Sphere to Rod Transition of Micelles Formed by Amphiphilic Diblock Copolymers of Vinyl Ethers in Aqueous Solution

Amphiphilic block copolymers of vinyl ethers containing 2-hydroxyethyl vinyl ether (HOVE) and partially deuterated n-butyl vinyl ether (NBVE) were synthesized by living cationic polymerization. Four block copolymers with the same hydrophilic length but different hydrophobic lengths were prepared. The internal structures of the micelles formed by these copolymers in aqueous solution were investigated by the contrast variation method of small-angle neutron scattering (SANS) measurement. The molar volume of HOVE was estimated to be quite small from the dependence of the forward scattering intensity on the contrast, which comes from the hydration effect in the micellar shell. The SANS data were well described by the theoretical form factor of a core-shell model. The micellar shape was strongly dependent on the hydrophobic chain length of the block copolymer. The polymer with the shortest hydrophobic chain was suggested to form spherical micelles, whereas the scattering curves of the longer hydrophobic chain polymers showed the q^-1 dependence, reflecting the formation of rodlike micelles. These scattering curves could be described well by a sphere-rod coexistence model. The volume fraction of the rodlike micelle was found to increase with increasing hydrophobic chain length.