Gel-permeation chromatography (GPC) is a type of size-exclusion chromatography (SEC) that allows for determining molar mass distribution of a polymer in a solution in ranges 10³ – 10⁶ g/mol. It provides such parameters of a polymer as:

• Dispersity Ð (outdated polydispersity index PDI), characterizing the broadness of the distribution of the molecules’ molar masses in a polymer

• Number-average molar mass M_n

• Weight-average molar mass M_w

• Z-average molar mass M_z

• Viscosity-average molar mass M_v

The molar mass distribution influences the mechanical properties of a polymer, its behavior in a solution, its interactions in a mixture, the rate of degradation, the formation of particles and many other properties. GPC can help you, for example, to determine the results of your synthesis, the degradation rates, to estimate the composition of a polymer blend, or to quality control your material source.

A single GPC analysis requires only 20 mg of a polymer. However, for the sake of manipulation, possible errors, or repetitive measurements, we would ask for at least 100 mg of a polymer. The sample must be provided dry in a tightly closed container. Please inform us, if the sample is sensitive to light, moisture or anything else and pack it accordingly. To be analyzed, the polymer has to be soluble in tetrahydrofuran (THF), chloroform, or hexafluoroisopropanol (HFIP). If you don’t know it about your sample, ask us ! The samples cannot be recovered after the analysis

The report includes a table with instrument raw data; dispersity Ð, number-average molar mass M_n, weight-average molar mass M_w, z-average molar mass M_z, and viscosity-average molar mass M_v calculated against standard or universal calibration with the instrument software, a plot fraction vs. molar mass as a pdf-file calculated against standard or universal calibration.

Good solubility of a polymer in the chosen solvent is the key to reliable results in GPC. A theta-solvent is a solvent in which the polymer molecules behave like ideal chains. In other words, the orientation of each unit is not affected by the orientation of any other unit in a chain. This makes it possible to establish a unique relationship between the hydrodynamic volume of a molecule and its molar mass. In addition, the analytical solutions are prepared very thinly, which prevents the polymer molecules from interacting with each other.
The mobile phase or eluent is a pure solvent that is pumped through the GPC system at a constant rate. When the sample solution is fed, it is passed by the eluent through a chromatographic column filled with the stationary phase. This phase is a microporous gel with a wide distribution of pore sizes. The largest molecules pass through the column without entering the pores because they do not fit in spatially. Vice versa, the smallest molecules are able to penetrate into all pores and their path through the column is the longest. Therefore, they are the last to leave the column. In a GPC setup, polymeric molecules are separated according to their hydrodynamic volume as elution time.
At the exit from the column, the eluted mobile phase is analyzed with several detectors that determine concentration of polymers in it. Refractive index (RI) detector – the machine compared RI of the pure solvent with the RI of the eluted solution, by this analysing the concentration of polymeric molecules in real time. UV-detector – investigated the intensity of the selected UV-absorption band that can be characteristic to the investigated polymer.

The intensity versus elusion time is transformed into the molar mass distribution using a calibration curve. The calibration is done by measuring monodisperse standards, e.g. polystyrene. Comparison of the measured data to the polystyrene standards gives values of molar masses in the standard calibration. The separation of polymeric molecules with GPC follows the law for intrinsic viscosity of polymeric solutions [η] = KMα, where K and α are constants inherent to each polymer-solvent pair and M is the molar mass. Plotting a logarithmic [η](M) provides a universal calibration curve for the given column in the given solvent.