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 = ethylene oxide and PO = propylene oxide) form micelles in aqueous solution that have a hydrophobic core of PO blocks and a strongly hydrated shell of EO blocks. The influence of electrolyte and surfactant on the aggregation behavior of three Poloxamers (F127, F88 and P123) was investigated by differential scanning calorimetry (DSC) and T-jump measurements. The micelle formation is influenced both by addition of electrolyte and of surfactant. The critical micellization temperature of the Poloxamer decreases linearly with increasing weight fraction of electrolyte 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 disappears with addition of surfactant. The adsorption of the anionic surfactant sodium dodecyl sulfate (SDS) starts at a lower concentration than the adsorption of the cationic surfactant N-dodecyl-N,N,N-trimethyl ammonium bromide (DTAB), whereas the latter is adsorbed to a larger extent. For all investigated Poloxamers one relaxation time tau in the millisecond range is obtained. For constant temperature the relaxation time decreases with the total concentration of block copolymer. For constant concentration of block copolymer the relaxation time decreases with increasing temperature and increasing surfactant concentration. The relaxation process can be described by the Aniansson-Wall mechanism and values for the association rate constant for the poloxamers were 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 Poloxamer, which determines the structure of the micelles. The thicker the shell of the micelle (i.e. the EO block length), the smaller the association rate constant k(+). Keeping the EO block length constant and decreasing the PO block size again increases k(+). Now the hydrophobic part of monomeric Poloxamer controls the penetration through the EO shell.