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Three methods of predicting dynamic viscosity of branched polyethylenes from size exclusion chromatography (SEC), refractive index/light-scattering detection data were examined: relating parameters in the Cross viscosity equation to molecular weight averages, use of a mixing rule, and a method based on the similarity of the cumulative molecular property distributions to a dimensionless viscosity versus frequency plot. The use of accurate SEC data was emphasized. The third “curve similarity” method provided the most promising results. The use of cumulative g′ (the molecular contraction factor based on intrinsic viscosity) distributions, in addition to cumulative molecular weight distributions, enable very useful sample comparisons.

Equations are presented for calculating molar mass averages and molar mass distributions from matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) data and from size exclusion chromatography (SEC) data. The utility of polydispersity is examined as an indicator of the expectation of MALDI-TOF MS mass discrimination effects. Cumulative distributions are found to be rich in information for comparing the two techniques and are easily obtained from both SEC and MALDI-TOF MS data. Analyses of a series of narrow molar mass distribution poly(methyl methacrylate) (PMMA) standards and one polydisperse sample have been performed with both methods. MALDI-TOF MS did not detect dimer and trimer in the PMMA samples, and it often indicated lower amounts of high-molar-mass polymers than did SEC. The results showed that the distribution breadth, as evidenced by the standard deviation of the distribution (calculated from the polydispersity and number-average molar mass), correlated well with the molar mass range observed in the MALDI-TOF MS spectra, whereas the polydispersity alone did not. Ratioing the extremes in the molar mass concentrations measured with the SEC differential refractometer, which were necessary to adequately define molar mass distributions, showed that detector dynamic range values as high as approximately 370,000 were required for the polydisperse samples. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 627–639, 2005

The object of this study is to develop multidetector size-exclusion chromatography (SEC) methods to determine the number of arms per molecule across the molecular size distribution of star-branched polymers. An empirical fit between the intrinsic viscosity molecular contraction factor g‘ and the number of arms f is used as an alternative to converting g‘ values to root mean square radii ratios used by random walk models. The quantitative analysis of star polymer distributions by SEC is then reduced to understanding factors unique to the accurate measurement of g‘ across the molecular size distribution. Two methods of analyzing SEC data are then tested: (1) the “conventional method” utilizing values of weight-average molecular weight and intrinsic viscosity at each retention volume and (2) the method of component chromatograms. The latter is a new method useful when only a few different types of branching are present. It depends on fitting each detector's chromatograms as the sum of component chromatograms. Plotting the intrinsic viscosity of the branched polymer versus that of the linear polymer at the same molecular weight was useful for diagnosing problems. The conventional method was defeated by axial dispersion in the narrow chromatograms and the homogeneity of branching in the samples. The component chromatogram method avoids the axial dispersion problem but its value depends on how accurately the component peaks reflected the true situation. In this study, the method provided the most reasonable values when component peaks were grouped together. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 552–570, 2002

With the increasing popularity of evaporative interfaces, detection using Fourier Transform Infrared (FTIR) spectrometry in the mid-infrared region is becoming more important in size-exclusion chromatography (SEC). FTIR spectrometry is a powerful, and potentially very widely applicable, method for obtaining chemical functional group information for each molecular size fraction. Quantitative evaluation of polymer composition across the SEC chromatogram can provide more accurate characterization of heterogeneous polymer samples for problem solving and for material specification. The evaporative interface removes the SEC mobile phase at the exit of the column and deposits the eluting polymer as a continuous film stripe or as a series of discrete films on infrared transparent substrates. Initially this detection approach was used only for qualitative analysis. More recently, it is being used quantitatively. Previously we demonstrated that the quality of the film generated by the evaporative interface was critical to determining the suitability of the resulting FTIR spectra for quantitative analysis. In a continuation of this work, the objective of this paper is to develop a procedure for obtaining valid quantitative results for polymer blends with the interface. Experimental topics include improving the quality of polymer films by post-SEC treatments, off-line FTIR calibrating using other means to obtain high quality polymer films, and utilizing in-line SEC detectors in calibration. Interpretation aspects focus upon peak fitting of FTIR spectra, linear regression, partial least squares, and data pre-processing. PLS prediction with internal calibration using the second derivative of solvent-annealed film spectra was found to provide the best compromise between processing time, accuracy and precision.

Local polydispersity is the term describing the variety of molecules present at the same retention volume in size exclusion chromatography (SEC) analysis. In the analysis of a linear homopolymer, local polydispersity is generally attributed to the effect of axial dispersion: it can cause molecular size variety (i.e., imperfect resolution) at each retention volume and thus local polydispersity in the molecular weight. In the analysis of polymer blends (copolymers and branched polymers), it is possible to have local polydispersity, even when the resolution is perfect, because molecules of different compositions (or degrees of branching) can have the same molecular size in solution. Conventional SEC interpretation assumes no local polydispersity if the axial dispersion effects are negligible. Three methods are currently available for detecting local polydispersity by using a combination of differential refractive index, light scattering, and viscometer detectors: the chromatogram comparison method, the conventional calibration curve comparison method, and the universal calibration comparison method. Here we experimentally assess these three methods using polymer blends and emphasize the chromatogram comparison method. All three are shown to be useful for assessing triple detector systems, but they are capable of detecting local polydispersity due to molecular heterogeneity only for very large differences in specific refractive index increments in the blend components. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 370–383, 2001

Traditional methods of evaluating fractionation in size exclusion chromatography (SEC) include number of theoretical plates, resolution indices and calculation of molecular weight averages. Most recently, fractionation evaluation has become evaluation of local polydispersity where “local polydispersity” is molecular variety at a particular retention volume. The three published methods for determining local polydispersity using triple-detector SEC are examined. All have two major uncertainties: the degree to which the observed local polydispersity affect the whole polymer molecular weight averages and the origin of the local polydispersity. New methods of answering these questions are examined using the SEC analysis of polymer blends. One method utilizes a plot where the area under the curve is the total number of moles of polymer. Calculation of this curve with equations containing different assumptions provides the needed significance test over the range where all three detectors have sufficient...

Size exclusion chromatography (SEC) is now a common polymer analysis method employed in polymer reaction engineering studies. However, this once simple technique is no longer very simple. In this overview we selectively review the literature relevant to the practical use of SEC to obtain quantitative results. Fractionation, detection, calibration, resolution correction and system development are examined in turn. It becomes evident that, at this stage in SEC development, each of these areas is advancing very rapidly. This means that valuable additional quantitative information on polymer molecular properties is now obtainable.However, this additional information is accompanied by significant additional sources of error and uncertainty. The original SEC with only a differential refractive index detector is currently the most reliable and precise system, as well as being the most limited in what information it provides. New methods of linking results from such simple systems with multi-detector “res...

Local polydispersity refers to variety in the types of molecules present at the same retention volume in an analysis by size exclusion chromatography (SEC). Such variety is undesired because it can be a major source of inaccuracy in SEC interpretation. A rapid, practical, method for detecting the presence of local polydispersity is presented. In this method, data from a differential viscometer (DV) and light scattering (LS) detector are used together with a universal calibration curve to generate a differential refractive index (DRI) chromatogram for the sample, while assuming that it does not exhibit local polydispersity. This reconstructed DRI chromatogram is compared to the actual DRI chromatogram. It is shown that any significant difference between the two indicates the presence of local polydispersity. Plots of residuals (the difference between the heights of the two DRI chromatograms vs. retention volume) allow the significance of the local polydispersity to be assessed and the retention volume range encompassing the local polydispersity to be defined.

The objective of this work is to obtain a simple method for detecting local polydispersity. Local polydispersity is the presence of a variety of different types of molecules at the same retention volume in SEC. One source of local polydispersity is axial dispersion. However, the topic of this paper is the detection of local polydispersity which is independent of axial dispersion effects This “perfect resolution” local polydispersity can occur because SEC separates on the basis of molecular size in solution and thus for complex polymer molecules, such as copolymers or branched polymers, a variety of combinations of molecular weight and composition can produce the same molecular size. In conventional SEC interpretation, it is assumed that with high resolution columns local polydispersity is absent. Highly misleading analyses can result if this assumption is invalid. Two very simple methods were developed in this work. The first method enabled polystyrene-poly(dimethyl siloxane) blends to be examine...

Size-exclusion chromatography using halogenated aromatic solvents such as trichlorobenzene at 145°C is widely used for industrial polyolefins. Such high temperature operation requires special instrument design and causes many operational problems. In this work, a new eluting solvent, methyl cyclohexane, has been successfully used at 90°C for the size-exclusion chromatography of industrial (high-molecular-mass) grades of polypropylene and polyethylene. Sample preparation involves dissolution in decalin at temperatures as high as 140°C, followed by dilution in methyl cyclohexane at 90°C previous to injection into the SEC column running with methyl cyclohexane as the mobile phase. In addition to permitting operation at 90°C, methyl cyclohexane is less toxic than the usual solvents and has a differential refractive index sensitivity advantage as well. It also provides new opportunities for ultraviolet, fluorescence, and infrared detection for functionalized polyolefins. However, one disadvantage is that polystyrene adsorbs from this solvent on styrene-based packings. Thus, narrow fractions of polyisobutylene (PIB) were used in place of polystyrene for universal calibration. Another disadvantage is that methyl cyclohexane is more flammable than halogenated aromatic solvents. A quantitative assessment of the new solvent system is in progress. Initial results are promising and are presented in this paper.

The peak shape changes in differential viscometer (DV) chromatograms, shown in Part I of this series, were investigated. Chromatograms were obtained with narrow-molecular-weight distribution standards of increasing molecular weight. Peak shape changes were not observed when the DV detector was operated in series with the differential refractive index (DRI) detector rather than in parallel. Flowrate variations during elution of each sample in the parallel detector configuration appear to be the reason for the changes. At high molecular weights (≥460,000), polydispersity of the standards also contributed. Molecular weight averages of broad-molecular-weight distribution polymers were not affected by the detector configuration used. With a series configuration, a new method of determining interdetector volume and interpreting narrow-molecular-weight distribution polymers is presented.

Interdetector volume accounts for the time delay between detectors when more than one detector is used in size exclusion chromatography (SEC). In this work, interdetector volumes are determined from chromatograms for concentration detectors in series and in parallel, as well as for a parallel differential viscometer (DV)—differential refractometer (DRI) combination. The three methods examined for accomplishing this determination (peak apex, multipoint, and centroid methods) all provided equivalent results for truly monodisperse, low-molecular-weight compounds in all cases. For the DV-DRI combination, the interdetector volume obtained for the latter two methods increased with increasing molecular weight. This result was attributed to increasing peak skewness observed for DV chromatograms of narrow-molecular-weight standards as molecular weight increased. This occurred despite the fact that their normalized DRI chromatograms were all superimposable. In addition to helping to explain current widespr...

A simple method is proposed for using a size-exclusion chromatograph equipped with both concentration and light-scattering detectors to calibrate other chromatographs having only concentration detectors. The method is developed and demonstrated for poly(ethylene terephthalate) in methylene chloride/dichloroacetic acid. It is shown that, in addition to circumventing the need for a light-scattering instrument on other chromatographs to be used for the analysis of PET, precision and accuracy of results are improved over those obtainable with light-scattering detection. The method uses averaging of the light-scattering detector data to establish a correlation between the molecular weight of the polymer of interest and the molecular weight of polystyrene at each retention volume. This correlation can then be used for other instruments employing the same mobile phase and only a concentration detector. Although the method assumes the validity of the universal calibration curve, the actual curve is not required, nor are Mark–Houwink constants or intrinsic viscosities. © 1994 John Wiley & Sons, Inc.

It is now theoretically possible to obtain absolute accurate values of number-average molecular weight of complex polymers (e.g., branched polymers or copolymers) using size exclusion chromatography (SEC) with only a detector that measures the difference between the eluting polymer solution viscosity and the viscosity of the pure mobile phase (a differential viscometer [DV] detector). However, both precision and accuracy of these “DV Mn” values are of concern. In this work, the precision of NBS 706 polystyrene was found to be two to three times worse for the DV Mn than for the conventionally calculated Mn. Also, regarding accuracy, the DV Mn values were affected by the location of the universal calibration curve along the retention volume axis (a problem intimately associated with the problem of specifying the correct interdetector volume), the sensitivity of the DV detector to low molecular weights present in the sample, and axial dispersion. Each of these sources of error are examined in turn and two methods of calculating Mn values are proposed. © 1994 John Wiley & Sons, Inc.

A systematic evaluation of two types of light-scattering detectors for size exclusion chromatography (SEC) was completed. The two detectors were the low-angle laser light scattering photometer (LALLS) and the multiangle laser light-scattering photometer (MALLS). Instrument evaluations were performed at both room (30–40°C) and high (135–145°C) temperatures using the polystyrene standard, NBS 706, at room temperature and the polyethylene standard, SRM 1476, at high temperature. Results of the evaluation showed that when experimental uncertainties were taken into account LALLS and MALLS demonstrated equivalent precision and accuracy for molecular weight determination. The main source of inaccuracy found (particularly for SRM 1476) was the sensitivity difference between the light-scattering and the concentration (DRI) detectors; i.e., the DRI detector was unable to measure very low concentrations of very high molecular weight material present in SRM 1476, whereas the light-scattering detectors respond strongly. It was shown that for LALLS the overall weight-average molecular weight (Mw) for the whole polymer calculated using an equation that did not require the DRI detector output circumvented this sensitivity problem while assuming that the low angle used was sufficiently close to zero. Use of this equation for MALLS is possible by extrapolating data from all angles used to obtain a light-scattering chromatogram at zero angle. However, this possibility was not examined here. A particular advantage of MALLS over LALLS is that MALLS can provide the z-average root mean square radius (commonly referred to here and in other light-scattering literature as the “radius of gyration”) values from the same data as those used to obtain molecular weight values. Although the radius of gyration values at each retention volume were not as precise as the corresponding weight-average molecular weights, at room temperature, precision was better than 2% for a significant portion of the chromatogram. © 1993 John Wiley & Sons, Inc.

Ortho-chlorophenol (OCP) has previously been reported as one component in mixed solvent systems for the size exclusion chromatography of poly (ethyiene terephthalate) (PET)1,2, In order to avoid the problems involved in maintaining mixed solvents, a technique has been developed to analyze PET in OCP alone. The system has been calibrated using narrow polystyrene and broad PET standards. Problems addressed include the high viscosity and high refractive index of OCP, solvent purity, sample preparation, and sample degradation. The system has been used to investigate the hydrolysis and degradation of PET during extrusion processes.

The time required for polymer molecules to pass from one detector to another is a critical parameter for interpretation of multidetector size exclusion chromatography data. A method of utilizing numerical optimization to determine this quantity is presented. The chromatography system included a differential refractive index detector (DRI) in combination with a low-angle laser light scattering photometer (LALLS) and a differential viscometer (DV). The optimization method requires a conventional calibration curve (in terms of intrinsic viscosity rather than molecular weight if the DV detector is used) and the raw data from analysis of a broad molecular weight distribution linear homopolymer standard. A cubic polynomial and a least square cubic spline were used to fit the conventional calibration curve. In interpretation of the DV data, it was necessary to use a least square cubic spline to fit the conventional calibration curve. Results obtained are “effective” in that they intrinsically provide ag...

Nanogels are highly branched, swellable polymer structures with average diameters between 1 and 100nm. Size-exclusion chromatography (SEC) fractionates materials in this size range, and it is commonly used to measure nanogel molar mass distributions. For many nanogel applications, it may be more important to calculate the particle size distribution from the SEC data than it is to calculate the molar mass distribution. Other useful nanogel property distributions include particle shape, area, and volume, as well as polymer volume fraction per particle. All can be obtained from multi-detector SEC data with proper calibration and data analysis methods. This work develops the basic equations for calculating several of these differential and cumulative property distributions and applies them to SEC data from the analysis of polymeric nanogels. The methods are analogous to those used to calculate the more familiar SEC molar mass distributions. Calibration methods and characteristics of the distributions are discussed, and the effects of detector noise and mismatched concentration and molar mass sensitive detector signals are examined.

Quantitative evaluation of polymer composition across the SEC chromatogram can provide more accurate characterization of heterogeneous polymer samples for problem solving and for material specification. To this end Fourier-transform infrared spectroscopy (FTIR) with solvent–evaporation interfaces has become a very powerful detector for size-exclusion chromatography (SEC). The solvent–evaporation interface removes the mobile phase at the exit of the chromatograph and deposits the separated molecular sizes as polymer films on infrared transparent substrates. Quantitative interpretation of the FTIR spectra obtained from these films has recently been found to be best accomplished by using partial least squares. In this paper, polystyrene and poly(methylmethacrylate), alone, as blends, and a copolymer were analyzed in a SEC equipped with an evaporative interface. Molecular weight effects, wavelength selection, the effect of averaging spectra on results, and selection of the best data preprocessing method were investigated. General methods of evaluating these variables were developed to arrive at conditions for this particular “model” situation in order to provide a basis for the analysis of more complex polymers.

Quantitative size exclusion chromatography (SEC) was considered a system with the following components: sample preparation, fractionation, detection, calibration, and resolution correction. Four systems were evaluated: I was 3 columns with “conventional single detector interpretation”; II was 4 columns with concentration correction and detector assessment; III was concentration correction applied to the data of I; IV was two development “mixed bed” columns. Analysis of polystyrene standards included calculation of their molecular weight averages and use of the Glockner “T” measure of resolution as well as “specific resolution index.” Systems I and III provided the best high molecular weight results. System IV allowed 12–20 min analysis times and provided a highly linear calibration curve with very good reproducibility; however, it showed significantly worse resolution at high molecular weights. Plots of molecular weight averages determined from SEC versus their known true values were particularly useful. Concentration correction using the Rudin model moved different concentration data toward a common universal calibration curve and generally lowered molecular weight averages. Narrow polystyrene standards required relatively high concentrations for precise molecular weight averages and therefore, their averages were not good indicators of the need for concentration correction. Analysis of polypropylene samples corroborated the lower high molecular weight resolution of System III. Concentration correction did significantly change the polypropylene molecular weight distribution but did not affect the result of the kinetic model fitting.

A kinetic model for the free-radical-initiated molecular weight degradation of polypropylene was further developed. The model has a single variable parameter, the initiator efficiency, f. Assumptions were detailed, new comparisons with experimental data presented, and model sensitivity to the value of f evaluated. The model was found to provide a good description of both molecular weight distribution and molecular weight average data from degradations carried out in a single-screw extruder at 200 and 220°C. Data at 0.04 wt% initiator feed concentration were fit and the resulting f value used to predict results at 0.01 wt% and 0.02 wt%. In accordance with observation, the model predicted that temperature would have no effect on the molecular weight of the extrudate because the comparatively long (≥ 2.8 min) residence time in the extruder permitted degradation reactions to go to completion. The model predictions were found to depend upon the change in molecular weight distribution rather than the absolute value of the distribution data. Predictions were therefore unaffected by concentration correction in size exclusion chromatography interpretation. The model was determined to have a low sensitivity to the value of f. Hence, f was estimated to only ± 25% and it is anticipated that attempts to use the model for comparing different initiators would be limited by this characteristic. Also, because the minimum residence time in the extruder was 2.8 min, the model has yet to be tested against data at times less than this value.

Polypropylene was analyzed by size exclusion chromatography (SEC) at 145°C using a single-differential refractometer detector. The objective was to provide data for characterization of polypropylene degradation during a reactive extrusion process. Two antioxidants [tetrakis(methylene(3,5-di-tert-butyl-4-hydroxyhydrocinnamate)) methane (I) and octadecyl 3,5-di-tert-butyl-4-hydroxyhydro cinnamate (II)] were tested for their ability to prevent thermal degradation of the polypropylene during sample preparation. The use of 0.20 wt% of (I) was effective during the 36–48 h required to completely dissolve the samples in trichlorobenzene for SEC analysis. “Reshaping” of the chromatograms by resolution correction demonstrated that, while the molecular weight averages were changed by 8% because of axial dispersion, most of the individual heights of the distributions were changed by less then 2%. Tail heights of the distributions were more affected but were also shown to be highly imprecise. Selecting individual heights of the distributions rather than molecular weight averages therefore minimized axial dispersion error and also circumvented errors in molecular weight averages originating from dilution of distribution tails below detector sensitivity limits. Various forms of distributions were examined and the equations linking the chain length distribution predicted by polymerization kinetic models to the SEC chromatogram are presented. The analytical method developed provided precise data for kinetic modeling. However, absolute accuracy requires further assessment.