. The density of states indicates that this structure should have metallicity.     

An automatic coarse-graining and fine-graining simulation method: application on polyethylene

Multiscale modeling of a polymeric system is a challenging task in polymer physics. Here we introduce a bottom-up and then top-down scheme for the simulation of polyethylene (PE). The coarse-grained numerical potential for PE is derived through an automatic updating program by mapping its radial distribution function (RDF) from the Lowe-Andersen temperature controlling (LA) simulation onto the one from detailed molecular dynamics (MD) simulation. This coarse-grained numerical potential can be applied in larger systems under the same thermodynamic conditions. We have tested the reliability of the derived potential in two ways. First, the blends of different linear low-density polyethylene (LLDPE) with high-density polyethylene (HDPE) have been simulated in LA with the coarse-grained numerical potentials and reasonable results are obtained. Moreover, Rouse scaling behavior is reproduced for monodispersed polymeric systems with different chain lengths. The atomistic details of the beads can be reintroduced into the coarse-grained HDPE and LLDPE/HDPE models, followed by a few MD runs to alleviate the local tension induced by this fine-graining procedure. The equilibrated large atomistic system can then be used for further studies.     

Stability of two-dimensional tessellation ice on the hydroxylated beta-cristobalite (100) surface

Monolayer adsorbed water on the beta-cristobalite (100) surface is studied via classical molecular dynamics simulations. The ordered two-dimensional (2D) tessellation ice structure (i.e., the four-membered and the eight-membered rings appear alternatively) is justified at low temperatures in the simulations. The stability of this possible new ice phase is further investigated by heating the system from 5 to 300 K. An order-disorder structural transition is observed between 100 and 200 K, featuring the melting process of the tessellation ice. This process is characterized by the water oxygen-oxygen radial distribution function, the coordination number, the distance vector between the center of mass of the oxygen and the hydrogen atoms in water, the mean square displacement of oxygen in water, and the vibrational density of state. The above techniques show consistency on that the order-disorder transition temperature of the 2D tessellation ice is far below 300 K. The 2D tessellation ice structure is also obtained via density functional calculations with different generalized gradient approximations. By comparing the calculated adsorption and the lateral energies between different methods, we find that the melting temperature of the specific 2D ice structure is strongly method dependent. Therefore, further experimental works are urged to justify this possible new ice phase and probe its stability.     

Computer simulation studies of the miscibility of poly(3-hydroxybutyrate)-based blends

By means of the molecular dynamics simulation method, the miscibility of poly(3-hydroxybutyrate)/polyethylene (PHB/PE) blend has been investigated. Two glass transition temperatures of the PHB/PE are found by scrutinizing its volume-temperature curve, and this result is qualitatively in agreement with the experimental results. To further analyze the miscibility of poly(3-hydroxybutyrate)-based blends, the Flory-Huggins parameters of PHB/PE, poly(3-hydroxybutyrate)/poly(ethylene oxide) (PHB/PEO), poly(ethylene oxide)/polyethylene (PEO/PE) have been calculated via a Monte Carlo scheme, and the morphology of the PHB/PEO and the PHB/PE blend has been simulated using dissipative particle dynamics method. The time evolution of dividing interface for PHB/PEO/PE blend shows a dynamic phase separation process. All these results indicate that PHB and PEO tend to mix together, whereas PE aggregates to form PE-rich domains in the PHB/PE and PHB/PEO/PE blends.     

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.     

Synthesis and crystal structures of ruthenium(II) complexes with polypyridyl: [Ru(bpy)2(AFO)](ClO4)2 · H2O and [Ru(dmp)2 (AFO)](ClO4)2 · 1/2DMF · 1/2MeCN

Two ruthenium(II) complexes with polypyridyl ligands, [Ru(bpy)₂(AFO)](ClO₄)₂ · H₂O (1) and [Ru(dmp)₂ (AFO)](ClO₄)₂ · 1/2DMF · 1/2MeCN (2) (bpy = 2,2′-bipyridine; dmp = 2,9-dimethyl-1,10-phenanthroline; AFO = 4,5-diazafluoren-9-one; DMF = N,N-dimethylformamide), were synthesized and characterized by elemental analyses, i.r. and u.v.-vis. spectra. The structures of the two complexes were determined by single crystal X-ray diffraction techniques. To relieve ligand interaction, the coordination sphere is distorted so as to form specific angles (δ) between the polypyridyl ligand planes and coordination planes (N-Ru-N). This revised version was published online in June 2006 with corrections to the Cover Date.