Determination of Molar Masses of Polymers II

Polymer samples typically consist of macromolecular chains with a distribution of lengths, resulting in a range of molar masses rather than a single discrete value. Conventional descriptors such as the number-average molar mass and weight-average molar mass quantify this distribution but do not fully capture polymer behavior in solution..

The viscosity-average molar mass provides a more realistic description of polymer behavior in solution because it accounts for the enhanced contribution of larger chains to flow resistance. Larger macromolecules occupy greater hydrodynamic volume and create more significant disturbances in solvent flow. Intrinsic viscosity is defined as the extrapolated specific viscosity at zero concentration and reflects the effective volume occupied by a polymer coil.

The dependence of intrinsic viscosity on molar mass is described by the Mark–Houwink relationship, which uses system-specific constants to relate viscosity to molecular size. The constant ‘K’ reflects polymer–solvent interactions, while the exponent ‘a’ provides structural insight into chain conformation. Values of a near 0.5 indicate flexible coils in theta solvents, values between 0.6 and 0.8 correspond to expanded coils in good solvents, and higher values suggest increasingly rigid or rod-like structures.

Intrinsic viscosity is experimentally determined using capillary viscometers by measuring flow times of polymer solutions at different concentrations. Gel permeation chromatography, also known as size exclusion chromatography, provides detailed molar mass distribution profiles. Polymer chains are separated as they pass through columns packed with porous beads. Larger molecules elute first because they cannot enter smaller pores, while smaller molecules penetrate the pores and elute later.