DYNAMIC SHEAR RHEOLOGY OF HIGH-MOLECULAR-WEIGHT POLYDIMETHYLSILOXANES- COMPARISON OF RHEOMETRY AND ULTRASOUND

The viscoelastic properties of three linear polydimethylsiloxanes (PDM S) of high molecular weights are investigated using rheometrical as we ll as ultrasonic tests over a large range of temperatures. Classical s hear rheometrical measurements are carried out in the low frequency ra nge from 10(-1) to 10(2) rad s(-1) between -50 and + 20 degrees C. The frequency range is enlarged using the time-temperature superposition principle, allowing coverage of about 6-7 decades of pulsation. Ultras onic tests use an inclined incidence wave reflection technique to meas ure the complex shear mechanical impedance from 1.5 to 25 MHz, between -10 and -50 degrees C. Rheometrical and ultrasonic experiments are th en combined for the three PDMSs at the same reference temperature. The y give the reduced shear elastic and loss moduli for reduced frequenci es covering 10 decades. A discrete relaxation time spectrum is first d educed from the master curve in each case. More accurate predictions m ay be obtained using a molecular weight distribution and BSW (Baumgaer tel-Schausberger-Winter) model for polydisperse systems. (C) 1998 Else vier Science B.V. All rights reserved.