PNIPAm-PEO-PPO-PEO-PNIPAm Pentablock Terpolymer: Synthesis and Chain Behavior in Aqueous Solution

The double thermosensitive and narrow dispersed PNIPAm 110 -PEO 100 -PPO 65 -PEO 100 -PNIPAm 110 pentablock terpolymer was synthesized by the typical atomic transfer radical polymerization (ATRP) method with N -isopropylacrylamide (NIPAm) as the monomer and modified poly(ethylene oxide) 100 −poly(propylene oxide) 65 −poly(ethylene oxide) 100 (PEO 100 -PPO 65 -PEO 100 ) block copolymer as the macroinitiator. Microdifferential scanning calorimetry (micro-DSC) data showed that the pentablock terpolymer exhibited two low critical solution temperatures (LCSTs) at 31 and 34 °C in the aqueous solution, which can be attributed to the thermal phase transition of the PPO block and PNIPAm block, respectively. The chain conformation of the pentablock terpolymer in aqueous solution was then studied in detail by using a combination of static and dynamic laser light scattering (SLS-DLS). The SLS-DLS results indicated that the loose “associates” and single coil chains coexisted in the aqueous solution at the low temperature, where the PEO, PPO, and PNIPAm blocks were soluble in water. These phenomena were inconsistent with those observed in other PEO-containing block copolymer systems. At the high temperature above the LCSTs of PPO and PNIPAm blocks (38−60 °C), the pentablock terplymer chains formed large and stable core−shell micelles with collapsed PPO and PNIPAm cores and swollen PEO shells. The TEM and cryo-TEM experiments provided visual images, which confirmed the formation of loose “associates” at 21 °C and large stable micelles at 38 °C. Increase of concentration hindered the formation of “associate” at low temperature, but the micelles formed at high temperature were almost independent of the solution concentration investigated.