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Build-Up of a Thermoreversible Polymer Network : from Early Local to Late Collective Dynamics
Thermoreversible polymer gels exhibit slow - logarithmic - mechanical strengthening. As is the case for many disordered systems, although this aging process is handwavingly ascribable to the “complexity of the energy landscape”, a more detailed understanding remains elusive. In order to gain some insight into the mechanisms of network build-up and aging, we resort to an original experimental approach : We first crosslink gelatin chains with a controlled density of irreversible, covalent bonds. A subsequent quench triggers the formation of reversible, triple-helix crosslinks. We analyze the time evolution of the shear modulus of this hybrid gel and its dependence on covalent crosslink density. We are thus able to distinguish between 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 crosslinks play a negligible role, (ii) a late, logarithmic aging one, which is preserved, though slowed down, in the presence of irreversible crosslinks. We discuss the implication of our results in terms of the switch from an early, local dynamics to a late, cooperative long-range one.