The linear viscoelastic behavior of highly filled polydimethylsiloxane measured in shear and compression

Abstract

The frequency and time dependent moduli of up to 60% glass bead filled polydimethyl siloxane (PDMS) were studied for four different molecular weight materials with a rotational, compressional, and lubricated squeezing flow rheometer. Measurements of the storage and loss modulus taken with a conventional rotational instrument show good qualitative and quantitative agreement with those taken with a newly developed oscillatory compressional rheometer for PDMS of the lowest molecular weight studied, and show trends similar to those reported previously for similar materials. The agreement between the two instruments is better for both unfilled and 35% filled material of a higher molecular weight. Good agreement was also observed between measurements of the relaxation modulus in lubricated squeezing flow and measurements of the storage modulus with the new compressional device for a high molecular weight PDMS at all filler levels. The modulus derived from oscillatory measurements showed good quantitative agreement with that measured in lubricated squeezing flow for identical materials. The effect of polymer molecular weight on the relative behavior of the loss and storage modulus with increasing filler content was also studied. It was shown that as the polymer molecular weight increases, the ratio of the loss to storage modulus becomes nearly independent of the filler volume fraction over the frequency range studied. The effect of an increase in filler amount on the elasticity of the material was shown to depend both on the polymer molecular weight and the frequency, and is explained in terms of the influence of the filler on the Deborah number of the system.