Influence of thermal history on the structural and mechanical properties of agarose gels

Using a multitechnique approach, two temperature domains have been identified in agarose gelation. Below 35 degrees C, fast gelation results in strong, homogeneous and weakly turbid networks. The correlation length, evaluated from the wavelength dependence of the turbidity, is close to values of pore size reported in the literature. Above 35 degrees C, gelation is much slower and is associated with the formation of large-scale heterogeneities that can be monitored by a marked change in the wavelength dependence of turbidity and visualised by transmission electron microscopy. Curing agarose gels at temperatures above 35 degrees C, and then cooling them to 20 degrees C, produces much weaker gels than those formed directly at 20 degrees C. Dramatic reductions in the elastic modulus and failure strain and stress are found in this case as a result of demixing during cure. An interpretation, based on the kinetic competition between osmotic forces (in favor of phase separation) and elastic forces (that prevent it) is proposed.