Rheological study of the formation of a viscoelastic film at the air-water interface in L64 triblock copolymer aqueous solutions

The formation of a film at the air-water interface in aqueous solutions of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) copolymers (commercially known as pluronics or poloxamers) and the effect this film can have not been studied so far. This thesis provides a first comprehensive study of the film formed in aqueous solutions of pluronic L64, and provides a more general understanding of the formation of films at the air-water interface in solutions with surface active components.;Rheology measurements on different geometries and surface tension measurements provide evidence of the formation of a polymer film at the air-water interface in solutions of L64 in D2O. Oscillatory shear measurements show a sudden increase in the apparent moduli at a certain temperature and composition which previous studies have attributed to a percolation phenomenon. Proof is provided that this increase in apparent moduli is due to the formation of a film at the interface, and not due to a percolation phenomenon. The surface effect is so big that the Boussinesq number in Cone and Plate geometries and even in Couette geometries is high enough to completely dominate the rheology measurements. A simple addition model is derived to determine the surface moduli from the apparent moduli measured in bulk rheology measurements. An apparent 2D phase diagram is determined in the concentration range of 5 to 25 wt% L64 in D2O. Two distinct regions are observed: A high modulus film region 1 and a low modulus film region 2. Strain amplitude and frequency sweeps are used to characterize the microstructure of the film in both regions. Both regions show different properties with viscoelastic characteristics. Steady shear results show that high shear rates completely break down the structure in film region 1, but don't affect the film in region 2. The kinetics of the formation of the film are investigated and show that the formation of the film is a slow kinetic process with nucleation properties that can take from 10 minutes to a couple hours depending on the temperature.;Interfacial effects can have a big effect on bulk rheology measurements that cannot be neglected, especially for surface active solutes. In oscillatory shear measurements they can lead to an overestimation of the moduli and in steady shear measurements they can lead to an apparent yield stress and an apparent shear thinning behavior. By using multiple geometries the interfacial contribution of rheology measurements can be detected and taken into account.