Effect of charged segment length on physicochemical properties of core-shell type polyion complex micelles from block ionomers

The influence of the charged segment length on the physicochemical properties of polyion complex (PIC) micelles formed from the mixture of poly(ethylene glycol)-b-poly(alpha,beta-aspartic acid) [PEG-P(Asp)] with poly(ethylene glycol)-b-poly((L)-lysine) [PEG-P(Lys)] or poly((L)-lysine) [P(Lys)] was evaluated in this study. The hydrodynamic radius (Rh) and apparent weight-averaged molecular weight (M-w,M-app) Of the PIC micelles were determined from the dynamic and static light scattering to calculate the association number, core radius (R-core), micellar radius (R-m), corona thickness (R-corona), and PEG density at the core/ corona interface (PhiPEG). Changes in R-core and Phi(PEG) of the PIC micelles with a change in the charged segment length of the block copolymer were explained by considering the balance between the interfacial energy and conformational entropy of polymer strands associated in the core-shell type PIC micelles. Balance among these two parameters (R-core and Phi(PEG)) seems to play a substantial role in determining micellar size, the association number, and, thus, M-w,M-app of the micelles. Furthermore, Phi(PEG) was shown to depend only on the length of the P(Asp) segment in PEG-P(Asp), regardless of the composition of the counter polycations (PEG-P(Lys) and P(Lys)). This result suggests that Phi(PEG) might be the most crucial parameter to determine the physicochemical characteristics of the PIC micelles.