Low Frequency Rheological Testing for Hydrolyzed Fraction Identification in Resins
Low frequency oscillatory rheology unmask hydrolyzed resin fractions by measuring zero-shear viscosity drop and storage modulus slope collapse near zero frequency.
Relaxation Spectrum Inversion
Meaning
A non-monotonic reorganization of relaxation times in a polymer melt subjected to extreme deformation rates represents a complex viscoelastic anomaly. This behavior, known as relaxation spectrum inversion, arises when the high-shear rates cause the shorter polymer chains to relax slower than expected relative to the longer chains. It occurs primarily in highly branched polymers and blends under high stress.
In processing, this inversion alters the predicted swell of the extrudate and the stability of the melt.
Physical Origin
Molecular chain entanglement and branch-point dynamics govern this behavior. Under steady shear, the chains are stretched and oriented along the flow direction. If the shear rate exceeds the rate of thermal motion, a relaxation spectrum inversion can occur as the branched nodes become temporary lock-points.
This changes the distribution of relaxation times from a smooth decay to a structured profile. The result is a melt that retains elastic memory for longer than predicted by linear rheology.
Flow Abnormality
The altered relaxation times affect how the polymer behaves as it exits the extrusion die. In a typical extrusion run, the polymer relaxes quickly, resulting in a predictable extrudate swell. When a relaxation spectrum inversion occurs, the delayed relaxation causes the polymer to swell unevenly or develop a rough surface finish.
This can lead to dimensional deviations in extruded profiles and makes the wall thickness of blown films difficult to control.
Diagnostic Analysis
Capillary rheometry and rotational tests are used to detect this behavior. Laboratory technicians run frequency sweeps at different strain levels to observe how the storage and loss moduli behave under shear. A shift in the relaxation curve indicates the onset of the inversion.
This diagnostic is used to screen resin batches before they reach the extrusion line. By identifying these issues early, processors can adjust the die geometry or lower the line speed to avoid surface defects on the product. Moulders also use this data to modify the gate size in injection molds to reduce shear rates and prevent part warping.
This measurement is useful when blending different post-industrial recycled resins that may have unknown branching levels.