Hydrophobically Modified Associating Polyacrylamide Solutions:  Relaxation Processes and Dilational Properties at the Oil−Water Interface

Abstract

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.

Aggregation behaviors of gelatin with cationic gemini surfactant at air/water interface

“…The higher the oscillation frequency is, the larger possibility the film of gelatin has to show negative phase angle. This phenomenon agrees with that in the literature [34]. The reason why the negative phase angle occurs is that the oscillation frequency becomes so high that the fast exchange of molecules cannot compensate the interfacial concentration change caused by area change.…”

“…It is reported that the negative phase angles have been obtained for the interfacial film at which the slow relaxation process dominates the dilational viscoelasticity, such as films containing high molecular weights and aggregations. On the contrary, the positive phase angles appear for the film at which the fast exchange of molecules between interface and bulk solution dominates the dilational viscoelasticity [34]. So it can be deduced that the film becomes more viscous at high C 12 C 2 C 12 concentration, which increases the fast exchange of C 12 C 2 C 12 molecules between the interface and the bulk.…”

The interfacial dilational properties of hydrophobically modified associating polyacrylamide studied by the interfacial tension relaxation method at an oil–water interface

“…The sample was polyacrylamide hydrophobically modified with 2-phenoxylethyl acrylate (POEA) prepared by micellar copolymerization technique. The synthesis and characterization of the copolymers are reported elsewhere [37,38]. It was dissolved in deionized water (resistivity > 18 M cm −1 ) to a certain concentration and used as water phase.…”

“…Both processes become faster with an increase in the bulk concentration far less than c * and then change little till c * . However, there is only one main relaxation process dominating the dynamic dilational properties above c * , which is believed to be related to the associations formed by hydrophobic microdomains [38].…”

“…The characteristic time of both the fast and the slow processes passed through a maximum with an increase in SDS concentration. It implied that the influence of SDS on the characteristic times of existed process plays a pivotal role in controlling dilational properties of polymer film at low SDS concentration and may increase the dilational elasticity, while the exchange of SDS molecules between monomers and mixed micelles in the interface dominates the film dilational properties at relative high SDS concentration and caused the very low value of dilational elasticity [38].…”