玻璃态高分子膜中有机溶剂传质行为研究

针对有机溶剂在玻璃态高分子膜中的渗透现象,提出基于物性推算法的膜分离材料选择方法.基于溶解-扩散机理模型,使用统计热力学和状态方程式为基础的GCLF-EOS模型进行溶解度计算、自由体积模型推算扩散系数,同时考虑填充型复合膜的基膜影响,建立膜渗透通量计算模型.本研究充分考虑有机溶剂对高分子的塑化效应,通过引入塑化固子,重新评价对传质过程有效的"空穴自由体积",改进现有的自由体积理论模型.利用芳香族溶剂苯的实验数据和理论预测比较,验证溶解性和扩散性理论模型的正确性.在此基础上对芳香族有机溶剂在玻璃态高分子膜中传递行为进行成功预测.     

高分子聚合物中溶剂扩散系数的预测

溶剂分子在高分子聚合物中的扩散现象广泛存在于日常生活和工业生产中,对其研究具有普遍意义.Vrentas-Duda模型以自由体积理论为基础,所有参数不需扩散实验测量,由溶剂和高分子的分子性质独立确定,可用于预测高分子聚合物中低相对分子质量溶剂扩散系数.研究Vrentas-Duda模型对典型高分子/溶剂体系中溶剂扩散系数的预测准确性,通过将预测值与实验值比较,评价该模型在不同高分子体系中的适用情况.对处于橡胶态的各向同性高分子聚合物体系,Vrentas-Duda模型能够准确计算除存在氢键以外多数体系的溶剂扩散系数;相对于溶剂互扩散系数而言,溶剂自扩散系数的预测精度更高.     

用状态方程改进的高聚物中溶剂扩散系数预测模型

将Simha-Somcynsky状态方程引入Vrentas-Duda自由体积理论模型,提出改进的高聚物-溶剂体系扩散系数模型,计算常压下橡胶态高聚物中有机溶剂扩散系数对浓度和温度的依存关系.利用高聚物结构单元的范德华体积导出高聚物自由体积分数表达式,通过Simha-Somcynsky方程求取高聚物体积以及对溶剂分子扩散有效的自由体积,避免原模型中繁琐的高聚物粘弹性实验测定和回归高聚物自由体积参数,提高了自由体积理论的预测能力.使用改进的模型预测了苯、甲苯、乙苯和三氯甲烷在聚苯乙烯、聚异丁烯和聚醋酸乙烯酯中的自扩散系数和互扩散系数,计算结果表明改进的自由体积模型具有较高的预测精度.     

残留溶剂在PMMA-CA聚合物膜中迁移及影响的多尺度分析

杂质对功能高分子聚合物理化性质的影响具有重要的学术价值与应用意义。今分别通过分子动力学(Molecular Dynamics,MD)及密度泛函(Density Functional Theory,DFT)等方法探索了聚合物玻璃态和橡胶态下溶剂扩散系数的不同以及残留溶剂对聚合物分子解离能的影响。根据自由体积理论,对聚甲基丙烯酸甲酯-丙烯酰胺杯芳烃(PMMA-CA)的玻璃化温度进行了分子动力学(MD)模拟,得到的自由体积与温度的关系曲线显示其玻璃化温度为395K。考察了玻璃态和橡胶态下溶剂扩散系数的不同,MD模拟得到的均方位移(Mean Square Displacement,MSD)曲线显示,聚合物在玻璃态下溶剂的自扩散系数远低于橡胶态下。通过密度泛函方法计算残留溶剂分子对杯芳烃解离能的影响,结果表明膜制备过程中残留的溶剂分子有利于杯芳烃的解离,但其影响比MMA分子共聚要弱得多。     

用Simha-Somcynsky状态方程改进高分子中溶剂扩散系数模型

将Simha-Somcynsky状态方程和Vrentas-Duda的自由体积理论模型相结合，提出改进的高分子中溶剂扩散系数模型.改进的模型利用状态方程和WLF理论定义的自由体积分数确定扩散过程的有效自由体积，避免原模型中繁琐的黏弹性实验测定.对苯、甲苯、乙苯、氯仿在聚苯乙烯、聚异丁烯和聚醋酸乙烯酯中的扩散系数的计算结果表明，改进模型的预测值与实验值取得较好一致，且自扩散系数的预测精度比原模型高.同时，改进的模型能够反映压力对扩散系数的影响.溶剂扩散系数随外界压强增加而减小，当溶剂浓度较低时，压强的影响非常显著，扩散系数可相差几个数量级.随着溶剂浓度升高，扩散系数接近常压时的扩散系数，压强影响可以忽略.     

基于Simha-Somcynsky状态方程的高分子-溶剂体系扩散系数模型

将Simha-Somcynsky状态方程引入到Vrentas-Duda扩散系数模型，建立基于状态方程的溶剂在高分子中扩散系数模型。与原模型相比，改进的模型避免进行高分子的黏弹性实验测定，模型中高分子的相关参数仅由状态方程的特征温度和特征体积确定，提高了模型的预测能力。对苯、甲苯、乙苯、氯仿在聚苯乙烯、聚异丁烯和聚醋酸乙烯酯中的扩散系数计算结果表明，改进模型的预测值与实验值吻合较好。     

涂料干燥过程中溶剂扩散系数的预测与测试方法

基于改进的自由体积模型,计算了不同温度下和溶剂质量分数的聚甲基丙烯酸甲酯(PMMA)?丙酮体系中溶剂的扩散系数,并通过称重法和热重分析法进行验证.结果表明,溶剂扩散系数的预测值与称重法的测试值吻合程度较高,证明改进的自由体积模型可以较好地预测涂料中溶剂的扩散系数.     

3种芳香烃在聚乙烯膜中的无限稀释扩散系数

基于Hadj-Romdhane和Danner色谱过程的数学模型 ,采用反相气相色谱法在 348.2～ 36 3.2K温度范围内测定了苯、甲苯和乙苯 3种芳香烃溶剂在聚乙烯膜中的无限稀释扩散系数 .实测数据的关联结果表明 ,采用空穴自由体积替代Vrentas-Duda自由体积理论方程中自由体积项的修正方程 ,能很好地描述溶剂分子的无限稀释扩散系数随温度的变化关系 .同时在修正方程的基础上建立了一般化自由体积方程 ,并对该方程的预测能力进行了探讨     

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。     

非电解质溶液扩散系数的理论研究评述

许多过程都涉及扩散控制的传质,相应的扩散速率计算对过程工程的精确量化具有重要意义.本文简述了扩散速率的理论表述模型,并着重介绍了非电解质溶液内扩散系数的理论分析计算途径.分子动力学模拟正处于发展中,并用于估算自扩散系数,但进入实用化阶段尚需时日,唯象模型仍为最重要的理论计算手段.作为最常用的方法,由无限稀释浓度下的扩散系数以及合适的插值方法可计算特定浓度下的扩散系数;自由体积理论是半经验性模型,可以计算自扩散系数,并借助混合规则计算相互扩散系数;最后,对扩散系数的理论研究作了展望.     

Estimation of solvent diffusion coefficient in amorphous polymers using the Sanchez-Lacombe equation-of-state

A modified free-volume model was proposed to predict the solvent diffusion coefficient in rubbery polymers without knowledge of any diffusion data. With the introduction of the Sanchez-Lacombe (SL) equation-of-state (EOS) into the Vrentas-Duda model, this model is an attempt to bridge the gap between the thermodynamic and transport properties of polymer solutions. The free volume provided by polymers for solvent diffusion can be estimated solely using the parameters of the SL EOS characteristics and the polymer glass transition temperature; thus the proposed model avoids the need to use polymer viscoelastic data in determination of polymer free-volume parameters. The other parameters in the Vrentas-Duda model remain applicable. Calculated results of solvent self- and mutual-diffusion coefficients of four common solvents in two polymers indicated that the modified model can give reliable predictions. In addition, it can reflect the effect of pressure on solvent diffusivity for concentrated polymer solutions.     

Free volume theory and nonlinear thermoviscoelasticity

The rheological behavior of polymers in the neighborhood of the glass transition is investigated in the framework of the free volume theory of nonlinear viscoelastic behavior. Free volume theory as normally applied above the glass transition is modified to account for the effect of the residual volume of vacancies below the glass transition; this modification is accomplished by modeling the changes in the state of the polymer as the sum of viscoelastic changes and a random disturbance deriving from the thermal collisions between molecule segments. The changes in mechanical properties in passing across the glass transition follow from the freezing-in of relaxation mechanisms and of free volume; the model, which also incorporates a time-dependent coefficient of thermal expansion under isobaric conditions, does not require additional parameters other than those characterizing the rubbery state. The pressure dependence of the glass transition is found to be in qualitiative agreement with measurements on PVAc, while the ratio of the glassy and rubbery heat capacities is found to coincide with the ratio of the equilibrium bulk compliances in the glassy and rubbery domains.     

聚合物-溶剂体系中能量对溶剂扩散的影响

The Vrentas-Duda free-volume theory has been extensively used to correlate or predict the solvent diffusion coefficient of a polymer/solvent system.The energy term in the free volume diffusion equation is difficult to estimate,so the energy term was usually neglected in previous predictive versions of the free volume diffusion coefficient equation.Recent studies show that the energy effect is very important even above the glass transition temperature of the system. In this paper, a new evaluation method of the energy term is proposed,that is the diffusion energy at different solvent concentrations is assumed to be a linear function of the solvent diffusion energy in pure solvents and that in polymers under the condition that the solvent in infinite dilution.By taking consideration of the influence of energy on the solvent diffustion,the prediction of solvent diffusion coefficient was preformed for three polymer/solvent systems over a wide range of concentrations and temperatures.The results show an improvement on the predictive capability of the free volume diffusion theory.     

Estimation of diffusion coefficients for trace amounts of solvents in glassy and molten polymers

Two methods based on the free-volume theory of transport are developed for the estimation of diffusion coefficients for trace amounts of solvents in amorphous polymers. The first method uses diffusivity data for a polymer–solvent system above the glass transition temperature to estimate the temperature dependence of the mutual diffusion coefficient below this temperature. In the second method, mutual diffusion coefficients are estimated for a particular polymer–solvent system both above and below the glass transition temperature using no diffusivity data for the system. The predictions of the proposed theory are compared with diffusivity data for the n-pentane–polystyrene and ethylbenzene–polystyrene systems.     

Prediction of solvent‐diffusion coefficient in polymer by a modified free‐volume theory

Several versions of free‐volume theory have been proposed to correlate or predict the solvent diffusion coefficient of a polymer/solvent system. The quantity of free volume is usually determined by the Williams–Landel–Ferry (WLF) equation from viscosity data of the pure component in these theories. Free volume has been extensively discussed in different equation‐of‐state models for a polymer. Among these models, the Simha–Somcynsky (SS) hole model is the best one to describe the crystalline polymer, because it describes it very approximately close to the real structure of a crystalline polymer. In this article, we calculated the fractions of the hole free volume for several different polymers at the glass transition temperature and found that they are very close to a constant 0.025 by the SS equation of state. It is quite consistent with the value that is determined from the WLF equation. Therefore, the free volume of a crystalline polymer below the glass transition temperature (T_g) is available from the SS equation. When above the T_g, it is assumed that the volume added in thermal expansion is the only contribution of the hole free volume. Thus, a new predictive free‐volume theory was proposed. The free volume of a polymer in the new predictive equation can be estimated by the SS equation of state and the thermal expansion coefficient of a polymer instead of by the viscosity of a polymer. The new predictive theory is applied to calculate the solvent self‐diffusion coefficient and the solvent mutual‐diffusion coefficient at different temperatures and over most of the concentration range. The results show that the predicted values are in good agreement with the experimental data in most cases. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 428–436, 2000     

Predicting ability of free-volume theory for solvent self-diffusion coefficients in rubbers

The effect of solvent size on the diffusion process is studied for various solvents with natural rubber and polybutadiene in terms of the free-volume theory. The importance of energy effects on the diffusion of penetrants in rubbers is examined. The critical molar volume of the polymer jumping unit is correlated with its glass transition temperature over the range 172 K to 305 K. The correlation shows a linear relationship between these two properties and can be used to predict one of the most sensitive free-volume parameters. Using this parameter in conjunction with the Vrentas-Duda free-volume theory, solvent self-diffusion coefficients in rubbers are then predicted over wide ranges of concentration and temperature. For all the systems, the predictions are comparable with experimental data. © 1996 John Wiley & Sons, Inc.     

Effect of glass transition on the concentration dependence of self‐diffusion coefficients

The free‐volume theory of diffusion is used to predict the effect of the glass transition on the concentration dependence of the solvent self‐diffusion coefficient at constant temperature. The theoretical prediction is in agreement with experimental data. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 1682–1684, 2003     

Sorption and diffusion of alcohols in amorphous polymers

The effects of polymer composition and penetrant molecular size on the solubility and diffusivity of alcohol vapors in a series of well characterized isoprene-methyl methacrylate copolymers and their corresponding homopolymers has been investigated at room temperature. The rate of sorption behavior changes progressively from Fickian to non-Fickian, to Case II to “Super Case II” transport with increasing methyl methacrylate (MMA) content in the polymers. The equilibrium solubility of the alcohols increases linearly with increasing penetrant molecular size for polymers which are above their glass transition temperature and decreases for polymers which are below their T_g. The solubility also initially increases as an approximately linear function of MMA content in the copolymers. At about 55 mole percent MMA, the sorbed concentration either levels off or passes through a maximum depending on the size of the penetrant. The apparent “diffusion coefficients” (D) decrease with increasing molecular volume of the penetrants. An exponential dependence was found between these two variables for PMMA. These “diffusion coefficients” also decrease exponentially with increasing MMA content in these polymers. However, at 55 mole percent MMA the copolymer undergoes a rubber to glass transition at the temperature of the experiments. On this basis, it is suggested that the hindered chain segmental motion contributes to the sorption process in addition to strictly thermodynamic considerations. Free volume theory can be used to explain the mechanism of diffusion through the rubbery polymers while the “hole” theory can be applied to explain the transport of the penetrants through the glassy polymers.