Separate-Gradient and Continuous-Gradient Langmuir–Blodgett Films

The classic Langmuir–Blodgett (LB) film technique is an advanced and powerful tool to prepare highly ordered ultrathin films at the molecular and atomic levels; however, it cannot totally reveal the aggregation mechanism and conformation change of Langmuir monolayers. In this work, the separate-gradient and continuous-gradient LB film methods are proposed to resolve these problems. Two kinds of gradient LB monolayer films of an asymmetric block copolymer polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) were successfully prepared, and their structures were compared with those of conventional single LB films by atomic force microscopy (AFM). At the air/water interface, initial nontypical aggregates/micelles of PS-b-PMMA spontaneously form, and each copolymer molecule forms several connected intramolecular PS-rich domains (small cores) tailed/surrounded with a PMMA block/shell. Upon monolayer compression, the small PS cores coalesce or connect into large cores or rods/networks through compression-driven morphological evolution. Combining with the continuous-gradient LB films, the time-consuming “ex-situ” AFM can be regarded as a “quasi-in-situ” approach to study interfacial structures/aggregation behavior. The above two gradient LB film methods are great contributions to the development and complementary to the classic LB film technique.