Interfacial rheology and conformations of triblock copolymers adsorbed onto the water–oil interface

Abstract

The conformation and the dilatational properties of three non-ionic triblock PEO-PPO-PEO (where PEO is polyethyleneoxide and PPO is polypropyleneoxide) copolymers of different hydrophobicity and molecular weight were investigated at the water-hexane interface. The interfacial behavior of the copolymers was studied by combining dilatational rheology using the oscillating drop method and ellipsometry. From the dilatational rheology measurements the limiting elasticity values, E(0), of the Pluronics as function of surface pressure, Π, and adsorption time were obtained, i.e. E(0)(t) and E(0)(Π). Here, it is shown that E(0)(t) depends on the number of PEO units and on the bulk concentration, showing maximum and minimum surface elasticity values which indicate conformational changes in the interfacial layer. Furthermore, in the framework of the polymer scaling law theory, conformational transitions were discussed in E(0) vs. Π plots. In a dilute regime (Π<14 mN m(-1)) at the water-hexane interface, E(0)=2Π fits well all the data, which indicates a two-dimensional "stretched chain" conformation. Increasing Π, two other interfacial transitions could take place. The different behavior of Pluronic copolymers could be also described by the local minima of E(0), which depends on the hydrophobicity of the copolymers. Conformational transitions observed by interfacial rheology were compared to ellipsometric data. Experimental results were discussed and explained on the basis of two- and three-dimensional copolymer structure taking into account that PPO chains could be partially immersed in hexane and water.

Interfacial characterization of Pluronic PE9400 at biocompatible (air–water and limonene–water) interfaces

“…This agrees with the experimental findings discussed above suggesting the formation of a more compact layer at the limonene-water interface at high interfacial coverage. This could be the result of PPO segments penetrating into the limonene in agreement with recent work which demonstrates the penetration of PPO segments into the hexane phase by ellipsometric measurements [28]. The same value of a at both interfaces indicates that the attractive interaction between adsorbed polymer molecules is similar at the air-water and the limonene-water interface.…”

“…This type of behavior has previously been reported for pluronics adsorbed at the air-water interface and at the hexane-water interfaces but not at the limonene-water interface [20,27]. The occurrence of this kink before the cmc is likely to be due to a conformational change at the interface in which the polymer changes its orientation from 2D flat conformation to a 3D brush or mushroom one as reported in the literature for other Pluronics [20,21,[27][28][29][30]. Fig.…”

“…Also, since it has been reported previously that the interfacial elasticity of these copolymers is a unique function of the interfacial pressure (i.e. interfacial coverage) [16,28], harmonic oscillations were performed throughout the whole adsorption process for a given concentration of PE9400 in the bulk. These values are also plotted in Fig.…”

“…The surface properties of some pluronics have been studied in the literature by means of different techniques such as dynamic and equilibrium surface tension, ellipsometry, surface rheology and neutron reflectivity at the air-water and hexane-water interfaces [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30]. Some of these studies show the occurrence of conformational changes within the adsorbed layer of Pluronics at the interface when increasing the amount of adsorbed polymer, suggesting that the conformation of Pluronic molecules changes from a 2D flat conformation to a 3D brush/mushroom one at the interface [16,17,20].…”

“…Hence, the interfacial characterization of PE9400 at the limonene/water interface validates the previous work in a more realistic and practical scenario. The experimental results have been analyzed in the framework of a previously reported model originally developed for protein adsorption which has recently been used as a model for Pluronics adsorption at the hexane-water interface [28]. Hence, this work provides a systematic study showing a sequence of surface, dynamic, rheological and desorption [29,37] properties of Pluronics.…”