Viscoelasticity is a fundamental characteristic of polymers. Polymers exhibit both viscous and elastic characteristics depending on chemical composition, temperature, and time.
Viscous behavior is characterized by deformation under stress with no recovery. There is no resiliency, no energy stored within the substance to allow it to recover its original shape (e.g. grease). Elastic behavior is characterized by deformation under stress with complete and almost immediate recovery (e.g. rubber).
Dynamic mechanical analysis (DMA) is used to evaluate the viscoelastic properties of polymers and generate modulus versus temperature curves. In the typical fixed frequency experiment a polymer bar is mounted in the fixture and the driver arm imparts a sinusoidal deformation while the following arm tracks the phase relationship between the applied strain and the resultant stress. This viscoelastic behavior is time and temperature dependent. We can control the frequency of the deformation (time), and can collect data from -150oC through the glass transition of the polymer.
We can also determine the creep and stress-relaxation behavior of polymer systems: (ASTM D2990) CREEP is the continuous deformation or straining of a polymer under constant stress or loading. It is caused by the flow of molecules to positions of lower stress and is not reversible.
STRESS-RELAXATION is the decaying of stress within a polymer under constant strain.
Both creep and stress-relaxation are important properties in plastics design. |