Abstract
In order to develop studies of viscoelastic and steady-flow properties of high-polymer melts, a self-recording rheometer has been produced which, by a minor exchange in the driving system, can employ the principle of a torsionally oscillating rheometer, a forced vibration torsion pendulum, or a Couette type viscometer as the measuring device for melts and solutions of high polymers. The oscillations and rotation of the outer and/or inner cylinders during measurement are changed into electrical potential by means of a differential transfomer device and after being amplified recorded on a X-Y recorder. The thermostat has been designed to keep temperature constant from room temperature to 300°C. Inert gas such as nitrogen flows under a slight positive pressure through a spiral pipe in the thermostat over the surface of the sample in order to minimize chemical degradation at high temperatures. The inert gas stabilizes the temperature and facilitates temperature control.
By using many torsion wires and bobs with different diameters, a very wide range of rheological properties can be measured. The frequency and number of revolution of the cup range from 1/256 to 8cps and from 0.122 to 240rpm, respectively. The key point of accurate measurements of viscoelasticity consists in a suitable choice of torsion wire and bob so as to give a couple of comparatively large values of phase angle and amplitude ratio.
After preliminary tests on silicone oil, the dynamic properties and the apparent viscosity of low-density polyethylene (Tenite 826F, Mn=26000) have been measured at 200°C. The dynamic viscosity as a function of angular freqency coincides with the apparent viscosity as a function of rate of shear, showing agreement with the theoretical prediction of DeWitt.
