engleski

Effects of Anionic Surfactant on the Morphology of Amphiphilic Diblock Copolymer Self-Assembled Aggregates

Important class of synthetic systems for creating self-assembled nanostructures are diblock copolymers. Due to its amphiphilic nature they form aggregates in selective solvents ranging from micelles and vesicles to continuous network structures. One of the most extensively studied block copolymers is a series of poly(butadiene-b-ethylene oxide)(PB-PEO).Self-assembling of PB-PEO in water is sensitive to the presence of many additives. A series of investigations revealed that the addition of surfactants induces a structural change of mixed micelles, that is, the sizes and shapes of mixed micelles change as functions of the surfactant-to-copolymer concentration ratio. In the present study we have studied the effects of anionic surfactant (sodium dodecyl sulphate, SDS) on the morphology of symmetric PB-PEO (2.0-b-2.0) in dilute regime. Due to its very low crtitical micelle concntration (cmc ≈ 0.01%wt) PB-PEO self-assembles into micelles (the PB block constitutes the core and the PEO block the hydrophylic corona) in very dilute solutions. Spherical and more or less elongated ellipsoidal aggregates with interior consisting of wormlike micelles were formed at cPB-PEO = 0.2 wt%. Aggregates are polydisperse showing a bicontinuous morphology. It seems that larger aggregate are formed by fusion of two spherical aggregates with diameter of ≈ 144 nm (aggregate size is ≈ 284 nm and thickness is ≈ 144 nm). SDS addition induced a series of morphological changes involving the onset of aggregates instability at the lowest SDS concentration and complete transformation to spherical mixed micelles at the highest concentration. Morphological transition is a continuous one with wormlike and spherical micelles existing over the broad SDS concentration range. The morphology of the self-assembled diblock copolymer aggregates is determined primarily by a force balance among three contributions: the core-chain stretching, the interfacial tension between the core and the solvent, and interactions inter corona. Formation of mixed aggregates by incorporation of the hydrophobic tail of SDS into PB-PEO core promotes structural changes leading to an increase in the polymer core-chain stretching and core size. This is accompanied by location of the sulfate head groups at the core-corona interface and an increase in the corona charge density. The increase in the repulsion among the corona chains leads to a decrease in the aggregate size in order to decrease total interfacial area and energy between the core and water. All of these factors alter the force balance controlling the morphology of the aggregates leading to structural changes from aggregated wormlike to spherical micelles. To our knowledge there have been no systematic studies of surfactant influence on this type of aggregated wormlike micelles.

diblock copolymer; surfactants; self-assembly

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