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 = ethylene oxide and PO = propylene oxide) from micelles in aqueous solution that have a hydrophobic core of PO blocks and a strongly hydrated shell of EO blocks. The critical micellization temperature (cmt) of the poloxamers 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 found that addition of SDS can suppress the micellization of F127 completely. A simple model is proposed which describes the suppression of polymer micelles by ionic surfactant. The surfactant binds cooperatively on the block copolymer molecules, and the hydrophobic block is thereby made hydrophilic. At saturation conditions four to five SDS molecules bind to one F127 molecule. The bound amount of SDS increases somewhat with increasing polymer concentration. At higher concentration (w greater than or equal to 20 wt %), pure F127 forms a cubic gel with increasing temperature. The gel region increases with increasing F127 concentration. Addition of SDS to a fixed F127 concentration decreases the 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 Phi decreases below 0.53, the critical value for hard sphere crystallization.