Meaning
Dynamic mechanical testing procedures apply sinusoidal deformation to polymer melts to capture linear viscoelastic spectra. Utilizing oscillatory shear rheometry enables rheologists to measure storage modulus and loss modulus across a range of angular frequencies and temperatures. The method governs material characterization within the linear viscoelastic region where stress response remains proportional to applied strain amplitude.
Non-linear responses occurring at high strain amplitudes fall outside the scope of standard dynamic oscillatory testing protocols.
Viscoelastic Response
Sinusoidal strain signals drive parallel plate or cone-and-plate geometries containing the molten polymer sample. In oscillatory shear rheometry, the phase shift angle between applied strain and measured stress separates elastic energy storage from viscous dissipation. A phase angle of zero degrees indicates a purely elastic solid, whereas ninety degrees represents a purely viscous liquid.
Polymer melts exhibit intermediate phase angles that shift with oscillation frequency, providing insight into melt elasticity and relaxation time distributions.
Melt Processing
Processing behavior during extrusion and injection molding depends directly on dynamic melt viscosity curves. Data gathered through oscillatory shear rheometry helps process engineers predict shear thinning behavior and melt elasticity in processing dies. High storage modulus values at low frequencies correlate with strong melt strength, preventing parison sag during blow molding operations.
Low complex viscosity at high shear rates reduces injection pressure requirements, enabling complete mold filling in thin wall applications.
Structure Characterization
Frequency sweeps at constant temperature reveal structural features including molecular weight distribution and polymer branching architecture. Complex viscosity curves obtained from oscillatory shear rheometry show distinct terminal Newtonian plateaus that shift with average molecular weight. Broad molecular weight distributions broaden the transition region between Newtonian and shear thinning regimes.
Rheological characterization allows resin formulators to detect crosslinking or long chain branching without solvent dissolution procedures.
