Ultrasonic Characterization of Polymers under Simulated Processing Conditions

Abstract

In contrast to usual materials, polymers are characterized by the non-equilibrium nature of their structure. Polymers are viscoelastic in that their mechanical properties depend not only on pressure (p), volume (V) and temperature (T), but also time (t). Viscoelasticity determines processability, while thermodynamic history during processing influences end-use properties. It is therefore important to perform mechanical measurements in conditions representative of those for manufacturing. This cannot be achieved with usual methods; ultrasonics, however, provides a unique means to realize such experiments. Using this approach, we describe an apparatus which measures sound velocity (v) and attenuation (a) and also incorporates simultaneous monitoring of the specific volume (V) to characterize the thermodynamic state of the material. The instrument allows close control of the thermodynamic history in the temperature range from T = −150 °C to T = 300°C and pressures up to p = 2.0 kbars. Results for the velocity, attenuation and specific volume are translated in terms of a dynamic complex modulus for the description of viscoelasticity. The experiment constitutes an essential step toward on-line measurements for process control in the plastics industry.