Determination of the molecular mass distribution of synthetic polymers by size-exclusion electrochromatography

The performance of size-exclusion electrochromatography (SEEC) for the mass distribution analysis of synthetic polymers was studied and compared to conventional, pressure-driven size-exclusion chromatography (SEC). Electroosmotic flow control, within-day, day-to-day and column-to-column repeatability were determined for SEEC with respect to retention and separation efficiency. It was shown that by using the retention ratio instead of the migration time, the precision of the mass distribution calculations is sufficiently high, and that similar distributions were obtained for a sample analyzed by pressure-driven SEC and by SEEC. Furthermore, hexafluoroisopropanol was demonstrated to be a new and potent solvent for SEEC. It was used for the separation of narrow polymethylmethacrylate standards and several commercially important polymers such as polycarbonate, polycaprolactam and poly(ethylene terephthalate), using UV detection in the deep UV region (195-230 nm).     

Quantitative FTIR detection in size-exclusion chromatography

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.     

Simulation of size-exclusion chromatography distribution coefficients of comb-shaped molecules in spherical pores comparison of simulation and experiment

Simulations of the distribution coefficients of linear polymers and regular combs with various spacings between the arms have been performed. The distribution coefficients were plotted as a function of the number of segments in order to compare the size exclusion chromatography (SEC)-elution behavior of combs relative to linear molecules. By comparing the simulated SEC-calibration curves it is possible to predict the elution behavior of comb-shaped polymers relative to linear ones. In order to compare the results obtained by computer simulations with experimental data, a variety of comb-shaped polymers varying in side chain length, spacing between the side chains and molecular weights of the backbone were analyzed by SEC with light-scattering detection. It was found that the computer simulations could predict the molecular weights of linear molecules having the same retention volume with an accuracy of about 10%, i.e. the error in the molecular weight obtained by calculating the molecular weight of the comb-polymer based on a calibration curve constructed using linear standards and the results of the computer simulations are of the same magnitude as the experimental error of absolute molecular weight determination.     

Size Exclusion Chromatography Calibration Assessment Utilizing Coupled Molecular Weight Detectors

Abstract

This paper shows that the usual method of representing an SEC calibration curve by a single polynomial curve may often be inadequate with new high resolution columns. Data points wind about the fitted line. The significant magnitude and systematic nature of these deviations clearly appear when a plot of residuals is derived from the conventional calibration curve and expressed in terms of the percent error in molecular weight. The deviation of the calibration data from the fitted line was approximately 10% for the conventional molecular weight and intrinsic viscosity calibration curves. It became 20% for the universal calibration curve. LALLS and DV detectors were used together with the DRI detector to provide evidence that the calibration curve deviations were due to the column packings and not due to some other cause (e.g., vendor values of molecular weight). Use of a polynomial fit to a portion of the curve corresponding to the retention volume range of an unknown was used to show the significant improvement in results which occurred when the calibration variations were taken into account. At present, use of many individual narrow standards is necessary to elucidate the effect.     

Application of off-line size-exclusion chromatographic fractionation-matrix assisted laser desorption ionization time of flight mass spectrometry for proanthocyanidin characterization

In this work, we wanted to devise a reliable method to characterize polymerized forms of tannins, their structural information and mass distribution. Size-exclusion chromatography (SEC) is a chromatographic technique used to determine molecular mass (weight) distributions of polymers. One important step in the data treatment is the modeling of the calibration curves. Polystyrenes (PS) are standards usually used because no commercial procyanidin (PC) standards are available. An off-line coupling of SEC and MALDI was carried out to measure differences between polystyrenes and procyanidins. Thus, a new calibration curve was established; from 1000 to 8000 Da, there is a good correlation between the MALDI and PS calibration curves, in this field the PS calibration is correct and enables true mass determination. For masses above 8000 Da, PS calibration overestimates the real molecular weight of PC, overestimation of 53%. And for masses below 1000 Da, PS calibration underestimates their real molecular weight (10-15%). This means that to truly characterize PC, calibration based on PC standards is required.     

Corrections for Instrumental and Secondary Broadening in the Chromatographic Analysis of Linear Copolymers

Abstract

The effects of instrumental broadening and of polymer/solvent/packing interactions (secondary broadening) are separately investigated in the context of chromatography experiments, for the characterization of linear copolymers with two types of repeating units. The problems associated to the estimation of: a) the joint molecular weight distribution - chemical composition distribution (MWD-CCD) through orthogonal chromatography; and of b) the average MWD - average CCD through standard size exclusion chromatography (SEC) with dual detection are considered. The main difficulty with the secondary broadening correction, is the calibration for this effect. In the case of standard SEC with dual detection, a simple solution was found to the instrumental broadening problem, that involves a direct extension of the calibration and deconvolution techniques developed for linear homopolymers and mass detectors.     

Studies on high-performance size-exclusion chromatography of synthetic polymers. I. Volume of silica gel column packing pores reduced by retained macromolecules

Macromolecules, which stay adsorbed within the active size-exclusion chromatography (SEC) column packings may strongly reduce effective volume of the separation pores. This brings about a decrease of retention volumes of the non-retained polymer samples and results in the increased apparent molar mass values. The phenomenon has been demonstrated with a series of poly(methyl methacrylate)s (PMMA) and a polyethylenoxide (PEO) fully retained by adsorption within macroporous silica gel SEC column from toluene or tetrahydrofuran, respectively. The non-retained probes were polystyrenes (PS) in toluene and both PS and PMMA in THF eluents. The errors in the peak molar mass values determined for the non-retained polymer species using a column saturated with adsorbed macromolecules and considering calibration curves monitored for the original "bare" column packing assumed up to several hundreds of percent. Errors may appear also in the weight and number averages of molar masses calculated from calibration dependences obtained with columns saturated with adsorbed macromolecules. Moreover, the SEC peaks of species eluted from the polymer saturated columns were broadened and in some cases even split. These results demonstrate a necessity not only to periodically re-calibrate the SEC columns but also to remove macromolecules adsorbed within packing in the course of analyses.     

Branched-polymer separations using comprehensive two-dimensional molecular-topology fractionation x size-exclusion chromatography

Branching has a strong influence on the processability and properties of polymers. However, the accurate characterization of branched polymers is genuinely difficult. Branched molecules of a certain molecular weight exhibit the same hydrodynamic volumes as linear molecules of substantially lower weights. Therefore, separation by size-exclusion chromatography (SEC), will result in the co-elution of molecules with different molecular weights and branching characteristics. Chromatographic separation of the polymer molecules in sub-microm channels, known as molecular-topology fractionation (MTF), may provide a better separation based on topological differences among sample molecules. MTF elution volumes depend on both the topology and molar mass. Therefore co-elution of branched molecules with linear molecules of lower molar mass may also occur in this separation. Because SEC and MTF exhibit significantly different selectivity, the best and clearest separations can be achieved by combining the two techniques in a comprehensive two-dimensional (MTFxSEC) separation system. In this work such a system has been used to demonstrate branching-selective separations of star branched polymers and of randomly long-chain-branched polymers. Star-shaped polymers were separated from linear polymers above a column-dependent molecular weight or size.     

The obstruction factor in size-exclusion chromatography. 1. The intraparticle obstruction factor

We report the first of a series of studies on the obstruction factor γ in size-exclusion chromatography (SEC). Here, using narrow dispersity polymer standards we examine how the intraparticle obstruction factor γ(p) depends individually on a number of analyte properties, column characteristics, and user-defined parameters. Far from being constant, γ(p) is seen to vary with analyte molar mass and solvent, as well as with the pore size and particle size of the column packing material, sometimes in seemingly counterintuitive manner. Over the limited temperature range accessible to our equipment, however, no statistically significant change in γ(p) with temperature was discovered. The results presented should be applicable to forms of packed column chromatography other than SEC. The latter technique, however, presents a convenient test bed for quantitative determination of the obstruction factor, due to minimized sorptive mass transfer and longitudinal diffusion contributions to band broadening in most forms of SEC.     

Mathematical Modelling and Computer Simulation of Linear Polymer Degradation: Simple Scissions

Summary: Degradation of a polymer in a reactor by the degrading agent(s) follows a distinct pattern, primarily influenced by structural integrity and reactor environment. This distinct pattern is recorded in the changes in the evolved molecular weight distribution (MWD) or polymer chain length distribution (PCLD) curve characteristics from the initial intact state. Modern size exclusion chromatography (SEC) is the best laboratory‐based method that can clearly provide these plots in the form of chromatogram; however, detailed molecular information is not available. The nature of molecular destruction can be well‐characterised if the distinct MWD shift patterns can be simulated to fingerprint the different chain scission dynamics. This is investigated by our current research using the power of computer simulation techniques to gain insight into the polymer ageing processes. One such technique for studying simple decay processes is presented here, and the results are compared with experimental findings. The concept of a binary tree scission model is introduced to show chain rupture as a sequence of probabilistic events and as a non‐linear function of time. Two new mathematical algorithms, an iterative Monte Carlo structured probability scheme and a semi‐iterative algebraic exact statistical formulation method, are investigated to implement this model and simulate the evolution of resultant temporal MW distribution. The latter, an innovative approach to mathematical modelling, has the potential to generate a statistically perfect instant MWD decay curve. A statistical comparison of the product yield is presented from the data obtained using a wide variety of simulated scission regimes to determine the sources of variability.

Simulated MWD lateral shift for percent cut scission model showing deviation from the initial MWD (red) over degradation time zones T_j(0 ≥ j ≤ 9) with bimodal and curve broadening effect due to accumulation of varied percent cut range 5–30%.     

Characterizing property distributions of polymeric nanogels by size-exclusion chromatography

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.     

Separation of complex branched polymers by size-exclusion chromatography probed with multiple detection

Size-exclusion chromatography (SEC) separates polymers by hydrodynamic volume (the universal calibration principle). Molecular weights can be determined using viscometry (relying on universal calibration) and light scattering (independent of universal calibration). In the case of complex branched polyacrylates with tetrahydrofuran as eluent, universal calibration is valid, although the separation in term of molecular weight is incomplete: a given elution slice contains a range of molecular weights, described in terms of a 'local polydispersity'. The local polydispersity index decreases when the number of branches per chain increases and complete separation is reached for highly branched chains.     

Multi-elemental analysis of solidified mineral melt samples by Laser-Induced Breakdown Spectroscopy coupled with a linear multivariate calibration

Laser-Induced Breakdown Spectroscopy (LIBS) has been successfully applied for multi-elemental analysis of solidified mineral melt samples containing several oxides present in various concentrations. The plasma was generated using a Nd:YAG laser and the spectra were acquired using an Echelle spectrometer, coupled to an ICCD detector, which covers a spectral range from 200 to 780 nm. Using a set of 19 calibration samples, we first established univariate calibration curves for the major elements (Al, Fe, Mg, Ca, Ti and Si). We found out that the presence of matrix effects makes such a model, traditionally used in LIBS, not satisfying for quantitative analysis of such samples. Indeed, no sufficiently linear trends can be extracted from the calibration curves for the elements of interest considering all the samples. Instead, a much more robust calibration approach was obtained by considering a multivariate model. The matrix effects are then taken into account by correcting the spectroscopic signals emitted by a given species due the presence of the others ones. More specifically, we established here a calibration model using a 2nd order polynomial linear multivariate inverse regression. The capability of this approach was then checked using a 2nd set of samples with an unknown composition. A good agreement was observed between the analysis provided by X-ray fluorescence (XRF) and the LIBS measurements coupled to the multivariate model for the unknown samples.     

Contactless conductivity detection of synthetic polymers in non-aqueous size-exclusion electrokinetic chromatography

A capacitively coupled contactless conductivity detection (CCD) system has been applied for the detection of neutral synthetic polymers in capillary size-exclusion electrokinetic chromatography (SEEC). Polystyrene standards, that were used as a model compounds, were separated on a capillary column packed with porous 10 microm silica particles with an electrokinetically driven mobile phase, and detected by CCD and UV detection simultaneously. Mass-calibration curves for polystyrene were constructed. Satisfactory results were obtained for the linearity, the run-to-run repeatability (<0.2% for the relative retention and <4% for the peak area) and the robustness of the detector. One of the major issues in this preliminary study was to investigate the origin of the peaks observed for the polystyrene standards. The effect of the molar mass of the polystyrenes on the sensitivity was small. Therefore, the signals obtained could not be explained as the result of an increased viscosity and a decreased solution conductivity of the solute zone. An alternative hypothesis is suggested, and recommendations for further research are given.     

Association behaviour of lignins and lignin model compounds studied by multidetector size-exclusion chromatography

SEC elution curves of spruce milled wood lignin (MWL) and guaiacyl lignin polymer models (G-DHPs) in N,N-dimethylformamide (DMF) exhibited a bimodal elution profile. Light scattering measurements indicated that these elution profiles were due to association effects between the molecules. This became apparent from the determination of high molar masses in the range 10(5)-10(8) g/mol. To study this effect, MWL and DHP were fractionated by precipitation in tetrahydrofuran (THF). The THF-insoluble fractions were found to be the fractions corresponding to the apparent high molar mass part of the DMF elution profiles. The THF-soluble fractions proved to be the less-associated fractions, with lower apparent molecular mass. The individual fractions proved to be rather stable in DMF. Accordingly, the bimodal elution profiles of the starting materials were not the result of an equilibrium between associated and molecular dispersed molecules but of different structures exhibiting a specific and stable association pattern. The different fractions were further characterised by SEC in THF after acetylation to determine molar masses in molecular disperse solutions.     

Monte Carlo simulation of size exclusion chromatography for branched polymers formed through free‐radical polymerization with chain transfer to polymer

The elution curves of size exclusion chromatography (SEC) for branched polymers formed through free‐radical polymerization that involves chain transfer to polymer were theoretically investigated by using a Monte Carlo method. We considered two types of measured molecular weight distribution (MWD), (1) the calibrated MWD relative to standard linear polymers, and (2) the MWD obtained by using a light scattering photometer (LS) in which the weight‐average molecular weight of polymers within the elution volume is determined directly. It was found that the calibrated MWD clearly underestimates the high molecular weight tail, and the measured distributions are narrower than the true MWD. On the other hand, the present simulation results showed that the LS method gives reasonable estimates of the true MWDs. The mean square radius of gyration of the polymer molecules having the same molecular weight was also investigated. The radii of gyration showed clear deviation from the Zimm‐Stockmayer equation^[1] because of the non‐random nature of branched structure and the difference in the primary chain length distribution.     

Determination of fragrance allergens in cosmetics by size-exclusion chromatography followed by gas chromatography-mass spectrometry

A method using size-exclusion chromatography (SEC) combined with gas chromatography-mass spectrometry (GC-MS) was developed for the quantitation of 24 restricted allergenic fragrance compounds in cosmetic samples. To achieve reproducible results fragrance calibration has to be performed with propyl acetate as a solvent containing a constant proportion of matrix components. With the exception of hydroxycitronellal (66+/-5%) all compounds showed good recovery rates in the range of 90-120%. The mean accuracy (relative error) was 1+/-10% for all 24 compounds in five spiked creams (10 mg/kg per allergen) and 8+/-34% in a reference sample (4-15 mg/kg). The biggest benefit compared to other methods is the flexible clean up with SEC which allows the determination of a large range of compounds in difficult matrices with GC-MS.     

Off-line size-exclusion chromatographic fractionation-matrix-assisted laser desorption ionization time-of-flight mass spectrometry for polymer characterization. Theoretical and experimental study

The parameters affecting the fractionation performance in size-exclusion chromatography (SEC) of broad polymer samples were investigated. Some equations were derived which enable the prediction of polydispersity (PD) in an SEC fraction. Good agreements were obtained between the calculated data and the experimental values. Based on these equations, SEC fractionation conditions were optimized. In the off-line SEC-matrix-assisted laser desorption ionization (MALDI) time-of-flight (TOF) mass spectrometry (MS), two different modes can be employed, i.e., using MALDI-MS to provide an absolute calibration curve for SEC, or using SEC as a sample preparation step for MALDI-MS measurements. It was demonstrated that it is more reliable to use the latter combination, because most problems inherent in SEC can be circumvented. Some guidelines for the optimization of off-line SEC fractionation-MALDI-TOF-MS were given. It was found that under optimized conditions normally only a few SEC fractions are already sufficient to separate a highly polydisperse sample into portions of low PD that can accurately be measured by MALDI-TOF-MS.