Heat-induced self-assembling of thermosensitive block copolymer. 1. Small-angle neutron scattering study

The sharp and thermoreversible morphological transitions of poly(2-ethoxyethyl vinyl ether-b-2-hydroxyethyl vinyl ether) (poly(EOVE-b-HOVE)) in aqueous solutions have been investigated by means of small-angle neutron scattering. The poly(EOVE-b-HOVE), synthesized by living cationic polymerization, had the number-average degree of polymerizations of the constituent blocks of 200 and 400, respectively, and a polydispersity index of 1.07. The poly(EOVE-b-HOVE) underwent a disorder-order transition around 17-21 degreesC, depending on the polymer concentration. This is a micellization with a poly(EOVE) core of ca. 20 nm in radius and a poly(HOVE) corona. When the polymer concentration was high enough, e.g., 17 wt %, a macrolattice formation with body-centered cubic symmetry was also observed. This transition took place in a narrow temperature window of less than a degree centigrade. The origin of this sharp transition was found to be a dissociation of hydrophobically associated structure, such as an iceberg structure. The enthalpy involved in this dissociation was estimated to be 2.05 kcal/mol of an EOVE monomeric unit, which is about twice as large as that of a well-known thermosensitive polymeric system, i.e., poly(N-isopropylacrlyamide) aqueous solutions.