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Functional thin films with hierarchical assembly down to molecular level are desirable to build complex scaffolds or nanodevices. Small molecules have rich chemical functionalities and can assemble into well-defined structures at the molecular level. Block copolymer (BCP)-based supramolecules may synergistically combine the processibility and self-assembly of BCPs with many advantages of small molecule. Ikkala and ten Brinke developed a versatile methodology to construct coil-comb supramolecules by attaching small molecules to the polymer side chain via hydrogen bonding, electrostatic interactions or metal coordination. The small molecules assemble within the microdomain and the supramolecules produced a rich library of hierarchical structures in the bulk spanning multiple length scales. By varying the small molecules attached to the BCP, new functionalities can be incorporated at will without chemically modifying the BCPs. These supramolecules can also direct the assemblies of nanoparticles with exceptional spatial precision and generate responsive naoncomposites.

In thin films, it is desirable to simultaneously control the macroscopic alignment of both the BCP microdomains and the small molecules to achieve macroscopic responses of functional small molecules. Understanding the phase behavior of supramolecules in thin films also provides information necessary to direct the nanoparticle assemblies in thin films. In thin films of conventional coil-coil BCPs, the microdomain orientation is mainly determined by the interfacial interactions between each component with the underlying substrate, the surface tension of each component and the incommensurability between the film thickness and the equilibrium period of the BCP morphology. Preferential interactions of either block with the substrate will force a parallel orientation of the microdomains. Orientation of the microdomains normal to the surface often depends on a delicate balance between the interfacial interactions of each component with the substrate and an incommensurability between the period of the BCP and the film thickness.

In the case of BCP-based supramolecules, however, the assembly process is more complicated. Upon attaching the small molecules to one block of the BCP, the architecture of the supramolecules changes to a coil-comb type that forms hierarchical structures. The ordering, orientation and the spatial distribution of the small molecules change the chain configuration of the comb block, the interactions between each component, and the interfacial interactions between each component with the underlying substrate, the entropic penalty associated with deforming the comb block of the supramolecule, and potentially the phase behavior of the supramolecules in thin films. For some supramolecules thermal annealing can only be implemented over a very narrow temperature range, due to the possible evaporation of the small molecules. Thus, solvent annealing has been the method of choice to achieve supramolecular ordering in thin films. The presence of solvent mediates the interactions between the components comprising the supramolecule and their interfacial interactions, and, therefor, must be taken into consideration. Although the solvents improve the mobility of each component, the structures obtained by solvent annealing are in a non-equilibrium state. Various parameters listed above will affect the local free energy minimum, thus the stability and life-time of the non-equilibrium states.

We systematically investigated the effects of interfacial interactions, film thickness and the small molecule loading on the macroscopic alignment of BCP-based supramolecules in thin films. We showed that the morphologies where BCP microdomains were oriented normal to the surface are in a kinetically trapped state. The perpendicular-to-parallel reorientation process strongly depends on the strength of the surface field, the chemical nature of the surface and the film thickness. The re-orientation of the BCP microdomain occurs more rapidly for films on substrates that have strong preferential interfacial interactions with the PS coil block. Such effects are considerably reduced for films on a surface with favorable interactions with the comb block. This can be attributed to the coil-comb architecture of the supramolecule. As the film thickness increases above one equilibrium BCP period, Lo, the effects of surface fields diminish rapidly and the BCP microdomains can be macroscopically oriented normal to the surface on different substrates, regardless of the strength of the surface field. However, the underlying substrate has a strong influence on the macroscopic alignment of the comb block.

The supramolecular assemblies in thin films overcome the bottleneck imposed by interfacial interactions and film thickness to macroscopically align the BCP microdomains normal to the surface. Present studies investigated various energetic contributions influencing the assembly process in supramolecular thin films and identified parameters governing the stabilities of non-equilibrium states. This contribution clearly defines the processing window within which both the BCP microdomains and small molecule orientation can be manipulated. These studies are also valuable to direct hierarchical assemblies of nanoparticles in thin films.