Principles and potential of solvent gradient size-exclusion chromatography for polymer analysis

The properties of a polymeric material are influenced by its underlying molecular distributions, including the molecular-weight (MWD), chemical-composition (CCD), and/or block-length (BLD) distributions. Gradient-elution liquid chromatography (LC) is commonly used to determine the CCD. Due to the limited solubility of polymers, samples are often dissolved in strong solvents. Upon injection of the sample, such solvents may lead to broadened or poorly shaped peaks and, in unfavourable cases, to "breakthrough" phenomena, where a part of the sample travels through the column unretained. To remedy this, a technique called size-exclusion-chromatography gradients or gradient size-exclusion chromatography (gSEC) was developed in 2011. In this work, we aim to further explore the potential of gSEC for the analysis of the CCD, also in comparison with conventional gradient-elution reversed-phase LC, which in this work corresponded to gradient-elution reversed-phase liquid chromatography (RPLC). The influence of the mobile-phase composition, the pore size of the stationary-phase particles, and the column temperature were investigated. The separation of five styrene/ethyl acrylate copolymers was studied with one-dimensional RPLC and gSEC. RPLC was shown to lead to a more-accurate CCD in shorter analysis time. The separation of five styrene/methyl methacrylate copolymers was also explored using comprehensive two-dimensional (2D) LC involving gSEC, i.e. SEC × gSEC and SEC × RPLC. In 2D-LC, the use of gSEC was especially advantageous as no breakthrough could occur.

Identification and quantification of polymeric impurity in block copolymer by one-dimensional and two-dimensional liquid chromatography coupled to high-resolution mass spectrometry and evaporative light scattering detector

Identifying and quantifying polymeric impurities in a polymeric material is critical for understanding material quality and performance, but it remains a challenge requiring developing new characterization methods. In this work, a comprehensive two-dimensional liquid chromatography method with simultaneous evaporative light scattering and high-resolution mass spectrometry detection was developed to separate and identify a polymeric impurity in alkyl alcohol-initiated polyethylene oxide/polybutylene oxide diblock copolymer. Size exclusion chromatography was implemented in the first dimension, and gradient reversed-phase liquid chromatography using a large-pore C4 column was applied in the second dimension using an active solvent modulation valve as the interface to minimize the polymer breakthrough. The two-dimensional separation significantly reduced the complexity of the mass spectra data compared to the one-dimensional separation, and the combination of retention time and mass spectral interpretation led to the successful identification of the water-initiated triblock copolymer impurity. This identification was confirmed by comparison with the synthesized triblock copolymer reference material. A one-dimensional LC method with evaporative light scattering detection was employed to quantify the triblock impurity. The impurity level in three samples made with the different processes was determined to be in the range of 9-18 wt% using the triblock reference material as the standard.

Critical parameters of liquid chromatography at critical conditions in context of poloxamers: Pore diameter, mobile phase composition, temperature and gradients

At the borderline between size exclusion chromatography (SEC) and interaction chromatography (IC) there is a special mobile phase composition and temperature at which polymer chains become "chromatographically invisible". This point is termed as "chromatographic critical point" and chromatographic separations performed using these conditions are called "liquid chromatography at critical conditions" (LCCC). LCCC is a powerful technique in the analysis of functional polymers and block copolymers. At these so-called critical conditions molar mass discrimination of any specific homopolymer is suppressed rendering elution of whole range of molar mass at same elution volume. These conditions allow enhanced separation with regard to non-critical segment either in exclusion or interaction regime of the polymer chromatography. This article is intended to critically discuss different parameters that can be maneuvered to improve separation and in turn characterization of non-critical segment of block copolymers at LCCC. Different experimental parameters evaluated in this study include pore size of the column, mobile phase composition, temperature and gradients. These parameters can be adeptly adjusted to improve separation of non-critical segment while keeping the other segment close to critical conditions. Current study demonstrates that pore diameter and mobile phase are the only practical variable that can be used for improvement of characterization of non-critical block in the block copolymer while non-critical block is in exclusion regime. On the other hand, pore diameter of the column, temperature, solvent composition and gradients are important parameters that can be skillfully tuned for improvement of separation of non-critical block while non-critical block elutes in interaction regime. The above-mentioned variations are evaluated for di-block as well as tri-block copolymers of A-B-A and B-A-B type. Moreover, LCCC-IC is especially important for analysis of poloxamers.

Chromatographic characterization of amphiphilic di- and tri-block copolymers of poly(ethylene oxide) and poly(ε-caprolactone)

Amphiphilic di- and tri-block copolymers based on poly(ethylene oxide) as a hydrophilic segment and poly(ε-caprolactone) as a hydrophobic part are synthesized by the ring-opening polymerization of ε-caprolactone while using poly(ethylene glycol)s and methoxy poly(ethylene glycol)s of varying molar masses as macro-initiators. The synthesized block copolymers are characterized with respect to their total relative molar mass and its distribution by size exclusion chromatography. Liquid chromatography at critical conditions of both blocks is established for the analysis of individual block lengths and tracking presence of unwanted homopolymers of both types in the block copolymer samples. New critical conditions of polycaprolactone on reversed phase column are reported using organic mobile phase. The established critical conditions of polycaprolactone extended the applicable molar mass range significantly compared to already reported critical conditions of polycaprolactone in aqueous mobile phase. Block copolymers are also analyzed at critical conditions of poly(ethylene glycol). Complete analysis of the di- and tri-block copolymers at corresponding critical conditions provided a fair estimate of molar mass of non-critical block besides information regarding presence of homopolymers of both types in the samples.

Separation and identification of polymeric dispersants in detergents by two-dimensional liquid chromatography

Polymeric dispersants are an important ingredient in many consumer products. Their separations and identifications in final product formulation can be very challenging due to the presence of multiple polymeric dispersants at different levels and the presence of other polymeric and small-molecule components. In this study, using nearly comprehensive two-dimensional liquid chromatography (2D-LC), various water-soluble polymer and co-polymer dispersants were separated with aqueous size exclusion chromatography (SEC) in the first dimension (¹D) and gradient elution reversed-phase liquid chromatography (RPLC) in the second dimension (²D). Detection of the polymeric dispersants was accomplished by evaporative light scattering detector (ELSD). A large ID (8.0 mm) SEC column used in common one-dimensional SEC practices was directly adopted in the 2D setup for rapid method development. A close representation of fully comprehensive 2D separation was achieved even with 60% of ¹D eluent diverted to waste, demonstrating the flexibility and versatility of having SEC in ¹D for two dimensional separation of polymers. Important method parameters, such as ²D column dimensions and flow rate, gradient conditions, and buffer pH were studied. Practical aspects of routine industrial applications such as solvent consumption and analysis time were also considered. This method was exploited for quick identification of polymeric dispersants in commercial detergent samples. Nine detergent samples were screened and polymeric dispersants and additional polymer features were detected in the samples.

Application of two-dimensional chromatography to the characterization of macromolecules and biomacromolecules

Modern polymeric materials are heterogeneous with respect to different structural features, for instance molar mass, composition, and architecture. One-dimensional separation methods such as size-exclusion chromatography (SEC) are insufficient to fully resolve the multidimensional distributions of such complex materials. Therefore, two-dimensional separation methods are increasingly employed to characterize macromolecules. The present article describes in detail the advantages and experimental aspects of two-dimensional macromolecular separations. Selected examples will be discussed to explain the strategies used to separate macromolecules with respect to specific structural features.

Multi-dimensional separations of polymers

Synthetic polymers and comprehensive two-dimensional liquid chromatography (LC × LC) are a synergistic combination. LC × LC provides unique insights in mutually dependent molecular distributions. Synthetic polymers offer clear demonstrations of the value of LC × LC.