Journal of Colloid and Interface Science, Vol.560, 50-58, 2020
Mechanistic study of the formation of fiber-like micelles with a pi-conjugated oligo(p-phenylenevinylene) core
Hypothesis: Crystallization-driven self-assembly (CDSA) of block copolymers (BCPs) with a crystallizable core-forming oligo(p-phenylenevinylene) (OPV) segment can be a powerful strategy for the preparation of uniform fiber-like nanostructures containing a pi-conjugated core with controlled dimension and composition. However, the self-assembly landscape can be complex, and our understanding of the nucleation and growth processes in the CDSA of BCPs with a crystalline pi-conjugated segment is limited. Experiments: We used fluorescence spectroscopy and H-1 NMR to follow the self-assembly of oligo(p-phenylenevinylene)-b-poly(N-isopropyl acrylamide) (OPV5-b-PNIPAM(49)), a typical pi-conjugated-coil BCP, as solution of the BCP in ethanol is cooled from 80 degrees C to 23 degrees C and allowed to age, mu DSC to monitor the crystallization exotherm, and DLS and TEM to follow micelle growth. We see a striking difference in the experiments that monitor unimer in solution comparing to those that monitor micelle growth. We see nearly complete disappearance of unimer within 30 min upon cooling. In contrast, the micelles continue to grow, increasing in length by a factor of ten over the next several hours. We are able to exclude growth by end-to-end coupling. Findings: We propose a self-assembly mechanism in which short semi-crystalline rod-like micelles form upon cooling, accompanied by small amorphous aggregates. Unimers that dissociate from these aggregates subsequently deposit on the growing ends of the core-crystalline micelles. We also find that the length of the PNIPAM block affects the elongation kinetics of OPV5-b-PNIPAM. (C) 2019 Elsevier Inc. All rights reserved.