The Influencing Factors on the Ring-shaped Morphology in PI-b-P2VP Diblock Copolymer

Abstract

Recently, Chang et. al. reported ring-shaped micelles of highly uniform shape and size could be formed from polyisoprene-block-poly(2-vinylpyridine) (PI-b-P2VP) diblock copolymer in THF/ethanol solvent mixture (Angew. Chem. 2009, 121, 4664). Based on their work, we prepared the ring-shaped morphology films by adding P2VP selective solvent (ethanol) dropwise into a mixture of PI-b-P2VP block copolymer in their co-solvent tetrahydrofuran with a certain volume ratio of THF/ethanol and spin-coating the solution onto the silica wafers. We systematically investigated the influencing fac- tors on the ring-shaped morphology by tapping mode atomic force microscopy (AFM), including copolymer concentrations (0.01, 0.02, 0.2, 0.4 and 2 mg/mL), the ratios of THF to ethanol (from 100∶0, to 10∶200), solvent selectivity (ethanol, ace- tone, water selective for P2VP and toluene selective for PI), solvent evaporation rates (0.048, 0.15, 0.3 mL/h), solvent drop- ping sequence and the dropping rates of selective solvent (1.13 mL/h, 6.72 mL/h and injecting ethanol directly into the solu- tion). The existence of solvent selective for P2VP and the ratio of THF to ethanol were essential for the formation of rings. In order to analyze the structure of the ring morphology, chloroauric acid (HAuCl4) was selectively loaded into P2VP phase by complexation and detected by AFM. Combining these results with dynamic light scattering (DLS), static light scattering (SLS) and cryo-transmission electron microscopy (cryo-TEM), a possible formation mechanism of rings was proposed: with the addition of ethanol which is a selective solvent for P2VP, PI-b-P2VP gradually self-assembled into ring-shaped micelles with the outer corona composed of P2VP and the inner core constituted of PI block. On the basis of a series of research, it is possible to control the ring morphology by changing experimental conditions which is promising for patterning application. Keywords diblock copolymer; ring-shaped structure; influencing factors; mixed solvents; morphology evolution