pH-induced morphological transition of aggregates formed by miktoarm star polymers in dilute solution: a mesoscopic simulation study

Abstract

The self-assembly of miktoarm star polymers μ-A _i (B(D)) _j C _k in a neutral solution and the pH-responsive behaviors of vesicles and spherical micelles in an acidic solution have been investigated by DPD simulation. The results show that the self-assembled morphologies can be regulated by the lengths of pH-responsive arm B and hydrophilic arm C, leading to the formation of vesicles, discoidal micelles, and spherical micelles in a neutral solution. The dynamic evolution pathways of vesicles and spherical micelles are categorized into three stages: nucleation, coalescence, and growth. Subsequently, the pH-responsive behaviors of vesicles and spherical micelles have been explored by tuning the protonation degree of pH-responsive arm B. The vesicles evolves from nanodisks to nanosheets, then to nanoribbons, as the protonation degree increases, corresponding to a decrease in pH value, while the spherical micelles undergoes a transition into worm-like micelles, nanosheets, and nanoribbons. Notably, the electrostatic interaction leads the counterions to form a regular hexagonal pattern in nanosheets, while an alternative distribution of charged beads has been observed in nanoribbons. Furthermore, the role of the electrostatic interaction in the morphological transition has been elucidated through the analysis of the distribution of positive and negative charges, as well as the electrostatic potential for associates.

Fig. 1. Schematic illustration of the miktoarm star polymer μ-A_i(B(D))_jC_k with a pH-responsive arm. White, red, yellow, cyan, gray and green represent O, A, B, C, D and DH beads, respectively. The meaning of colors in this figure is suitable for all figures in this paper.

Fig. 2. Left, morphological phase diagram of μ-A_i(B(D))_jC_k in a neutral solution and corresponding morphologies represented by the symbols. Right, the morphologies and sectional images for three kinds of aggregates. (a)–(c) refer to the vesicle, disk, and sphere structures, respectively.

Fig. 3. The density profiles of four kinds of beads A, B, C and D with the radii around the mass center of vesicle at arm lengths k = 5: (a) j = 5; (b) j = 10; (c) j = 15. (d) The variation of R_g and the composition ratio ϕ of hydrophobic beads in miktoarm polymer with the arm length j.

Fig. 4. The variations of number of aggregates and mean gyration radius 〈R_g〉 with simulation step and the corresponding dynamic evolution: (a) vesicle formed at arm lengths at j = 15 and k = 5; (b) spherical micelle formed at arm lengths at j = 20 and k = 15. Solvents and beads O are omitted for clarity.

Fig. 5. Morphological transition of vesicle at different protonation degrees: (a) α_H⁺ = 20%; (b) α_H⁺ = 60%; (c) α_H⁺ = 100%. Solvents and beads O are omitted for clarity.

Fig. 6. Radial distribution functions at different protonation degrees when arm lengths j = 15 and k = 5: (a) g_A–B(r), pairs of beads A and B; (b) g_CI–DH(r), pairs of beads CI and DH.

Fig. 7. Morphological transition of spherical micelle at different protonation degrees: (a) α_H⁺ = 20%; (b) α_H⁺ = 60%; (c) α_H⁺ = 100%. Solvents and beads O are omitted for clarity.

Fig. 8. Radial distribution functions at different protonation degrees when arm lengths j = 20 and k = 15: (a) g_A–B(r), pairs of beads A and B; (b) g_CI–DH(r), pairs of beads CI and DH.

Fig. 9. The electrostatic potentials as a function of distance from their center of mass for associates at different protonation degrees: (a) the case of vesicle; (b) the case of spherical micelle.