Morphology Evolution in Slowly Dip-Coated Supramolecular PS-b-P4VP Thin Films

Block copolymer films obtained by dip-coating have been much less studied than those obtained by spin-coating, although it is an important industrial process. In our investigations of supramolecular diblock copolymer films of PS-P4VP (similar to 30 wt % P4VP) dip-coated from THF solutions, we initially found that films containing a monohydroxy-functionalized small molecule (SM), 1-naphthol (NOH), have a dot morphology, whereas those containing a monocarboxylic acid-functionalized small molecule, 1-naphthoic acid (NCOOH), have a stripe morphology. The COOH functionality is known to hydrogen bond more strongly to pyridine than OH. Yet the total amount of SM in the two types of films under given conditions is the same and PS-P4VP/NOH and PS-P4VP/NCOOH (equimolar SM:VP) in THF solution are both micellar, with the same spherical shape and size as PS-P4VP alone. On the other hand, the stripe morphology can be transformed to dot morphology by decreasing the dip-coating rate, the solution concentration or the SM:VP molar ratio, and vice versa. Vertical TEM indicates that the stripes correspond to horizontal cylinders, whereas the dots appear to be essentially spherical micelles. Although the films tend to be subject to terracing or dewetting, the average thickness of the films was observed to decrease toward a minimum with increase in dip-coating rate. This was related to the slow rates used, shown recently in sol-gel films to correspond to the so-called capillarity regime, and never before, to our knowledge, investigated in connection with its influence on block copolymer morphology. The minimum film thickness corresponds to the brush copolymer regime, which can be obtained almost uniformly for NOH-containing films, but is accompanied by stripe regions in the NCOOH-containing films investigated. Possible reasons for the differences in morphology evolution in films containing NOH vs NCOOH are discussed in the light of the experimental findings.