共聚高分子混合物的分子热力学模型

以硬球链流体的分子热力学模型为基础 ,引入方阱位能相互作用的贡献 ,建立了共聚高分子混合物的分子热力学模型 .模型中具有物理意义的链节参数 (链节数、链节直径和链节间的相互作用能 )由纯物质的pVT关系拟合得到 ,而用来校正交叉作用能和交叉碰撞直径的可调参数需由液液平衡的实验数据回归得到 .采用了相对简单的处理方法来确定这些可调参数 .对所选共聚高分子混合物的共存曲线、互溶窗和互溶图等相行为的关联结果令人满意 .     

Hydrophilic poly(ethylene oxide)–aramide segmented block copolymers

The present paper discusses block copolymers with segments of either poly(ethylene oxide), poly(propylene oxide), or mixtures of poly(ethylene oxide)/poly(propylene oxide) and monodisperse aramide segments. The length of the polyether segments as well as the concentration of polyethylene oxide was varied. The synthesized copolymers were analyzed by DSC, FTIR, AFM and DMTA. In addition, the hydrophilicity was studied.The crystallinity of the monodisperse aramide segments was found to be high and the crystals, dispersed in the polyether phase, displayed a nano-ribbon morphology. The PEO segments were able to crystallize and this crystalline phase reduced the low-temperature flexibility. The PEO crystallinity and melting temperature could be strongly reduced by copolymerization with PPO segments. By using mixtures of PEO and PPO segments, hydrophilic copolymers with decent low-temperature properties could be obtained.     

Modeling the phase behavior of H2S+n-alkane binary mixtures using the SAFT-VR+D approach

A statistical associating fluid theory for potential of variable range has been recently developed to model dipolar fluids (SAFT-VR+D) [Zhao and McCabe, J. Chem. Phys. 2006, 125, 104504]. The SAFT-VR+D equation explicitly accounts for dipolar interactions and their effect on the thermodynamics and structure of a fluid by using the generalized mean spherical approximation (GMSA) to describe a reference fluid of dipolar square-well segments. In this work, we apply the SAFT-VR+D approach to real mixtures of dipolar fluids. In particular, we examine the high-pressure phase diagram of hydrogen sulfide+n-alkane binary mixtures. Hydrogen sulfide is modeled as an associating spherical molecule with four off-center sites to mimic hydrogen bonding and an embedded dipole moment (micro) to describe the polarity of H2S. The n-alkane molecules are modeled as spherical segments tangentially bonded together to form chains of length m, as in the original SAFT-VR approach. By using simple Lorentz-Berthelot combining rules, the theoretical predictions from the SAFT-VR+D equation are found to be in excellent overall agreement with experimental data. In particular, the theory is able to accurately describe the different types of phase behavior observed for these mixtures as the molecular weight of the alkane is varied: type III phase behavior, according to the scheme of classification by Scott and Konynenburg, for the H2S+methane system, type IIA (with the presence of azeotropy) for the H2S+ethane and+propane mixtures; and type I phase behavior for mixtures of H2S and longer n-alkanes up to n-decane. The theory is also able to predict in a qualitative manner the solubility of hydrogen sulfide in heavy n-alkanes.     

Liquid–liquid equilibria of copolymer mixtures based on an equation of state

Liquid–liquid equilibria of copolymer mixtures were studied by an equation of state (EoS) for chain-like fluids. The equation consists of a reference term for hetero-nuclear hard-sphere chain fluids developed by Hu et al. where the next-to-nearest-neighbor correlations have been taken into account; and a perturbation term from Alder et al.’s square-well attractive potential. The segment parameters, including number of segments, segment diameter and interaction energy between segments, are obtained by fitting pVT data of pure homopolymer. For the case of different species in the same copolymer, the interaction parameters for unlike segment pairs are obtained by fitting pVT data of pure copolymer. For the interaction between segment of homopolymer and different species in copolymer, the parameters are treated as adjustable by fitting liquid–liquid equilibria data. In the latter case, the difference between different species in a copolymer is simply neglected as an approximation. Therefore, in general, only one pair of adjustable interaction parameter is determined from LLE data. To model miscibility maps of copolymer mixtures having two or three kinds of species, the interaction parameters are obtained from the boundary between miscible and immiscible regions. The EoS used in this work can correlate phase behavior including coexistence curves, miscibility windows and miscibility maps.     

Phase behaviour of fluorocarbon and hydrocarbon mixtures: interpretation of type II and type III behaviour in terms of a ‘hard-sphere + attractive force’ equation of state

Christou, G., Morrow, T., Sadus, R.J. and Young, C.L., 1986. Phase behaviour of fluoro-carbon and hydrocarbon mixtures: interpretation of type II and type III behaviour in terms of a ‘hard-sphere + attractive force’ equation of state. Fluid Phase Equilibria, 25: 263–272.Experimental measurements of the upper critical solution temperature of octadecafluorooctane + n-alkanes are reported. These measurements and those discussed by Schneider (1983) on the phase behaviour of tetrafluoromethane + alkanes are compared with results obtained from the one-fluid model using a ‘hard sphere + attractive term’ equation of state.Interaction parameters, ξ, obtained from the comparison together with values from the literature on other fluorocarbon + hydrocarbon mixtures show trends which are briefly discussed.     

Monte carlo simulations of polymer surfaces in a cubic lattice, 1 development of a simulation algorithm and first results

A method is presented which allows to simulate surface properties of a model polymer system (in addition to bulk properties). The decisive parameter which controls the equilibrium concentration of empty sites within the bulk phase as well as the surface properties is the attractive energy operating between nonbonded neighbouring polymer segments. In effect, by varying this parameter between -0,5 and -1,5 the volume fraction of vacancies in the bulk phase changed from 0,183 to 0,003. All the parameters defined in order to characterize the “roughness” of the surface indicate that the surface gets smoother and smoother with increasing attractive segment-segment interaction and, accordingly, also with decreasing bulk equilibrium concentration of vacancies.     

Adsorption of Chain Molecules

We analyze the influence of chain length on the adsorption isotherm using the framework of lattice theory. Each molecule is represented as a chain of segments occupying separate sites in the lattice. Adsorption equilibria (particularly adsorption isotherms) are analyzed for one-component and two-component mixtures of chain molecules. Copyright 1999 Academic Press.     

Modeling phase equilibria for acid gas mixtures using the CPA equation of state. Part II: Binary mixtures with CO2

In Part I of this series of articles, the study of H₂S mixtures has been presented with CPA. In this study the phase behavior of CO₂ containing mixtures is modeled. Binary mixtures with water, alcohols, glycols and hydrocarbons are investigated. Both phase equilibria (vapor-liquid and liquid-liquid) and densities are considered for the mixtures involved. Different approaches for modeling pure CO₂ and mixtures are compared. CO₂ is modeled as non self-associating fluid, or as self-associating component having two, three and four association sites. Moreover, when mixtures of CO₂ with polar compounds (water, alcohols and glycols) are considered, the importance of cross-association is investigated. The cross-association is accounted for either via combining rules or using a cross-solvation energy obtained from experimental spectroscopic or calorimetric data or from ab initio calculations. In both cases two adjustable parameters are used when solvation is explicitly accounted for. The performance of CPA using the various modeling approaches for CO₂ and its interactions is presented and discussed, comparatively to various recent published investigations. It is shown that overall very good correlation is obtained for binary mixtures of CO₂ and water or alcohols when the solvation between CO₂ and the polar compound is explicitly accounted for, whereas the model is less satisfactory when CO₂ is treated as self-associating compound.     

A new method of semigrand canonical ensemble to calculate first-order phase transitions for binary mixtures

First-order phase transitions of binary mixtures at the given pressure (P) and temperature (T) are studied by taking into account the composition fluctuations. Isothermal-isobaric semigrand canonical ensemble is adopted to find the relations among the total number of molecules, the composition fluctuations and Gibbs free energy density. By combining two identical subsystems of mixtures successively, the free energy density is transformed until being stable and its linear segments represent phase transitions. A new method is developed to calculate the phase equilibriums of binary mixtures. The method handles multiple types and number of phase equilibriums at single time and its solutions are physically justified. One example is shown for calculating the phase diagram of binary Lennard-Jones mixture. It demonstrates that the fluctuations of the total number of molecules in mixtures are fundamental behind phase transitions and the van der Waals loops in Gibbs free energy are reasonable.     

A new three-parameter cubic equation of state for calculation physical properties and vapor–liquid equilibria

A new three-parameter cubic equation of state is presented by combination of a modified attractive term and van der Waals repulsive expression. Also a new alpha function for the attractive parameter of the new EOS is proposed. The new coefficients of alpha function and the other parameters of the attractive term are adjusted using the data of the saturated vapor pressure and liquid density of almost 60 pure compounds including heavy hydrocarbons. The new EOS is adopted for prediction of the various thermophysical properties of pure compounds such as saturated and supercritical volume, enthalpy of vaporization, compressibility factor, heat capacity and sound velocity. Following successful application of the new EOS for the pure components, using vdW one-fluid mixing rules, the new EOSs are applied to prediction of the bubble pressure and vapor mole fraction of the several binary and ternary mixtures. The accuracy of the new EOS for phase equilibrium calculation is demonstrated by comparison of the results of the present EOSs with the PT, PR, GPR and SRK cubic EOSs.     

Equation of state for square-well chain molecules with variable range II. Extension to mixtures

An equation of state (EOS) developed in our previous work for square-well chain molecules with variable range is further extended to the mixtures of non-associating fluids. The volumetric properties of binary mixtures for small molecules as well as polymer blends can well be predicted without using adjustable parameter. With one temperature-independent binary interaction parameter, satisfactory correlations for experimental vapor–liquid equilibria (VLE) data of binary normal fluid mixtures at low and elevated pressures are obtained. In addition, VLE of n-alkane mixtures and nitrogen + n-alkane mixtures at high pressures are well predicted using this EOS. The phase behavior calculations on polymer mixture solutions are also investigated using one-fluid mixing rule. The equilibrium pressure and solubility of gas in polymer are evaluated with a single adjustable parameter and good results are obtained. The calculated results for gas + polymer systems are compared with those from other equations of state.     

Mass spectrometry of DNA mixtures by laser ablation from frozen aqueous solution.

We report time-of-flight mass spectra of test mixtures of six single-stranded DNA segments. The segments range in size from 8 to 60 nucleotides (molecular weight range 2413 to 18,602 Da). The best mass spectra were obtained by pulsed laser ablation of thin frozen films of an aqueous solution of the mixture from an oxidized copper substrate. These mass spectra are dominated by the molecular-ion peak for each DNA segment, and show little evidence of fragmentation, peak broadening or cluster formation. In contrast, mass spectra obtained using UV laser ablation from an anthranilic acid matrix yield broad peaks with evidence of fragmentation, and DNA segments longer than 26 nucleotides are difficult to detect.     

Influence of gel-phase microdomains and lipid rafts in lipid monolayers on the electron transfer of a lipophilic redox probe: dioctadecylviologen

Gel-phase microdomains and lipid rafts form spontaneously in monolayers of lipid mixtures of dioleoylphosphatidylcholine (DOPC), palmitoylsphingomyelin (PSM) and cholesterol (Chol), self-assembled on mercury. The influence of microdomains on the electron transfer properties of 2 mol% dioctadecylviologen (DODV), incorporated in these lipid monolayers, was investigated by cyclic voltammetry. In pure DOPC, the DODV molecules tend to aggregate, giving rise to strong attractive lateral interactions. With an increase in the PSM mole fraction in DOPC/PSM binary mixtures, the edges of the resulting gel-phase microdomains act as docking sites for the DODV molecules, decreasing lateral interactions and modifying the DODV redox properties. A similar behavior is shown by lipid rafts formed by adding Chol to the above binary mixtures. By varying the DOPC/PSM molar ratio, the midpoint between the peak potentials of the DODV reduction and oxidation peaks shifts in parallel with the surface dipole potential of the lipid mixture. This behavior indicates that the formal (half-reduction) potential of a redox pair, as measured versus a given reference electrode, may include a surface dipole potential if one or both members of the redox pair are embedded in a medium different from the bulk phase containing the reference electrode.     

Prevention of protein adsorption by flexible and rigid chain molecules

The ability of tethered polymer layers to reduce the non-specific adsorption of proteins is studied using a molecular theory. The protein adsorption isotherms are calculated for flexible and rigid molecules as well as for mixtures. It is found, in agreement with earlier predictions, that flexible polymers are more effective in preventing protein adsorption. The interactions of the polymer with the surface are shown to be very important in determining the ability of the polymer layer to reduce the adsorption of proteins. Further, it is found that one can tune the adsorption of a certain protein conformation by changing the interactions between the surface and the polymer segments or the composition in the case of mixtures. It is found that the optimal layers to obtain large reduction of protein adsorption and availability of functional groups for binding are obtained by using mixtures of flexible and rod-like molecules. The role of the polymer-surface interactions is shown to be different for the kinetic control of protein adsorption as compared to thermodynamic control. The application of the findings as guidelines for the molecular design of biocompatible materials is discussed.     

Mobile phase effects on retention on a new butylimidazolium-based high-performance liquid chromatographic stationary phase

A new HPLC stationary phase based on n-butylimidazolium bromide has been characterized by a linear solvation energy relationship (LSER) approach in the binary acetonitrile/water mobile phases. The retention properties of the stationary phase were systematically evaluated in terms of intermolecular interactions between 28 test solutes and the stationary phase. The results and further comparisons with conventional reversed phase system confirm that retention properties are similar to phenyl phases in acetonitrile/water mixtures. The results obtained with acetonitrile/water mixtures are also compared with results obtained using methanol/water mixtures.     

Adsorption of oligomers on the polymer-tethered surfaces

A density functional study of adsorption of oligomers on weakly attractive surfaces modified with grafted chains is presented. The chain molecules are modeled as freely jointed tangent spheres. The segments interact via Lennard-Jones (12-6) potentials. Two types of substrates are considered - a neutral surface and the Lennard-Jones (9,3) surface. The mechanism of adsorption is discussed. Depending on the system characteristics, the primary, secondary and ternary adsorption is found. The effects of selected factors on adsorption and separation of mixtures are analyzed.     

A study of the phase behavior of a simple model of chiral molecules and enantiomeric mixtures

We present a study of the solid-fluid and solid-solid phase equilibrium for molecular models representative of chiral molecules and enantiomeric mixtures. The models consist of four hard sphere interaction sites of different diameters in a tetrahedral arrangement with the fifth hard sphere interaction site at the center of the tetrahedron. The volumetric properties and free energies of the pure enantiomers and binary mixtures were calculated in both fluid and solid phases using isobaric Monte Carlo simulations. The models exhibit essentially ideal solution behavior in the fluid phase with little chiral discrimination. In the solid phase the effects of chirality are much greater. Solid-fluid phase behavior involving the pure enantiomer solids and also racemic compounds was calculated. The calculations indicate that, depending on the relative sizes of the hard sphere interaction sites, packing effects alone can be sufficient to stabilize a racemic compound with respect to the pure enantiomer solids.