Ultrasonic relaxation studies of chain entanglement effects in poly(dimethylsiloxanes)

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Abstract
Ultrasonic relaxation data covering a frequency range from 1–1000 MHz, and a temperature range of 303 to 333 K are reported for poly(dimethylsiloxanes) with a molecular weight ( text-decoration:overline M n) variation of 1 × 104 to 8 × 104. The experiments were conducted with a view to determining the effect of chain entanglements on the longitudinal and shear dynamic relaxation processes; five important features have emerged. First, the normal mode and segmental relaxation processes are clearly resolved in the overall acoustic attenuation spectrum. Second, the effect of entanglement on the normal mode motions as observed above a critical molecular weight is to reduce the rate of the shear process before reducing the rate of volume relaxation. Consequently the ratio ηv/ηs is both frequency and molecular weight dependent. Third, the critical molecular weight (and so the effect of entanglement) is dependent on temperature, so that the variance in shear and volume characteristics occurs in a specific molecular weight–frequency–temperature domain. Fourth, a negative decrement in adiabatic compressibility is observed in the same molecular weight–frequency–temperature domain. Finally, at the temperatures used in this work, the segmental process is very rapid, the rotation cannot be described by formation of a transition state by thermal excitation of a predominant ground state and the speed of rotation becomes virtually independent of temperature.

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