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, su bscripts denote number-average chain lengths) with B blocks terminated by h ydroxy groups (denoted EmBnH) were methylated to provide copolymers having the same chain length and composition but with B blocks terminated by metho xy groups (denoted EmBnM). Micelle properties of the M copolymers were dete rmined by dynamic and static light scattering (hydrodynamic radius, associa tion number, thermodynamic radius) and the values obtained compared with th ose for the H copolymers, most of which had been published previously. The results for copolymer E18B10M in solution at 40 degreesC were consistent wi th the formation of worm-like micelles, the micelles of the other copolymer s being spherical, including E18B10H in solution at 40 degreesC and E18B10M in solution at 30 degreesC. For micelles of the B-18 copolymers, methylati on 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 t hermodynamic radius by ca. 10%. The explanation of these effects takes into account the increased hydrophobicity of the methylated B blocks, the highl y 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 copo lymers with hydroxy-ended B blocks. For copolymers forming spherical micell es, the effect of methylation on association number is equivalent to raisin g the temperature of the solution by ca. 10 degreesC. For micelles of copol ymer E18B10, the effect of methylation is to lower the temperature of the s phere-to-worm transition from 40-50 degreesC (E18B10H) to 30-40 degreesC (E 18B10M).