Viscometry & Mark-Houwink

SEC-IV utilizes intrinsic viscosity and empirically determined Mark-Houwink parameters to determine polymer molar masses. The Mark-Houwink parameters depend on the polymer, solvent and conformation.

Though not as rigorous as SEC-MALS, Universal Calibration (UC) is commonly utilized in the analysis of molecular weights of linear polymers. UC determines hydrodynamic volume from retention time on the column, and calculates molar mass at each elution volume from the ratio of hydrodynamic volume and intrinsic viscosity (this analysis is less accurate than SEC-MALS because it ignores the possibility of non-ideal sample-column interactions, and requires reference standards compatible with the mobile phase).

The ViscoStar on-line differential viscometer and Optilab differential refractometer operate in tandem along with ASTRA software to analyze molar mass and intrinsic viscosity distributions of a polymer sample by means of SEC-IV or UC.

These techniques are particularly useful when the refactive indices of the polymer and solvent are closely matched, making the polymer essentially invisible since it will not scatter appreciable quantities of light (dn/dc~0).

Measuring Mark-Houwink parameters

The relationship between molar mass and intrinsic viscosity for a given type of polymer, indicative of molecular conformation, is described by the Mark-Houwink parameter. This value may be obtained in two ways:

by purely viscometric means, using UC to characterize both molar mass and intrinsic viscosity.

by SEC-MALS-IV-RI wherein molar mass is determined by the combination of a DAWN MALS detector and Optilab, while intrinsic viscosity is determined with a ViscoStar and Optilab. The analysis is carried out in the ASTRA software.