Microstructure Evolution And Nonlinear Rheological Behavior Of Acetylated Cellulose Nanofibrils Hydrogel

Cellulose nanofibrils?CNF?with biocompatibility and excellent mechanical properties have wide application prospect in food and medicine industry.However,CNF tend to aggregate in water due to strong hydrophilic property,which makes the structure difficult to control.The acetylation of CNF,while retaining the basic properties,introduces hydrophobic groups to the surface of fibrils,and leads to the improvement in regulation of its hydrogel.In this thesis,we prepared acetylated CNF?ACNF?by chemical modifications and high-pressure homogenization.Based on the effects of Ca²⁺and methylcellulose?MC?on the ACNF surface properties,we investaged the micro-structure and dynamic rheological properties of hydrogel,and explored the relationship between gelation behavior and mechanical properties of ACNF.?1?Effects of Ca²⁺and MC on the surface properties of ACNF were investigated by Fourier transform infrared spectroscopy?FTIR?and Zeta potential analyzer.Furthermore,the aggregation and relaxation time obtained by fitting of ACNF were studied by dynamic light scattering?DLS?.Then the relationship between ACNF interactions and its aggregation were established.The results show that aggregation of ACNF is caused by shielding surface charge at the presence of Ca²⁺.Aggregation and relaxation time of ACNF increase with increasing Ca²⁺concentration.MC is binded to ACNF through hydrogen bonding,which increases the hydrophobic effect but cannot cause aggregation.?2?The effects of Ca²⁺with different concentration and MC with different molecular weights on sol-gel behavior of ACNF near the critical gelation concentration were investigated by confocal laser scanning microscope?CLSM?and small-angle X-ray scattering?SAXS?.It was found that fractal dimensions were increased with the increasing concentration of ACNF and Ca²⁺by the fitting of SAXS intensity profiles.It is indicated that the accumulation mode of ACNF on the nanometer scale has changed.When ACNF concentration is lower than the critical point,MC promotes sol-gel transition of ACNF suspension,making the structure more uniform and continuous.When ACNF concentration is high than critical point,the structure becomes uneven at the presence of MC.?3?Large amplitude oscillation shear test was carried out to explore the structural transformation of hydrogel.The correlation between linear rheological properties and yield behavior and microstructure changes was discussed.And the relationship between the third harmonics harmonics I_3/1 and the nonlinear parameter Q(=I_3/1/?²)and strain amplitude was obtained by Fourier transform rheology.Also,the relationship between stress and strain and strain rate was studied by Lissajous curves.The formation of different microstructures of ACNF is promoted by Ca²⁺and MC,resulting in changes in viscoelastic modulus and linear viscoelastic regions as well as yield behavior.The mechanism and process of the yield behavior can be further investigated by nonlinear parameters(I_3/1 and Q).The intensity of parameters I_3/1 and Q and the deformation of Lissajous curves show that Ca²⁺accelerates the nonlinear viscoelastic transition of the gel,while MC has opposite effect mainly due to the difference in microstructure.In addition,the increase in temperature can change the contribution proportion to gelation of hydrogen bonding and hydrophobic interactions,and thereby modify the rheological properties of the hydrogel.