Gelation Kinetics, Fractal Structure And Improvements Of Hydrogels Formed From Thermosensitive Microgels

Thermosensitive poly(N-isopropylacrylamide)(PNIPAM) microgel is hydrophilic and highly swollen bellow its volume phase transition temperature. However, it becomes hydrophobic and sharply collapses above the temperature. Dispersion of collapsed microgels loses colloidal stablility in electrolyte solution and forms hydrogel simultaneously. The novel hydrogel has two networks hierarchically. It can be tissue engineering materials such as injectable cell scaffold.Under conditions of total physiological salinity, represented by NaCl, PNIPAM microgel dispersions can gel isothermally. At constant NaCl concentration, gelation time tg decreases with microgel concentration increasing. At constant microgel and NaCl concentration, tg decreases with temperature increasing. The gelation kinetics can be described by Tobitani-Ross-Morphy Model.PNIPAM microgel dispersion can gel at free physiological Ca2+concentration, which is rather low. Its gelation kinetics deviates from the prediction of Tobitani-Ross-Morphy model. The reason is gel’s formation assisted by bridging and specific absorption of Ca2+when electrostatic repulsion remains.Study of fractal structure of hydrogel can be done by comparing two models of scaling theory. According to the model of Shih et al., the hydrogel falls into the strong-link regime. However, the resulted backbone fractal dimension yielded unrealistic values, suggesting inapplicability of this model. According to Wu-Morbidelli model, the hydrogel falls into the transition regime. As temperature increases, fractal dimension df decreases from～2.5to～1.8. When NaCl concentration increases, df decreases from～2.1to～1.7. The variation of df indicates aggregation change from reaction-limited, even with rearrangement, to diffusion-limited mechanism. It stems from changes of interaction between microgels.Strength of hydrogel can be linearly increased by increasing concentration of PNIPAM microgel, concentration of NaCl or temperature with different mechanisms. Strengthening can also be done by large strain. The reason is fractal backbone stretches and stress transmission changes. Gelation of microgel dispersions still requires NaCl in blends of linear polymer poly(sodium4-styrenesulfonate) or artificial clay Laponite. Physical blending suppresses syneresis but decreases gel strength.In the present thesis, gelation kinetics, fractal structure and improvements of a novel hydrogel are studied. It contributes to the application of the materials and study of heat-induced globular protein gelation.