(355b) Stretching, Buckling, and Bending of Composite Biopolymer Gels

Composite polymer gels can exhibit strain softening and stiffening properties due to the different deformation modes within each constituent matrix. Here, we use a combination of confocal imaging and rheometry to investigate composite 0.05 wt% agarose hydrogels composed of 0.038 wt% fluorescent dendritic chitosan nanocolloids dispersed in a water/glycerol solvent. A sol-gel transition occurs when the agarose is cooled below the gel point, during which the chitosan nanocolloids self-assemble into a complex architecture within the agarose. The composite displays increased stiffness while still retaining high fracture strains. Interestingly, the presence of soft dendritic networks results in a complex stiffening-to-softening transition in response to various oscillatory and steady shear strains. The increased stiffness of the biopolymer composite is likely due to the formation of a dual network hydrogel in which chitosan microfibers form a sacrificial load-bearing network within the agarose matrix. The dendritic chitosan network might also suppress non-affine deformation within softer agarose matrix. At intermediate strains, redistribution of internal stress occurs through temporary disruption in the chitosan network which could explain the observed softening properties. At higher strains, the background agarose matrix dominates the strain response as indicated by network stiffening until fracture. We use a confocal rheometer to visualize the heterogeneous yielding and breakdown of the inter-connected cluster network of chitosan microfibers to determine the microstructural evoluation during shear. Findings from this study suggest that the chitosan micronetworks elongate and stretch during strain stiffening, while reversible bond breakage and reformation between the agarose and chitosan give rise to softening.