Thermal diffusion and molecular weight calibration of poly(ethylene-co-vinyl acetate)s by thermal field-flow fractionation

Copolymer characterization is accomplished with respect to measurement of thermal diffusion coefficient (D_T) and molecular weight determination by thermal field-flow fractionation. The examined copolymers are the eight poly(ethylene-co-vinyl acetate)s [P(E-V)] having different compositions of vinyl acetate ranging from 25 to 70% and the molecular weight from 110,000 to 285,000, and three polyvinyl acetate standards as component homopolymer. The carrier solvents are tetrahydrofuran, toluene, and chlorobenzene which have different viscosities and thermal conductivities. Measured D_T values vary from 1.36 × 10^?8 to 5.97 × 10_?8 cm²/(s . K) which are dependent on the composition of copolymers and types of carriers. These values increase linearly with the increase of weight percent of vinyl acetate. It is possible to estimate D_T values of polyethylene from the extrapolated intercept in the plots of D_T vs. vinyl acetate wt % of copolymer. Tetrahydrofuran is found to be the appropriate carrier solvent for the separation of P(E-V) copolymers since D_T varies greatly with the increase of wt % in THF. Attempts are made to correlate the measured retention data with molecular sizes of copolymers for the construction of the molecular weight calibration curve. Good correlations (r² ≥ 0.931) are found in which D/D_T values of polymers vary inversely with the product of hydrodynamic volume by weight ratio of vinyl acetate. Based on this relationship, the unknown molecular weight of copolymer sample can be determined from component homopolymers for which standards are readily available. © 1995 John Wiley & Sons, Inc.     

Small-angle neutron scattering studies of dynamics and hierarchical pattern formation in binary mixtures of polymers and small molecules

We present the dynamics of the composition fluctuations and pattern formation of two-component systems in both single-phase and two-phase states as studied by time-resolved small-angle neutron scattering and light scattering. Two-component systems to be covered here include not only dynamically symmetric systems, in which each component has nearly identical self-diffusion coefficients, but also dynamically asymmetric systems, in which each component has different self-diffusion coefficients. We compare the dynamic behaviors of the two systems and illuminate their important differences. The scattering studies presented for dynamically asymmetric systems highlight that stress–diffusion coupling and viscoelastic effects strongly affect the dynamics and pattern formation. For dynamically symmetric systems, we examine the universality existing in both polymer systems and small-molecule systems as well as new features concerning the time evolution of hierarchical structures during phase separation via spinodal decomposition over a wide range of wave numbers (up to four orders of magnitude). For both systems, we emphasize that polymers provide good model systems for studying the dynamics and pattern formation. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3027–3062, 2004     

High temperature thermal field-flow fractionation of polyethylene and polystyrene

In this paper the high-temperature thermal field flow fractionation method is exploited for the analysis of polyethylene (PE). The experimental apparatus set-up, obtained by simply modifying a commercial instrument, is presented. The numerical procedure for deriving retention calibration plot versus molecular weight is discussed with reference to the specific polymer-solvent pair, PE-o-dichlorobenzene (ODCB), here employed. Different methods for computing the physicochemical data set of the solvent, necessary for calibration, are compared. The selectivity of the checked PE-ODCB system proves comparable with respect to the values currently found in thermal field-flow fractionation (ThFFF) analysis. Differences are found between PE and polystyrene (PS) analysis in the same solvent. The conditions for high temperature ThFFF operation in PE analysis and their advantages are discussed with respect to the standard SEC technique for PE, PS, and PE-PS copolymer analysis. Molecular weight distributions obtained by ThFFF of two PE commercial samples agree with those obtained by SEC. © 1995 John Wiley & Sons, Inc.     

Nonlinear viscoelastic diffusion in concentrated polystyrene/ethylbenzene solutions

Nonlinear gradient-driven diffusion was studied in concentrated polystyrene (PS)/ethylbenzene (EB) solutions using vapor sorptions with a finite driving force. The nonlinear sorptions were carried out on thin films (≅2.05, 3.50 μ thick) at conditions where non-Fickian, “viscoelastic” effects appear. These data were modeled with the nonlinear diffusion equation studied by Tang. Four dimensionless material parameters in the model were determined from a limited amount of linear-response, differential sorption data on PS/EB mixtures measured in the same range of experimental conditions as for the nonlinear sorptions. The nonlinear model successfully predicts the observed nonlinear response either above or below the glass transition (T_g). In order to simultaneously capture the nonlinear response both above and below T_g, the abrupt change in the concentration dependence of physical properties at T_g must be accounted for. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2103–2119, 1997     

Dynamics of end-grafted polystyrene brushes in theta solvents

The dynamics of the concentration fluctuations in end-grafted polystyrene brushes in a theta solvent (cyclohexane) are probed by evanescent wave dynamic light scattering at different wavevectors q and temperatures. When the solvent quality changes from marginal to poor, the relaxation function C(q, t) exhibits strong effects as compared with the smooth variation of the brush density profile. From a single exponential above 50 °C, C(q, t) becomes a two-step decay function. The fast decay is still assigned to the cooperative diffusion albeit slower than in the good solvent regime whereas the slow nonexponential and nondiffusive process might relate to microsegragated and/or chain dynamics in the present polydisperse brush. The relaxation function of the present three brushes with different grafting density reveals similarities and disparities between wet brushes and semidilute polymer solutions. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3590–3597, 2006     

A thermodynamic approximation for the estimation of velocity correlation coefficients in binary liquid mixtures

An equation is derived using the relations of equilibrium thermodynamics to estimate limiting distinct diffusion coefficients, (or velocity cross-correlation coefficients) in binary non-electrolyte mixtures. It is tested against 13 mixtures. Good agreement is obtained with distinct diffusion coefficients derived experimentally in the case of near deal systems. For non-ideal mixtures the calculated values appear to follow the experimental coefficients to some extent. For this reason, the thermodynamic approximation is not as suitable for a reference equation as a structural approximation previously derived.     

Sorption and diffusion of carbon dioxide in wood‐fiber/polystyrene composites

In this study, sorption and diffusion of carbon dioxide (CO₂) in wood‐fiber/polystyrene composites were investigated. The effects of gas pressure and fiber content on the solubility and diffusion coefficients were evaluated. A statistical analysis indicated that pressure is more important than fiber content in determining the solubility and diffusivity of CO₂. An increase in saturation pressure causes an increase in the solubility and diffusion coefficients, whereas inclusion of the fibers decreases both of these properties. Models were developed to predict the uptake and diffusion coefficients of CO₂ in the composite samples as functions of pressure and fiber content. A theoretical model based on Henry's law and the Langmuir equation compared favorably to the experimental data for CO₂ solubility. This dual mode model also described both the transient sorption and desorption data, but only if the concentration‐dependent value of diffusivity was treated as a history‐dependent parameter. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 723–735, 2002     

Solubility of benzil in binary alkane + dibutyl ether solvent mixtures. Comparison of predictive expressions derived from the nearly ideal binary solvent model

Experimental solubilities are reported for benzil dissolved in six binary mixtures containing dibutyl ether with hexane, heptane, octane, cyclohexane, methylcyclohexane, and 2,2,4-trimethylpentane at 25°C. Results of these measurements are compared to the predictions of equations developed previously for solubility in systems of nonspecific interactions. The most successful equation in terms of goodness of fit involved a volume fraction average of the excess Gibbs energies relative to the Flory-Huggins model, and predicted the experimental solubilities in the six systems studied to within an overall average absolute deviation of 3.4% and with a maximum deviation of 6.0%.     

Composition and molecular weight analysis of styrene-acrylic copolymers using thermal field-flow fractionation

Thermal field-flow fractionation coupled with online multiangle light scattering, differential refractive index and quasielastic light scattering (ThFFF-MALS/dRI/QELS) was used to simultaneously determine the molecular weight (MW) and composition of polystyrene-poly(n-butyl acrylate) (PS-PBA) and polystyrene-poly(methyl acrylate) (PS-PMA) copolymers. The online measurement of the normal diffusion coefficient (D) by QELS allowed calculation of the copolymer thermal diffusion coefficient (D(T)) of sample components as they eluted from the ThFFF channel. DT was found to be independent of MW for copolymers with similar compositions and dependent on composition for copolymers with similar MW in a non-selective solvent. By using a solvent that is non-selective to both blocks of the copolymer, it was possible to establish a universal calibration plot of DT versus mole fraction of one of the monomer chemistries comprising the copolymer. PS-PBA and PS-PMA linear diblock polymers were determined to vary in composition from 100/0 to 20/80 wt% PS/acrylate and ranged in MWs between 30 and 360 kDa. The analysis of a PS-PBA miktoarm star copolymer revealed a polydisperse material with a weight percent PBA of 50-75% and MW ranging from 100 to 900 kDa. The presented ThFFF-MALS/dRI/QELS method allowed rapid characterization of polymers with MW and chemical distributions in a single analysis.     

Osmotic compressibility in dilute polystyrene‐cyclohexane and polystyrene‐methylcyclohexane solutions: Correlation of polystyrene virial coefficients with solvent quality

Osmotic compressibilities were determined as a function of temperature for solutions of polystyrene in cyclohexane and methylcyclohexane by static light scattering, the measurements extending well below the theta temperature, T = θ. Virial coefficients extracted from the data are compared with literature values obtained by light scattering, membrane osmometry, and phase equilibrium measurements, and successfully correlated over wide ranges of molecular weight, temperature, and solvent quality. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 184–196, 2001     

Rapid detection of compositional drift of polydisperse copolymers using thermal field-flow fractionation and multi-angle light scattering

Summary The use of thermal field-flow fractionation (ThFFF) with multi-angle light scattering (MALS) for the rapid detection of compositional heterogeneity in random copolymers is demonstrated. Soret coefficients were directly calculated from the ThFFF retention times while the MALS detector provided the polymer's radius of gyration (R _g) distribution. FromR _g, the diffusion coefficient (D) could be calculated and this allowed, in combination with the Soret coefficient, the calculation of the thermal diffusion coefficient (D _T). It was shown that theD _T distribution can serve as a measure for the chemical composition distribution of random styrene acrylonitrile copolymers. Comparison of ThFFF-MALS results with literature data from ThFFF-hydrodynamic chromatography (HDC) cross-fractionation experiments showed a fair agreement.     

Extraction and separation of lanthanides from aqueous chloride and nitrate media using mixtures of binary extractants based on secondary and tertiary amines

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Phase behaviour of star-shaped binary mixtures of triphenylbenzenes,triphenylboroxines and triphenyltriazines

ABSTRACT

Star-shaped nonaalkoxy-1,3,5-triphenyltriazines were prepared and their mesomorphism was investigated by differential scanning calorimetry (DSC), polarised optical microscopy (POM) and X-ray diffraction (WAXS and SAXS). Compounds with chain lengths C₉ – C₁₂ displayed enantiotropic hexagonal columnar mesophases over a wide range of temperature. Their similarity in molecular size and mesomorphism prompted us to study their binary mixtures with previously reported planar 1,3,5-triphenylboroxines and propeller-shaped 1,3,5-triphenylbenzenes. All three compounds are fully miscible in their hexagonal columnar mesophases, but mixtures of planar with propeller-shaped compounds generated columnar stacks that contain only one type of molecule to avoid their shape incompatibility. However, the two types of columnar stacks were fully miscible which circumvented macroscopic phase separation.     

Electrochemical study of 18-crown-6-Tl+ complexes in binary solvent mixtures

Summary The formation constants,K _S, of the 18-crown-6 complex with thallium(I) ion were studied by polarographic measurements in binary mixtures of acetonitrile, acetone, tetrahydrofuran, and dimethylsulfoxide with water, as a function of the solvent mole fraction. In all the cases, the variation of the stability constant can be described by the empirical relation logK _S=a[(–1)/(2+1)]+b where stands for relative permittivity of a given mixture anda andb mark the regression coefficients. The values ofa calculated for four series of binary mixtures showed correlation with the Gutmann donor numbers of the neat organic solvents which form the mixture.On leave from the Department of Chemistry, Jingzhou Teacher's College, Jingzhou, Hubei, China     

Studies of particle-particle interactions using polystyrene latices and time-average light scattering

Dilute dispersions of monodisperse negatively-charged polystyrene latex particles, radii 161 Å, have been examined by time-average light scattering at various latex volume fractions and electrolyte concentrations. The latter were varied from the low value produced by maintaining mixed bed ion-exchange resin beads in the systems (ca. 10^–5 mol dm^–3) to the value of 5×10^–3 mol dm^–3 obtained by the addition of sodium chloride. From angular scattering measurements determinations of the structure factors were made; these were produced as a consequence of the particle-particle interactions in the system. By extrapolation of the structure factor to zero scattering angle, values were obtained for the osmotic compressibility and hence the osmotic pressure of the systems as a function of the latex volume fraction. It was found that the experimental data obtained could be interpreted in terms of a hard-sphere model for the particle-particle interaction. Good agreement was obtained provided that the particles were assigned a hard-sphere radius which was determined by the electrostatic repulsion between the particles.     

Kinetic modeling of the thermal degradation of polyethylene and polystyrene mixtures

One possible process for recovering valuable chemical and petrochemical products from plastic waste is the stepwise thermal degradation of polymer mixtures. This potentially allows the step by step simultaneous separation of the different product fractions generated by the polymers of the blend. The aim of this paper is to investigate the effect of the mixing scale of the polymers and their interactions in the melt. Several thermogravimetric analyses were performed on small samples of polyethylene (PE) and polystyrene (PS) mixtures. Two types of operating conditions were adopted: the first one is a dynamic analysis with a linear increase of the temperature over time, the latter consists of two sequential isothermal steps. The experimental results confirm that if the mixing scale is poor, the decomposition of each polymer behaves independently of the presence of the other one. Conversely, when the mixing of the two polymers reaches the molecular scale, a co-pyrolysis takes place with partial interactions. A two phase system is assumed: one phase characterized by a larger PS fraction, the other one by a prevailing PE amount. In order to properly predict the kinetic interactions typical of the mixed phases, it was necessary to extend the detailed kinetic model already developed and validated for the single polymers. The resulting two phase model gives a satisfactory explanation of several experimental data from the thermal degradation of PE–PS mixtures.     

X-ray structural studies on microscopic mixing in binary mixtures of dimethyl sulphoxide with acetonitrile and N,N-dimethylformamide

The liquid structures of binary acetonitrile (AN)–dimethyl sulphoxide (DMSO) and N,N-dimethylformamide (DMF)–DMSO mixtures were investigated by the X-ray scattering method. Comparison of the X-ray scattering data of AN–DMSO liquid mixtures with those of neat AN and DMSO revealed that the intermolecular AN–DMSO interactions are practically not detected; that is, the X-ray scattering data of the liquid mixtures are well reproduced by summing up those of neat AN and DMSO weighted by their mole fractions. The same applies for DMF–DMSO mixtures. Thus, each component solvent molecule independently forms self-assembled clusters in the liquid mixtures, the structures of which are the same as those in the neat liquids. The clusters are mixed to form macroscopically homogeneous liquid mixtures. The thermodynamic quantities on mixing process for the AN–DMSO, DMF–DMSO and AN–DMF systems in the literature are well elucidated on the basis of the microscopic structure of the liquid mixtures.     

Study of the dissolution process of polystyrene in concentrated cyclohexane solution by MNR relaxation

 ¹³C-NMR relaxation times of polystyrene (PS) chains in its theta solvent, cyclohexane, have been measured at different temperatures. It was found that relaxation of carbon nuclei of the side-chain-phenyl groups and those of main chains have remarkably different temperature-dependent relaxation behaviors in the solvent. A two-step model for the dissolution process is proposed. According to the model, swelling of the polymer below θ temperature corresponds mainly to the gradual dispersion of the side-chain phenyl groups; while the complete dissolution above θ temperature corresponds mainly to the gradual dispersion of the main chains at a molecular level. These dispersions reflect the fact that cohesional interaction among side-chain-phenyl rings or main chains are weakened by solvent molecules, which shows the existence of the cohesional entanglements among polymer chains. The results of T ₁(C) are confirmed by the biexponential dependence of ¹H-NMR spin–spin relaxation on temperature. Received: 2 July 1997 Accepted: 21 October 1997     

Enthalpies of mixing of binary and ternary mixtures containing diisopropylether,benzene and cyclohexane at 303.15 K

Enthalpies of mixing H have been measured for liquid binary mixtures of diisopropylether (DIPE)+benzene or cyclohexane and for liquid ternary mixtures diisopropylether+benzene+cyclohexane at 303.15 K and constant pressure using a C80 calorimeter. A Redlich-Kister type equation was used to correlate experimental results.