INTERACTION OF ABA BLOCK-COPOLYMERS WITH IONIC SURFACTANTS - INFLUENCE ON MICELLIZATION AND GELATION

Block copolymers of the poloxamer type EO(x)PO(y)EO(x) (with EO = ethy lene oxide and PO = propylene oxide) from micelles in aqueous solution that have a hydrophobic core of PO blocks and a strongly hydrated she ll of EO blocks. The critical micellization temperature (cmt) of the p oloxamers is strongly influenced by cosolutes such as surfactants. The interaction of F127 (EO(97)PO(69)EO(97)) with the anionic surfactant SDS was investigated by small-angle neutron scattering (SANS), static light scattering, and differential scanning calorimetry (DSC). It is f ound that addition of SDS can suppress the micellization of F127 compl etely. A simple model is proposed which describes the suppression of p olymer micelles by ionic surfactant. The surfactant binds cooperativel y on the block copolymer molecules, and the hydrophobic block is there by made hydrophilic. At saturation conditions four to five SDS molecul es bind to one F127 molecule. The bound amount of SDS increases somewh at with increasing polymer concentration. At higher concentration (w g reater than or equal to 20 wt %), pure F127 forms a cubic gel with inc reasing temperature. The gel region increases with increasing F127 con centration. Addition of SDS to a fixed F127 concentration decreases th e gel region, until the gel completely disappears. The ''melting'' of the gel is a result of the suppression of the poloxamer micelles. With increasing surfactant concentration the hard sphere volume fraction P hi decreases below 0.53, the critical value for hard sphere crystalliz ation.