  We probe the mechanisms at work in the build-up of thermoreversible gel
networks, with the help of hybrid gelatin gels containing a controlled density
of irreversible, covalent crosslinks (CL), which we quench below the physical
gelation temperature. The detailed analysis of the dependence on covalent
crosslink density of both the shear modulus and optical activity evolutions
with time after quench enables us to identify two stages of the physical
gelation process, separated by a temperature dependent crossover modulus: (i)
an early nucleation regime during which rearrangements of the triple-helix CL
play a negligible role, (ii) a late, logarithmic aging one, which is preserved,
though slowed down, in the presence of irreversible CL. We show that aging is
fully controlled by rearrangements and discuss the implication of our results
in terms of the switch from an early, local dynamics to a late, cooperative
long-range one.
