Self-Assembly of Linear Amphiphilic Pentablock Terpolymer PAA(x)-PS48-PEO46-PS48-PAA(x)in Dilute Aqueous Solution

A series of linear amphiphilic pentablock terpolymer PAA(x)-b-PS48-b-PEO46-b-PS48-b-PAA(x) (A(x)S(48)O(46)S(48)A(x)) with various lengths x of the PAA block (x = 15, 40, 60, and 90) were synthesized via a two-step atom transfer radical polymerization (ATRP) using Br-poly(ethylene oxide)-Br (Br-PEO46-Br) as the macroinitiator, styrene (St) as the first monomer, and tert-butyl acrylate (tBA) as the second monomer, followed with the hydrolysis of PtBA blocks. The A(x)S(48)O(46)S(48)A(x) pentablock terpolymers formed micelles in dilute aqueous solution, of which the morphologies were dependent on the length x of the PAA block. Cryogenic transmission electron microscopy (cryo-TEM), dynamic light scattering (DLS), and zeta potential measurement were employed to investigate the morphologies, chain structures, size, and size distribution of the obtained micelles. The morphology of A(x)S(48)O(46)S(48)A(x) micelles changed from spherical vesicles with ordered porous membranes to long double nanotubes, then to long nanotubes with inner modulated nanotubes or short nanotubes, and finally, to spherical micelles or large compound vesicles with spherical micelles inside when x increased from 15 to 90. The hydrophobic PS blocks formed the walls of vesicles and nanotubes as well as the core of spherical micelles. The hydrophilic PEO and PAA block chains were located on the surfaces of vesicle membranes, nanotubes, and spherical micelles. The PAA block chains were partially ionized, leading to the negative zeta potential of A(x)S(48)O(46)S(48)A(x) micelles in dilute aqueous solutions.