Properties in Bulk Compression

In dynamic bulk viscoelastic measurements, the deformations are ordinarily exceedingly small and therefore in the linear range of behavior the change in free volume during a cycle of deformation is a very small proportion of the total free volume. However, a large constant hydrostatic pressure can be imposed if desired on the small periodic pressure changes, thereby altering the free volume and hence the relaxation times and the frequency scale of the viscoelastic dispersion. 

The most extensive measurements of dynamic bulk compression have been those of McKinney and Belcher on poly(vinyl acetate), covering ranges of frequency from 50 to 1000 Hz, temperature from 0° to 100°C, and superposed hydrostatic pressure from 1 to about 1000 atm. Both storage and loss bulk compliances, B and B , were obtained. All data were successfully combined by the method of reduced variables taking into account the substantial dependence of both Bg and Bg (the equilibrium and glasslike bulk compliances) on both T and P, as follows  

The equilibrium compliance Bg may be attributed to compression of the occupied volume plus collapse of free volume and the glasslike compliance Bg, which B approaches at high frequencies and low temperatures, is attributed to compression of the occupied volume only. The dependences of these quantities on T and P were evaluated from the data to be 

The conventional reduced curves plotted against reduced frequency are obtained by choosing cu as the independent variable in equation 6 and reducing B and B to To and Pq by equations 8-11, then plotting against ojat-p where = a 12 is given in terms of free volume by equations 49 and 58 of Chapter 11. For this purpose, / was taken as 4.8 X 10- deg , jS/ as 0.96 X 10- cm2/dyne, and/ = 0.025 at 17 C and 1 atm. The results have already been seen in Fig. 2-9 as the Classical example of bulk viscoelastic behavior. 

Dynamic bulk storage and loss compliances of poly(vinyl acetate) reduced to I atm and 1000 Hz by equation 6 and plotted against reduced temperature defined by equation 12. Bq and correspond to B, and Bg in the text. (McKinney and Belcher. ) 

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