Rheological Determination of Ultra High Molecular Weight Tails in Bimodal Resins
Low-frequency storage modulus and creep recovery accurately quantify ultra-high molecular weight tails in bimodal resins, ensuring stress crack resistance.
Double Reptation Model
Meaning
Physical theories describing the movement of entangled polymer chains provide a way to link molecular weight to macroscopic flow. The double reptation model accounts for the fact that an entanglement is lost when either of the two involved chains relaxes its ends. This approach improves upon earlier models by correctly predicting the viscosity of polydisperse resins where long and short chains interact.
Molecular Motion
Chains slide through tube-like constraints formed by their neighbors in a process called reptation. In the double reptation model, the relaxation of the surrounding tube itself is considered alongside the motion of the central chain.
Prediction Accuracy
Mathematical simulations of the storage and loss moduli benefit from the inclusion of these dual relaxation mechanisms. Engineers use the double reptation model to calculate the molecular weight distribution from rheological data obtained in a frequency sweep. This method is often more sensitive to the high molecular weight fraction than gel permeation chromatography.
Material Analysis
Resin producers use these calculations to monitor the consistency of catalyst performance during a production run. Because the double reptation model is sensitive to small changes in the chain length, it can detect the onset of branching before it affects the bulk density. This early detection allows for process adjustments that prevent the production of off-grade material.