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Definition of the instantaneous (shear) compliance J0 (or steady-state initial creep compliance):

Equation 6.7

J0 = 1 / G0 = γ0 / τ0

Unit: [1/Pa = Pa-1]. The following holds:

Equation 6.8

J0 = limt → 0 J(t)

J0 is the limiting value of the J(t)-function at the very beginning of the creep test, i. e., when t = 0.

G0 [Pa] is the instantaneous (shear) modulus (or steady-state initial shear modulus) representing the sum of the elastic behavior of the springs S1 and S2, and γ0 [%] is the instantaneous deformation, i. e. the limiting value of the deformation at the time point t = 0.

The value of γ0 is determined at the intersection of the γ-axis and the straight line which is fitted to the steady-state sector of the creep curve, showing the curve slope tanβ (see Figure 6.8).

It is the same straight line which is used to determine the value of zero-shear viscosity. To enable accurate determination of the values of J0 (or G0, respectively), at least 10 % of the measuring points of the entire creep curve should occur within this steady-state sector.

Both parameters, J0 and G0, are coefficients to characterize the elastic behavior of a material. Polymers with similar structures but a higher molar mass M are showing lower J-values or higher G-values, respectively. For non-flowing materials, e. g. for chemically strongly crosslinked elastomers such as hard rubbers, the calculated J-values obtained would approach towards a very low value, or the G-value towards a very high value, respectively. Therefore, these kinds of materials would be absolutely rigid, showing hardly yielding.