Intrinsic viscosity in conjunction with size exclusion chromatography (SEC-IV) has long been used by the polymer community to characterize molar mass. The two primary methods are Mark-Houwink analysis (using empirical values of Mark-Houwink coefficients from the literature) and Universal Calibration, which is appropriate for all linear, random coil polymers. Though these techniques are less rigorous than MALS, SEC-IV may be the optimal means of characterizing materials not amenable to SEC-MALS analysis:
- When the solvent and solute have the same refractive indices (making dn/dc=0), the polymer does not scatter light
- When the polymers fluoresce under excitation by the MALS laser, the fluorescence greatly increases the apparent scattering signal producing erroneous molecular weights. While a fluorescence blocking filter may be incorporated into the DAWN HELEOS II, this does not always provide a sufficient solution to strongly fluorescing polymers.
The ViscoStar III differential viscometer coupled with an Optilab T-rEX differential refractometer to any GPC system provides superior measurements of intrinsic viscosity for characterization of all types of polymers. The ASTRA software not only analyzes polymers based on the Mark-Houwink equation and Universal Calibration technique but, with a MALS detector, can even calculate Mark-Houwink coefficients.
But these instruments can do much more! By simply adding a DAWN HELEOS II, transform a basic SEC-IV setup into a powerful SEC-MALS-IV polymer characterization station, to analyze from first principles:
- Absolute determination of molar mass and size regardless of conformation
- Branched polymers
- Measurement of Mark-Houwink coefficients ab initio