Modulating living crystallization-driven self-assembly behaviors of oligo(p-phenylene ethynylene)-containing block copolymers and micellar stability by solvent and corona-forming chain length

Abstract

Living crystallization-driven self-assembly with separated nucleation and growth stages enables the formation of uniform fiber-like micelles with precise length/composition. The kinetic stability of formed micelles in the whole micellar elongation process is one of the most important prerequisites to realize the living characteristic of CDSA, while reports on systematic investigations on structural and experimental effects on the kinetic stability of micelles are rare. Herein, we synthesize a series of OPE₇-based block copolymers (OPE₇-b-PNIPAM₈, OPE₇-b-PNIPAM₂₂ and OPE₇-b-PNIPAM₄₇, OPE = oligo(p-phenylene ethynylene), PNIPAM = poly(N-isopropylacrylamide), and the subscripts represent the polymerization degree of each block), consisting of the same crystalline core-forming OPE₇ block, but different corona-forming PNIPAM segments of varying chain lengths. These BCPs are used as a model to probe the influence of the PNIPAM chain length and solvent on the kinetic stability of micelles and CDSA behaviors. We found that the initially formed micelles and seed micelles can be kinetically frozen with the decrease in PNIPAM chain length or the solubility of BCPs upon adding water. Interestingly, it was found that OPE₇-b-PNIPAM₈ followed an atypical micellar elongation mechanism in the self-seeding process in ethanol/water (v/v = 90/10). The diamond-like seed micelles followed an end-to-end coupling manner along the regions of acute angles of micelles to form necklace-like micelles with annealing temperatures from 35 °C to 60 °C, while ribbon-like micelles were formed at 70 °C. Both OPE₇-b-PNIPAM₂₂ and OPE₇-b-PNIPAM₄₇ obey typical micellar elongation mechanisms in the self-seeding process to give uniform fiber-like micelles of controlled length under similar conditions. In addition, the resistance of OPE₇-b-PNIPAM₂₂ seed micelles toward dissolution upon heating increased with the decrease in PNIPAM chain length and the increase in solvent polarity. In particular, the resistance of seed micelles, a key parameter in determining self-seeding behaviors, can be evaluated by the difference in Hansen solubility parameters between BCPs and solvents.