Advances in Polymer Characterisation – Multi-Detection GPC

Gel-permeation chromatography (GPC) is an essential polymer characterisation tool. It generates the information required by polymer scientists to tailor a polymer’s properties to its end use requirements by controlling its molecular properties, since the two are inextricably linked. For example, increasing a polymer’s molecular weight can increase its strength but there are many other effects that the molecular properties can have on the bulk ones. Table 1 gives an example of the connections although the real links will be specific to each polymer under study.
Gel Permeation Chromatography (GPC) employs specialized columns to separate natural and synthetic polymers, biopolymers, proteins or nanoparticles on the basis of size.
As the sample is separated and elutes from the column, it can be characterized by a single concentration detector (Conventional Calibration) or series of detectors (Universal Calibration and Triple Detection).
The principle of GPC/SEC is the separation of molecules based on their hydrodynamic radius (Rh) or volume (Vh), not molecular weight. The separation process takes place in a column which is packed with porous, micro particulate material such as styrenedivinylbenzene gel, silica gel etc.
Because of their size, the larger molecules are excluded from some of the pores in the packing material and therefore elute faster through the column than the smaller molecules. In effect, the molecules are sorted by size, with the largest eluting first and the smallest last.
When the GPC/SEC separation is coupled with light scattering, viscometer and Concentration detectors together (triple detection), it will provide a distribution of absolute molecular weight, molecular size, and intrinsic viscosity as well as information on macromolecular structure, conformation aggregation and branching.
Depending on the choice of detectors, various types of calibrations and/or Calculations are employed to compute parameters like molecular weight (MW), molecular weight distribution (MWD), intrinsic viscosity (IV) or molecular density, hydrodynamic radius (Rh), and radius of gyration (Rg). It is also possible to obtain additional information on macromolecular structure, conformation, aggregation, branching and copolymer/conjugate composition.
Gel Permeation Chromatography/Size Exclusion Chromatography (GPC/SEC) is the technique of choice for rapid and reliable characterization of molecular weight and molecular structure for all types of macromolecules – proteins, natural polymers and synthetic polymers.
OMNISEC is the totally new GPC/SEC system from Malvern. It is a complete system for sample separation and detailed molecular characterization.
The technology has advanced, however, and with additional detectors comes a wealth of information. Light scattering detectors, whether SEC-MALS, RALS, or LALS (multi-angle, right-angle or low-angle light scattering), measure absolute molecular weight and molecular weight distribution with excellent accuracy. A viscometer allows measurement of molecular structure and branching, and multiple concentration detectors allow compositional analysis of copolymers. The combination of these detectors allows for complete characterization of polymers without the compromises of relative measurements or chromatography setup dependencies.
Developing novel polymers and achieving their maximum potential can only be achieved when the polymer is fully characterized.
A polymer characterisation study
Here we see a real example of polystyrene (PS), polymethylmethacrylate (PMMA), polycarbonate (PC) and polyvinylchloride. An overlay of duplicate RI chromatograms for each of the four samples with their respective measured molecular weights is shown in Figure 2, while overlays of the molecular weight distributions can be seen in Figure 3. The differences in elution volumes and in the molecular weights at the same retention volumes can clearly be seen. This shows how the different samples, each with their different structures, elute at different time points for a given molecular weight. Light scattering based measurements of molecular weight are immune to these differences, therefore the measured molecular weights are accurate regardless of the structural differences.
Table 2 gives the calculated data for the samples showing the absolute molecular weights calculated directly from the Right Angle Light Scattering (RALS) and Low Angle Light Scattering (LALS) detectors. Also included is Mark-Houwink (M-H) data, calculated from Molecular Weight and Intrinsic Viscosity data and giving assessment of polymer structure. For example, a linear and a branched polymer of the same molecular weight will have different intrinsic viscosities and hydrodynamic radii. Comparison of the parameters generated here in the form of plots such as the Mark Houwink plot allows characterisation of polymer branching etc. sic viscosities, and so different M-H values.
Conclusions
The OMNISEC platform provides high quality, information-rich polymer GPC/SEC data. The molecular weight distribution, polydispersity, intrinsic viscosities, hydrodynamic size and Mark-Houwink parameters have all been calculated for 4 highly useful polymers. The newly designed viscometer allows simple access to powerful intrinsic viscosity and M-H data in greater detail than ever before. This data is of utmost importance to all polymer chemists who want to understand polymer differences or trends in terms of molecular weight and structural changes independently from each other.
By controlling the level of branching, or substitution of a polymer, scientists and manufacturers can control the physical properties of their materials with greater precision. The result of this improved control is higher quality and a higher value product with fewer production failures. In research, it means more successful syntheses, greatly improved data quality and faster publication.
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