Viscoelastic and Mechanical Characterization of Hydrophilic Polyurethane Dispersions and Gel
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Hydrophilic polyurethane (PU) forms colloidal dispersion and can aggregate as gel under gravity and shear induced sedimentation process. Small-amplitude steady state and oscillatory shear flow experiments were used to systematically investigate the rheological behavior as a function of solid content, ionic strength and solvent property for hydrophilic PUs. The solid content and solvent property found to significantly affect the PU dispersions at high weight fraction (50 wt/%) and at a high ionic strength (4M NaCl). The complex viscosity (η*) was found to be well described by Cross-Model using a non-linear regression analysis. The rheological characterization of dispersion system shows gel like viscoelastic behavior and can be considered as micro-gel dispersions. Upon aggregation and sedimentation of the dispersed micro-gel under gravity (S2) and shear force (S1) the system shows the morphology of conventional gels. The dynamic oscillatory rheology of PU micro-gel, i.e. S1 and S2, shows anomalous behavior as a function of solid content at low solid content. Microstructure of PU gels were analyzed using fractal theory of 3D gel network. Both Shish et al. or Wu-Morbidelli theory were failed to capture 3D fractal characteristics. However, Peppas-hydrogel theory seems to predict microstructure in terms of mesh-size and cross-linking density and can provide a qualitative prediction of the drug entrapment behavior was possible to correlate with the results.