Subin Damodaran & Regina Roemling discuss branching analysis of polymers through triple-detection SEC/GPC.
Branching considerably impacts the material properties of polymers, such as flexibility, impact resistance and processability, making them highly adaptable across a wide array of applications. To understand the structure, shape and conformation of polymers in solution, size exclusion chromatography (SEC) together with advanced detectors such as multi-angle light scattering (MALS) and viscometry proves indispensable.
In a triple-detection SEC setup, the refractive index (RI) detector gauges concentration, MALS provides true molecular weight and radius of gyration (Rg), while the viscometer measures intrinsic viscosity (IV). The intrinsic viscosity of polymers reflects how dense and how flexible polymer chains are in dilute solutions. The lower the IV, the more compact the polymer is while rigid polymer chains show a higher IV than flexible random coils. A polymer with an extended shape or low density in solution will show a higher intrinsic viscosity than a polymer with a compact structure or high density. A branched polymer will have a more compact structure compared to a linear polymer, resulting in a reduced IV and Rg.
When combined with light scattering measurements, the MW and IV distributions provide a wealth of structural information about polymers, including: conformation (random coil, sphere, rod); density of the backbone; presence of branching (number of branches, branching frequency);hydrodynamic radius (Rh); and Mark-Houwink parameters.
The viscometer also allows the determination of MW by universal column calibration for comparison with legacy methods or for special applications to polymers with unfavorable optical properties (e.g. weak scatterers, light absorption, or fluorescence).
Tosoh recently launched the LenS3 MALS-V, an integrated light scattering and viscometry dual detector (Fig. 1). This detector offers a simple and compact solution for triple detection measurements when combined with a concentration detector (RI or UV). In the example depicted in Fig. 2, SEC on a TSKgel GMPWXL column (13 µm, 7.8 mm ID x 30 cm L) using an EcoSEC Elite GPC system with RI coupled to the LenS3-V detector to enable triple detection was used to determine the structure of a high molecular weight dextran. The RI chromatogram showed a very small shoulder at the high molecular weight region whereas the LALS signal showed a high intense peak. This indicates that the shoulder consists of very high molecular weight dextran. The slope of IV trace (yellow) becomes smaller in the section of the very high molecular weight LALS signal. This indicated that the high molecular weight dextran has a higher degree of branching
– a more compact structure – than the lower molecular weight dextran.
In the given example, higher MW dextrans tend to exhibit increased branching on their backbone, leading to the formation of a more compact structure in solution. In practice, elucidating structural changes in the low MW and low Rg region requires a light scattering instrument with high sensitivity and capable of detecting very slightly anisotropic scattering. Additionally,
the triple detection analysis allowed us to quantify the branching. Combined with the EcoSEC Elite GPC system and TSKgel columns, the new LenS3 MALS-V offers a complete, cost-effective, and high-performance approach for triple detection. Just like with light scattering data, the SECview software includes seamless viscometry data acquisition and processing to get the most out of advanced detection GPC/SEC analyses.
Subin Damodaran & Regina Roemling are with Tosoh Bioscience.