Cross Fractionation of Styrene-Butadiene Copolymer

A sample of styrene-butadiene copolymer was fractionated by successive precipitation (one-direction fractionation) in cyclohexane/isooctane and benzene/methyl ethyl ketone systems. The chemical composition and molecular weight distributions of the sample were constructed from the fractionation data. The results obtained in both systems were nearly identical for the chemical composition distribution, but were different for the molecular weight distribution. The same sample was fractionated by cross fractionation using both solvent/nonsolvent systems. Comparing the results of cross fractionation with the results of one-direction fractionation, the first gave broader molecular weight and chemical composition distribution curves than the second. However, only cross fractionation showed that the chemical composition distribution curve has a long tail not only at the right side but also at the left side of the distribution maximum.

The superiority of cross fractionation over one-direction fractionation seems clear from the present work. It is also clear that even if the chemical composition distribution curves obtained by one-direction fractionation in different systems are identical with one another, the curves do not always show the true distribution.     

The Fractionation and Properties of Gluten Proteins

Abstract

Gluten protein may be separated into fractions of differing molecular weight distributions by a successive extraction procedure using urea solutions and dilute sodium hydroxide solution. Fractions containing mainly low molecular weight protein decrease dough strength and mixing stability but increase dough plasticity. Fractions rich in high molecular weight protein have the opposite effect. Doughs or glutens behave as uncross-linked systems in which entanglement coupling appears to play an important role.     

Fractionation of partly hydrolyzed polyvinyl acetate

Vinyl acetate–vinyl alcohol copolymers were fractionated in order to obtain their degree of hydrolysis distributions. In the method employed for fractionation, the differences in molecular weight of copolymer did not affect the fractional separation. Degree of hydrolysis distributions was found to be broad, with a pronounced maximum at a low degree of hydrolysis. Viscosity measurements were performed both for the precipitated fractions and unfractionated polymer. The Huggins constant was found to increase with a decrease in the degree of hydrolysis of polyvinyl acetate. These results were interpreted in terms of a polymer molecular association. From values of Huggins constants, comparative information about copolymer “blockiness” is also established.     

Fractionation of chitosan by ultrafiltration

It was shown that low-molecular chitosan could be fractioned by ultrafiltration. The types of membranes suitable for this purpose were determined. The effect of ionic strength and the solvent nature on the molecular weight and polydispersity of the obtained samples was studied. The possibility of obtaining chitosan with low polydispersity was shown.     

Fractionation of Cellulose

Cellulose samples with molecular weight distributions that are considerably narrower than those of the natural products can be obtained by at least three fundamentally different routes. (i) Synthesis of easily soluble derivatives, fractionation by means of well-established methods and subsequent regeneration, (ii) selective extraction of short chains from activated cellulose, using solvents of suitable marginal quality, and (iii) partition of the homologs between two coexisting phases formed by the demixing of homogeneous solutions. All three methods can be applied successfully. However, the efforts in terms of labor and required solvent differ considerably. Most of the experiments were performed with the following three cellulose samples: Avicel (M _w = 30 kg mol^–1, U = (M _w/M _n)–1 = 2.0), Solucell (M _w = 230 kg mol^–1, U = 1.8), and Stockstadt (M _w = 320 kg mol^–1, U = 5.7). Options (ii) and (iii) emerged most promising for large scale fractionation. The mixed solvent consisting of DMAc and LiCl turned out to be particularly versatile in both cases. In the pure state it can be used for incremental extraction (yielding quick access to orienting information on the width of the molecular weight distribution) as well as for one-step extraction. In combination with suitable precipitants (like acetone) it enables the realization of the coexistence of two liquid phases required with route (iii). One obstacle for fractionation that all methods share is the high viscosity of cellulose solutions. With the last method it is possible to mitigate this limitation considerably by the use of spinning nozzles for the mixing of feed and extracting agent.     

Fractionation of polyethylene by gel permeation chromatography

The performance of the gel permeation chromatography (GPC) technique was studied with several commercial samples of linear high density polyethylene. A comparison of these data was made with those obtained from the elution column technique based on fractional dissolution. GPC gave reproducible fractionation data in the molecular weight range of 10³–10⁶. These data were obtained from two columns in series with nominal capacity of 1–10⁴ and 1–10⁶A. Polyethylene fractions of known molecular weights are required as calibration standards and for many commercial polyethylenes it is necessary to increase sensitivity at molecular weight higher than 10⁶.     

Fractionation of styrene-methyl methacrylate copolymer

Styrene-methyl methacrylate copolymer was fractionated by the column elution and coacervate extraction techniques. Different solvent-nonsolvent combinations were used; both batchwise and continuous column elution fractionations were carried out. Successful resolution of fractions according to molecular weight was achieved. The effect of the solvent-precipitant pair on the fractionation efficiency, as tested by gel permeation chromatography, is discussed. The results of continuous column elution depend significantly on the elution rate.     

Fractionation and Size Distribution of Water Soluble Polymers by Flow Field-Flow Fractionation

Abstract

Flow field-flow fractionation (flow FFF) is introduced as a chromatographic-like method with a potential for separating and characterizing water soluble polymers. The theory of the method is summarized, showing that one gets a size distribution curve based on the Stokes diameter, d. Problems in interpreting the elution profile in both flow FFF and gel permeation chromatography are discussed in the light of complications arising from electrostatic chain expansion in polyelectrolytes.

The experimental approach is described using a channel of 2.00 ml volume. Sulfonated polystyrenes of three different molecular weights are separated from one another with and without added salts. The dependence of retention on sample size is shown to be least in the salt solution, indicating that this is most suitable for analytical work.

The sodium salts of polyacrylic acid are also investigated. Distinct elution profiles are noted for two of these polydisperse polymers. Size distribution curves for the 2,000,000 MW sample curves are obtained from, the elution profiles and are shown to be independent of experimental variations. Finally, fractions are collected after separation and rerun through the coloumn, showing a reasonable confirmation of the expected fractionation effect.     

Thermodynamic Relationship for Fractionation of Ternary Copolymers

ABSTRACT

Thermodynamic equations for fractionation of terpolymers were derived, and from them it was obvious that triple or dual cross-fractionation should be carried out to determine the distributions of chemical composition and molecular weight of terpolymers.     

Fractionation of styrene–acrylonitrile copolymers

Three types of commercial styrene–acrylonitrile copolymer were fractionated by coacervate extraction and by column-elution techniques. Both methods were studied with two different solvent–nonsolvent pairs. Glass wool was used as the support material in the column. Fractionation by the coacervate extraction method was studied with benzene–triethylene glycol as a solvent–nonsolvent system at 60°C and with dichloromethane–triethylene glycol at 25°C. Column elution was carried out with acetone–methanol as the solvent–nonsolvent system at 30°C, and with dichloromethane–methanol at 20°C. Results of excellent reproducibility were obtained by these two methods. Characterization of fractions involved determination of both the molecular weight and chemical composition. It was established that the fractionation of the samples tested was dependent upon molecular weight only. The two methods described above are compared. Each gives an efficient procedure for fractionation of styrene–acrylonitrile copolymers.     

Fractionation of linear polyethylene during bulk crystallization under high pressure

Two linear polyethylene fractions (M_η, 11,260 and 100,000) and mixtures of these fractions have been isothermally crystallized from the melt under pressures up to 3000 atm. Characterization of individually crystallized fractions with transmission electron microscopy indicates that pressure can be used to produce a crystallite whose thickness is a measure of the chain length within it. Although the high molecular weight fraction yields spherulites containing individually varying lamellae thicknesses, the maximum thickness of each lamella is a measure of the chain length within it. Both electron micrographs and differential thermal analysis results show that crystallization of homogeneous mixtures of the high and low molecular weight fractions under high pressure results in a distinct fractionation and segregation according to molecular weight.     

Fractionation,purification and concentration of dextran solutions by ultrafiltration

Dextran, a glucose polymer, is used as a blood plasma volume expander and for the production of iron dextran. For the removal of low molecular weight material from dextran aqueous solutions, and the purification and concentration of these solutions, two laboratory-scale and two large-scale ultrafiltration units were used with several membranes.The results showed that low molecular weight dextrans and soluble impurities can be removed successfully from dextran solution by ultrafiltration. Dextran solutions were concentrated from 20 g l^?1 up to 350 g l^?1.     

Using Fractionation and Diffusion Ordered Spectroscopy to Study Lignin Molecular Weight.

Recent reports demonstrate that applications of the biopolymer lignin can be helped by the use of a fraction of the lignin which has an optimal molecular weight range. Unfortunately, the current methods used to determine lignin's molecular weight are inconsistent or not widely accessible. Here, an approach that relies on 2D DOSY NMR analysis is described that provides a measure of lignin's molecular weight. Consistent results were obtained using this well-established NMR technique across a range of lignins.     

The Fractionation of Copolymers by Chemical Composition

Abstract

Phase diagrams for the system of methyl ethyl ketone, cyclo-hexane, and styrene-acrylonitrile copolymer were determined. The phase diagrams indicate that the copolymer may be fractionated by chemical composition in this system. Discussions of the thermodynamics are also presented, to show that copolymers can effectively be fractionated into fractions of different compositions if a system can be found in which the difference between the Flory interaction parameters (x parameters) of two constituents of the copolymer with solvent is sufficiently large. Theoretically, the fractionation of copolymer must always occur to a certain extent, depending both on chemical composition and molecular weight. The composition fractionation results of styrene-acrylonitrile copolymers are given to confirm the discussions.     

Synthesis and characterization of branched polystyrene. Part II. Fractionation

Fractionations have been carried out by a column-elution method on mixtures of linear and branched polystyrene. These had been prepared by anionic polymerization followed by a coupling reaction to form star-shaped branched polymers. Therefore, the components of the mixture were each of narrow distribution, and their molecular weights differed from each other as multiples of the original single chain length. For single chains in the 25,000–30,000 molecular weight range, separations were accomplished in which the multiples (number of branches) were 1, 2, 4, and 6. By this means, virtually monodisperse samples of the branched polymers could be isolated.     

Fractionation of Poly(butyl methacrylate) by Molecular Topology Using Multidetector Thermal Field‐Flow Fractionation

Thermal field‐flow fractionation (ThFFF) is an interesting alternative to column‐based fractionation being able to address different molecular parameters including size and composition. Until today it has not been shown to be able to fractionate polymers of similar molar masses and chemical compositions by molecular topology. The present study demonstrates that poly(butyl methacrylates) with identical molar masses can be fractionated by ThFFF according to the topology of the butyl group. The influence of the solvent polarity on the thermal diffusion behavior of these polymers is presented and it is shown to have a significant influence on the fractionation of poly(n‐butyl methacrylate) and poly(t‐butyl methacrylate). Fractionation improves with increasing solvent polarity and solvent polarity may have a greater influence on fractionation than solvent viscosity. It is found that the thermal diffusion coefficient, D_T, as well as the hydrodynamic diameter, D_h, exhibit increasing trends with increasing solvent polarity. The solvent quality has a significant influence on the fractionation. It is found that cyclohexane, being a theta solvent for poly(t‐butyl methacrylate) but not for poly(n‐butyl methacrylate), significantly improves the fractionation of the samples by decreasing the diffusion rate of the former but not the latter.

     

Fractionation of apple procyanidins by size-exclusion chromatography.

Oligomeric constituents of apple procyanidins were fractionated by size-exclusion chromatography using a TSKgel Toyopearl HW-40F column. The best separation was obtained using a mobile phase of acetone-8 M urea (6:4; adjusted to pH 2) at a flow-rate of 1.0 ml/min. In this chromatographic system, the use of 8 M urea in the mobile phase resulted in a molecular sieve effect without any surface affinity interaction between the gel beads and the procyanidin molecules. Each fraction obtained was examined by reversed-phase high-performance liquid chromatography and time-of-flight mass spectrometry. The order of elution of the procyanidins from the column was coincident with their degree of polymerization.     

Hydrodynamic Fractionation of Macromolecules. I. A Simple Theory

A new mechanism for separation in gel permeation chromatography (CPC) or gel filtration has been developed based upon the postulate that flow occurs through the gel phase. On the basis of the hydrodynamic nature of the separation mechanism, the new name Hydrodynamic Fractionation is proposed. A theory has been formulated for the case of equal pore size in the gel phase and equal sized spaces between the beads. This theory predicts the elution volume vs molecular weight curve. The flow-rate dependence of the separation peaks is investigated.

The theoretically predicted relationship between molecular weight and elution volume compares very well with the gel permeation experiments using glass beads as packing probably because these packings correspond most closely to the assumptions in the theory. In the cross-linked polymer packing where a large distribution of pore sizes exists the theory does not fit well; however, it does predict the general shape of the curve. The flow-rate dependence of the separation peaks is experimentally the same as qualitatively predicted. Also the experimental findings of equilibrium experiments were correlated with the corresponding GPC data using the present theory.     

Fractionation of a polymer using a preparative-scale continuous chromatograph

Summary The construction and operation of a liquid preparativescale continuous chromatograph is described. The chromatographic bed is arranged in a series of interconnected vertical columns and is rotated whilst one end of the tube bundle is closed by a stationary plate through which liquids enter and leave the bed. The problem of maintaining an efficient seal is described. The chromatograph was commissioned by continuously fractionating dextran on porous silica beads with water as eluent. Results show that as the feed concentration was increased, the fractionation obtained deviated markedly from that expected from basic GPC theory, but that the relative molecular mass distribution of the feed was still narrowed with feed concentrations of 20% w/v.     

Crystallization Elution Fractionation of LLDPEs Made with Metallocene Catalysts

Summary: Temperature rising elution fractionation (TREF) and crystallization analysis fractionation (CRYSTAF) fractionate semicrystalline polymers according to their crystallizabilities from dilute solution and have been widely used to measure the CCD of LLDPE. A new fractionation technique, known as crystallization elution fractionation (CEF), has been developed recently. The main difference between CEF and TREF and CRYSTAF is that the crystallization cycle in CEF is performed dynamically under solvent flow in a long column that contains an inert support material. In this paper, several metallocene-LLDPE resins have been analyzed by CEF to investigate the effect of cooling cycle parameters, comonomer fraction, polymer molecular weight, and blend cocrystallization on the fractionation. This new technique can be used to obtain CCDs with better resolution and in shorter times than TREF and CRYSTAF.