Association Behavior of Poly(ethylene oxide)-Poly(propylene oxide) Alternating Multiblock Copolymers in Water toward...

Thermal changes in the association behavior of poly(ethylene oxide)-poly(propylene oxide) alternating multiblock (PEO-PPO AMB) copolymers in water are investigated by the use of transmittance and light scattering measurements. Two PEO-PPO AMB copolymers with a different weight fraction of PEO, (EO₂₂₀PO₃₃)₈ and (EO₆₈PO₃₃)₉, are prepared. The weight-average molecular weights of (EO₂₂₀PO₃₃)₈ and (EO₆₈PO₃₃)₉ estimated by static light scattering measurements are 1.3 × 10⁵ and 4.1 × 10⁴ g mol^-1, respectively. The number of PEO-PPO repeating pairs is over 8. It is found that the aqueous solution of (EO₂₂₀PO₃₃)₈ undergoes phase separation with a lower critical solution temperature (LCST) of around 58 °C at 0.3 wt %. For the aqueous (EO₆₈PO₃₃)₉ solution, the LCST is estimated to be ca. 42 °C. The critical solution concentration for (EO₆₈PO₃₃)₉ is not clear because of a small concentration dependence of T_c at a higher concentration range. Dynamic light scattering measurements indicate that a micellelike aggregate is formed below the LCST. From the Debye plot, it is elucidated that the second virial coefficient, A₂, starts going down at around 32 °C for (EO₂₂₀PO₃₃)₈ and below 15 °C for (EO₆₈PO₃₃)₉. The A₂ value of (EO₂₂₀PO₃₃)₈ approaches 0 near 50 °C, whereas that of (EO₆₈PO₃₃)₉ approaches 0 at around 35 °C. At a high temperature, the attractive interaction among the copolymers becomes dominant, thereby inducing the formation of micellelike aggregates.