| Abstract: | A new device has been designed for the measurement of the die swell of extruded polymer melts. According to the proposed procedure, samples can be collected, annealed, and photographed. The device may be used in conjunction with any capillarytype rheometer. It can accommodate simultaneously as many samples as it is needed. The samples are suspended in a thermostated liquid, carefully selected for each polymer. The liquids must be thermally stable and of proper density and thermodynamic and interfacial properties. The device was used in conjunction with the Instron capillary rheometer, ICR. Three types of polymer were tested: polystyrene (PS), polyethylene (PE), and a semirigid poly(vinyl chloride) formulation (PVC). The swelling of the extrudates was followed for ca. 40 min; the equilibrium dimensions were usually reached within the first 2 min. Parallel with these measurements, the samples were tested in the Weissenberg rheogoniometer (WR) recording both shear and normal stresses. For PS and PE, the flow curves determined in these two rheometers overlapped, while they differed for PVC. The swell ratio, Bexp = D/D0 (where D and D0 are the equilibrium diameter of the extrudate and diameter of the capillary, respectively), was converted to recoverable shear strain, s, as follows. First, Bexp and s were determined in ICR and WR, respectively, for a PS sample over wide and overlapping ranges of rate of shear. This experimental dependence was found to follow Tanner's theoretical relation. Consequently, this relation was used to compute s from Bexp for all the other samples. Excellent agreement was observed between the s values calculated from Bexp and s values determined in WR. |
