New Insight into Agarose Gel Mechanical Properties

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Abstract
The current study focuses on the effects of the molecular weight on the mechanical behavior of agarose gels. The small strain rheology and large strain deformation/failure behavior of three different molecular weight agarose gels have been examined, with the results expressed in term of molar concentration. For small deformation strains, the gelation temperature at low concentrations and the critical concentration for gel formation are strongly affected by the molecular weight. In addition, the elasticity of the network is also very sensitive to this parameter. It has been demonstrated that the experimental gelation cure curves can be superimposed on a universal gelation master curve, independent of the cure time. This would indicate self-similarity of the network at different scales, irrespective of concentration. A relationship between the elastic modulus and the molecular weight has been extracted from these results, where the molecular weight dependence exhibits a power law exponent of 2.42. For large deformation strains, the Poisson ratio has been estimated to be 0.5 for each of the agarose types examined, which indicates that these gels are incompressible. The strain at failure is largely dependent on the molecular weight, and is essentially independent of the biopolymer concentration. This result highlights the fact that the strain at failure is sensitive to the connectivity distances in the gel network. However, the failure stress and Young's modulus of agarose gels show a dependence on both concentration and molecular weight. The observations regarding Young's modulus are in good agreement with those found for small deformation strain rheology for the shear modulus. One of the primary advantages of using the lowest molecular weight agarose is that higher molar concentrations can be reached (more molecules per unit volume). However, the mechanical response of agarose gels is very sensitive to the molecular weight at fixed molar concentration, and if the present results are extrapolated 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 is based on the influence of the “connectivity” at long distances, which influences the strain at failure (when the strain at failure is zero, the system is not connective).