Phase-distribution chromatography (PDC) of polystyrene

A chromatographic technique is described where the stationary phase is a layer of a very high molecular polystyrene fraction (M = 10⁷) on glass beads (ballotines). The mobile phase is cyclohexane passing the column at a constant temperature below the theta-temperature. A polystyrene sample of sufficiently low molecular weight (M ≤ 10⁶) injected as a small plug at the top of the column is fractionated because the distribution between the mobile and the stationary phase depends on the molecular weight. Since the large molecules preferentially remain in the stationary phase, the smaller molecules leave the column first. The fractionation effect is inverse to that found in GPC experiments. The separation efficiency is rather good and can be described by a simple thermodynamic theory.     

Conformational transitions in ultrahigh-molecular-mass polymers and their manifestation in chromatography on monolithic columns

For a monolithic capillary column based on polydivinylbenzene, a change in the elution profile of polystyrene standards with variation in the eluent-flow rate is studied. It is shown that, for polymers with molecular masses up to 3 × 10⁶, the elution profile does not depend on the flow rate. For higher molecular mass polymers at low flow rates, there is a single almost Gaussian peak that splits into two peaks that move to different sides from the initial peak with an increase in the flow rate. A peak with a smaller retention time (peak I) rapidly attains the limiting elution time, and later on, its retention is independent of the eluent-flow rate. In contrast, the retention time of the other peak (peak II) continuously increases with an increase in the flow rate of the mobile phase, so that, at high flow rates, this peak is retained for a longer time than the low-molecular-mass marker. The intensity of peak I decreases, while the intensity of peak II increases with an increase in the eluent-flow rate; the ratio of their intensities tends toward a certain limiting value (≤1). The observed profile of elution of ultrahigh-molecular-mass polymers assumes the presence of a dynamic equilibrium similar to that existing in the case of first-order reversible reactions. For the assumed equilibrium, the rate constants of direct and back reactions are determined. It is found that these constants are close to the inverse maximum relaxation time of a polymer molecule. The character of the transformations of the polymer molecule in the chromatographic column is discussed.     

Unusual signal enhancement in reversed-phase chromatography of polystyrene monitored by UV detector

Summary Gradient elution of polystyrene standards on reversed phase C18 columns by methanol/tetrahydrofuran or methanol/dichloromethane mixtures yielded a strange effect of the molecular weight of the sample on the specific peak are (mAUs per g sample injected). The effect did not occur when pure dichloromethane was used as an eluent. Further to this, dependence of specific peak area on flow rate was observed in gradient elution with methanol/tetrahydrofuran mixtures. It was found that these effects were due to polymer elution at the verge of precipitation. Depending on the dwell time of the sample in the column, opalescence occurred that added to the UV signal used for monitoring the elution.     

Direction of temperature gradient for normal-phase temperature gradient interaction chromatography in polystyrene fractionation

Temperature gradient interaction chromatography (TGIC) is a powerful technique for molecular weight fractionation of polymers, in which the interaction strength is controlled by varying the column temperature. In the present paper, the effects of the sign of the temperature dependence of the retention and the direction of the temperature gradient (raising or lowering) on TGIC in the normal-phase mode were studied for the molecular weight fractionation of polystyrene samples in organic mobile phases. It was found that a positive temperature gradient was effective in the system consisting of amino-modified silica (NH(2)) column and the eluent mixture of tetrahydrofuran and n-hexane where retention decreased with increasing temperature. A negative temperature gradient was effective for the systems consisting of a bare-silica column//chloroform/n-hexane and NH(2)-column//chloroform/n-hexane, where retention increased with increasing temperature. Increasing retention with increasing temperature has been found, so far, only for a water-soluble polymer (PEO) in an aqueous mobile phase in RP-TGIC.     

Chiral Separation of Nateglinide and its L Enantiomer on a Molecularly Imprinted Polymer-Based Stationary Phase

A new chiral stationary phase for nateglinide (N-(trans-4-isopropylcyclohexylcarbonyl)-D-phenylalanine) based on a molecularly imprinted polymer has been prepared by non-covalent imprinting. For chromatographic analysis the effects on the separation of mobile phase composition, flow rate, and temperature were investigated, and the optimum conditions for HPLC were shown to be: mobile phase, acetonitrile; flow rate, 0.5 mL min^?1; temperature, 25 °C. It was shown that the nateglinide-imprinted polymer was capable of recognizing the enantiomeric difference between nateglinide and its L enantiomer, whereas the non-imprinted polymer had no such ability. Scatchard analysis was used to investigate the binding characteristics of the nateglinide-imprinted stationary phase; this indicated that two classes of binding site were present in the imprinted polymer. The equilibrium dissociation constant (K _D) and the apparent maximum number (Q _max) of high- and low-affinity binding sites were 3.7 × 10^?4 mol L^?1 and 11.38 μmol g^?1, and 1.81 × 10^?3 mol L^?1 and 27.73 μmol g^?1, respectively.     

Void volume determination and experimental probes in reversed phase chromatography

Summary In reversed-phase liquid chromatography with n-alkyl bonded silica, the dead volume (V₀) of the column is theoretically indeterminate owing to adsorption of organic modifier on n-alkyl chains and of water on silanol groups. With binary mobile phases, retention volumes of the mobile phase components and of their deuterated species are relaeed to the adsorption isotherms and V₀ by equations which can be solved with some assumptions on the adsorbed layer composition. Methanol-water and acetonitrile-water systems are studied. As the experimental excess isotherm shows a linear part in the concentration range 50–80% in organic modifier, the hypothesis of an adsorbed layer of constant composition in this range is possible. When increasing the water content of the mobile phase, adsorption of water occurs up to saturation of silanol groups. Then the assumption of a constant water content for a mobile phase having more than 50% of water is applied. With the hypothesis of a constant adsorbed content of organic modifier when the eluent has more than 80% of organic modifier, V₀ and the absolute isotherms are calculated over the entire range of mobile phase composition. Experimental retention behavior of the mobile phase components are totally explained by these V₀ determinations. The retention times of commonly used V₀ markers are compared with V₀ values. It is shown that, when buffering the eluent, no visible effect on the distribution equilibrium is observed, so that injection of concentrated potassium nitrate is a convenient method to measure V₀. With a few solutes with are UV detectable it is possible to measure V₀ whatever the mobile phase composition in methanol-water and acetonitrile-water systems.     

Semi-micro size-exclusion chromatography: Molecular-weight measurement of poly (ethylene terephthalate) and separation of oligomers and prepolymers

Summary Several polystyrene gels of different pore sizes were packed into a 500 mm×2.1 mm I.D. column. Semi-micro size-exclusion chromatography (SEC) using these columns was carried out with a system consisting of a triple piston pump, a micro loop injector and a flow cell with 1.0-μl cell volume constructed for semi-micro HPLC, because the dead volume of the injector and the cell volume of flow cell for conventional HPLC caused a significant loss in column efficiency. The effects of sample amount, injection volume and mobile phase flow rate on column efficiency and retention volume were examined and the optimized operational variables of the sample amount (below 500 μg), the injection volume (less than 15 μl) and the flow rate range (30–70 μl/min) determiend for semi-micro SEC. Oligostyrene, epoxy resin, phenol-formaldehyde resin and phthalates were analyzed by the optimized semi-micro SEC system under the given conditions. In addition, molecular weight distribution of four different poly(ethylene terephthalate) films was successfully measured by using a mixture of chloroform and hexafluoroisopropanol as the eluent.     

Effects of molecular weight distribution in drag reduction and shear degradation

A reduction of frictional drag in turbulent flow was obtained in benzene by using three monodisperse polystyrene samples having weight-average molecular weights of 1.8, 4.1 and 7.1 × 10⁶. By testing these polymers individually and in mixtures, data were obtained for samples with known molecular weight distributions. The drag reduction of these samples was studied as a function of polymer concentration and flow rate so that a generalized picture of the effects of polydispersity could be obtained. These results are used to help explain much of the behavior that was observed for polystyrene and other polymers. This includes the fact that the polystyrene samples exhibit a remarkably high resistance to the loss of drag reduction via degradation in turbulent flow. Such experiments indicate that drag reduction and degradation depend strongly on molecular weight distribution. Thus a molecular level interpretation of experimental results cannot be made unless the effects of the distribution are considered.     

Influence of operation conditions on the microencapsulation of PCMs by means of suspension-like polymerization

Microcapsules containing PRS® paraffin wax (core) and a polystyrene shell were prepared by suspension-like polymerization. The influence of reaction temperature, stirring rate, and mass ratio of paraffin wax to styrene on the properties of phase change materials microcapsules was studied. The reaction temperature had not a significant effect on the size of the microcapsules but an increase of molecular weight and a narrow molecular weight distribution of polystyrene shell were observed when reaction temperature was increased. An exponential relationship between the stirring rate and the mean particle diameter in number has been found. It was observed that paraffin is difficultly encapsulated when the paraffin/polymer mass ratio was higher than 2.00, as a consequence of a shortage of polymer that could not completely cover the amount of paraffin added. However, when a large proportion of monomer was employed, the polymer tended to polymerize inside the droplets during the microencapsulation process forming complex inner structures. The microcapsules obtained have an interesting energy storage capacity of 153.5 J/g that makes them suitable for different applications.     

The effect of 4-vinylcyclohexene on the degradation of polystyrene and on the polymerisation of styrene

It has previously been postulated that the inhibition of polystyrene degradation observed in the initial stages of the isothermal degradation of polystyrene-polybutadiene blends is partly due to reaction of polystyryl radicals with 4-vinylcyclohexene (4VCH) evolved from the polybutadiene. This hypothesis has been tested by examining the effect of small amounts of 4VCH on polystyrene degradation and on the free radical polymerisation of styrene. The results provide support for the hypothesis: there is an induction period in the degradation and the polymerisation is retarded. From molecular weight measurements on polymers made in the presence of various amounts of 4VCH, a chain transfer constant in styrene polymerisation at 60°C of 117 × 10^?5 has been calculated.     

Diffusion-controlled vinyl polymerization. II. Limitations on the gel effect

It is frequently observed that the gel effect in vinyl polymerizations decreases in intensity at about the 50-70% conversion level. This is apparent in both rate and molecular weight data. It is postulated that there should be a limitation on the decrease in the termination rate constant to explain this effect. As an extension of the general theory of chain length dependent termination behavior, a general treatment of the gel effect with a limiting value of the termination rate constant is presented. A specific model is proposed for this limited rate constant which is based on the simple consideration that as translational movement of macroradicals becomes increasingly difficult the contributions made by segmental motion derived solely from the propagation reaction will be the prevailing mechanism. The termination reaction changes from chain length dependent to chain length independent during this transition period.     

Kinetics of formation of charge transfer complexes of poly(vinyl pyridines) with iodine, 7,7′8,8′-tetracyanoquinodimethane and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone by electronic spectroscopic studies

The kinetics of formation of charge transfer complexes of poly(4-vinyl pyridine), poly(2-vinyl pyridine), and poly(2-vinyl pyridine-co-styrene) with iodine, 7,7′,8,8′-tetracyanoquinodimethane and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone has been studied using electronic absorption spectroscopy. The charge transfer complexes of analogous low molecular weight donors, namely 2- and 4-picolines with the same set of acceptors have also been investigated for comparison. The composition and the equilibrium constants of the charge transfer complexes have been obtained. The equilibrium constant of the polymeric complexes is found to be higher than that of analogous complexes of the low molecular weight donors. A new method for determining the rate constants of the association and dissociation of the equilibrium involving the charge transfer complex formation has been proposed. The rate constants vary with the concentration of the acceptor. It indicates that the charge transfer complexes undergo a further reaction and hence the observed rate constants are not true but apparent rate constants. The charge transfer complexes have also been investigated by electron paramagnetic resonance spectroscopy.     

逆流法对凝胶色谱峰加宽效应的研究

在以无孔玻璃珠为填料的色谱柱上研究了流动相中的分离能力和峰加宽效应。得到一定的分离能力。根据Kelley和Billmeyer描述流动相中峰加宽效应的理论,塔板高度和溶液的分子量有关而和分子量分布无关。但实验表明,对于具有相同分子量,但分子量分布不同的样品峰宽不同。苯的逆流峰加宽因子h稍大于直流峰加宽因子h′。产生这个差别的原因可以用在逆转过程中流速场受到干扰来解释。 在以多孔硅胶为填料的柱中对一系列不同分子量、分子量分布和化学结构的样品考察了逆流峰加宽因子,h,和淋出体积,Ve,之间的关系。实验表明h和Ve之间的关系具有普适性,这个结果和Tung的一致。在多孔填料柱上对苯同样得到h>h′。这个结果意味着用逆流法得到的h来校正GPC体系造成的峰加宽稍嫌不够。 只有对于分子量分布很窄的样品,其逆流淋出曲线的形状才非常接近高斯分布。     

Decoupling Study of Melt Polycondensation of Polycarbonate: Experimental and Modeling of Reaction Kinetics and Mass Transfer in Thin Films Polycondensation Process

During the melt polycondensation process of polycarbonate, reaction and mass transfer are deeply coupled owing to relatively high melt viscosity. In this work, the polycondensation reaction kinetics and mass transfer behavior of volatile phenol are decoupling studied in detail by using thin‐film experiments with 250–280 °C, 10–1000 Pa and 0.085–0.68 mm film thickness. A realistic apparent rate model coupled the reaction kinetics with thermodynamic equilibrium and diffusion behavior is developed to describe the polycondensation process, while the diffusion characteristic of small molecule (phenol) is further obtained based on penetration theory. The obtained polycondensation equilibrium constant ranges from 0.3 to 0.55, while the activation energy and pre‐exponential factor of temperature‐dependent diffusion coefficients of phenol are 87.9 kJ mol⁻¹ and 5.08 × 10² m² s⁻¹, respectively. It is also observed that the overall apparent rate of polycarbonate (PC) polycondensation process increases with higher temperature, lower pressure, and thinner film thickness. Coupling the reaction kinetics with mass transfer, the predictions of the realistic apparent rate model are in quite satisfactory agreement with experimental data.     

The separation of diastereoisomers of polystyrene oligomers in reversed phase HPLC

Summary The separation of diastereoisomers from oligomers of low molecular weight polystyrene was achieved using a carbon clad zirconia stationary phase and an acetonitrile mobile phase. The selectivity of the C18-methanol system separated the polystyrene oligomers based on molecular weight while the carbon clad zirconia surface in combination with an acetonitrile mobile phase allowed the expression of the isomeric sample dimensionality. Consequently, full utilisation of the different retention mechanisms on each surface greatly improved the isomeric separation from oligomers of low molecular weight polystyrene.     

Light-scattering studies of phase-demixing of polystyrene/poly(o-chlorostyrene) blends

The phase separation of blends of polystyrene and poly(o-chlorostyrene) has been studied by observing the changing small-angle light-scattering profile with time. As a blend is heated to a temperature at which it undergoes phase separation, a light-scattering maximum is observed which grows in intensity and moves to smaller angles with time. This maximum is associated with a characteristic spacing which increases at a rate which becomes greater at higher temperature or with lower molecular weight. This spacing varies with a power of time as might be expected for domain growth occurring by a viscous flow mechanism. The integrated scattering intensity (invariant) is found to increase initially with time and then remain constant, as is characteristic of phase separation followed by phase ripening.     

HPLC of synthetic polymers characterization of polystyrenes by high performance precipitation liquid chromatography (HPPLC)

Summary The advantages and disadvantages of high performance precipitation liquid chromatography have been demonstrated for polystyrene homopolymers. Depending on the mobile phase composition at the dissolution point of the polymeric sample and surface properties of the stationary phase, elution is governed either by a solution process or by adsorption. A contribution by adsorption was noticed on silica as well as on reversed phases based on silica with a normal phase gradient of increasing polarity (heptane to dichloromethane). Elution was solely governed by solubility of the polymers on both types of stationary phase for polystyrenes with a molecular weight above 35 000 and reversed phase gradient of decreasing polarity (methanol to dichloromethane). Under these conditions an identical dependence of elution solvent composition on sample size was found as for turbidity titrations. Due to differences in the velocity of the eluent front and the polymeric sample with porous stationary phases the polymers can be eluted as colloidal solutions Non-porous stationary phases are superior in this respect because the velocities of eluent and solutes are identical.     

Single electron transfer‐living radical polymerization of methyl methacrylate catalyzed by ytterbium powder

Zerovalent ytterbium (Yb) powder is firstly used as a catalyst in single electron transfer‐living radical polymerization of methyl methacrylate initiated by carbon tetrachloride in N, N‐dimethylformamide (DMF) and dimethyl sulfoxide, respectively. Polymerization proceeds in a “living”/controlled way as evidenced by kinetic studies and chain extension results, producing well‐defined polymers with controlled degree of polymerization and narrow molecular weight distribution. The apparent activation energy of polymerization in DMF is accounted to be 36.2 kJ/mol, and the energy of equilibrium state is calculated to be 13.9 kJ/mol. An increase in the concentration of Yb(0) yields a higher monomer conversion. It is observed that polymerization rate experiments a rapid increase in the presence of more polar solvent water, and increasing in the content of H₂O results in an increase in the apparent rate constant of polymerization, and a decrease in the molecular weight distribution. The reaction rate and molecular weight increase along with the decrease of DMF content. The effect of Yb(0) powder content, different ligands and concentration of initiator on the polymerization is also investigated. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Efficiency and Resolution in Countercurrrent Chromatography

Abstract

Efficiency and resolution have been studied with a CCC apparatus classified by Y. Ito¹ as a HDES type J, and known as the HSCCC or the Multilayer Coil Separator Extractor. It has been shown that the efficiency decreases when increasing the linear velocity of the mobile phase in the column, which can be done by increasing the flow rate, or by decreasing the volume of the mobile phase in the column. The two ways of changing the linear velocity are not equivalent, since, at constant linear velocity, the efficiency increases when the volume of the mobile phase is well balanced with that of stationary phase. For each flow rate there is a minimum value for the volume of the mobile phase, and the relationship is linear. Working under these conditions, changing both the flow rate and the volume of the mobile phase leads to an efficiency which goes through a minimum at medium flow rates.

The resolution does not show this minimum: it is best at low flow rates and low volumes of mobile phase, and its variations are more important at low flow rates. This means that, if the resolution is good at moderate flow rates, it will remain good at higher flow rates. Combining three equations, it has been possible to predict the resolution when working with the minimum volume of mobile phase for a given flow rate.     

Continuous process for ATRP: Synthesis of homo and block copolymers

Continuous ATRP of MMA was carried out in a flow tubular reactor with varying flow rate, temperature, and [monomer]/[initiator] ratios. Changing the flow rate directly relates to the reaction time. This process produces polymer continuously with the conversion increasing with decreasing flow rate. The molecular weight (relating to the flow rate) increases linearly with conversion which is also observed when the [monomer]/[initiator] ratio was changed. The effect of altering the reaction temperature was studied and the apparent activation energy of the propagation reaction of MMA in this system was calculated to be ∼56.9 kJ mol⁻¹, close to the values reported previously. Preparation of diblock copolymers is also reported with varying comonomers and the conversion, and SEC results suggested that this continuous system is an excellent and facile way to have a continuous ATRP process.