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 degreesC were consistent with the formation of worm-like micelles, the micelles of the other copolymers being spherical, including E18B10H in solution at 40 degreesC and E18B10M in solution at 30 degreesC. For micelles of the B-18 copolymers, methylation reduced the values of all properties by ca. 10%. For micelles of the B-9- B-10 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 B-18 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 degreesC. For micelles of copolymer E18B10, the effect of methylation is to lower the temperature of the sphere-to-worm transition from 40-50 degreesC (E18B10H) to 30-40 degreesC (E18B10M).