  Thermoreversible hydrogels are commonly prepared by cooling down to ambient
temperature, aqueous polymer solutions first brought to a boil. The incubation
time of the polymer solution at such a high temperature is traditionally kept
to a minimum to minimize its impact on the subsequent gelation. Here we study
the effect of a prolonged heating of a 1.5\% w/w agar solution at
$T=80^{\circ}$C, well above the gelling temperature. The incubation time
$\mathcal{T}$ of the polysaccharide solution is varied from a few hours up to
five days. We show that the agar solution ages as the result of both the
hydrolysis and the intramolecular oxidation of the polysaccharides. As a
consequence, both the viscosity and the pH of the solution decrease
continuously during the incubation period. Furthermore, samples withdrew at
different incubation times are cooled down to form gels which structure and
mechanical properties are systematically determined. Cryo-microscopy and X-ray
diffraction experiments reveal that agar gels formed from solutions of
increasing incubation times, display a coarser microstructure composed of
micron-sized foils which result from the condensation of the polysaccharides
and contrast with the fibrous-like microstructure of gels prepared from a fresh
agar solution. Along with structural changes, a prolonged incubation time of
the polymer solution at $T=80^{\circ}$C leads to weaker agar gels of
significantly lower shear elastic modulus. Moreover, extensive
macro-indentation experiments coupled to direct visualization show that
increasing the incubation time of the agar solution up to a few days decreases
the yield strain of the gel, while the rupture scenario turns continuously from
brittle to ductile-like. Our study suggests that the incubation time of agar
solutions at high temperature could be used to tune the mechanical properties
of agar-based gels.
