Oscillatory Shear Rheology Of Laponite / Poly(Ethylene Glycol) Suspensions

Laponite is a synthetic hectorite. When it is dispersed in water, the suspension undergoes liquid-to-solid transition and exhibit complex phase diagram. The rheology of Laponite suspension shows thixotropic behavior with both time dependency and nonlinear viscoelasticity. In the present work interparticle interaction in Laponite suspension was controled by adding NaCl and poly(ethylene glycol) (PEG), and the gelation and yielding process of the samples were studied by oscillatory shear rheolmetry in order to relate the linear and nonlinear viscoelasticity with the structure of the sample. The present work was divided as following parts.Firstly, Laponite/PEG suspensions with different PEG concentrations were prepared and their gelation process under linear viscoelastic condition was investigated by small amplitude oscillatory shear (SAOS). The samples were preshear to a reproducible state before SAOS time sweep experiment, which provide data of storage and loss modulus G’ and G’’ versus observation time tw. The strain rate in the preshear process affect the starting structural state of the subsequent gelation process. The larger the preshear strain rate, the further the samples were driven away from the gel state. The same sample after different strain rates of preshear reaches similar modulus after long time (800 s) of gelation, but the short time rate of gelation increases with increasing preshear strain rate. With constant preshear strain rate, the G’, G’’ versus tw curves from samples of different PEG concentrations can be shifted into a mastercurve (G’, G’’ versus tw/tp). Considering the steric effect of PEG adsorbed on the Laponite particle surface, a quantitative relationship between the shift factor tp and the interparticle interaction potential U(h) was found with the help of the colloidal coagulation kinetic theory. Samples with different NaCl or samples after different preshear shear rates can also construct mastercurves similarly, which suggests the universality in the gelation process of Laponite/PEG suspension.Secondary, G’, G’’ versus frequencyωresults of the same sample at different observation time during the gelation process was obtained by time-resolved viscoelastic spectrum technique. Results of different times can be shifted along the frequency axis into a mastercurve (G’, G’’ versus ?ω). The shifting factor ? reflects the dependence of relaxation timeτof the sample on observation time tw in the gelation process. The mastercurves of samples of different PEG concentrations can shifted into a super mastercurve (G’, G’’versus ?’?ω), with a new shift parameter ?’which depends on PEG concentration. The ? versus tw results obtained from samples with different PEG concentrations can be shifted horizontally along the tw axis using the shift factor tp obtained from the SAOS time sweep experiment, and vertically along the ? axis using the shift factor ?’obtained from the super mastercurve, into a mastercurve of ?’? versus tw/tp. This mastercurve reflects the common dependence of structural relaxation timeτof the samples on the observation time tw among samples of different PEG concentrations. It shows a two-step gelation feature. In the first step the relaxation time increase exponentially with observation time; in the second the relaxation time increase linearly with observation time. This result is consistent with previous reported gelation and glass transition process in various systems.Thirdly, the time-dependent nonlinear viscoelastic behavior of Laponite/PEG gels in the yielding process were investigated by large amplitude oscillatory shear (LAOS). The dependence of various quantitative parameters of LAOS with observation time tw was obtained by LAOS time sweep experiment. The yielding process of Laponite/PEG gels depends not only on the strain amplitude but also the observation time and the structural state of the sample has reached before yielding. Samples with fuller developed structure exhibit delayed yielding behavior; these samples stay constant in fundamental modulus G’1, G’’1 as well as in relative nth harmonic intensities In/1 for a period of time which depends on the strain amplitude. Samples with less developed structure, in contrast, yields directly from the begining of LAOS expreiment. Under LAOS condition, Laponite/PEG suspensions after preshear undergo gelation and structural destruction simultaneously. Due to the competing results of the two processes, the samples generally exhibit gelation followed by yielding. The extent of the gelation a sample reaches before yielding depends on its PEG concentration and the strain amplitude of LAOS. Laponite/PEG gels reaches stationary state after yielding under LAOS. The stationary flow is highly nonlinear viscoelastic, which was measured by In/1 and quantitative Lissajous curve parameters GM, GL, ?M and ?L. A pair of new LAOS parameters NE and NV were proposed which extract only and all the higher harmonics of the LAOS response. The difference between samples with different PEG concentration and molecular weight and NaCl concentrations can be distinguished with the help of these quantitative parameters of LAOS. A physical mechanism was proposed to explain the experimental results of Laponite/PEG suspension, and a structural kinetic model for thixotropy was used to simulate part of the LAOS response of Laponite/PEG suspension and compared with the experimental results.