SYNTHESIS OF ABA TRIBLOCK IONIC COPOLYMERS AND THEIR SELF-ASSEMBLY IN SOLUTION

Triblock copolymers PNPMA-b-PS-b-PNPMA with 146 styrene (St) units and 8, 20 and 36 p-nitrophenyl methacrylate (NPMA) units were prepared by ATRP of St using a, a'-dibromo-p-xylene as initiator, and following ATRP of NPMA. Their molecular weights and narrow molecular weight distributions (M-w/M-n = 1.03 similar to 1.11) were determined by H-1-NMR and GPC. The anionic copolymer PMAA-b-PS-b-PMAA was synthesized by hydrolysis of PNPMA-b-PS-b-PNPMA, and the cationic copolymer PNPMAAm-b-PS-b-PNPMAAm was prepared by the amino substitution of the same PNPMA-b-PS-b-PNPMA. The complete substitution of NPMA groups was verified by FT-IR spectra. Equal molar PMAA-b-PS-b-PMAA and PNPMAAm-b-PS-b-PNPMAAm both having the same chain lengths were dissolved in common solvent THF with 20% deionized water, and the solution was dialyzed against water to induce the self-assembly of copolymers. The aggregates formed from copolymers with shorter hydrophilic blocks (8 or 20) were deposited in the self-assembly. The aggregates obtained from copolymers with 36 hydrophilic units were studied by TEM and SEM. Polydisperse spheres were observed containing an asymmetrically placed single void space, which has broken through the surface (so-called bowl-shaped aggregates) The size of the bowl-shaped aggregates was measured by DLS to be (565 +/- 300) nm. The possible mechanism was discussed. Highly swollen large spheres composed of reverse micelles of hydrophilic cores and PS shells first formed. The diffusion of the solvent inside the large sphere cause bubbles, and they coalesced to a large bubble which broke through the surface of the large sphere that induced the formation of bowl-shaped aggregates.