A Dilatometer to Measure the Influence of Cooling Rate and Melt Shearing on Specific Volume

Abstract

Abstract We developed a dilatometer to investigate the specific volume of polymers as a function of pressure (to 100 MPa), temperature (to 260 oC), cooling rate (to 100 oC/s), and shear rate (to 80 1/s). The dilatometer is based on the principle of confined compression and comprises of a pressure cell used in combination with a tensile testing machine with rotation capability. The design of the pressure cell is a mixture of a traditional ‘pistondie type’ dilatometer and a Couette rheometer, i. e. piston and die make up an annular shaped sample spacing. Typical dimensions of annular samples are: inner radius ri = 10.5 mm, outer radius ro = 11.0 mm, height h = 2.5 mm, and a typical mass of about 60 to 70 mg. Silicon grease is used to reduce loss of hydrostatic pressure in the sample due to friction occurring between the solidifying sample and the dilatometer wall. Specific volume measurements at low cooling rate using an isotactic polypropylene (i-PP) are compared with measurements performed using a commercial bellows type dilatometer, showing relative differences in the range of 0.1 to 0.4%. Finally, results for an isotactic polypropylene are presented showing a profound influence of cooling rate and melt shearing on the evolution of specific volume.