We have investigated the heat-induced self-association in water of a block copolymer (PIPOZ-b-PEOZ) comprising two thermosensitive blocks: a poly(2-isopropyl-2-oxazoline) block (degree of polymerization 71) and a poly(2ethyl-2-oxazoline) block (degree of polymerization 38) using differential scanning calorimetry (DSC), static light scattering (SLS), dynamic light scattering (DLS), and small-angle X-ray scattering (SAXS) together with visual observation on the macroscopic and microscopic scales. The dehydration temperatures of the PIPOZ and PEOZ blocks in water are 43 and 54 °C, respectively. When heated abruptly to 50 °C where PIPOZ-b-PEOZ is amphiphilic, the copolymer first forms star micelles that further aggregate to form large concentrated phase droplets, and finally the droplets coalesce into a bulk liquid phase, having a copolymer concentration as high as 0.8 g/cm 3 . When heated abruptly to 70 °C, where both blocks are hydrophobic, the copolymer solution also separates into two liquid phases, consisting of phase-separated polymer-rich micrometer or submicrometer size droplets dispersed in a polymer-poor liquid phase, but the droplets do not coalesce into a liquid bulk phase. We discuss the role of the more hydrophilic PEOZ block in the macroscopic phase separation behavior. No microphase separation takes place in the concentrated phase of the aqueous PIPOZ-b-PEOZ block copolymer solution.

We have investigated the self-assembly in dilute aqueous solutions of a mixture of an anionic−neutral block copolymer (AP) and a cationic−neutral block copolymer (MP) by changing the added sodium chloride (NaCl) concentration C S or electrostatic interactions among oppositely charged blocks, by direct observation, optical and electron microscopies, and small-angle X-ray scattering. The ratio of the charged to neutral block chain lengths was ca. 10, and the total copolymer concentration and the mixing ratio (the mole fraction of the MP charge unit in the total charge units) of AP and MP were fixed to be 0.005 g/cm 3 and 0.6, respectively. With decreasing C S from 2 to 0 M, we have found reentrant one-phase, two-phase, one-phase transitions in the aqueous solution of the AP−MP mixture. The two-phase to one-phase transition at C S ∼ 0.5 M arises from the competition between the macroscopic phase transition and micellization, which is the first observation in dilute block copolymer solutions. Moreover, we have found a micelle morphology transition from the bilayer vesicle to the cylindrical micelle with further decreasing C S from 0.5 M to lower than 0.05 M.

The hydration state and dynamics of plain and chemically modified cyclodextrins (CDs) in aqueous solution were investigated by using dielectric relaxation measurements at 25 degrees C over a wide frequency range up to 20 GHz, which is the relaxation frequency of pure liquid water molecules. The obtained dielectric relaxation spectra were decomposed into two major and one minor relaxation modes with relaxation times of approximately 8.3, 20-25, and 1000-2500 ps, respectively, depending on the CD species. The two major modes, fast and medium, were attributed to a rotational relaxation process of water molecules belonging to the bulk (free) state and an exchange of water molecules hydrated to CDs owing to hydrogen bond formation. The hydration numbers of the CDs strongly depend on the number of hydroxy (OH) groups controlled by chemical modification such as methylation. Increasing the number of methoxy or 2-hydroxypropoxy groups increases the hydration number of CD molecules, and results in higher solubilities of the chemically modified CDs than those of the plain CDs. The minor, slow mode was assigned to overall rotational relaxation for CDs with finite permanent dipole moments, which also depends on the number of OH groups.

The hydration state of poly(ethylene oxide)s (PEOs) in aqueous solutions was investigated using dielectric relaxation measurements at 25 degrees C over a frequency range up to 20 GHz, which is the relaxation frequency of water molecules in a bulk state. The dielectric relaxation spectra obtained indicated decomposition into two major and one minor relaxation modes with relaxation times of 8.3, 22, and 250 ps, respectively. The two major modes were attributed to rotational relaxation of water molecules belonging to the bulk state and water molecules hydrogen bonded to ethylene oxide (EO) monomer units. The number of hydration water molecules per EO unit depended on the molar mass of PEO (M) and reached a constant value of 3.7 at M > 1500, which agrees with the value obtained by other experiments.

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