AGGREGATION OF POLY(ETHYLENE OXIDE)-POLY(PROPYLENE OXIDE)-POLY(ETHYLENE OXIDE) TRIBLOCK COPOLYMERS IN THE PRESENCE OF SODIUM DODECYL-SULFATE IN AQUEOUS-SOLUTION

NMR chemical shift measurements and fluorescence quenching have been combined in a study of the aggregation behavior and the aggregate properties of two poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (EPE) triblock copolymers in the presence of sodium dodecyl sulfate, at 20 and 40-degrees-C. The block copolymers differed in the ethylene oxide blocks, one (L64) containing a total of 26 EO units on the two blocks and the other (F68) 156 units; both had 30 PO units in the hydrophobic middle block. Neither of the EPEs form micelles by themselves at the low temperature but form mixed micelles with SDS, even at SDS concentrations as low as 1 mM. The aggregation numbers obtained from fluorescence quenching studies indicate that about 3 SDS molecules per EPE are needed to "glue" together the PO blocks into a hydrophobic core at the low temperature. The micelles formed at low SDS concentrations are small, with aggregation numbers down to 15 for SDS, associated with 4-5 EPEs. At high SDS concentrations the micelles approach pure SDS micelles. L64 forms micelles at 40-degrees-C. These are large but decrease rapidly in size with the addition of SDS. The C-13 chemical shift for the methyl carbons in the PPO block are progressively shifted more upfield in the transformation from the large L64 micelles to the small mixed ones; the shift in neat PPO is even more upfield and is close to the shift for the methyl groups in the PPO block of F68 unimers in aqueous solution. The shifts are probably indicative of a change from an extended conformation in the large micelles to much more coiled conformations in the small and unimer micelles as well as in the neat PPO liquid. The shifts of the methylene carbons in the PEO blocks go downfield on addition of SDS to both of the EPEs in aqueous solution, much in the same way as when SDS is added to PEO solutions. The change of the shifts of the individual carbons in the alkyl chain of SDS on addition of the EPEs to SDS micelles follows a similar pattern for both EPEs. Except for the strong upfield shift for the C1 carbon, which is also obtained when PEO interacts with SDS, they are all in the downfield direction and much stronger than the change given by only PEO. An interesting periodicity is observed with particularly small shifts for carbons 4, 7, and 10, 11.