KINETIC AND CALORIMETRIC INVESTIGATIONS ON MICELLE FORMATION OF BLOCK-COPOLYMERS OF THE POLOXAMER TYPE

Block copolymers of the Poloxamer type EO(x)PO(y)EO(x) (where EO = eth ylene oxide and PO = propylene oxide) form micelles in aqueous solutio n that have a hydrophobic core of PO blocks and a strongly hydrated sh ell of EO blocks. The influence of electrolyte and surfactant on the a ggregation behavior of three Poloxamers (F127, F88 and P123) was inves tigated by differential scanning calorimetry (DSC) and T-jump measurem ents. The micelle formation is influenced both by addition of electrol yte and of surfactant. The critical micellization temperature of the P oloxamer decreases linearly with increasing weight fraction of electro lyte and the decrease is independent of the cation size (Li+, Na+, K+) . The DSC peak that is due to the micelle formation of the Poloxamer d isappears with addition of surfactant. The adsorption of the anionic s urfactant sodium dodecyl sulfate (SDS) starts at a lower concentration than the adsorption of the cationic surfactant N-dodecyl-N,N,N-trimet hyl ammonium bromide (DTAB), whereas the latter is adsorbed to a large r extent. For all investigated Poloxamers one relaxation time tau in t he millisecond range is obtained. For constant temperature the relaxat ion time decreases with the total concentration of block copolymer. Fo r constant concentration of block copolymer the relaxation time decrea ses with increasing temperature and increasing surfactant concentratio n. The relaxation process can be described by the Aniansson-Wall mecha nism and values for the association rate constant for the poloxamers w ere determined. The evaluated rate constants are in the range of 0.2 x 10(7)-2 x 10(7) l mol(-1) s(-1) and are not diffusion controlled. The micellization kinetics depends on the molecular structure of the Polo xamer, which determines the structure of the micelles. The thicker the shell of the micelle (i.e. the EO block length), the smaller the asso ciation rate constant k(+). Keeping the EO block length constant and d ecreasing the PO block size again increases k(+). Now the hydrophobic part of monomeric Poloxamer controls the penetration through the EO sh ell.