Fragmentation Of Fiber-Like Micelles With A Pi-Conjugated Crystalline Oligo(P-Phenylenevinylene) Core And A Photocleavable Corona In Water: A Matter Of Density Of Corona-Forming Chains

Although fiber-like micelles with a crystalline core prepared by living crystallization-driven self-assembly show promising applications in biomedicine, undesirable fragmentation of fiber-like micelles in water usually occurs, which is a frustrating hindrance toward the exploration of their applications. To deepen our understanding on the fragmentation of fiber-like micelles with a crystalline core, we herein developed a strategy to prepare uniform fiber-like micelles with tunable densities of corona-forming chains and systematically examined the influence of the density of corona-forming chains on the resistance toward micellar fragmentation in water. A block copolymer consisting of a crystalline oligo(p-phenylenevinylene) (OPV5) segment, a hydrophilic poly(N-isopropylacrylamide) (PNIPAM(40)) block, and a photocleavable o-nitrobenzyl (ONB) junction (OPV5-ONB-b-PNIPAM(10), the subscripts represent the (mean) number of repeat unit of each block) was first synthesized. Subsequently, uniform fiber-like micelles with an L-n of 396 nm composed of an OPV core and PNIPAM corona chains were prepared by the self-seeding approach. By taking advantage of photocleavable ONB junctions and varying UV irradiation time, uniform fiber-like micelles with different densities of PNIPAM chains tethered on the surface of micelles were obtained in ethanol. By employing these fiber-like micelles as a model, the influence of density of PNIPAM chains on the resistance toward fragmentation of micelles in water was investigated. The results showed that pristine micelles without removal of corona chains significantly fragmented with the decrease of length from about 396 to 134 nm in water, whereas micellar fragmentation could be effectively retarded by decreasing the grafting density of the PNIPAM corona-forming block. For the micelles with the removal of about 73% of PNIPAM chains, almost no micellar fragmentation was observed after aging in water for 15 days. By taking advantage of the structural similarities existing between bottom-brush polymers and fiber-like micelles with a crystalline core, we proposed that as the density of PNIPAM chains decreased, more space would be provided for remaining PNIPAM chains to adjust their conformation. Consequently, the tension on the OPV core induced by the stress of corona layers of PNIPAM chains decreased.