Determining Molar Masses

Analysen von Polymeren

Rheological properties may depend on the molar mass distribution – particularly in the case of elastomers. Research and Development is not the only area that can benefit from a sound understanding of a polymer’s molecular structure; molar mass distribution is also an objective criterion when it comes to product assessment in other areas of application. The countless different molar masses of a polymer can be separated from each other using gel permeation chromatography (GPC). The molar mass in relation to the concentration, or molar mass distribution, is ultimately obtained using suitable detectors. This figure is then used to calculate the known molar mass averages.

For GPC, the polymer must be dissolved in a solvent without any chemical reaction, meaning the polymer chains are left unchanged. The molecules form a ball with a radius dependent on the molar mass. These molecules pass through a highly porous column as part of the GPC process. The driving force is a solvent with a specific flow velocity. The molecules diffuse into pores at different rates and therefore require different throughput times. Large molecules cannot diffuse into most pores and remain in the fast solvent stream. Small polymers constantly diffuse into pores and gaps and therefore require more time. The characteristic used as the basis for separation is therefore the hydrodynamic radius. A concentration-dependent detector then picks up the signal and calculates the concentration. A suitable polystyrene calibration can be used to obtain the molar mass for this purpose.

So far, we’ve been dealing with traditional GPC, but CURRENTA Analytics also has other combination methods at its disposal – most notably light scattering detection, which is a method of determining absolute molar mass. The polymer particles scatter to form a characteristic pattern, which is recorded from multiple angles. The molar mass, the second virial coefficient and the radius of gyration can all be calculated using an appropriate mathematical model. Conclusions can be drawn about the shape of the polymer by comparing the molar mass with the radius. Another helpful detection method is online viscosimetry, which can be used to determine an intrinsic viscosity for each molar mass. Plotting these two factors side by side makes it possible to make statements about short and long chain branching.

Identifying long chain branching using GPC viscosimetry
Identifying long chain branching using GPC viscosimetry

Most plastics do not consist of just one single polymer. In addition to additives, complex polymers feature different chemical compositions, sizes and shapes. The aim of the HPLC-GPC combination is to reflect the heterogeneity of these samples by producing a three-dimensional graphic result. The sample is separated on an HPLC column according to its chemical composition, then on a GPC column according to its molar mass or hydrodynamic radius. One way this method could be applied would be for characterizing the copolymer nitrile butadiene rubber (NBR). Depending on how it’s produced, a sample will contain a different proportion of acrylonitrile (ACN). The following two diagrams show both the molar mass and the ACN content, making it possible to distinguish copolymers.

Separating NBR by chemical composition (HPLC) and molar mass (GPC)
Separating NBR by chemical composition (HPLC) and molar mass (GPC)

At CURRENTA Analytics, we have the option of adapting the chromatography system, the solvent and the analytical column to the sample. Since the sample must be dissolved in a solvent, we are not only free to select the solvent, but we also have the option of performing GPC at temperatures of up to 160 °C. This kind of “high-temperature GPC” (HT-GPC) is often used for polyolefins. In addition, combined IR spectroscopy can also be used for chemical identification.

Finally, it’s worth mentioning the special separation method “asymmetrical flow field-flow fractionation” (AF4) once again. In this process, the polymer is held on a membrane by a second orthogonal solvent stream. Depending on their size and diffusion coefficient, the molecules reach the end of the channel at different speeds and are subsequently detected. The particles do not go through a conventional stationary phase, which is why the biggest advantage of this method is the range of applications it covers. For instance, it can be used to measure nanoparticles and even larger gel or agglomerate particles, in addition to polymers.

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