Measurement of the absolute molecular weight and molecular weight distribution of polyolefins using low-angle laser light scattering

By connecting a low-angle laser light scattering photometer (LALLSP) on line with a high-temperature gel permeation chromatograph (GPC), molecular weight distributions were determined for three polyethylene standards (two high density and one low density) without resorting to column calibration procedures. In addition, the absolute weight-average molecular weight and second virial coefficients of four polyethylene standards (three high density and one low density) were obtained using low-angle laser light scattering. The molecular weights measured were in excellent agreement with those specified by the National Bureau of Standards. For the low density material, a microgel fraction was detected by the on-line LALLSP but not by the conventional GPC detector. This resulted in a z-average molecular weight of over 3 × 10⁶ daltons, an order of magnitude greater than that determined by conventional GPC techniques.     

The Instrument Spreading Correction in GPC. II. The General Shape Function Using the Fourier Transform Method with a Nonlinear Calibration Curve

Abstract

The instrument spreading function suggested in Part I of this series is investigated for use with the Fourier transform method for generating corrected elution volume chromatograms. The instrument spreading parameters are obtained using linear theory on narrow molecular weight distribution standards, as indicated in Part I. The corrected chromatogram is then combined with a nonlinear molecular weight calibration curve which was fit with a function suggested by Yau and Malone to generate true values of the number- and weight-average molecular weights.

The instrument spreading function is shown to qualitatively and quantitatively describe the dispersion, skewing, and flattening effects ordinarily found in GPC chromatograms due to imperfect resolution by the GPC columns. The Yau-Malone function is shown to be a very useful function for fitting nonlinear molecular weight vs elution volume calibration data. Although the Fourier transform method is shown to work well with analytically generated data, it is shown that a number of numerical problems must be overcome before it can quantitatively produce corrected elution volume chromatograms. Some of these numerical problems are discussed.     

Gel permeation chromatography. VIII. A study of the effect of column arrangement of resolution at normal and overloading concentrations

In the determination of molecular weight distributions by GPC, the traditional column arrangement is such that the fractionation process proceeds from a high-permeability limit to a low-permeability limit column. We report computer comparisons of data obtained from columns in their normal ordering (high- to low-permeability limit), reverse ordering, and random ordering. The columns had a permeability limit range from 1 × 10⁶ Å down to 1 × 10³ Å, and the polymers had a molecular weight range of 1.8 × 10⁶ down to 2.1 × 10³. The concentrations used varied from 0.05% up to 0.5%. The data show significantly different results, with the random arrangement the preferred ordering. A qualitative model for the separation mechanism is presented to account for the improvement in resolution. Additional data are presented which show that serious errors (as high as 45%, depending on concentration) will be encountered in GPC studies, unless the calibration curve is obtained at the same relative concentration as the samples, with definite overloading effects occurring at the higher concentrations. A new method of curve fitting was used in the higher molecular weight region to give meaningful calibration curves.     

The Instrument Spreading Correction in GPC. I. The General Shape Function Using a Linear Calibration Curve

Abstract

A general shape function is proposed for describing the instrumental spreading behavior in gel permeation chromatography (GPC) columns due to axial dispersion and skewing effects. The general shape function contains statistical coefficients which describe the axial dispersion, skewing, and flattening of ideal monodisperse standards. A method denoted as the “method of molecular weight averages” is used to derive equations to correct GPC number- and weight-average molecular weights and intrinsic viscosities calculated from linear molecular weight calibration curves. The validity of these equations is experimentally verified with data for polystyrene, polybutadiene, and polyvinyl chloride polymers in tetrahydrofuran. The physical significance of the correction equations and their statistical coefficients is discussed in relation to the observed GPC chromatograms. Application of this shape function to the numerical Fourier analysis method for correcting differential molecular weight distribution (DMWD) curves is outlined. Also, a method is presented for obtaining corrected DMWD curves from a fitted molecular weight calibration curve corrected for instrument spreading by use of the hydrodynamic volume concept.     

Characterization of poly[di(carboxylatophenoxy)-phosphazene] by an aqueous gel permeation chromatography

A water-soluble phosphazene polyelectrolyte, poly[di(carboxylatophenoxy)phosphazene] (PCPP), was characterized using aqueous gel-permeation chromatography (GPC) with concentration (UV and RI) and molecular weight sensitive (multiangle laser light-scattering) detectors. Agreement was observed between the weight-average molecular weights determined by GPC with a light-scattering detector, conventional GPC using fractionated narrow PCPP standards, and also by static light-scattering measurements. The effect of chromatography conditions, such as ionic strength of the mobile phase, column resolution, and injection volume was investigated. Mark–Houwink constants of PCPP in aqueous solution (phosphate buffer, pH 7.4, 0.42MNaCl) were determined. The validity of the universal calibration curve and the occurrence of a secondary nonexclusion mechanism of separation in aqueous GPC of phosphazene polyelectrolytes are discussed. © 1996 John Wiley & Sons, Inc.     

Correlating mooney viscosity to average molecular weight

Copolymers of styrene and butadiene of the same chemical conposition, but widely different molecular weight distributions, were characterized by gel permeation chromatography. A very good correlation was obtained between the logarithm of the Mooney viscosity and the logarithm of the geometric mean of the number- and weight-average molecular weights. Instead of this molecular weight average, one can equally well use the very convenient “Av log M,” which is a weight-average molecular weight, using a logarithmic molecular weight scale. This correlation makes it possible to predict the Mooney viscosity from GPC data with a precision of about 15% (± one standard deviation), independent of the molecular weight distribution of the polymer. The obtained correlation was much better than with either weight-average molecular weight or viscosity-average molecular weight with α = 0.67.     

Recoverable compliance behavior of high-density polyethylenes

Eight samples of high-density polyethylene with weight-average molecular weights ranging from 5.5 × 10⁴ to 17.3 × 10⁴ have been studied. In addition to GPC molecular weight characterization, the recoverable compliance, the shear viscosity, and the extrudate swell were determined at temperatures between 138 and 200°C. The range of the maximum creep stresses ranged from 60 to 1840 dynes/cm². The creep recovery response was in the linear or near-linear range. The results are interpreted in the light of the anomalous results of Mendelson and Finger.     

Fractionation of linear polyethylene with gel permeation chromatography. IV

Reproducibility was examined on the GPC fractionation of linear polyethylenes. The experiences over the period of two and a half years were used. Calibration was done with the same batches of narrow distribution polystyrene standards. A control sample of broad-distribution polyethylene was run at each calibration. The reproducibility over ten calibrations with this control was in terms of standard deviation of ca. ±10% for the number-average and ca. ±15% for the weight-average chain length. The fractionation data of 36 commercial resins were corrected by using the control samples as an additional standard. The correction was very effective in decreasing the scattering of the data in the intrinsic viscosity–molecular weight plot, especially when the viscosity average was used for the molecular weight expression.     

Light scattering characterisation of polymers II. Analyses of high molecular weight polystyrene

For light scattering analysis of an extremely high molecular weight polymer with large size in solution, use of mixtures of a given polymer with its low molecular weight homologue is described to have great advantages. With such bimodal mixtures as test samples, one can alter the unknown form of the angular variations of scattered lights to a form to be expected and widen the range of the linearity with sin² (ø/2), where ø is the scattering angle. According to the considerations, a sample of linear polystyrene was successfully analysed on a commercial apparatus, Fica 50 to have the weight-average molecular weight of 27 million.     

Composition of Butadiene-Styrene Copolymers by Gel Permeation Chromatography

Abstract

An ultraviolet detector was added to the GPC making it possible to determine both molecular weight distribution and composition of butadiene-styrene copolymers as a function of molecular weight. After calibration, several types of commercial and experimental copolymers were analyzed. SBR was found to have a very uniform composition over most of the range of molecular weight. However, the styrene content decreased at the high end of the distribution. Representative samples of anionically polymerized copolymers were also examined. In general, the composition of these polymers varied more than SBR. Usually, the styrene content was high at low molecular weight and decreased as molecular weight increased. Several experimental copolymers that were first metalated and then grafted with styrene were studied. Incomplete grafting as well as the presence of low molecular weight homopolystyrene was readily detected. This appears to be a particularly sensitive method for studying composition of copolymers.     

Polymerization of ethylene using a nickel α-diimine complex covalently supported on SiO2–MgCl2 bisupport

Bisupported catalyst for ethylene polymerization was prepared by mixing alcohol solution of MgCl₂ with pretreated SiO₂ in heptane, and further treating with bis(4-(4-amine-3,5-diisopropylbenzyl)-2,6-diisopropylphenylimino) acenaphtheneNiBr₂ (abbreviated as NiLBr₂) solution. The bisupported catalyst could be used to polymerize ethylene with high activity using alkylaluminum halides as inexpensive cocatalysts. According to high-temperature GPC, the weight-average molecular weights of the polymers obtained ranged from 2.15 × 10⁵ to 9.27 × 10⁵, with molecular weight distributions of 3.12–4.23. By adjusting the polymerization temperatures, the products with good morphologies could be obtained. The resultant polyethylenes were confirmed by ¹³C-NMR to contain significant amounts not only of methyl but also of ethyl, propyl, butyl, amyl, and longer side chains (longer than six carbons).     

Comparaison des méthodes de fractionnement par chromatographie de perméation et par gradient d'elution pour la détermination des répartitions moléculaires des polypropylénes

Molecular weight distributions for polypropylene samples have been determined by a permeation fractionation method (GPC). Porous silica beads were used as a packing material for the columns. The set of columns allows a good separation of the polypropylene macromolecular chains in a range of molecular weights from 5000 to 1.5 × 10⁶, and the thermal and mechanical stabilities of these beads are very good. The calibration has been carried out with fractions of polypropylene of narrow molecular weight distribution prepared by a large-scale column fractionation. The molecular weights M?_w and M?_n and the ratios M?_w/M?_n calculated from the GPC curves show, in general, good agreement with the ones calculated from the column fractionation curves. However, the M?_w/M?_n ratios are always highter in the case of GPC fractionation. This could be due to diffusion phenomena.     

Gel permeation chromatography: Dispersion effects on molecular weight monitor-installed gel permeation chromatograph

Computer simulation was carried out to examine the performance of a molecular weight (MW) monitor-installed gel permeation chromatograph (GPC), by taking account of the effects of limited column resolution according to Tung's phenomenological scheme. Efficiency of GPC fractionation was discussed also in the same light. For simulated GPC fractionation results of model polymers having log-normal distribution, various average MW's and MWD functions were calculated from the data obtained by the MW monitor method as well as the conventional MW calibration methods, and compared with the given true values. The MW monitor method generally tends to predict narrower distributions than the true ones, as opposed to the conventional calibration methods which usually predict broader distributions. For certain simple cases, semiquantitative relation between the extent of column resolution and these deduced average MW's was derived. The efficiency of GPC fractionation (as judged, for example, by the polydispersity of recovered fractions) is limited by such factors as fraction size, column resolution, and polydispersity of the original sample itself.     

Polyether polyols as GPC calibration standards for determination of molecular weight distribution of polyether polyols

Average molecular weights (M_n, M_w and M_p) are important characteristics of oligomers and polymers, and therefore there is a need to have a precise and reliable determination method. A gel permeation chromatography (GPC) coupled with a single refractive index detector was used to determine the molecular weight distributions of commercial polyether polyols calibrated against a series of polyether polyols with known molecular weights and low polydispersity. Results of these GPC analyses were compared to the ones calibrated against the commercially available polystyrene (PS) standards. The number‐average molecular weights (M_n) obtained with GPC using polyether polyols calibration were closer to the theoretical values than the M_n obtained using PS as calibration standards. Hence, these GPC analyses using polyether polyols as calibration standards can provide reliable determination of molecular weight distribution of polyether polyols and can be potentially applied to natural oil‐based polyols, including palm oil‐based polyols. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42698.     

Molecular Characterization of Styrene-Butadiene-Styrene Block Copolymers (SBS) by GPC, NMR, and FTIR

Summary This paper deals with the characterization of styrene-butadiene-styrene (SBS) block copolymers at molecular level through different techniques. SBS molecular weight distributions (MWD) were measured by Gel Permeation chromatography (GPC) in tetrahydrofuran (THF) solvent and using a calibration curve based on mono-dispersed polystyrene standards; afterwards, the relative molecular weights obtained by GPC were adjusted using a correction factor based on Mark-Houwink-Sakurada parameters for SBS in THF. Quantitative characterization of polybutadiene (PB) structures in SBS was made by solid-state ¹³C Nuclear Magnetic Resonance (NMR). A novel method using Fourier transform infrared spectroscopy (FTIR) has been developed for characterizing the SBS composition. This method covers the determination of composition in SBS ranging from 20 to 75 wt% of polybutadiene (PB) and was performed based on a FTIR calibration curve, which was prepared using mixtures of polystyrene (PS) and polybutadiene (PB) of known compositions.     

Synthesis of well‐defined glycoconjugate polyacrylamides via preactivated polymers prepared by ATRP

Poly(N‐acryloxysuccinimide) (polyNAS) with narrow molecular weight distributions (MWD) applicable for the preparation of well‐defined glycoconjugate polyacrylamides were successfully prepared by atom transfer radical polymerization (ATRP). The structures of polyNAS were characterized by ¹H‐NMR and GPC. GPC results showed that the molecular weight polydispersity indices (PDI) range from 1.17 to 1.29. The molecular weights could be calculated based on ¹H‐NMR results but GPC results of polyNAS by using 0.01M LiBr/DMF did not give accurate molecular weights, probably because of the complex interaction in the system. The effects of free N‐hydroxysuccimide produced in the polymerization processes on the free‐radical concentrations and apparent initiation efficiencies of ATRP were discussed. Well‐defined glycoconjugate polyacrylamides (i.e., with narrow molecular weight distributions and designed glycoconjugate degrees) were prepared by substituting N‐oxysuccimide units with galactosamine followed by reaction of ethanolamine. The galactose conjugate degrees were determined by ¹H‐NMR and the total substitutions of N‐oxysuccimides were verified by ¹H‐NMR and FTIR. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 189–194, 2005     

Molecular weights of styrene–maleic anhydride copolymers

A dual-calibration method for the determination of molecular weights and molecular weight distribution of styrene–maleic anhydride copolymers (S/MA) by gel permeation chromatography (GPC) is introduced. It might be applicable to copolymers of other type. A linear relationship of intrinsic viscosity [η] and weight-average molecular weight (M?_w) for unfractionated S/MA in tetrahydrofuran (THF) at 25°C can be expressed by the equation The maleic anhydride content of the copolymers ranges from 5 to 50 mole-%, and the M?_w range is from 2 × 10⁴ to 7 × 10⁶. The plot of log [η] M?_w versus GPC elution volume of the S/MA copolymers falls on the same curve as that of the polystyrene standards in THF.     

Characterization of coal-derived humic substances with the aid of low-pressure gel permeation chromatography

Low-pressure gel permeation chromatography (GPC) has proven to be a valuable tool for determining the molecular weights of coal-derived humic substances (HS). A 40×0.9 cm (ID) column packed with Sephadex gel (typically with Sephadex G-100) in combination with alkaline borate-based mobile phase allows an almost complete fractionation of typical HS from young brown coals. After a calibration with a set of globular proteins, a molecular-weight distribution can be determined and average molecular weights can be calculated. The determination of the weight-average molecular weight, M_w, seems to be more reliable than the determination of the number-average molecular weight, M_n. Limitations of the proposed method (and size-exclusion methods in general) were discussed in detail, and some sources of uncertainties were identified.     

Quantitative Determination of Plasticizers in Polymeric Mixtures by GPC

Abstract

Organic plasticizers were added to plastics to improve flow and reduce brittleness by lowering the glass transition temperature. The amount of plasticizer added to the base resin determined its efficiency in bringing about these desired changes in properties. Analytical gel permeation chrornatography (GPC) was utilized to quantitatively determine the amount of organic plasticizers in poly(styrene) mixtures. The internal standard method was applied to the determination of triethyleneglycol-dibenzoate and tricyclohexylcitrate over the concentration range of 5.0 to 30.0 wt% in poly(styrene). Linear calibration curves and excellent precision between measurements was demonstrated over the concentration range investigated. GPC analysis has the advantage over spectrophoto-metric techniques in its ability to separate low molecular weight plasticizers: from higher molecular weight resins. In addition to the potential of making quantitative measurements from the detected peak, the associated material can be separated from the polymer, collected, and separately analyzed by UV or IR techniques.     

Extension of GPC Techniques

Abstract

Most of the common GPC gels exhibit pore sizes which are too small to separate very extended molecules, e.g., native cellulose in solution. Several gel types were tested with vinyl polymers of molecular weights up to 10⁷ and with cellulose nitrate. Large pore size Styragel with acetone as solvent proved to be the most favorable gel system yielding effective separations for polymers with coil diameters in solution up to 4000 Å.

The evaluation of GPC runs usually requires a separate calibration procedure. We attempted to determine the molecular weight of the eluate directly as it leaves the column. This is done by a special automatic viscometer that allows measurement of the viscosity of small cuts (e.g., 0.5–1 ml) of the effluent volume. A set of six capillary viscometers are loaded and unloaded continuously. The details of the apparatus are described and examples of the performance reported.