Size Exclusion Chromatography (SEC) of Complex Polymers-Methods of Correction for Imperfect Resolution

Abstract

Herein is reported a review and extension of recently published methods of correction for imperfect resolution (inadequate peak separation and or excessive peak broadening) for the SEC of linear homo and uniform copolymers and complex polymers such as polymers with long chain branching and nonuniform copolymers. Emphasis Is placed on analytical correction methods and absolute detector systems such as the low angle laser light scattering photometer and viscometer in series with a mass detector. Analytical correction methods and absolute detector systems play a vital role in the SEC characterization of complex polymers.     

Proposed Instrumental Spreading Shape Function (ISF) as Applied to Size Exclusion Chromatography (SEC)

Abstract

Herein is proposed a new statistical ISF. This function which is gaussian and symmetric in shape, is an infinite series. The series is described by two parameters, instrument peak dispersion coefficient, σ², and a new coefficient, a polyplatykurtic coefficient A_k. The two parameters combine to form a peak broadening parameter, X. This new shape function seems to fall within the expectation of several authors in the past (1–11). This series was obtained during studies involving aqueous SEC with series of dextran standards. This paper is a continuation of a series of papers on these investigations (12,13)     

Size Exclusion Chromatography of Polymers With Random Tetrafunctional Branching

Abstract

The behaviour of polydisperse branched copolymers of methyl methacrylate with a small amount of randomly situated tetrafunctional ethylenedimethacrylate units was investigated by means of size exclusion chromatography (SEC). A procedure has been suggested for the conversion of apparent values of molecular parameters of real polymer branched systems (M_{n, app}; M_{w, app} obtained from SEC data by calibration of the separation system using a linear polymer) into actual values. This was made possible by off-line coupling of SEC and viscometry. The branching was characterized by the weight average number of branching sites in the macro-molecule, m_w, and the branching index, y.     

Size Exclusion Chromatography of Poly(vinylpyrrolidone)

Abstract

Poly(vinylpyrrolidone) and poly(ethylene oxide) separate by hydrodynamic volume on Toyo Soda TSK-PW columns in a mixed solvent mobile phase of 50:50 (v/v) MeOH/H₂O containing 0.1M LiNO₃. From this separation a single universal calibration curve based on hydrodynamic volume [η]M can be obtained. Accurate weight average molecular weights of PVP were obtained by both SEC/LALLS and universal calibration showing good agreement between the two methods. SEC/LALLS overestimates the number average molecular weight for broad distribution polymers due largely to the lack of sensitivity of the LALLS detector to the low molecular weight portion of the polymers, while the universal calibration method slightly underestimates the number average molecular weight as compared to osmometric values.     

Chromatography of Suspensions - Analytical Corrections for Axial Dispersion

Abstract

An analytical method is proposed to correct the signal from a turbidity detector for axial dispersion in the chromatography of particle suspensions. An earlier analytical method due to Hamielec and Singh [1] is limited to light scattering in the Rayleigh regime. The solution reported herein is derived using the more general Mie light scattering theory and enables the correction of various moments of the particle diameter frequency distribution function. The axial dispersion phenomenon is described by the well known integral equation where the instrumental spreading function is assumed Gaussian. The solution is applicable for both linear and nonlinear particle diameter-retention volume calibration curves.     

Elution Behaviors and Rationalized Calibration for Low-Molecular-Weight Polymers in Size Exclusion Chromatography

Abstract

Molecular weight relationships among oligostyrene, n-hydrocarbon, epoxy resin, p-cresol novolak resin, and oligoethylene glycol having the same retention volume were discussed using SEC gels of different pore sizes in chloroform and tetrahydrofuran. Gel capacity and the maximum number of components resolvable increased with the use of chloroform except the case of epoxy resin. Different elution behaviors of oligomers in different eluents make it difficult to use molar volumes or effective chain lengths as calibration parameters. The influence of the pore size and shape of the gel on the elution order among oligomers was negligible except some cases. Molecular weight conversion equations for several oligomers based on molecular weight of oligostyrene or n-hydrocarbon were derived. These equations make it possible to use oligostyrene or n-hydrocarbon as a reference standard when molecular weights of oligomers are measured.     

Study of the Degradation of High Molecular Weight Polystyrene in the Course of Size Exclusion Chromatography (SEC)

Abstract

The possibility of degradation of large macromolecules in their motion through a porous medium in the course of SEC decreases considerably the range of application of this highly productive and highly selective analytical method for the determination of the MWD of polymers. In fact, the MWD determined by SEC with the existence of degradation is mainly the distribution according to degradation fragments and does not reflect the initial molecular characteristics of the polymer.

The dependence of the degree of degradation of maccromolecules on the main parameters of the chromatographic experiment: the rate of the eluent flow, the size and shape of sorbent particles and the size of their pores was studied in detail. A non-monotonic change in the degree of degradation with increasing flow rate of the eluent was detected.

It is shown that the degree of degradation increases with decreasing particle size and the size of their pores. A new concept of the degradation of high polymers in SEC is formulated. It is related mainly not to the existence of local rate gradients in the interparticle volume but, rather, to the entire combination of hydrodynamic and adsorption effects accompanying the process of interaction between the macromolecules and the surface of sorbeiit particles. Practical recomendations are given for carrying out SEC, including that of super-high molecular weight samples under “non-degrada- tive” condritions.     

Size Exclusion Chromatography of Polyamides

Abstract

We report here a new chromatographic method for characterization of polyamides and polyether-block-amide copolymers by Size Exclusion Chromatography. First, it concerns absolute characterization of polyamide probes by classical light scattering and Ubbelohde viscometry measurements and the choice of narrow distribution standards to calibrate the GPC system. The GPC analysis of polyamides 6, 11, 12 and copolymers is based upon the dual detection refractometer/on-line continuous viscometer at 130°C using columns packed with cross-linked polystyrene gels, benzyl alcohol as mobile phase and polytetrahydrofuran standards for calibration. The main results are absolute average molecular weights and distribution curve, intrinsic viscosity and viscosity law. In addition, viscometric data produced evidence of long-chain branching occuring beyond a limiting molecular weight in some polyamide 12 samples.     

A Calibration Technique for Size Exclusion Chromatography

Abstract

A modified Weibull distribution function is shown to be useful in calibrating the molecular size separation capabilities of both rigid and swellable gel packing materials. Two parameters are used in this function which are related to a packing material's micropore volume distribution. The calibration curves of a set of different packing materials connected in series were predicted from the Weibull calibration functions obtained for each individual packing material.     

Band Broadening Function in Size Exclusion Chromatography of Polymers: Review of Some Recent Developments

Summary: This article reviews some recent developments on the determination of the Band Broadening Function (BBF) in Size Exclusion Chromatography (SEC) of polymers. It was carried out in the frame of the IUPAC Project: “Data Treatment in Size Exclusion Chromatography of Polymers”. The correction for band broadening (BB) is important for quantitative determinations of the molar mass distribution (MMD) of narrow-distributed (or highly multimodal) polymers, and of derived variables such as kinetic parameters. In the narrow range of a molar mass standard, the BBF is uniform and of positive skewness. In a broad chromatographic range, the BBF is non-uniform and skewed; and it can be adequately represented by an exponentially-modified Gaussian function (EMG) of 2 parameters that vary slightly with elution volume: an increasing Gaussian variance and a decreasing exponential decay. Additionally, the total BBF variance remains almost constant if not close to the total exclusion limit. The following methods for determining BBF parameters are reviewed: a) a direct method based on assuming Poisson-distributed MMDs; b) a direct method based on measuring the mass- and molar mass chromatograms of narrow standards; c) a theoretical method based on a stochastic model that is equivalent to the Giddings-Eyring model; and d) a theoretical method based on a deterministic model obtained through an extension of the classical van Deemter expression. Ideally, the correction for BB requires a robust numerical inversion algorithm. However, alternative simplified solutions are also possible.     

Characterization of Xylansulfate by Size Exclusion Chromatography

Abstract

A sulfated, low molecular weight beech xylan was fractionated preparatively on Sephadex G-50 into ten fractions with molecular weights from 17 000 to 1 800. The degree of sulfation, refractive index increment, molecular weight, and specific optical rotation of each fraction was determined. The fractions were then used to calibrate a Sephacryl S–200 gel column for the molecular weight distribution analysis of xylansulfate.

The molecular weights, M _W, obtained by SEC for samples of xylansulfate were in good agreement (within 4 ± 1%) to the values derived by low angle light scattering.     

Theory of Multiple‐Detection Size‐Exclusion Chromatography of Complex Branched Polymers

SEC separates complex branched polymers by hydrodynamic volume, rather than by molecular weight or branching characteristics. Equations relating the response of different types of detectors are derived including band broadening, by defining a distribution function N′(M,V_h), the number of chains with molecular weight M and hydrodynamic volume V_h. While the true molecular weight distribution of complex polymers cannot be determined by SEC, irrespective of the detector used, the formalism enables multiple detection SEC data to be processed to both analyze the polymer sample and reveal mechanistic information about polymer synthesis. The formalism also shows how the true weight‐ and number‐average molecular weight, and , can be obtained from correct processing of the hydrodynamic volume distributions.

     

Size Exclusion Chromatography of Poly(vinylpyrrolidone): I. The Chromatographic Method

Abstract

The optimum size exclusion chromatographic (SEC) method for poly-(vinylpyrrolidone) (PVP) was found to be based upon a stationary phase of diol derivatized silica of pore sizes 3000, 500, and 75Å and a mobile phase of 50:50 (v/v) MeOH/H₂O containing 0.1M-LiNO₃. Sample recovery under identical conditions varied for the commercial packings investigated and was found to be inversely related to molecular weight. The latter phenomenon was rationalized on the basis of a limited number of active substrate sites available for binding. Methanol was found to be a more effective mobile phase modifier than either dimethyl formamide or acetonitrile apparently due to its ability to function as a proton donor in hydrogen bonding with PVP. Chromatographic evidence for the existence of semipolyampholyte character in PVP is presented. A procedure for the construction of a column set log-linear in calibration and of extended dynamic range is described and is based upon hydrodynamic volume theory.     

Multimode Retention Behavior of Azaarenes in Size Exclusion Chromatography with Poly(Divinylbenzene)

Abstract

The size exclusion retention behavior of azaarenes on poly(divinylbenzene) is described. With dichloromethane as the mobile phase a strong nonsize effect was evident and retention was found to be governed by the pK_a of the eluate. By changing the mobile phase to N-methylpyrrolidinone, retention became independent of pK_a and components eluted in order of decreasing molecular weight. Some degree of nonsize behavior was still evident and separation proceeded through a multimode mechanism. LogMW for both the azaarenes and PAH. However, in the case of the azaarenes, a less than ideal correlation coefficient (-.705) suggests that retention is not purely size-dependent.     

A New Packing for Aqueous Size Exclusion Chromatography Polyvinylpyrrolidone-Coated Silica

Abstract

Adsorption of polyvinylpyrrolidone on silica in aqueous medium is studied. Adsorption isotherms, thermogravimetry and elementary analysis show that a complete surface coverage is reached with large pore microbeads. The coating stability and its ability to prevent non-steric effects when used as packing in exclusion chromatography are demonstrated. Four different pore size packings were prepared for chromatographic evaluations. It is shown that water soluble polymers like polyethylene oxide, polyvinylalcohol, polyacrylamide and hydroxyethylcellulose can be chromatographied in pure water according to the steric exclusion mechanism.     

Computer Assisted Data Acquisition and Analysis for Size Exclusion Chromatography

Abstract

Two programs, ACEACQ and ACEANN, provide data acquisition and analysis for Size Exclusion Chromatography (SEC). ACEACQ is used for data logging, real-time smoothing and data reduction by boxcar averaging. ACEACQ requires that traditional SEC hardware be coupled with a Hewlett Packard data logger and microcomputer. A Mettler electronic balance is connected to the computer in a novel fashion to continuously monitor the mass of solvent eluted during an experiment. ACEANN provides analysis for the data collected by ACEACQ. Calculations include molecular weight averages, molecular hydrodynamic diameter averages and column separation efficiency parameters. Significant reductions in analysis time have been realized along with improvements in accuracy of data acquisition, analysis and experimental reproducibility.     

An Experimental Evaluation of a New Commercial Viscometric Detector for Size-Exclusion Chromatography (SEC) Using Linear and Branched Polymers

Abstract

Herein are reported some findings on the application of a new type of continuous automatic viscometer, in parallel with a differential refractometer, as a detector system for SEC. A universal calibration is required for the instrument and two methods of construction are applicable. The first is the customary peak-position calibration using polymer standards of narrow molecular-weight distribution and the second uses a single broad standard of known [Mbar]_w and [Mbar]_n. The two types of calibration are shown to give nearly-identical values of molecular weight when used to process chromatograms obtained from various linear homopolymer standards of varying chemical composition. These values compare favourably with those quoted by the suppliers of the polymer standards. One of the more powerful features of this instrumentation, namely its potential for estimating accurate molecular weights of branched polymers, is demonstrated by analysis of a series of branched polyvinylacetates prepared by a conventional bulk, free-radical polymerisation procedure. The calculation of the degree of chain branching is discussed. Another particular feature of the viscometer detector, its ability to indicate the presence of low concentrations of high-molecular-weight impurity in polymer samples, is also shown.     

Characterization of Polymer Architectures by Multidetection Size Exclusion Chromatography

Summary: This paper discusses the analysis of star-branched and dendritic polyisobutylenes (PIBs) using branching parameters based on radii of gyration R_g,z and hydrodynamic radii R_h,z. R_g,z and R_h,z were measured by Size Exclusion Chromatography equipped with a Wyatt Technology Viscostar viscometer (VIS), a Wyatt Optilab DSP RI detector, a Wyatt DAWN EOS 18 angle Multiangle Light Scattering (MALS) detector and a Wyatt Quasi-elastic Light Scattering QELS detector. Branching parameters were calculated from these measurements and compared with those computed for these architectures.     

An Analysis of Imperfect Resolution in the Chromatography of Particle Suspensions

Abstract

Here-in is reported a general method of correction for imperfect resolution in the chromatography of particle suspensions. It overcomes most of the limitations of previously reported methods. A non-linear particle diameter - retention volume calibration curve and a generalised spreading function are considered. Moment equations are developed for two types of general detectors; the first type includes a refractive index detector and a turbidity detector with Rayleigh scattering while the second type includes a tubidity detector with Mie scattering. The moment equations were applied to the analysis of chromatograms of narrow Dow polystyrene latices measured by size exclusion chromatography.