Effect of end group on the micelle properties of diblock copolymers of ethylene oxide and 1,2-butylene oxide

Copolymers of ethylene oxide and 1,2-butylene oxide (E18B10, E43B9, E40B10, E90B10 , E96B18 and E184B18 , E = oxyethylene unit, B = oxybutylene unit, subscripts denote number-average chain lengths) with B blocks terminated by hydroxy groups (denoted EmBnH) were methylated to provide copolymers having the same chain length and composition but with B blocks terminated by methoxy groups (denoted EmBnM). Micelle properties of the M copolymers were determined by dynamic and static light scattering (hydrodynamic radius, association number, thermodynamic radius) and the values obtained compared with those for the H copolymers, most of which had been published previously. The results for copolymer E18B10M in solution at 40°C were consistent with the formation of worm-like micelles, the micelles of the other copolymers being spherical, including E18B10H in solution at 40°C and E18B10M in solution at 30°C. For micelles of the B18 copolymers, methylation reduced the values of all properties by ca. 10%. For micelles of the B9–B10 copolymers, the effect of methylation was to reduce the hydrodynamic radius by ca. 10%, but to increase the association number by ca. 25% and thermodynamic radius by ca. 10%. The explanation of these effects takes into account the increased hydrophobicity of the methylated B blocks, the highly stretched state of the B18 blocks in their micelles, and the probability that water will concentrate at the centre of the cores of micelles of copolymers with hydroxy-ended B blocks. For copolymers forming spherical micelles, the effect of methylation on association number is equivalent to raising the temperature of the solution by ca. 10°C. For micelles of copolymer E18B10, the effect of methylation is to lower the temperature of the sphere-to-worm transition from 40–50°C (E18B10H) to 30–40°C (E18B10M).