A dilatometer to measure the influence of cooling rate and melt shearing on specific volume

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 80 C/s), and shear rate (to 77 1/s). The dilatometeris based on the principle of con¯ned 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 atraditional 'piston-die 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, outerradius ro = 11.0 mm, height h = 2.5 mm, and a typical mass of about 60 - 70 mg. Silicon grease is used to reduce loss of hydrostatic pressure in the sample due to friction occurring between the solidifying sampleand 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 di®erences in the range of 0.1-0.4 %. Finally, results for an isotactic polypropylene are presented showing a profound influence of cooling rate and melt shearing on the evolution of specific volume