T g , the thermal effect on the data is greater. Another approach taken that was successful for temperatures around T g was to
shift only the tan delta data as the temperature dependence should be neutralized in that data. Both approaches produced
similar results in the region near T g but it was nearly impossible to shift the data far from T g as the thermal dependence was
much more pronounced than any horizontal shift. A Æ45
master curve with a vertical shift and a 0
master curve with no
vertical shift are shown in Fig. 14.2.
The master curve is much cleaner when vertical shifting is employed, when it is ignored the isothermal frequency sweeps
stack up far away from T g (Fig. 14.3).
Both shift factors have a reference temperature of 150
C and while they are different, the difference could be attributed to
the difference in T g seen between the two orientations. A 25
C difference can account for the shift of three orders of
magnitude on either the rubbery or glassy end.
Fig. 14.2 Storage modulus master Curves with and without vertical shifting
Fig. 14.3 Shift factors for 0
coupons and Æ45
coupons
14 Time Temperature Superposition Shift Factors for Fabric Composites
97