Association properties of a triblock copolymer of ethylene oxide and styrene oxide, S4E45S4, in aqueous solution

The block copolymer S4E45S4 has been prepared by sequential anionic copolymerisation of ethylene oxide followed by styrene oxide starting from a difunctional initiator. The notation E denotes an oxyethylene unit and S an oxyphenylethylene unit. Clouding of aqueous solutions was studied across the concentration range 0.01-40 wt.%. Micellar properties were investigated by static and dynamic light scattering: at 15°C the mass-average association number of the micelles was 24. Second virial coefficients, from both static and dynamic light scattering, were found to be negative, consistent with significant micellar linking. The critical micelle concentration (c.m.c.) of the copolymer in solution at 15°C was very low, ca. 0.013 g dm-3. Comparison of this c.m.c. with published values for BnEmBn triblock copolymers (B = oxybutylene) shows the hydrophobicity of an S unit to be three times that of a B unit. Extension of the comparison to oxyethylene-oxypropylene (E-P) block copolymers leads to the scale of hydrophobicities S : B : P = 12 : 4 :1.