Review of gel permeation chromatography

Gel permeation chromatography (g.p.c.) with organic solvents is of immense value in the characterisation of plastics and resins. Topics reviewed here are column packings and packing techniques, instrumental features, theories of the separating process and of instrumental dispersion, the use of g.p.c. to measure molecular size and hydrogen bonding, the applications of g.p.c. to low molecular weight polymers, applications to high molecular weight polymers, work on glass beads (as opposed to organic beads) and high resolution g.p.c.     

Calibration procedures in gel permeation chromatography

Procedures for the determination of the log molecular weight vs. elution volume calibration relation are reviewed for linear homopolymers. The calibration curve is readily established with narrow molecular weight distribution fractions. For broad molecular weight distribution fractions the peak molecular weight at the peak elution volume of a fraction's gel permeation chromatogram has to be calculated from average molecular weights. When well-characterised fractions of the polymer requiring analysis are unavailable, a calibration curve can be established by procedures which assume that the hydrodynamic volume of a polymer molecule in solution controls fractionation in gel permeation chromatography. These universal calibration procedures require information on Mark-Houwink viscosity constants or polymer unperturbed dimensions. The validity of hydrodynamic volume as the universal calibration parameter is discussed with special reference to the goodness of the solvent for a polymer and polymer polarity. Examples are given of the various calibration procedures which are employed in the determination of molecular weight distribution and average molecular weights for poly(methylmethacrylate), polyethylene and polyisoprene.     

High performance gel permeation chromatography of polystyrene

Microparticulate crosslinked polystyrene packings in short columns have been investigated with high performance liquid chromatography instrumentation. Reliable molecular weight data for six polystyrene standards having narrow molecular weight distributions and for a polystyrene having a broad distribution have been obtained by optimizing the injection procedure, using a constant flow pump, and incorporating an internal standard into each injected solution. Experimental determinations of the dependence of the polydispersity for polystyrene standards on eluent flow rate and polymer diffusion coefficient were in agreement with a relation predicted from theoretical considerations of chromotogram broadening. Because of the dependence of chromatogram broadening on polystyrene molecular weight, high efficiency separations for high polymers were only obtained at low eluent flow rates. For low polymers, high efficiency separations may be performed at fast eluent flow rates. It was concluded that accurate molecular weight distributions can only be determined from chromatograms obtained at low eluent flow rates, which was supported by experimental measurements of polydispersity on polystyrene sample prepared by a radical polymerization at low monomer conversion. A differential weight distribution calculated from an experimental chromatogram for the polydisperse polystyrene determined at the lowest eluent flow rate (0.1 cm³min^?1) was compared with distributions predicted theoretically for polystyrenes prepared by radical polymerization. It was concluded that the experimental distribution contained a small contribution from chromatogram broadening and that most of the radicals in the polymerization of styrene terminated by combination.     

Studies of oligomers by gel permeation chromatography

The photoinitiated reactions of bromotrichloromethane and of carbon tetrabromide on styrene, and of bromotrichloromethane on methyl methacrylate have been studied. The products have been analysed by gel permeation chromatography and the transfer constants for the first few steps in the polymerisations have been determined; some Arrhenius parameters have also been found. The processes may provide a synthetic route to a number of oligomers which are difficult to form by other methods.     

The gel permeation chromatography of phenolic compounds

Gel permeation chromatography, using polystyrene gel and tetrahydrofuran as solvent, has been applied to hydroxybenzophenones, esters of salicylic acid, alkylphenols, alkylated methylenediphenols, and phenol–formaldehyde condensation products. The difference between the calculated molecular volume of these phenolic compounds and that obtained by actual determination with GPC has been ascribed to tetrahydrofuran solvation of the phenolic hydroxyl group. Furthermore, it has become clear that THF solvation is affected by the steric hindrance of ortho-substituted phenol and by inactivation of the phenolic hydroxyl group resulting from internal hydrogen bonding.     

Mathematical analysis of gel permeation chromatography data

The purpose of this paper is to show how a theoretical molecular weight distribution can be found which would result in a given gel permeation chromatogram. Also, to show how the method can be applied to blending problems where one desires a given molecular weight distribution.     

Differential gel permeation chromatography for quality control

For the direct comparison of similar polymers, as, for example, for quality control, differential gel permeation chromatography provides a simple, sensitive technique that is relatively insensitive to operational variables. One polymer is chosen as a standard and a solution of that polymer is used as the eluent in an otherwise conventional GPC. Differential chromatograms of slightly different polymers are both positive and negative with respect to the baseline. Positive portions represent an excess and negative portions a deficiency as compared to the standard. As in conventional GPC, the elution volumes of the differences indicate the molecular size ranges of the differences. The algebraic sum of the differences is zero. Examination of the raw curves with only general information about calibration and operating conditions tells nearly as much about the differences as carefully executed conventional GPC with complex data reduction.     

Characterization of nonderivatized cellulose by gel permeation chromatography

The development of a gel permeation chromatography (GPC) system for the analysis of nonderivatized cellulose in the DP range of technical cellulose products is reported. In contrast to traditional GPC, where cellulose derivatives have to be prepared, in this work the cellulose solvent cadoxen was applied for the determination of the molecular weight distribution using a special analysis system. Nuclear magnetic resonance studies of cadoxen solutions indicated no complex formation between cadoxen and cellulose. Therefore, to avoid precipitation of cellulose from the prepared solution, cadoxen was also used as eluent in the chromatographic system. For this reason a stationary phase had to be found which is stable under the given strong alkaline conditions. The utilization of Fractogel TSK as column material resulted in the separation of dissolved cellulose showing a DP_v between 400 and 2000 with good reproducibility. The application of very sensitive detectors together with a GPC program allows the characterization of unknown cellulose samples of different origin within a few hours.     

Separation of coal-derived liquids by gel permeation chromatography

Coal-derived liquids, obtained from pilot plants and bench-scale reactors, have been separated by gel permeation chromatography into aromatic, phenolic, and asphaltenic fractions, where asphaltenes and long-chain hydrocarbons are in the same fraction. The Chromatographie system uses 10 nm μStyragel columns and solvents such as tetrahydrofuran (THF) and toluene. The separation is reasonably clean and almost devoid of overlapping. The saturated hydrocarbons are separated from the asphaltenes by vacuum distillation. Aromatic, phenolic and aliphatic fractions are characterized by high-resolution gas chromatography—mass spectrometry. The phenolic fraction contains alkylated phenols, indanols, and naphthols. The aromatic fraction is composed of alkylated benzenes, indans, naphthalenes and small amounts of multi-ring aromatics such as alkylated fluorenes and pyrenes. Most of the long-chain hydrocarbon fraction is of straight-chain alkanes ranging from tetradecane to tetratetracontane. Some branched alkanes, such as pristane and phytane, are also present. If olefins are present in the sample they also separate with the long-chain hydrocarbon fraction. Although various analytical data such as i.r., n.m.r., molecular size distribution and elemental composition of asphaltenes have been obtained, the chemical characterization is not complete. The gel permeation Chromatographie separation technique, as discussed in this paper, is very useful for fast analysis of any coal-derived liquid.     

Comments on data treatment in gel permeation chromatography

The effects of variables on treatment of the gel permeation chromatogram are reported. Variables investigated include (1) the molecular weight distribution of polymers for preparing the calibration curve, i.e., the logarithm molecular weight–elution count relationship, (2) nonlinearity of the calibration curve, and (3) fluctuation of the baseline. The deviation of the calibration curve prepared by the polymer having broad molecular weight distribution was evaluated in detail by assuming log-normal distribution function for the distribution. The polymer having a D value less than 1.3 was recommended for this purpose. Generally, the shape of the chromatogram is fairly different from that of the true molecular weight distribution curve when the calibration curve is not linear over the entire range of interest. By fitting polynomials to the calibration curve, the chromatogram was sufficiently converted to the molecular weight distribution curve. The apparent difference between them was removed. Slight deviation of the baseline from the true one gave rise to obvious error in the calculated molecular weight and its distribution, especially for the sample having a broad distribution.     

Analysis of phenol-formaldehyde resols by gel permeation chromatography

The chromatographic analysis of resol solutions in tetrahydrofuran solvent, by means of a set of columns packed with crosslinked polystyrene gels, has been carried out with adequate resolving power for a clear-cut qualitative or semiquantitative differentiation between various types of resols to be practicable. The resulting chromatograms, which show the distribution of different constituents by molecular size, could be interpreted by the use of reference substance and by calibrating the system with a number of compounds of known structures. The method has been used to investigate the way in which various reaction parameters (nature of catalysts, proportion of starting material, treatments undergone by the resols) affect the composition of resols. Different commercial products have been thus characterized. It has also afforded an insight into the progress of the polycondensation reaction as a function of time and helped to state the reactivity of different groups and unblocked ring positions. Thus, an hydroxymethyl group appears to be more reactive in the para than in the ortho position. Otherwise the reactivity of unblocked ring positions would be enhanced by an hydroxymethyl group in the ortho rather than para position.     

Characterization of commercial polyvinylbutyral by gel permeation chromatography

Fractions of commercial polyvinylbutyral “Movital” were investigated by the “off-line” combination of gel permeation chromatography and viscometry. Parameters of the Mark–Houwink equation were determined for polyvinylbutyral in tetrahydrofuran at 25°C. Real values of the molecular parameters were obtained by the evaluation of chromatographic data using the principle of universal calibration.     

A new instrument for preparative gel permeation chromatography

A preparative liquid-phase chromatograph was built for the purpose of obtaining sufficiently large quantities of very narrow fractions of different polymeric species, such as polystyrene, PVC, polybutadiene, and polyethylene. This apparatus allows the fractionation of approximately 20 g of polymer per day; the fractions so obtained have polydispersities of about 1.1 over a very wide range of molecular weights. Polydispersities of less than 1.01 were obtained after recycling of the sample.     

Characterization of nylon 66 by gel permeation chromatography

The quantitative characterization of nylon 66 of various polydispersities has been carried out by gel permeation chromatography (GPC), using m-cresol solvent at 130°C. A Q-factor value of 13.9 for nylon 66 has been validated, by limiting viscosity-number determinations for the particular solvent/temperature combination described above. Using this value together with a simple correction technique for viscous fingering and unsymmetrical dispersion, the practical quantitative characterization of linear nylon 66 has been achieved. Construction of a universal calibration curve, based on hydrodynamic volume, gave a straight-line relationship for polystyrene fractions and nylon 66 samples covering a broad range of polydispersity values.     

Infrared detection of polymers in gel permeation chromatography

A gel permeation chromatograph equipped with an on-line Grubb-Parsons infrared spectrometer is described. The versatility, specificity, sensitivity, and limitations of such an infrared detector are discussed with particular reference to spectrometer specification, eluent absorbance, and solute absorbance. A stable baseline is produced when this detector is operated at high temperatures, e.g., for the separation of polyethylene in o-dichlorobenzene at 135°C. Individual functional groups in a chemically inhomogeneous solute, such as a copolymer, may be monitored by repeated injections of the solute, changing the wavelength setting between separations. This procedure is illustrated with AB poly(styrene-b-t-butyl methacrylate) block copolymer in trichloroethylene at 35°C.     

Analysis of brake fluids by gel permeation chromatography

The non-volatile component of a brake fluid can be identified by comparing the gel permeation chromatogram of the fluid with those of a range of possible non-volatile components.