Molecular Weight Determination

The molecular weight of a polymeric or biological material usually has a direct effect on its mechanical properties, thermal behavior, and viscosity characteristics.  There are a variety of analytical techniques used to characterize molecular weight in polymers, each providing useful information about the influence of molecular weight on the measured properties. CPG offers molecular weight analysis using GPC, Dilute Solution or Intrinsic Viscometry (IV) and Melt Flow Index (MFI).

Gel Permeation Chromatography (GPC)

GPC is the most commonly used technique for determination of molecular weight. In GPC, a dilute solution of the analyte in question is prepared using a good solvent for the analyte to ensure good dissolution and expansion of the polymer chains in solution. The solution is injected into a packed column containing a distribution of pore sizes which separate the polymer chains by size. Exiting the column, the size-separated chains are effectively ‘counted’ by a refractive index detector according to their elution time from the column. A series of calibration standards (polymers with known molecular weight) are also run through the column, which allows correlation of the elution time to a molecular weight. This analysis allows the calculation of a molecular weight distribution, along with molecular weight moments (number-, weight-, and z-averaged molecular weights).

CPG can perform GPC on water-soluble polymers and natural materials, as well as organic-soluble materials, even at elevated temperatures. GPC is particularly useful in assessing degradation behavior in materials, as well as lot-to-lot variation in raw resin.

In addition to the standard refractive index detector, CPG can also perform multi-angle light scattering to provide absolute molecular weight, and viscometry, to provide a universal calibration method. The coupling of light scattering with viscometry also allows determination of degree of branching in branched polymers.

Relevant standards

ASTM D6474 Standard Test Method for Determining Molecular Weight Distribution and Molecular Weight Averages of Polyolefins by High Temperature Gel Permeation Chromatography

ASTM D5296 Standard Test Method for Molecular Weight Averages and Molecular Weight Distribution of Polystyrene by High Performance Size-Exclusion Chromatography

ISO 13885-1 Binders for Paints and Varnishes -- Gel Permeation Chromatography (GPC) -- Part 1: Tetrahydrofuran (THF) as Eluent

Dilute Solution or Intrinsic Viscometry (IV)

Another series of test methods similar to the concepts of GPC, in that they test dilute polymer solutions, are lumped into the category of dilute solution viscometry or intrinsic viscometry. Like GPC, dilute solution viscometry characterizes the molecular weight of a material according to its size in solution, which in turn affects its viscosity (a larger chain results in a higher viscosity). The solution is allowed to flow through a fine capillary in a viscometer, and the flow time is measured relative to the flow time of the neat solvent. The results are normally reported in terms of relative or inherent viscosity. If a series of concentrations are measured, the intrinsic viscosity can be calculated (the viscosity extrapolated to zero concentration), which can then be used to determine a viscosity-averaged molecular weight using Mark-Houwink parameters for the polymer-solvent system used.

Relevant standards

ASTM D445 Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (and Calculation of Dynamic Viscosity)

ASTM D4603 Standard Test Methods for Determination of Solution Viscosities of Polyamide (PA)

ASTM D789 Standard Test Methods for Determination of Solution Viscosities of Polyamide (PA)

ASTM D2857 Standard Practice for Dilute Solution Viscosity of Polymers

ASTM D1243 Standard Test Method for Dilute Solution Viscosity of Vinyl Chloride Polymers

ASTM D1601 Standard Test Method for Dilute Solution Viscosity of Vinyl Chloride Polymers

ASTM D4020 Standard Specification for Ultra-High-Molecular-Weight Polyethylene Molding and Extrusion Materials

ASTM D3616  Rubber—Determination of Gel, Swelling Index, and Dilute Solution Viscosity

ISO 1628 Determination of the Viscosity of Polymers in Dilute Solution Using Capillary Viscometers

Melt Flow Index (MFI)

MFI is perhaps the most commonly reported parameter relating to molecular weight for thermoplastics, as it is a relatively simple test to perform, requires no solvents, and is directly relatable to melt processing behavior. In MFI tests, the solid resin is placed into a heated capillary, and extruded through the die at the end of the capillary under the influence of a specific weight on the plunger pushing the molten resin through the capillary. The mass of material extruded in a given amount of time is measured and reported as the MFI under those test conditions (temperature and mass). The MFI can be correlated to the viscosity-averaged molecular weight, although it is not reported as a molecular weight.

Relevant standards

ASTM D1238 Standard Test Method for Melt Flow Rates of Thermoplastics by Extrusion Plastometer

ISO 1133-1 Determination of the Melt Mass-Flow rate (MFR) and Melt Volume-Flow Rate (MVR) of Thermoplastics