New insight into agarose gel mechanical properties

The current study focuses on the effects of the molecular weight on the mec hanical behavior of agarose gels. The small strain rheology and large strai n deformation/failure behavior of three different molecular weight agarose gels have been examined, with the results expressed in term of molar concen tration. For small deformation strains, the gelation temperature at low con centrations and the critical concentration for gel formation are strongly a ffected by the molecular weight. In addition, the elasticity of the network is also very sensitive to this parameter. It has been demonstrated that th e experimental gelation cure curves can be superimposed on a universal gela tion master curve, independent of the cure time. This would indicate selfsi milarity of the network at different scales, irrespective of concentration. A relationship between the elastic modulus and the molecular weight has be en extracted from these results, where the molecular weight dependence exhi bits a power law exponent of 2.42. For large deformation strains, the Poiss on ratio has been estimated to be 0.5 for each of the agarose types examine d, which indicates that these gels are incompressible. The strain at failur e is largely dependent on the molecular weight, and is essentially independ ent of the biopolymer concentration. This result highlights the fact that t he strain at failure is sensitive to the connectivity distances in the gel network. However, the failure stress and Young's modulus of agarose gels sh ow a dependence on both concentration and molecular weight. The observation s regarding Young's modulus are in good agreement with those found for smal l deformation strain rheology for the shear modulus. One of the primary adv antages of using the lowest molecular weight agarose is that higher molar c oncentrations can be reached (more molecules per unit volume). However, the mechanical response of agarose gels is very sensitive to the molecular wei ght at fixed molar concentration, and if the present results are extrapolat ed to very low molecular weight, it can be suggested that below a limiting molecular weight a percolating network will not be formed, as suggested by the Cascade model (Carbohydr. Polym. 1994, 23, 247-251). This speculation i s based on the influence of the "connectivity" at long distances, which inf luences the strain at failure (when the strain at failure is zero, the syst em is not connective).