Sorption and diffusion measurements in ternary polymer-solvent-solvent systems by means of a magnetic suspension balance—Experimental methods and correlations with a modified Flory-Huggins and free-volume theory

Diffusion and sorption data of methanol and toluene in the ternary system methanol-toluene-poly(vinylacetate) (pvac) were measured by means of a magnetic suspension balance (MSB). Ternary diffusion coefficients were determined from sorption kinetics with a method of Crank [1975. The Mathematics of Diffusion, second ed. Claredon Press, Oxford] classically used for binary polymer solvent systems. Assumptions and limitations for this method are discussed. Sorption data of methanol and toluene in the ternary system were correlated with a modified Flory-Huggins theory taking four instead of three interaction parameters into account and compared with predictions of UNIFAC-FV. All four binary Flory-Huggins interaction parameters were correlated as a function of concentration, determined from binary polymer-solvent sorption measurements and from solvent-solvent equilibrium data. The modified Flory-Huggins approach for ternary systems predicts the two limiting cases for the binary polymer-solvent as well the two solvent-solvent equilibria. Diffusion coefficients determined from sorption kinetics measurements were correlated with free-volume theory predictions of Vrentas et al. [1984. Self diffusion in polymer-solvent-solvent systems. Journal of Polymer Science, Polymer Physics Edition, 22, 459-469] and compared with a few available literature data from Surana et al. [1998. Diffusion and equilibrium measurements in ternary polymer-solvent-solvent systems using inverse gas chromatography. Industrial and Engineering Chemistry Research, 37, 3203-3207]. Cross diffusion terms are not taken into account. Diffusion and sorption data, determined with this gravimetric method, were used in model simulations with modified Flory-Huggins and free-volume theory to predict drying curves of ternary methanol-toluene-pvac solutions. The results were compared with previously published experimental results performed by means of Inverse-Micro-Raman-Spectroscopy (IMRS) from Schabel [2004. Trocknung von Polymerfilmen - Messung von Konzentrationsprofilen mit der Inversen-Mikro-Raman-Spektroskopie. Ph.D. Thesis, Universität Karlsruhe (TH), Germany].     

Liquid Membrane Separations of Metal Cations Using Macrocyclic Carriers

Abstract

The selectivity in water and methanol solvents of macrocyclic crown ether ligands toward univalent and bivalent cations is well known. Incorporation of these ligands into chloroform liquid membranes separating water and salt solution phases results in a system showing selective cation transport. The cation transport rates of single cations across these liquid membranes have been correlated with equilibrium constant values for cation-macrocycle interaction in methanol. This correlation has been extended to binary cation mixtures of Cs⁺ with Li⁺, Na⁺, K⁺, and Rb⁺. A model for cation transport from these cation mixtures has been reduced to an equation which gives good agreement between measured and predicted transport rates across our liquid membranes.     

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.     

聚乙烯/碳黑复合导电材料的正电子谱学研究

在高密度聚乙烯（HDPE）中掺入碳黑（CB）可以显著提高其导电性能,而且碳黑含量的变化对导电性有很在影响。本文用正电子湮没技术研究了这种复合体系中自由体积随碳黑含量的变化规律,证实了碳黑颗粒处于非晶区的观点,并结合电阻率测量和差热分析的实验结果探讨了渗流阈值以及结晶度等问题。     

EFFECT OF POLYMERIZATION DEGREE ON MICROSTRUCTURE OF POLYETHYLENE GLYCOL STUDIED BY POSITRON ANNIHILATION

Positron lifetimes in polyethylene glycol (PEG) polymer have been measured as a function of polymerization degree N (N = 25 - 500). It is observed that all the positron parameters are strongly dependent on N when N≤250, and then tend to constant levels when N≥250. The variations of the longest-lived component (I3,τ3) show that the size of free- volume holes has a minimum at N - 100 and the hole concentration decreases with N. These trends may be due to the variations in crystallinity, the chain ends, entanglements and the Van der Waals interaction between segments. The variations of the second component (I2,τ2) with N reflect the changes of defect properties in crystalline regions of PEG.     

甲基两烯酸甲酯-甲基丙烯酸共聚物/梯形聚苯基硅倍半氧烷原位共混物的正电子寿命谱研究

用正电子湮没技术研究了甲基丙烯酸甲酯－甲基丙烯酸共聚物（Ｐ（ＭＭＡ－ＭＡＡ））／梯形聚苯基硅倍半氧烷（ＰＰＳＱ）原位共混体系的自由体积孔穴尺寸及浓度与其组成的关系。具有两种不同配比的该共混物的正电子寿命温度谱表明：该共混物有两种结构转变，即：Ｔｂ、Ｔｇ，因含５％ＰＰＳＱ的共混试样Ｃ５中的自由体积分数较大，其Ｔｐ、Ｔ均小于含１％ＰＰＳＱ的共混试样．Ｃ１。研究了热历史对Ｐ（ＭＭＡ－ＭＡＡ）／梯形ＰＰＳＱ共混物结构的影响。还发现正电子平均寿命与自由体积分数有着类似的随温度变化的规律     

Diffusion of organic solvents in isobutylene-based polymers

Diffusion behavior of several organic solvents in polyisobutylene (PIB) and in poly(p-methylstyrene-co-iso-butylene) (PMS-BR) with different monomer ratios has been studied. The experiments have been conducted over a temperature range of 50 to 100°C using a conventional gravimetric Sorption technique. The PMS-BR copolymers contained 2, 7, and 15 weight percent p-methylstyrene, respectively. Although employing temperatures were far above the glass transition temperatures of polymers, the diffusion coefficients are correlated well with the Vrentas-Duda free-volume theory. For all the solvents, the PIB shows the highest diffusivity while the copolymer with the 15% p-methylstyrene gives the lowest value. This behavior can be explained by the amount of fractional free-volume present in a system.     

Characterization of photodegradation of a polyurethane coating by positron annihilation spectroscopy: correlation with cross-link density

Photodegradation of a polyurethane coating by Xe arc-lamp irradiation as a function of time is studied by positron annihilation spectroscopy. Doppler broadening energy spectra (DBES) of annihilation irradiation is measured as a function of incident positron energy (0–30 keV). A significant decrease in the S-defect parameter from DBES is observed as a function of irradiation time. This is interpreted as a loss of free-volume and holes during the photodegradation process. The cross-link density in the same system is measured by a solvent (methylene chloride) swelling method and is found to increase as a function of irradiation time. A direct correlation between the increase in cross-link density and the decrease in the S-defect parameter from the DBES data is observed. These results are used to discuss the photodegradation process in terms of chemical and physical changes due to UV-irradiation in protective polymeric systems.     

Effect of humidity and temperature on polymer electrolyte membrane (Nafion 117) studied by positron annihilation spectroscopy

Polymer electrolyte fuel cells (PEMFC) have emerged as portable power source for future transportation and home based systems. Over the past years perfluorosulfonated cation exchange membranes have been used as solid polymer electrolyte in fuel cells, however, humidity control and water management have remained major issues in the area of stack development. Local drying that results in the changes in the local conductivity behavior and physical failure of the membrane is one of the key issues to be addressed in detail. It is therefore necessary to estimate these changes precisely to evaluate the membrane behavior during the fuel cell operation. Positron annihilation spectroscopy offers a very sensitive probe to the determination of polymer free-volume as well as electronic state of the polymer. In the present study we studied Nafion 117 with varying temperature and humidity using positron annihilation spectroscopy. The ortho-positronium lifetime (o-Ps, i.e., τ₃) which gives the size of the free-volume holes inside the polymer was determined. It was found that the o-Ps lifetime increases for relative humidity between 40% and 60% and thereafter it decreases in the temperature range of 62-95 °C. When the inversion point at which the o-Ps lifetime is the maximum was plotted with actual vapor pressure, it was observed that it has a logarithmic behavior with the ordinate intercept of 1 ns. This indicates that at higher temperature the membrane has larger free-volume holes; and also at higher temperature a larger vapor pressure is essential to saturate the membrane. Above this point the clusters expand and the free-volume hole size decreases. This also indicates the optimum vapor in the cell to be maintained during the fuel cell operation. The studies also indicated that there is a correlation between o-Ps lifetime and the hydrogen crossover current density under the similar temperature and humidity condition.     

Simulation analysis of drying of ternary polymeric solution coatings

A simulation study on multicomponent polymeric solvent casting process was carried out using known different models and their relative performance of the model assessed in terms of residual parameters and solvent concentration profiles in the coating film. While all the models were found to predict nearly identical concentration profile for highly volatile solvents, significant variation in the profiles was observed in case of the less volatile solvents. Estimated values of the preexponential factors (D₀₁ and D₀₂) exhibited marginal differences implying perhaps a material balance deviation of the predictive models. The effect of air flow rate used for the drying process has been realized through its influence on the skinning behavior, low flow rate being the preferred while for maintaining the appropriate level of skinning.