Mechanical properties of particle-filled polymer gels
This thesis examines the role of particulate fillers in modulating the small and large deformation behavior of food protein gels. Particular emphasis was given to addressing the validity of established theoretical models intended to describe the relative change in elastic modulus of particle-filled composites (Er) as a function of filler volume fraction (ϕf). Such models are widely employed, but generally have limited capacity to describe physically realistic scenarios. Model composite systems were investigated to highlight potential discrepancies between theory and experimental observations. Glass microspheres were used as ideal rigid bound fillers in both heat-set whey protein isolate (WPI)/xanthan gum and acid-induced sodium caseinate/xanthan gum mixed biopolymer gels. All established theories failed to match the curve shape of Er with increasing ϕf in both systems, suggesting a fundamental discrepancy in the mathematical treatment of stress translation through the composite network. We demonstrated Er instead followed a power law scaling relation with ϕf, with the scaling behavior depending on gel type and filler size distribution. The effect of network structure was investigated using WPI-stabilized emulsion droplets in gelatin gels. Adjusting the electrostatic environment produced either a homogeneous or heterogeneous architecture; the latter being characterized by droplet-rich, protein dense domains. Er exhibited power law scaling with ϕf in both networks, but these produced distinct trends in the scaling behavior. The homogeneous gels produced a constant scaling factor with varying gelator concentration (cgel) arising from imperfect interfacial adhesion. The heterogeneous domains provided greater reinforcement, but the scaling factor decreased with cgel. We developed a coarse-grained modification to the fractal scaling model of colloidal gels which addresses stress translation through the network by introducing heterogeneity at the length scale of fillers. This approach extends the heterogeneous fractal scaling within colloidal aggregates to the macroscopic level and additionally incorporates fillers as mechanically-efficient junction zones in the load-bearing network. The model also naturally produces an interdependence between ϕf and cgel in eliciting reinforcement, as observed in the heterogeneous gelatin network. The impact of fillers on the large deformation and fracture behavior was also addressed, and could be interpreted based on their contribution to energy dissipation mechanisms.