Band Broadening Function in Size Exclusion Chromatography of Polymers: Review of Some Recent Developments

Two-dimensional separation of water-soluble polymers using size exclusion and reversed phase chromatography employing capillary-channeled polymer fiber columns

Polymeric materials are readily available, durable materials that have piqued the interest of many diverse fields, ranging from biomedical engineering to construction. The physiochemical properties of a polymer dictate the behavior and function, where large polydispersity among polymer properties can lead to problems; however, current polymer analysis methods often only report results for one particular property. Two-dimensional liquid chromatography (2DLC) applications have become increasingly popular due to the ability to implement two chromatographic modalities in one platform, meaning the ability to simultaneously address multiple physiochemical aspects of a polymer sample, such as functional group content and molar mass. The work presented employs size exclusion chromatography (SEC) and reversed-phase (RP) chromatography, through two coupling strategies: SEC x RP and RP x RP separations of the water-soluble polymers poly(methacrylic acid) (PMA) and polystyrene sulfonic acid (PSSA). Capillary-channeled polymer (C-CP) fiber (polyester and polypropylene) stationary phases were used for the RP separations. Particularly attractive is the fact that they are easily implemented as the second dimension in 2DLC workflows due to their low backpressure (<1000 psi at ∼70 mm sec^-1) and fast separation times. In-line multi-angle light scattering (MALS) was also implemented for molecular weight determinations of the polymer samples, with the molecular weight of PMA ranging from 5 × 10⁴ to 2 × 10⁵ g mol^-1, while PSSA ranges from 10⁵ to 10⁸ g mol^-1. While the orthogonal pairing of SEC x RP addresses polymer sizing and chemistry, this approach is limited by long separation times (80 min), the need for high solute concentrations (PMA = 1.79 mg mL^-1 and PSSA = 0.175 mg mL^-1 to yield comparable absorbance responses) due to on-column dilution and subsequently limited resolution in the RP separation space. With RP x RP couplings, separation times were significantly reduced (40 min), with lower sample concentrations (0.595 mg mL^-1 of PMA and 0.05 mg mL^-1 of PSSA) required. The combined RP strategy provided better overall distinction in the chemical distribution of the polymers, yielding 7 distict species versus 3 for the SEC x RP coupling.

Quasi-monodisperse polymer libraries via flash column chromatography: correlating dispersity with glass transition

Quasi-monodisperse (Đ = 1.005–1.040) poly(methyl) acrylate (PMA) polymer libraries are obtained via flash column chromatography separation of disperse (Đ = 1.130) PMA. The influence of dispersity on the glass transition is investigated.

Size-separation characterization of starch and glycogen for biosynthesis-structure-property relationships

Starch and glycogen are highly branched polymers of glucose of great importance to humans in managing and mitigating nutrition-related diseases, especially diabetes and obesity, and in industrial uses, for example in food and paper-making. Size-separation characterization using multiple-detection size-exclusion chromatography (SEC, also known as gel-permeation chromatography, GPC) is able to furnish substantial amounts of information on the relationships between the biosynthesis, processing, structure, and properties of these biopolymers, and achieves superior characterization for use in industrial product and process improvements. Multi-detector SEC is able to give much more information about structure than simple averages such as total molecular weight or size; the detailed information yielded by this technique has already given new information on important biosynthesis-structure-property reactions, and has considerable potential in this field in the future. However, it must be used with care to avoid artifacts arising from incomplete dissolution of the substrate and shear scission during separation. It is also essential in interpreting data to appreciate that this size-separation technique can only ever give size distributions, never true molecular weight distributions. Other size-separation techniques, particularly field-flow fractionation, require substantial technical development to be used on undegraded native starches.

Starch digestion mechanistic information from the time evolution of molecular size distributions

Size-exclusion chromatography [SEC, also termed gel permeation chromatography (GPC)] is used to measure the time evolution of the distributions of molecular size and of branch length as starch is subjected to in vitro digestion, including studying the development of enzyme-resistant starch. The method is applied to maize starches with varying amylose contents; the starches were extruded so as to provide an analogue for processed food. The initial rates of digestion of amylose and amylopectin components were found to be the same for high-amylose starches. A small starch species, not present in the original starting material, was formed during the digestion process; this new species has a slower digestion rate and is probably formed by retrogradation of longer branches of amylose and amylopectin as they are partially or wholly liberated from their parent starch molecule during the digestion process. The data suggest that the well-known connection between high amylose content and resistant starch arises from the greater number of longer branches, which can form the small retrograded species. The method is useful for the purpose of comparisons between different starches undergoing the process of digestion, by observing the changes in their molecular structures, as an adjunct to detailed studies of the enzyme-resistant fraction.