Effects of guest poly(ethyleneglycol) homopolymer on the order-to-disorder transition of hydrophobically modified poly(ethyleneglycol) (C12E25) micellar solution: PEG molecular weight dependence and SCF simulation

Effects of the molecular weight of guest poly(ethyleneglycol) (PEG) on the order-to-disorder transition of micellar aqueous solution of hydrophobically modified ethyleneglycol oligomer, poly(ethyleneglycol) monododecylether (C12E25) have been investigated by means of linear-viscoelastic measurements, and differential scanning calorimetry. Added PEG having a chain length similar to or even higher than that of micellar corona PEG can be incorporated into the micellar lattice of C12E25 aqueous solution with maintaining the body-centered-cubic (bcc) lattice structure. The order-to-disorder transition temperature decreases with increasing concentration of guest PEG at a fixed concentration of C12E25. The decrement of transition temperature is increased with increasing molecular weight of guest PEG. Self-consistent field (SCF) calculation shows that the higher molecular weight guest chains are more excluded from the micellar corona chains to be localized between the micellar particles, which enlarge the lattice distance more pronouncedly than the shorter guest chains do. The unstability of micellar lattice, i.e. the depression of transition temperature, may be explained in terms of guest chain localization in the lattice.