The second method requires that a polymer solution be
cooled slowly, while continuously measuring the storage
(G8) modulus, loss (G9) modulus and phase angle, d.2,3 The
temperature at which a frequency independent phase angle is
obtained is defined as Tgel . A less precise method defines
Tgel as the temperature at which tand51 (G85G9). This
method assumes that the solution comes to equilibrium at
each temperature, which is unlikely. Djabourov and
co-workers4 have shown that the helix content of gelatin gels
below the Tgel continues to increase for hundreds of hours.
Thus, the helix content in aggregating solutions just below
Tgel would also continue to evolve over long periods of time,
and gels are unlikely to reach equilibrium at each temperature.
Consequently, the value obtained for Tgel would depend
on the rate of temperature decrease. Furthermore, since tgel is
concentration dependent, this method would underestimate
Tgel for low concentration solutions more than for high concentration solutions, resulting in an apparent concentration
dependence of Tgel .