Weight Average Molecular Weight

Melt Flow

Larger polymer chains increase the entanglement density of the melt, resulting in a higher viscosity and a lower melt flow rate. Measuring the weight average molecular weight helps to predict how the resin will behave under shear during extrusion and moulding. This parameter is a primary indicator of whether the material can be processed on a given line.

Mechanical Strength

Tensile and impact properties of a polymer improve as the chain length increases, because the longer chains can absorb more energy before slipping past one another. When the average chain length falls below a specific threshold, the material becomes brittle and prone to cracking under load. This transition point is critical for structural parts used in demanding industrial applications.

In particular, blow moulding and film extrusion processes demand a high melt strength, which requires a resin with a high molecular weight to prevent the parison or bubble from collapsing.

Analytical Method

Gel permeation chromatography is the standard laboratory technique used to measure the molecular weight distribution of a polymer. This method separates the dissolved chains based on their size as they pass through a porous column. The resulting chromatogram provides the data needed to calculate both the weight average and the number average molecular weights.