Meaning
A small fraction of exceptionally long-chain molecules exists at the extreme high end of a polymer’s molecular weight distribution. Although present in low concentrations, the ultra-high molecular weight tail dominates the melt elasticity and flow behaviour of the resin during processing. This molecular fraction enhances melt strength, which is useful in film blowing and extrusion blow moulding where the melt must support its own weight.
It is controlled during polymerization and can increase the risk of melt fracture if it is too pronounced.
Polymer Structure
High entanglement density of these long chains increases the viscosity of the polymer. These chains become entangled with shorter chains, resisting deformation and flow. Identifying this fraction requires advanced gel permeation chromatography.
Melt Strength
In processes where a molten shape must be suspended, such as a parison in blow moulding, the long chains prevent the melt from sagging or tearing. The entanglements provided by the molecular tail act as physical network points that store elastic energy during extension. This elastic behavior allows processors to produce large, thin-walled containers with uniform thickness.
Processing Influence
While beneficial for melt strength, these long chains can lead to processing challenges in high-shear environments. They require longer relaxation times, which means that any orientation introduced during flow cannot be quickly dissipated before the polymer solidifies. This retained orientation can cause molded parts to warp or exhibit molded-in stresses that reduce impact resistance, which increases the scrap rate in production.
