Rheological Study of the Shape Transition of Block Copolymer-Nonionic Surfactant Mixed Micelles

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• 作者：David Lö ; f ; Karin Schill&eacute ; n ; Miguel F. Torres ; Alejandro J. Mü ; ller
• 刊名：Langmuir
• 出版年：2007
• 出版时间：October 23, 2007
• 年：2007
• 卷：23
• 期：22
• 页码：11000 - 11006
• 全文大小：120K
• 年卷期：v.23,no.22(October 23, 2007)
• ISSN：1520-5827

文摘

A rheological study of mixed micelles formed by PEO-PPO-PEO triblock copolymer P123 and nonionic surfactantC₁₂EO₆ in aqueous solutions has been carried out with the purpose of investigating the time dependence of a shapetransition of the mixed micelles and characterizing the shape before and after the transition. The rheology resultspresented in this report give clear evidence that the P123-C₁₂EO₆ mixed micelle grows and changes gradually in shapefrom spherical to elongated (rodlike) geometry with increasing temperature. These results are in accordance with theresults found in the parallel dynamic and static light scattering and calorimetrical investigation.^1,2 By using steady-staterheology, the time dependence of the sphere-to-rod transition of the mixed micelle system was carefully followed withtime and temperature as simultaneously recorded variables in the experiments. This was performed by a designed novelexperimental procedure. A temperature ramp was applied at a rate of 2.6 C/min from a temperature below to atemperature above the shape transition at a constant shear rate while the viscosity of the solution was measured. Theinvestigation was limited to two different compositions, surfactant-to-copolymer molar ratios (MR = n_C12EO6/n_P123)of 2.2 and 6.0 with varying total concentration from 1.5 to 21 wt % in comparison with the neat component. At lowconcentration, a slow transition was observed, which indicated that the mixed micelles are still growing into rods forseveral minutes after reaching the final temperature. At a total concentration of 4.0 wt % and above, the system reachedequilibrium quickly. A concentration-dependent kinetic process is therefore anticipated, which was also found in thetime-resolved static light scattering experiments previously performed (Löf, D.; Schillén, K.; Olofsson, G.; Niemiec,A.; Loh, W. J. Phys. Chem. B 2007, 111, 5911). At concentrations above 10 wt %, shear-thinning behavior wasobserved for the mixed solutions, which strongly suggests the extended shape of the mixed micelles after the shapetransition. The obtained zero-shear viscosity at the investigated molar ratios was found to be lower with higher molarratios, which indicates that the mixed micelles both in the spherical and in the rodlike state becomes smaller withhigher content of C₁₂EO₆. These results correlate well with the obtained results from the previous dynamic lightscattering measurements on the same system (Löf, D.; Schillén, K.; Olofsson, G.; Niemiec, A.; Loh, W. J. Phys. Chem.B 2007, 111, 5911).