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1.
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 Tg. 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.  相似文献   

2.
The diffusion of radioactively tagged n-hexadecane, n-dotriacontane, and a polybutadiene oligomer with molecular weight 1600 has been studied in 12 rubbery polymers. Diffusion coefficients were obtained from the theory for the thin smear method: for n-hexadecane and for n-dotriacontane (with one exception), in the form appropriate for a completely miscible polymer–penetrant pair, and for the oligomer in the form appropriate for slow entry of the penetrant across the penetrant–polymer interface. For the four flexible linear penetrants, n-dodecane, n-hexadecane, n-dotriacontane, and oligomer, the ratios of diffusion coefficients (or translational friction coefficients) are nearly the same in every polymer. It is concluded that these penetrants travel with similar segmentwise motions, although that is not the case with bulkier, more rigid penetrants. For the three normal paraffins, the friction coefficient is approximately proportional to molecular weight, but that for the oligomer is smaller than would be predicted on this basis.  相似文献   

3.
4.
The presence of low molecular weight molecules in a polymeric matrix often has a marked effect on material properties. Knowledge of specific penetrant distributions and component interactions is important for an elucidation of structure-property relationships, plasticization phenomena, and any modification of structure induced by the presence of penetrants. The sorption-mode characteristics of water, methanol, and ethanol in Nylon 6 films have been investigated by the application of the differential sorption method. The sorption and diffusion behavior were interpreted in terms of clustering theory with suitable account being made for penetrant molecular size and hydrogen-bonding capability. The examination of transport and mechanical properties of these films indicates a pronounced dependence of those properties on the concentration of penetrants. The effect of penetrant cluster formation at characteristic concentrations of sorbed penetrant is to decrease the concentration dependence of both diffusion and mechanical relaxation processes in the case of alcohols. The onset of water clustering apparently only affects the mechanical relaxation process. The samples were further characterized by DSC, X-ray diffraction and density measurements to detect any significant changes in the structure of Nylon 6 induced by the penetrant conditioning.  相似文献   

5.
The permeability of light gases in a series of different glassy polymers is analyzed through a thermodynamic‐based approach for solubility and diffusivity. The nonequilibrium thermodynamic model for glassy polymers describes the solubility of the different penetrants; diffusivity is given as the product of a mobility factor and a thermodynamic factor. The latter is predicted by the nonequilibrium lattice fluid thermodynamic model, while the mobility coefficient is determined using the experimental permeability data. For rather soluble penetrants (e.g., CO2), a plasticization factor is also accounted for, considering the mobility to depend exponentially on penetrant concentration, as often observed experimentally. The model is able to describe accurately the experimental behavior in a simple and effective way, considering only two adjustable parameters. The mobility coefficient is found to depend on the penetrant size (critical volume) and on the fractional free volume of the polymer matrix, following rather general and reasonable correlations. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2776–2788, 2015  相似文献   

6.
The effect of penetrant diffusion on the barrier properties of PVC geomembranes have been determined for the following organic environmental contaminants: benzene, dichloromethane, and trichloroethylene. The membranes experienced swelling to a degree which depends on the type of penetrant used. Some dissolution may occur, and in general we are dealing with Case II transport. The experimental work was performed with an ASTM-cell, a Cahn balance, and an in-house built gravity (G-) cell. Those instruments generated comparable data on breakthrough times. The obtained diffusion coefficients as well as the breakthrough times obeyed an Arrhenius-type relation over the temperature range studied. Liquid sorption of the various penetrants modified the geomembrane structure. Membrane surface pretreatment with different contaminants influences the subsequent transport of organic penetrants through PVC geomembranes. That is to say: the induced swelling, as result of membrane contact with one penetrant, is likely to alter the system free volume, allowing for a different rate of mass transport for subsequent penetrants. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 1189–1197, 1997  相似文献   

7.
Sorption/desorption kinetics and sorption equilibria have been determined for ethanol, toluene, and xylene vapors in a poly(dimethylsiloxane) membrane containing about 32 wt % silica resin at 25°C. Dependence of diffusion coefficient on vapor activity and sorption isotherms have been compared to identify the transport mechanisms of those penetrants in the PDMS membrane. The analysis of Zimm–Lundberg clustering functions showed that all three penetrants had a tendency to form clusters and ethanol molecules might be immobilized by the residual silanol groups within the silica resin in the membrane. The diffusion coefficient of toluene was roughly constant and that of xylene slightly decreased as increasing the vapor activity due to the competing effects of penetrant clustering and solvent swelling of polymer. The diffusion coefficient of ethanol versus activity exhibited a maximum due to the effects of ethanol immobilization and cluster formation. The freevolume effect by solvent swelling to diffusion was obscured by either penetrant clustering or immobilization for the three penetrant–polymer systems. © 1994 John Wiley & Sons, Inc.  相似文献   

8.
Evidence is presented that the apparent diffusion coefficient of one penetrant can be increased by competition from another penetrant for the same adsorption sites in polyester. The competition is governed by the relative diffusivities of the penetrants and their affinities for the adsorption sites.  相似文献   

9.
The diffusion behavior of nonionic penetrants in aqueous solution into nylon 6 was examined in the temperature range 5°–95°C. The Arrhenius plot of the diffusion coefficients is linear and its slope changes at 30–40°C higher than the glass transition temperature in water, as determined by dilatometry and viscoelastic measurements. The results are discussed in relation to the molecular size of the penetrant and the segmental motion of polymer chains.  相似文献   

10.
This work introduces a new numerical algorithm that can be used to analyze complex problems of penetrant transport. Penetrant transport in polymers often deviates from the predictions of Fick's law because of the coupling between penetrant diffusion and the polymer mechanical behavior. This phenomenon is particularly important in glassy polymers. This leads to a model consisting of two coupled differential equations for penetrant diffusion and polymer stress relaxation, respectively. If the polymer relaxation is the rate-limiting step, both the concentration and stress profiles are very steep. A new algorithm based on a finite difference method is proposed to solve the model equations. It features the development of a tridiagonal iterative method to solve the nonlinear finite difference equations obtained from the finite difference approximation of the differential equations. This method was found to be efficient and accurate. Numerical simulation of penetrant diffusion in glassy polymers was performed, showing that the integral sorption Deborah number is a major parameter affecting the transition from Fickian to anomalous diffusion behavior. © 1993 John Wiley & Sons, Inc.  相似文献   

11.
A methodology is proposed to relate the diffusion coefficient of small penetrant molecules in polymers to temperature, strain, and penetrant concentration. The approach used is based on well-known free volume theories. It is assumed that the transport kinetics is governed by the constant redistribution of the free volume, caused by the segmental motions of the polymeric chains. An expression for the diffusion coefficient is inferred from the temperature, strain, and penetrant concentration dependence of the free volume. The stress dependence of solubility is predicted from the Hildebrand theory. It is shown that the resulting constitutive equations exhibit many features desirable for joint durability studies. Finally, a non-Fickian driving force arising from differential swelling is included in the governing equations.  相似文献   

12.
含硅聚合物中小分子扩散行为的分子模拟   总被引:4,自引:1,他引:3       下载免费PDF全文
黄宇  刘庆林  张新波 《化工学报》2007,58(6):1359-1365
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。  相似文献   

13.
Uniform, submicron-diameter polystyrene (PS) and poly(methylmethacrylate) (PMMA) microspheres were dilated by preswelling with pure organic vapors followed by rapid removal of the preswelling penetrant by protracted evacuation of the preswelling chamber to a pressure of 10?3 mm Hg. Aging of the preswollen polymers was carried out both in vacuum and in the presence of various penetrants at sorbed concentrations typically less than 2 wt%. Inferences about relaxations of the polymers were based upon changes in concentrations of the penetrants within the microspheres, at a given temperature and penetrant activity, which result from aging in vacuum or in the presence of penetrant. The kinetics of the relaxations were monitored by probing the expanded glasses with relatively low concentrations of penetrants. In general, the continuous presence of these low concentrations of probe molecules either arrested or retarded the ensuing relaxation as compared with the aging that occurred in vacuum. A series of lower monohydric alcohols, lower n-alkanes, and some other similar penetrants were used as probes to test explicitly and systematically the effects of size and structure of the penetrant contacting the polymer during aging on the decay of excess sorption capacity of the preswollen glassy polymers. Decay of excess sorption in the presence of penetrant was evident only when molecularly small penetrants with interactive functional groups, alcohols for example, were sorbed into preswollen PMMA. This result suggested that the relaxation occurring in the presence of penetrant involved specific interactions between the penetrants and the carbonyl groups in the PMMA.  相似文献   

14.
An accurate determination of a penetrant volume fraction in a swollen polymer is of crucial importance in a range of different technologies. Using optical methods, such as in-situ spectroscopic ellipsometry, it is possible to extract the thickness and refractive index of dry and swollen polymer films. The volume fraction of the penetrant can then be calculated from the change in thickness, or from the refractive index using effective medium approximations. For thermodynamically equilibrated and ideal swollen rubbery polymers, these calculations yield accurate results. However, for glassy polymers the influence of the excess free volume trapped within the polymer network during vitrification is rarely taken into account. In this work we investigate the effect of excess free volume in the calculations of penetrant volume fraction in a model glassy polymer – penetrant system. The influence of the excess free volume is included by extrapolating the properties of an equilibrium polymer matrix from above its glass transition temperature. The error between the approaches that do, and do not, take account for the non-equilibrium of the glassy polymer is quantified and the implications for other systems are discussed. The errors are shown to be very significant, especially when the dry polymer has a large excess free volume. Such materials are particularly relevant in membrane applications.  相似文献   

15.
A general theory of diffusion of large molecules in rubbery amorphous polymers is of interest for the scientific understanding and with regard to material design and process optimization. A broadly applicable model would be useful in developing controlled transport of plasticizers and other additives through polymeric substances. A diffusion model is presented which has been developed for large molecular penetrants above the Tg of the amorphous polymer allowing for required increase in redistribution of the free volume of the polymer structure, as well as the penetrant size and shape. Applicability of the model is demonstrated by comparing theoretically developed diffusion curves for DNOP and DNDP in PVC vs. their weight fractions at 82°C and 91°C. These theoretically derived plots are compared with experimental D vs. w1 curves for these systems generated at lower temperature.  相似文献   

16.
17.
The diffusion of radioactively tagged n-hexadecane, 1,7-heptanediol, and ethylene glycol has been studied in one polar and six nonpolar rubbery polymers. The penetrant–polymer pairs included some in which the two components were miscible in all proportions and some in which they were nearly incompatible (solubility of penetrant in polymer of the order of 1%). The theory for the thin smear method for measuring the diffusion coefficient was evaluated for incompatible pairs with two cases: case 2, when the rate-limiting step is diffusion through the polymer; and case 3, when the rate-limiting step is entry into the polymer across the penetrant–polymer interface. They are easily distinguished experimentally from each other and from case 1, which refers to completely miscible pairs. In examples of case 3, e.g., ethylene glycol with butadiene or styrene–butadiene rubbers, the half-time for saturating the polymer surface in contact with the penetrant was found to be several days at 25°C. The diffusion coefficients, when compared in four hydrocarbon rubbers, of n-hexadecane and 1,7-heptanediol were similar in magnitude even though in each rubber the hexadecane was soluble in all proportions and the diol only very slightly. The diffusion coefficient of ethylene glycol, despite its extreme thermodynamic incompatibility, appeared to be somewhat larger than those of the other penetrants.  相似文献   

18.
The diffusion coefficient at zero penetrant concentration D0 of dichloromethane, chloroform, carbon tetrachloride, cyclohexane, benzene, o-xylene, m-xylene, and p-xylene, and n-hexane in LDPE were measured at 25°C, using the desorption method. The D0 values obtained in this way are correlated with the size, shape, and chemical nature of the penetrant molecules. The temperature dependence of the diffusion coefficients of toluene and n-hexane in LDPE are also reported in the limited temperature range of 25–45°C. It indicates that, in spite of a size larger than that of toluene, n-hexane has a lower activation energy of diffusion.  相似文献   

19.
The desorption of mixtures of ethane and butane at atmospheric pressure from low-density polyethylene was investigated over the temperature range from 20 to 60°C. Desorbed penetrants were continuously trapped in glass tubes immersed in liquid nitrogen, and composition was determined as a function of time by means of gas chromatography. The ratio of the quantity of desorbed gas at any time t, qt, to the quantity at complete desorption, q, was used to determine diffusion coefficients and solubility constants. The diffusion coefficients for both ethane and butane increase with increasing butane concentration in the temperature interval investigated. The solubility of both penetrants can be correlated by Henry's law at 40, 50, and 60°C. However, at 20 and 30°C. the solubility constant for both penetrants increases with increasing butane concentration. This trend is consistent with experimental observations for single-component diffusion and solubility of several hydrocarbons in polyethylene, where increasing concentration of penetrant plasticizes the polymer, resulting in increasing diffusion coefficients and solubility constants.  相似文献   

20.
P. Musto  G. Mensitieri 《Polymer》2005,46(12):4492-4503
The diffusion of low molecular weight penetrants through films of a PMDA-ODA polyimide and a corresponding polyimide-silica bicontinuous nanocomposite (organic-inorganic hybrid) was monitored using time-resolved FTIR spectroscopy and gravimetric analysis. A reacting and a non-reacting penetrant, ammonia and water vapour, were used in the study in order to evaluate their relative interactions with the components of the polyimide-silica hybrid from sorption and diffusion measurements.In the experiments with water vapours the sorption equilibrium was measured with activity levels varying from 0.1 to 0.75. The state of water molecules was examined on the basis of a simplified H-bonding association model.From the experiments using ammonia as the penetrant, it was possible to separate the physical diffusion component from the reaction-determined transport rate by using different sample thickness and penetrant pressure. A remarkable catalytic effect of the silica phase on the reaction of ammonia with the polyimide was observed.  相似文献   

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