注射保压过程中O2/N2分子在PMMA熔体内部的扩散行为

研究了注射成型保压过程中,不同工艺条件下O2/N2混合气体在聚甲基丙烯酸甲酯（PMMA）聚合物内部的扩散行为。针对注射成型工艺特点,将注射保压阶段的低分子扩散简化为不同温度条件下的准静态扩散行为;建立了包含O2/N2气体和10条聚合度为10的全同立构PMMA链团的扩散映射模型。经能量最小化及退火算法实现了扩散模型的能量初始化;基于COMPASS力场,实现了O2/N2混合气体在PMMA熔体内部扩散的分子动力学模拟实验。结果表明,O2和N2的扩散系数均随温度的升高而增大,且同一温度下,O2比N2更容易扩散;不同温度下O2和N2的均方位移函数（MSD）对数lg(MSD)与时间对数lg(t)关系曲线的斜率（n）都接近于1,符合Einstein扩散机制。     

CH4在PVDF中渗透行为及机理的分子模拟研究

采用巨正则蒙特卡洛法（GCMC）和分子动力学法（MD）相结合的方法模拟研究了典型气体CH₄在聚偏氟乙烯（PVDF）中的吸附扩散行为,探讨了温度及压力对气体吸附扩散能力的影响,分析了CH₄在PVDF中的吸附位点及扩散轨迹。模拟结果表明,CH₄在PVDF中的溶解系数、渗透系数随温度的升高先增大后减小,随压力的升高而增大;扩散系数和自由体积分数均随温度和压力的升高而增大;吸附过程中,CH₄在PVDF内呈现有选择性地聚集吸附,且多吸附于模拟晶胞中的低势能区;随后,PVDF内的CH₄分子在以空穴形式存在的自由体积之间进行扩散,温度越高、压力越大,扩散能力越强。     

含硅聚合物中小分子扩散行为的分子模拟

选择PCFF和COMPASS分子力场对橡胶态聚合物PDMS 和玻璃态聚合物PS1体系进行模拟。COMPASS力场模拟得到的体系密度,O2和N2在PDMS与PS1中扩散系数更接近实验值。在模型大小一定时,Group-based求和法中截断距离越长,耗用机时越长,但对计算结果改进不大;截断距离为1.3 nm时计算结果最好。Ewald方法耗时多而对计算结果却无明显改进。体系大小对扩散系数的计算值影响甚微。体积越小的分子,在聚合物中运动的范围越大,扩散系数越大。氧气和氮气分子在PDMS与PS1中运动轨迹不同,在PS1中氧气运动范围远大于氮气,而在PDMS中氧气运动范围稍大于氮气。小分子运动轨迹基本与聚合物自由体积分布对应,自由体积分数大,扩散系数也大。     

注射成型保压阶段气体吸附扩散行为分子动力学模拟

构建了包含4条聚合度为40的全同立构聚苯乙烯(iPS)、间规立构聚苯乙烯(sPS)和无规立构聚苯乙烯(aPS)等6种PS分子链晶胞模型,采用Monte Carlo法分析研究了氧气/氮气(O_2/N_2)在PS熔体中的平衡吸附情况,引入Compass力场对温度-压力耦合效应下O_2、N_2纯组分及混合组分在PS熔体中的扩散行为进行分子动力学(MD)模拟。结果表明,在不同结构的PS熔体中,O_2/N_2混合状态下可以提高气体的总体吸附作用,但却抑制了纯O2或纯N2的吸附能力;混合气体能在一定程度上提高气体的扩散能力;O_2/N_2在PS熔体中扩散能力的强弱受PS主链活跃性和溶剂气体分子体积的影响;气体在不同结构的PS熔体中的扩散行为是从非爱因斯坦扩散到爱因斯坦扩散的转变过程,且以跳跃式、孔穴扩散形态存在。     

超临界水中气体扩散系数的分子动力学模拟

采用分子动力学（MD）方法计算T＝703.2-763.2K,p=30-45MPa范围内,氧气和氮气在超临界水中的无限稀释扩散系数。计算结果表明,在超临界条件下的扩散系数较常温常压下要大1－2个数量级。扩散系数随温度的升高而增大,随压力的增高而降低,且氧的扩散系数稍大于氮的扩散系数。     

聚丙烯气体辅助注射成型冷却过程中温度分布的研究

通过对聚合物气体辅助注射成型冷却过程进行合理的假设与简化,对聚丙烯（PP）气体辅助注射成型冷却过程进行了实验与数值模拟研究。结果表明,注射氮气后,PP的冷却速度显著加快,并在气-熔界面处出现小范围的结晶平台。采用MATLAB软件对氮气辅助注射成型PP冷却过程中的温度分布进行了数值模拟,将计算所得模拟值与温度采集的实验值进行比较,发现在熔体降温阶段温度分布的模拟值与实验值吻合程度很高;在固相冷却阶段由于聚合物本身复杂特性以及气体的渗透效应,PP的模拟值略高于实验值,而氮气的模拟值低于实验值。     

不同气体流动模式下的气体辅助注射成型——隐式直接算法在温度场模拟中的应用

对气辅成型冷却过程的温度场进行了数据模拟,通过对持续注射氮气聚合物气体辅助注射成型(GAIM)冷却过程的合理假设,采用结晶温度区间的平均散热代替结晶潜热和第三类对流传热边界条件,使用MATLAB对高密度聚乙烯（PE-HD）GAIM冷却过程进行了数据模拟;通过实验测定了不同注射氮气压力下的流速并验证模拟温度场。结果表明：冷却温度场的模拟和实测值基本吻合,其模拟结果对于制件多层次结构的预测和气辅工艺参数的优化有重要的指导意义。     

注射成型发泡过程中温度和剪切速率对CO2扩散行为影响的分子动力学研究

为研究临界CO₂辅助聚乳酸（PLA）注射成型发泡过程中,温度和剪切速率对CO₂扩散行为影响。应用分子动力学模拟方法,以PLA和CO₂为研究对象,基于周期性边界条件和SA算法,引入CAMPASS力场,构建PLA/CO₂扩散模型,对模型进行能量最小化处理。从温度和剪切速率两个方面研究在注射成型发泡过程中PLA主链活跃性、体系能量响应、均方回转半径对CO₂分子在PLA中扩散行为的影响。结果表明,CO₂扩散系数在不受剪切力作用时,随着温度升高而增大,温度达到388 K时,CO₂扩散系数达到0.219 8×10^-5 cm²/s;受剪切力作用时,随着剪切速率增大而增大;温度为378 K,剪切速率为1 ps^-1时,扩散系数达到17.743 6×10^-5 cm²/s;PLA分子链所处环境温度不断提高,分子链能量升高,活跃性提高,CO₂分子扩散路径增多,从而提高CO₂扩散速率;分子链所受剪切力不断增大,分子链能量升高,活跃性提高,单个分子链从缠结网络中剥离出来,沿剪切力方向形成流动取向,从而提高CO₂扩散速率。     

PMMA/MWCNT复合材料的热降解动力学

采用溶液共混法制备了聚甲基丙烯酸甲酯/多壁碳纳米管（PMMA/MWCNT）复合材料,利用热重法研究了不同气氛下复合材料的热降解,并采用Flynn-Wall-Ozawa、Kissinger和Friedman等3种方法计算其动力学参数。结果表明,MWCNT的添加量为3 %时,在氮气和氧气中复合材料的初始降解温度较纯PMMA分别提高了54.62 ℃和70.4 ℃,最大热失重速率温度也有一定程度的提高,说明MWCNT能显著提高PMMA的低温热稳定性,尤其是在有氧环境中,而对高温热稳定改善不明显;采用Kissinger法、Flynn-Wall-Ozawa法和Friedman法计算得到的活化能（Ea）变化趋势一致,当MWCNT的添加量为3 %时,Ea较纯PMMA提高最多,在氮气中分别为45.99、95.10、72.46 kJ/mol,在氧气中分别增加53.42、120.63、110.41 kJ/mol;由Friedman法求解出复合材料的反应级数（n）在氮气中约为1.5,在氧气中约为0.9。     

炭膜分离CO_2/CH_4混合气的分子模拟

针对炭膜分离CO_2/CH_4混合气体的过程,分别采用Materials Studio和Lammps软件进行分子模拟,建立与炭膜孔结构相近的Z字形孔模型,通过实验数据验证了模型的可靠性,通过对CO_2/CH_4纯组分及混合气体在膜孔内的吸附和扩散过程的模拟得到分离系数并探讨气体分离机理。综合吸附与扩散过程的模拟结果表明:适当的低温和较小的孔径有利于实现CO_2/CH_4混合气体的分离;随着温度的升高,CO_2/CH_4的分离系数减小,而且膜孔径对分离系数的排序为0.670 nm1.005 nm1.340 nm;在温度为298 K、膜孔径为0.670 nm的操作条件下CO_2/CH_4的分离系数为20.1,与实验数据较吻合。研究结果可为优化炭膜制备提供指导,并为探讨分离过程机理提供依据。     

STUDY ON PERMEATION OF ACETONE/NITROGEN MIXED GAS THROUGH AL2O3 MICROPOROUS MEMBRANES: MOLECULAR DYNAMICS

In this work, by applying Materials Studio 2.2 software package, molecular dynamics (MD) was performed to investigate the dynamic processes of 1:4 acetone/nitrogen mixed gas permeating through different Al₂O₃ microporous membranes. Three systems were modeled by considering different box lengths, microporous sizes, and textures of Al₂O₃ membranes to compare different permeation behaviors. In each system, initial mixed gas contained 20 acetone molecules and 80 nitrogen molecules, and its density was set to 0.1 g/cm³. Analysis on the concentration profiles of nitrogen molecules (N2) and acetone molecules (Ace) in each system at different sampling times was implemented to discuss the permeation behaviors of smaller N2 and larger Ace. The results showed that adsorption and diffusion occurred synchronously but adsorption was dominant for acetone molecules and that the adsorption on the floor surface of the feed gas region more easily reached equilibrium (local equilibrium) than the diffusion and the adsorption on the ceiling of the initially vacuum region. Furthermore, for nitrogen, adsorption followed diffusion. Higher temperature is in favor of the enrichment of acetone on the floor surface of the feed gas region but against the adsorption of nitrogen. The adsorptive layer was found to be a double-layer with COMPASS force field and to be a monolayer with PCFF force field.     

STUDY ON PERMEATION OF ACETONE/NITROGEN MIXED GAS THROUGH AL2O3 MICROPOROUS MEMBRANES: MOLECULAR DYNAMICS

In this work, by applying Materials Studio 2.2 software package, molecular dynamics (MD) was performed to investigate the dynamic processes of 1:4 acetone/nitrogen mixed gas permeating through different Al₂O₃ microporous membranes. Three systems were modeled by considering different box lengths, microporous sizes, and textures of Al₂O₃ membranes to compare different permeation behaviors. In each system, initial mixed gas contained 20 acetone molecules and 80 nitrogen molecules, and its density was set to 0.1 g/cm³. Analysis on the concentration profiles of nitrogen molecules (N2) and acetone molecules (Ace) in each system at different sampling times was implemented to discuss the permeation behaviors of smaller N2 and larger Ace. The results showed that adsorption and diffusion occurred synchronously but adsorption was dominant for acetone molecules and that the adsorption on the floor surface of the feed gas region more easily reached equilibrium (local equilibrium) than the diffusion and the adsorption on the ceiling of the initially vacuum region. Furthermore, for nitrogen, adsorption followed diffusion. Higher temperature is in favor of the enrichment of acetone on the floor surface of the feed gas region but against the adsorption of nitrogen. The adsorptive layer was found to be a double-layer with COMPASS force field and to be a monolayer with PCFF force field.     

Changes in the structure and functional groups produced during the fluorination of mesophase microbeads

The mesocarbon microbead (MCMB) fluorides with different fluorine contents were obtained in the temperature range of 100–110 °C in the presence of the mixed gas containing fluorine and nitrogen. The MCMB fluorides were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy and elemental analysis. The results showed that the carbon lamination of the MCMBs was severely destroyed, which was indicated by a dramatic increase in the interlayer spacing of the resultant materials from 0.345 nm of the MCMBs to the range of 0.656 to 0.722 nm. Further studies on diversity of functional groups indicated that the whole fluorination reaction process comprises two steps: include a first diffusion of fluorine gas molecules inside the voids formed due to loose packing of MCMB molecules, and then fluorination. The two steps interacted with each other and jointly determined the apparent speed of the reaction. The original lamination structure of the MCMBs was completely destroyed for volume expansion of the resultant materials and diffusion of fluorine/nitrogen mixed gases. The final MCMB fluorides are themselves a conglomeration of particles with diameters ranging from 1 to 3 μm.     

Microporous Substructure of Porous Glasses: Dynamic and Equilibrium Approaches

Porous glasses produced by the leaching of two-phase alkali borosilicate glasses have been studied by dynamic and equilibrium methods for diagnostics of pore morphology. It has been detected the availability of microporous substructure with some kinds of adsorbing micropores of diameter 0.3–2 nm including ultramicropores of molecular size in porous glasses with transporting pores of mean diameter from 4 to 15 nm. The multimodal nanoporous structure of porous glasses detected by kinetic mass spectral method of gas diffusion diagnostics (DD-method) at low temperatures is consistent with the results obtained from analyzing equilibrium desorption isotherms of nitrogen, oxygen and argon at 77.5 K by different calculation techniques including an equilibrium method of gas desorption at low partial pressures (LPED-method). Micropore volume in porous glasses is equal to 6–18% from total pore volume. The dependence of nanoporous morphology of porous glasses on conditions of their production and composition has been established. The diffusion and equilibrium characteristics of different molecules (nitrogen, oxygen, argon) varying in molecule size and quadrupole moment value have been determined for primary and secondary porous substructures of porous glasses at liquid nitrogen temperature at the first time.     

Surface modification of poly(ethylene) in an rf downstream remote plasma reactor

Polyethylene (PE) was treated in a remote downstream reactor with oxygen, nitrogen, hydrogen, and mixed gas plasma. The effects of these treatments were investigated by contact angle goniometry and x-ray photoelectron spectroscopy (XPS). The oxygen plasma treatment allowed a rapid and efficient hydrophilation of the PE, although the samples were placed far outside the main discharge region. In hydrogen plasma treatment, only a small amount of polar functional groups are formed, while the nitrogen plasma results in a surface similar to that in an oxygen plasma treatment. Thirteen percent of oxygen was found by XPS in these samples, while only 3% of nitrogen was present. The mixed oxygen/hydrogen plasma treatment revealed that 0.1% oxygen in the process gas produced a detectable degree of oxidation. By experiments with quenched atomic oxygen, the singlet molecular oxygen (O½Δ) was found to be the major reactive species in the surface reaction in our system. The role and the source of radiation in this reaction is discussed. © 1993 John Wiley & Sons, Inc.     

Permeability of oxygen and nitrogen gases in ethyl cellulose solid films retaining cholesteric liquid crystalline order

Ethyl cellulose (EC) films that retain lyotropic and thermotropic cholesteric liquid crystalline order, and an amorphous EC film were prepared. The liquid crystalline order was identified by optical measurements. The comparative permeability of oxygen and nitrogen gases for three kinds of EC film was determined, and the applicability of the EC films that retained cholesteric liquid crystalline order to oxygen enrichment are discussed. The permeability of oxygen or nitrogen gas for the liquid crystalline films was lower than that for the amorphous ones. The activation energy for the permeability coefficient of oxygen gas was ca. 3.5 kcal/mol. The ratio of permeability coefficient for oxygen gas to that for nitrogen gas was less than 4. Interestingly, the permselectivity of oxygen and nitrogen gases for the liquid crystalline films was greater than that for the amorphous ones. © 1996 John Wiley & Sons, Inc.     

The barrier properties of polyethylene terephthalate to mixtures of oxygen, carbon dioxide and nitrogen

The diffusion and solubility of oxygen, nitrogen and carbon dioxide have been studied in amorphous and biaxially oriented films of polyethylene terephthalate (PET). To measure the sorption and desorption of each gas simultaneously in cases where mixtures of gases were studied, a mass spectrometer was used as a detector. It was found that the solubility and diffusion of nitrogen in PET were markedly affected by the presence of the other gases, oxygen and carbon dioxide with differences in detail between results for the amorphous and biaxially oriented films. It is of particular interest that the presence of oxygen reduces the solubility and increases the diffusivity of nitrogen.     

不可压热流体中气体传质扩散过程的数值模拟

控制流体流动中溶解的气体分子浓度能有效控制流动过程中的化学反应,而由热产生的自然对流能够加强气体分子的传递,因此研究气体分子在热流体流动中的扩散混合过程有重要意义。应用格子Boltzmann方法,耦合热效应和扩散效应,数值模拟了一个简化的容器中随着自然对流的发展,溶解的氧气分子在整个容器中的扩散过程。首先建立了二维9速模型的双扩散模型来模拟热量和质量的双扩散对流。为了考察不同自然对流流动对气体分子传递的影响,设计了3种不同给热条件,对不同热流动的形成过程和气体分子扩散过程进行了模拟,与文献结果吻合良好。通过详细分析热边界如何影响流动和传质过程,证实了模拟的速度场与文献数据差异的合理性,同时为控制气体传质过程提供给热条件的设计依据。     

Diffusion processes in lead-silicate glasses in gas heat treatment with nitrogen

The character of diffusion of oxygen and nitrogen in the surface layer of nitrated lead-silicate glass was investigated. The dependences of the diffusion coefficients on the composition of the glass and duration of gas heat treatment were established.