Dynamic Rheological Frequency Sweeps for High Molecular Weight Fraction Detection in Bimodal Polyolefins
Dynamic rheological frequency sweeps detect ultra-high molecular weight tails in bimodal polyolefins where standard melt flow rates fail.
Phase Angle
Meaning
Viscoelastic metric representing the time lag between the applied sinusoidal strain and the resulting stress indicates the balance of elastic and viscous behavior in a polymer melt. This phase angle, denoted as delta, ranges from zero degrees for a purely elastic solid to ninety degrees for a purely viscous liquid. This parameter is crucial for assessing how a polymer will store and dissipate energy during high-shear processing steps.
Viscoelastic Shift
Measuring this angle across a range of oscillation frequencies reveals the relaxation behavior of the polymer chains.
Dynamic Rheology
Rheologists calculate this value from the ratio of the loss modulus to the storage modulus. As the temperature of the melt increases, the phase angle generally increases as the polymer transitions to a more viscous state. This transition is plotted to determine the processing window of the resin.
Melt Defect
Molders monitor this parameter to prevent defects such as die swell or melt fracture in extrusion lines. If the phase angle is too small, the high elasticity of the melt leads to excessive swell and distortion as it exits the die. Adjusting the melt temperature shifts this value into the optimal range for stable extrusion, which stabilizes the output dimensions.