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

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-PNIPAM(8), OPE7-b-PNIPAM(22) and OPE7-b-PNIPAM(47), 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-PNIPAM(8) 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 degrees C to 60 degrees C, while ribbon-like micelles were formed at 70 degrees C. Both OPE7-b-PNIPAM(22) and OPE7-b-PNIPAM(47) 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-PNIPAM(22) 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.

Automated summary

Living crystallization-driven self-assembly (CDSA) enables the formation of uniform fiber-like micelles with precise length/composition. The kinetic stability of formed micelles is crucial for realizing the living characteristic of CDSA. In this study, a series of block copolymers with different chain lengths were synthesized to investigate the influence of chain length and solvent on the kinetic stability and self-seeding behaviors of micelles. The results showed that the micelles could be kinetically frozen and the elongation mechanism varied depending on the chain length and solvent polarity.