Morphological Control via Chemical and Shear Forces in Block Copolymer Self-Assembly in the Lab-on-Chip

• Published in: ACS nano 2013-02, Vol.7 (2), p.1424-1436
• Main Authors: Wang, Chih-Wei, Sinton, David, Moffitt, Matthew G
• Format: Article
• Language: English
• Subjects: Breakup; Coalescence; Coalescing; Cylinders; Micelles; Morphology; Nanostructure; Self assembly; Shear
• ISSN: 1936-0851; 1936-086X
• DOI: 10.1021/nn305197m

We investigate the effects of variation in chemical conditions (solvent composition, water content, polymer concentration, and added salt) on the morphologies formed by PS-b-PAA in DMF/dioxane/water mixtures in a two-phase gas–liquid segmented microfluidic reactor. The differences in morphologies between off-chip and on-chip self-assembly and on-chip morphological trends for different chemical conditions are explained by the interplay of top-down shear effects (coalescence and breakup) and bottom-up chemical forces. Using off-chip morphology results, we construct a water content-solvent composition phase diagram showing disordered, sphere, cylinder, and vesicle regions. On-chip morphologies are found to deviate from off-chip morphologies by three identified shear-induced paths: 1) sphere-to-cylinder, and 2) sphere-to-vesicle transitions, both via shear-induced coalescence when initial micelle sizes are small, and 3) cylinder-to-sphere transitions via shear-induced breakup when initial micelle sizes are large (high capillary number conditions). These pathways contribute to the generation of large extended bilayer aggregates uniquely on-chip, at either increased polymer or salt concentrations. Collectively these results demonstrate the broad utility of top-down directed molecular self-assembly in conjunction with chemical forces to control morphology and size of polymer colloids at the nanoscale.