Bulk compliance

Now, in rheological terminology, our compressibility JT, is our bulk compliance and the bulk elastic modulus K = 1 /Jr- This is not a surprise of course, as the difference in the heat capacities is the rate of change of the pV term with temperature, and pressure is the bulk stress and the relative volume change, the bulk strain. Immediately we can see the relationship between the thermodynamic and rheological expressions. If, for example, we use the equation of state for a perfect gas, substituting pV = RTinto a = /V(dV/dT)p yields a = R/pV = /Tand so for our perfect gas ... 

In other words, independently of the viscoelastic history in the linear region, the tensile compliance function can readily be obtained from both the shear and bulk compliance functions. For viscoelastic solids and liquids above the glass transition temperature, the following relationships hold when t oo J t) t/T[ [Eq. (5.16)], D t) = y Jt [Eq. (5.21)], and D t)J t)/ >. These relations lead to r 3t that is, the elongational viscosity is three times the shear viscosity. It is noteworthy that the relatively high value of tensile viscosity facilitates film processing. 

As indicated in another section, the response to an isotropic pressure as a step function of time gives the bulk creep compliance B(t). However, the response to a sinusoidal pressure gives the complex bulk compliance ... 

The bulk modulus K (= /H, the reciprocal of the bulk compliance) can be measured in compression with a very low height-to-thickness (h/i) ratio and unlubricatcd flat clamp surfaces. In pure compression with a high h/t ratio, and lubricated clamps, the compressive modulus (= /D, the reciprocal of the compressive compliance) will be measured. Any intermediate hjt ratios will measure part bulk and part compressive moduli. Hence it is vital for comparing samples to use the same dimensions in thermal scans and the same h/t ratio when accurately isotherming and controlling static and dynamic strains and frequencies. 

Bulk modulus B is usually calculated from measurements of bulk compliance (1/B) or compressibility. The usual expression may be written... 

F. Tensile (Bulk) Compliance, Tensile (Bulk) Modulus... 

As an example of bulk viscoelastic behavior, data for a poly(vinyl acetate) of moderately high molecular weight are shown in Fig. 2-9. Measurements by McKinney and Belcher of the storage and loss bulk compliance B and B" at various temperatures and pressures are plotted after reduction to a reference temperature and pressure of 50°C and 1 atm respectively (see Chapter 11). The complex bulk compliance is formally analogous to the complex shear compliance, but the two functions present several marked contrasts. 

FIG. 2-9. Storage and loss bulk compliance plotted logarithmically against frequency, for a poly(vinyl acetate) reduced to 50°C and I atm, as described in text. ... 

The inclusion of values in Table 1 l-III derived from dynamic bulk viscoelastic measurements implies the concept that the relaxation times describing time-de-pendent volume changes also depend on the fractional free volume—consistent with the picture of the glass transition outlined in Section C. In fact, the measurements of dynamic storage and loss bulk compliance of poly(vinyl acetate) shown in Fig. 2-9 are reduced from data at different temperatures and pressures using shift factors calculated from free volume parameters obtained from shear measurements, so it may be concluded that the local molecular motions needed to accomplish volume collapse depend on the magnitude of the free volume in the same manner as the motions which accomplish shear displacements. Moreover, it was pointed out in connection with Fig. 11 -7 that the isothermal contraction following a quench to a temperature near or below Tg has a temperature dependence which can be described by reduced variables with shift factors ay identical with those for shear viscoelastic behavior. These features will be discussed more fully in Chapter 18. 

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 ... 

Characteristic Data for Dynamic Bulk Compliance and Modulus... 

The components B and B" of the complex dynamic bulk compliance, or th components K and K" of the dynamic bulk modulus, were obtained in experiment... 

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