Self-Assembly of Thermosensitive Amphiphilic Pentablock Terpolymer PNIPAMx-b-PtBA90-b-PPO36-b-PtBA90-b-PNIPAMx in Dilute Aqueous Solution

A series of linear thermosensitive amphiphilic pentablock terpolymers PNIPAMx-b-PtBA90-b-PPO36-b-PtBA90-b-PNIPAMx (NxT90O36T90Nx) with various lengths x of PNIPAM block (x = 111, 107, 95, 58, 33, and 26) were synthesized via a two-step atom transfer radical polymerization (ATRP) using modified poly(propylene oxide) (PPO36) as the macroinitiator, tert-butyl acrylate (tBA) as the first monomer, and N-isopropylacrylamide (NIPAM) as the second monomer. The NxT90O36T90Nx pentablock terpolymers form micelles in dilute aqueous solution, whose structure and morphology are dependent on the length x of PNIPAM block and solution temperature. At 14 °C, below the lower critical solution temperature (LCST) of PNIPAM block, the morphology of NxT90O36T90Nx micelles changes from core–corona spherical micelles with homogeneous hydrophobic core to rodlike micelles, core–shell–corona spherical micelles with clear boundaries, core–shell–corona spherical micelles with blurry boundaries, and finally large compound spherical micelles with embedded hydrophilic chains in the hydrophobic region when x decreases from 111 to 26. This morphology change might be attributed to the different stretching factors of hydrophobic PPO and PtBA blocks in the micelles, which varied differently with length x of PNIPAM block and hence resulted in the change of compatibility of the two hydrophobic blocks. A scaling relationship between the length L0 of hydrophilic corona and the length x of PNIPAM block is found to be L0 ∼ x0.77±0.08, which falls in the range 0.5–0.86 theoretically predicted for the micelles of diblock copolymers. The size of hydrophobic regions increases with decreasing x. At 60 °C above the LCST of PNIPAM block, all NxT90O36T90Nx pentablock terpolymers form spherical micelles with uniform structure and size distribution in aqueous solution regardless of the length x of PNIPAM block.