New analysis method helpsdevelop improved polymers

1st April 2013

More accurate polymer analysis is helping develop improved polymers at a faster rate than ever before. Dr David Niehaus describes a new analysis method ­ the size exclusion chromatography cubed (SEC3) method which combines SEC,a laser light-scattering detector, a refractive index detectorand a viscometer detector to accurately characterise molecularweight, size, and branching.

Molecular weight distribution is a critical factor in the production of nylon-based polymers because it is directly tied to melt viscosity, a characteristic that is important both in processing and in the performance of the end product. In the past, chemists dissolved the experimental nylon polymers in hexafluoroisopropanol (HFIP) and used conventional size exclusion chromatography to determine their molecular weight.

This method was not as accurate as researchers needed because the broad molecular weight distribution of nylons and polyesters is difficult to match with a standard.

Universal calibration, a molecular weight calibration technique, was not a viable option because the HFIP solvent is highly polar while the size exclusion chromatography (SEC) column packings are non-polar. This mismatch allows the polymer to interact with the packing with the result that separation is not based exclusively on size.

As a result, chemists could not get exact molecular weights with this approach. At best it was comparative tool, showing that the molecular weight of one batch of polymer was higher or lower than that of another batch (Fig. 1).

Another problem with the conventional SEC method is that it does not provide any information on branching, which was often important. Nylon often branches, and branching frequently has a critical impact on polymer properties. Branching information is especially critical in designing a reactor or transfer line in order to help researchers determine whether individual processing steps are adding branching points.

Several years ago, DuPont researchers began evaluating the size exclusion chromatography cubed (SEC3) method. They configured a system consisting of a Model T60A instrument from Viscotek Corporation, Houston, Texas, consisting of dual viscometer and light scattering detectors and combined it with a Waters 410 refractive index detector and an autosampler.

The Viscotek viscometer-light scattering system also includes a four-channel data manager and the sophisticated TriSEC Windows-based data processing and graphics software. The chemists integrated the instruments, so polymer molecules elute from the SEC columns and are monitored simultaneously in real-time by all three detectors connected to the system. Adding these three detectors to the SEC technique provides a three dimensional approach. The first dimension is the chromatographic process that separates polymer molecules according to molecular size.

The second is the light scattering detector response that yields molecular weight. The third dimension comes from the viscometer detector, which gives a response, the intrinsic viscosity that is inversely proportional to molecular density. Together these variables provide a detailed picture of the molecular structure.

Intrinsic viscosity

The intrinsic viscosity has units of volume per unit mass and represents the specific volume of the molecules in solution ­ the inverse of the molecular density. The viscometer measures the intrinsic viscosity of the sample and the standard.

Since the molecular weight, polydispersity and refractive index change with concentration (dn/dc) of the standard are known, it is possible to calculate the molecular weight of the sample using a single calibration standard. This approach provides the absolute molecular weight, thus eliminating the assumption that the sample and standard have the same molecular structure. The light scattering detector eliminates the need to construct a column calibration curve. All that is required is an injection of a standard of known molecular weight. All three detectors look at this sample simultaneously.

Quantitative calculations using all three chromatograms yield the molecular weight distribution, molecular size distribution and the intrinsic viscosity (IV) distribution. The relationship among these variables is shown most usefully in a Mark-Houwink plot, which is a log-log plot of IV against molecular weight and Radius of gyration (Rg) vs molecular weight for a sample. This plot clearly indicates differences in branching even for samples with the same molecular weight. Being more compact in structure, branched molecules are seen to have much lower IV values than the linear molecules of the same molecular weight. Using a linear standard as a reference, it is possible to calculate the number of branches and branching frequency. The additional information provided by the SEC3 method is helping R & D chemists develop new polymers.

The ability to measure molecular weight distribution far more accurately than in the past helps researchers correlate physical properties to molecular structure with a higher degree of certainty. The net result has been a significant improvement in DuPont's understanding of polymer properties.

Dr David Niehaus is a research chemist with DuPont, Wilmington, Delaware, USA. Contact Viscotek Europe, Tel: +44 (0)1256 473000.





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