Morphology transition of polyion complex (PIC) micelles with carboxybetaine as a shell induced at different block ratios and their pH-responsivity

In this study, we investigated the morphology transition of polyion complex (PIC) micelles with the change in block ratio and pH-responsivity of PIC micelles or vesicles using entirely ionic diblock copolymers composed of carboxybetaine and ionic blocks. We used 2-((2-(methacryloylo-xy)ethyl)dimethylammonio)acetate (PGLBT) as carboxybetaine, poly(sodium styrenesulfonate) (PSSNa) as the anionic polymer, and poly[3-(methacrylamido)propyltrimethylammonium chlorid-e] (PMAPTAC) as the cationic polymer. The effect of pH on the PGLBT homopolymer and the PGLBT-containing diblock copolymer was examined by DLS, ELS, and transmittance, and a rapid change of state was observed between pH 4 and 2. At this pH, the carboxyl group of PGLBT was protonated to form a hydrogen bond in the molecule. Furthermore, at a lower pH, diblock copolymer behaved like a cationic polymer. The formation behavior of PIC micelles at different block ratios in the diblock copolymers was investigated by DLS, SLS, TEM, and AFM. PIC vesicles formed when the block ratio of ionic blocks to the PGLBT block was equal or larger (the content of PGLBT was 52% or less). On the other hand, PIC micelles were formed when the block ratio of PGLBT to ionic blocks was larger (the content of PGLBT was 68% or more). The pH-responsivity of PIC micelles was different from that of PIC vesicles. The size of PIC vesicles decreased by lowering pH and increased when the below pH 3. The behavior was the same as the change of state of PGLBT homopolymer with the change in pH. However, the size of PIC micelles increased by lowering pH from pH 6 to 3 and decreased at pH below pH 3. The PGLBT, which became the shell, changed its state with the change in pH and affected the aggregation number of micelles. Graphical Abstract