Hydrophobically modified associating polyacrylamide solutions: Relaxation processes and dilational properties at the oil-water interface

The interfacial dilational viscoelastic properties of hydrophobically associating block copolymer composed of acrylamide (AM) and a low amount of 2-phenoxylethyl acrylate (POEA) (<1.0 mol %) at the octane-water interfaces were investigated by means of two methods: the interfacial tension response to sinusoidal area variations and the relaxation of an applied stress. The dependencies of interfacial dilational modulus and phase angle on the polymer concentration were explored. The influence of sodium dodecyl sulfate (SDS) on the dilational viscoelastic properties of polymer solutions was studied. The results obtained by oscillating barriers method showed that the dilational modulus passed through a maximum value with increasing polymer concentration, while the phase angle decreased with increasing concentration below 200 ppm, then showed very low concentration dependence up to 3000 ppm, and increased dramatically above it. When SDS was added to the aqueous phase, the dilational modulus passed through a maximum with increasing SDS concentration, while the change of phase angle depended on the polymer bulk concentration. The results obtained by the relaxation of an applied stress show that two main relaxation processes exist in the interface at low bulk concentration below the critical aggregation concentration: one is the fast process involving the exchange of hydrophobic microdomains between the proximal region and distal region in the interface with a characteristic time value from several tens of seconds to several seconds at different bulk concentration; the other is the slow relaxation process involving conformational changes of polymer chain in the interface with characteristic time value from 1000 s to several tens of seconds, depending on the bulk concentration. However, there is only one main relaxation process controlling the dilational properties above c*: a fast relaxation process with the characteristic relaxation time of less than 1 s, which is believed to be related to the associations formed by hydrophobic microdomains. Anionic surfactant SDS can influence the dilational properties of polymer solutions by the following ways: first, SDS can absorb onto the interface and bind to the hydrophobic microdomains to change the characteristic times and contributions of the existed relaxation processes of polymer chains; second, SDS can provide a new fast relaxation process involving the exchange of SDS molecules between monomers and mixed micelles in interface. The information on relaxation processes obtained from interfacial tension relaxation measurements can explain the results from dilational viscoelasticity measurements very well. The negative phase angles have been obtained in some case. It is believed that the in-interface slow relaxation process, which sometimes dominates the dilational viscoelasticity of polymer film, is responsible for this phenomenon in our employed experimental method.