Quantitation in liquid chromatography of polymers: size-exclusion chromatography with dual detection

Characterizing Styrene Monomer and Oligomers by SEC/MALS/VISC/DRI

Worldwide polystyrene (PS) production in 2020 was approximately 27 million metric tons, distributed among many nations, making it one of the most heavily imported and exported chemicals. Commercially produced PS usually possesses a broad molar mass distribution, often with a substantial oligomeric component. The latter can significantly affect processing and end-use, in addition to having potentially hazardous health effects and to impacting the polymer's export classification by regulatory agencies. Quantitation of the oligomeric region of polymers by size-exclusion chromatography with concentration-sensitive and/or static light scattering detection is complicated by the non-constancy of the specific refractive index increment ( ∂ n / ∂ c ) in this region, which affects the calculated amount (mass fraction) of oligomer in a polymer, molar mass averages, and related conclusions regarding macromolecular properties. Here, a multi-detector SEC approach including differential refractometry, multi-angle static light scattering, and differential viscometry has been applied to determining the ∂ n / ∂ c of n -butyl terminated styrene oligomers at each degree of polymerization from monomer to hexamer, and also of a hexadecamer. Large changes in this parameter from one degree of polymerization to the next are observed, including but not restricted to the fact that the ( ∂ n / ∂ c ) of the monomer is less than half that of PS polymer at identical experimental conditions. As part of this study, the individual effects of injection volume, flow rate, and temperature on chromatographic resolution were examined. Incorporation of the on-line viscometer allowed for accurate determination of the intrinsic viscosity and viscometric radius of the monomer and oligomers.

Detection challenges in quantitative polymer analysis by liquid chromatography

Accurate quantification of polymer distributions is one of the main challenges in polymer analysis by liquid chromatography. The response of contemporary detectors is typically influenced by compositional features such as molecular weight, chain composition, end groups, and branching. This renders the accurate quantification of complex polymers of which there are no standards available, extremely challenging. Moreover, any (programmed) change in mobile-phase composition may further limit the applicability of detection techniques. Current methods often rely on refractive index detection, which is not accurate when dealing with complex samples as the refractive-index increment is often unknown. We review current and emerging detection methods in liquid chromatography with the aim of identifying detectors, which can be applied to the quantitative analysis of complex polymers.

Characterization of poly(methyl methacrylate)-graft-poly(styrene)s using various chromatographic techniques

Two graft copolymer samples of identical average composition were synthesized by grafting living polystyrene anions onto a broadly distributed PMMA backbone. Size exclusion chromatography (SEC) with only RI-detection, SEC with viscometry and light scattering detection, SEC with UV and RI dual detection, gradient chromatography and 2-dimensional chromatography were applied to compare the information that can be obtained by the different techniques. While only limited information was retrieved by conventional SEC or SEC with molar mass sensitive detection, SEC with UV and RI revealed different chemical heterogeneity of the samples. Using gradient chromatography and 2-dimensional chromatography it was possible to identify non-grafted side chains and unreacted parent PMMA besides the actual graft copolymer molecules. While in one sample a heavily grafted product was formed besides non-grafted PMMA, the second sample did not contain ungrafted PMMA but a graft product of lower grafting density. The different product distributions were explained by the different synthetic procedures.

Investigations of multiple detection of polymers

Theoretical and experimental aspects of multiple detection analysis of polymers are critically revised for size exclusion chromatography (SEC). In particular, different combinations of detectors, the importance of the injected mass and the influence of the tacticity of polymers are evaluated in respect to the accuracy of the weight fractions of the polymer components. It is also shown how overlapping detector responses of the chemical components will affect the accuracy of the weight fractions. The calculation of the weight fractions is performed with equations derived for n different chemical components and detectors by using the slopes and intercepts of the linear response equations. Several detector combinations of dual detection in SEC are evaluated with PS-b-PMMA diblock copolymers to determine the comonomer compositions and for the first time different combinations of triple detections are performed for the determination of the weight fractions of a blend of three homopolymers, respectively. The correct determination of the weight fractions of minor and main components of polymers is strongly affected by the chosen combination of detectors, the injected mass and the intercept of the response calibrations. It is shown how these conditions have to be changed to obtain correct quantifications of the weight fractions according to the experimental setups. The experimental results are approved with online SEC-(1)H/NMR where no response factors are required.

Deconvolution of overlapping spectral polymer signals in size exclusion separation-diode array detection separations by implementing a multivariate curve resolution method optimized by alternating least square

Peaks eluting from a size exclusion separation (SEC) are often not completely baseline-separated due to the inherent dispersity of the polymer. Lowering the flow rate is sometimes a solution to obtain a better physical separation, but results in a longer retention time, which is often not desirable. The chemometrical deconvolution method discussed in this work provides the possibility of calculating the contribution of each peak separately in the total chromatogram of overlapping peaks. An in-house-developed MATLAB script differentiates between compounds based on their difference in UV-spectrum and retention time, using the entire 3D retention time UV-spectrum. Consequently, the output of the script offers the calculated chromatograms of the separate compounds as well as their respective UV-spectrum, of which the latter can be used for peak identification. This approach is of interest to quantitate contributions of different polymer types with overlapping UV-spectra and retention times, as is often the case in, for example, copolymer or polymer blend analysis. The applicability has been proven on mixtures of different polymer types: polystyrene, poly(methyl methacrylate) and poly(ethoxyethyl acrylate). This paper demonstrates that both qualitative and quantitative analyses are possible after deconvolution and that alternating concentrations of adjacent peaks do not significantly influence the obtained accuracy.

Characterization of Commercial Polysorbates Using Different Chromatographic Techniques

Polysorbates – which are commercially available under the name Tween – are complex mixtures of different functionalities. Molar mass distribution and chemical composition of various Tween samples (20, 40, 60 and 65) were determined using size exclusion chromatography (SEC) with dual detection. A separation of different functionalities was achieved using liquid chromatography under critical conditions (LCCC) on typical reversed phase columns with methanol – water and acetone – water as mobile phase. The size of the hydrophilic part was determined after microwave-assisted methanolysis, which allowed also the identification of the fatty acids bound to the hydrophilic core.

Characterization of Nanocrystalline CdSe by Size Exclusion Chromatography

High-performance size exclusion chromatography (HPSEC) is a powerful tool for probing the size and size distribution of complex materials. Here we report its application to the analysis of cadmium selenide nanocrystals produced in organic solvents. If nanocrystal-column interactions are minimized, this method provides an accurate measure of nanocrystal hydrodynamic diameter directly in solution; such information is complementary to TEM in that it can measure the thickness of various capping agents. While the resolution of single-pass HPSEC is limited to 1 nm, we show here that recycling size exclusion chromatography can be applied to assess the fine details of a sample's distribution. Finally, semiconductor nanocrystals can be made a variety of shapes whose optical characteristics are difficult to distinguish. HPSEC can be applied to the general problem of shape separations which we demonstrate with a tetrapod material.

Liquid exclusion adsorption chromatography, a new technique for isocratic separation of nonionic surfactants. III. Two-dimensional separation of fatty alcohol ethoxylates

A quantitatively accurate mapping of lower fatty alcohol ethoxylates can be achieved using a combination of liquid chromatography under critical conditions as the first dimension and liquid exclusion-adsorption chromatography as the second dimension. With coupled density and refractive index detection in both dimensions, the contribution of preferential solvation can also be estimated. In most cases, however, the use of refractive index detection alone also yields satisfactory results.

Liquid exclusion–adsorption chromatography, a new technique for isocratic separation of nonionic surfactants

A new technique of liquid chromatography, which allows baseline separation of fatty alcohol ethoxylates with up to 15-20 ethylene oxide units under isocratic conditions allows an accurate quantitative analysis of single hydrophobic chain surfactants. Using density and refractive index detection, the accurate weight fractions of the individual oligomers are obtained. Moreover, the contribution of preferential solvation can be determined. With refractive index detection alone, good accuracy can also be achieved.