How a Small Change of Oligo(<i>p</i>-phenylenevinylene) Chain Length Affects Self-Seeding of Oligo(<i>p</i>-phenylenevinylene)-Containing Block Copolymers
Yinan Cui, Zhiqin Wang, Xiaoyu Huang, Guolin Lu, Ian Manners, Mitchell A. Winnik, Chun Feng
Abstract
Self-seeding of copolymers with a π-conjugated crystalline segment has been considered to be a versatile approach of living crystallization-driven self-assembly to generate uniform fiber-like nanostructures with a semiconducting core. The structure of a core-forming block, one of the most important factors affecting the self-seeding process, has not yet been systematically investigated. We synthesized three well-defined diblock copolymers of OBPVn-b-PNIPAM [OBPV = oligo(2,5-dibutyloxy-1,4-phenylenevinylene), PNIPAM = poly(N-isopropyl acrylamide), with n = 4, 5, 6, and the same corona-forming PNIPAM block. Here, the subscripts represent the number of repeat units of each block.] We then examined in detail their self-assembly behavior, especially under self-seeding conditions. OBPV4-b-PNIPAM49 formed short fiber-like micelles in methanol, but aggregated into ill-defined structures in ethanol and in isopropanol. In contrast, both OBPV5-b-PNIPAM49 and OBPV6-b-PNIPAM49 formed fiber-like micelles of uniform width and with lengths of about 1–2 μm in these solvents. Fiber-like micelles with a broad length distribution were formed by self-seeding of OBPV4-b-PNIPAM49 in methanol. Although uniform fiber-like micelles of controlled lengths can be generated via self-seeding of OBPV5-b-PNIPAM49 and OBPV6-b-PNIPAM49 in methanol, only short fiber-like micelles with a length of about 94 nm were obtained for OBPV6-b-PNIPAM49. Self-seeding of OBPV5-b-PNIPAM49 and OBPV6-b-PNIPAM49 in ethanol gave uniform micelles of controlled lengths. It is interesting to note that the resistance of seed fragments of OBPV6-b-PNIPAM49 toward dissolution is much stronger than that of OPV5-b-PNIPAM49 in these solvents, even though OBPV6 has only one more phenylene vinylene unit than OBPV5. These results demonstrate that the OBPV chain length significantly affects the self-seeding behavior of OBPV-based copolymers. Intriguingly, uniform comicelles with a heterogeneous core of OBPV5 and OBPV6 can be prepared by self-seeding of a mixture of OBPV5-b-PNIPAM49 and OBPV6-b-PNIPAM49, indicative of the great potential of synergistic self-seeding in the fabrication of semiconducting nanofibers with a heterogeneous core.