Evolution of Small Molecule Content and Morphology with Dip-Coating Rate in Supramolecular PS-P4VP Thin Films

In dip-coated thin films of supramolecular block copolymers where a small molecule (SM) preferentially interacts with one of the blocks, an important issue is whether or not the SM content in the films is the same as that in the dip-coating solution. Here, we quantitatively determined the SM content, using infrared spectroscopy in the attenuated total reflection mode (IR-ATR), in films of poly(styrene-b-4-vinylpyridine) (PS-P4VP) dip-coated from THF solutions containing naphthol (NOH) and naphthoic acid (NCOOH). Dip-coating was effected at very slow rates (0.5-6 mm/min), i.e., in the recently described capillarity regime, where film thickness decreases with rate. It was found that, for both SMs, the SM/VP ratios in the films are the same under given conditions and evolve identically with dip-coating rate, starting from much less than, and then increasing toward, the SM/VP solution ratio. This behavior was related to THF being a hydrogen-bond competitor and to consequent diffusion of the SM back into solution during the capillarity-dominated dip-coating process. The increase in the SM/VP ratio correlates with a morphology evolution from spherical to (in-plane) cylindrical to lamellar in the NCOOH-containing films. The more limited morphological evolution in the films containing NOH, which forms a weaker H-bond with VP than NCOOH, was attributed mainly to greater partitioning of NOH in the PS phase.