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Background and project goal:
Synergistic assemblies of block copolymers (BCPs) and small molecules combine the processibility and self-assembly of block copolymer with the rich chemical functionality and molecular assembly of small molecule. This combination can lead to hierarchical functional materials. BCP-based suprmolecules can be readily constructed by non-covalently linking small molecules to the polymer side chains. In a thin film, these BCP-based supramolecules can potentially generate a great array of structures and morphologies not accessible to traditional BCPs. In combination with standard lithographic processes and the ability to control the microdomain orientation and long-range ordering, the potential applications of BCP-based supramolecules are enhanced even further. For this proposed study, we aim to investigate the influence of interfacial interactions, film thickness, small molecule concentration and the small molecule packing on supramolecular assemblies in thin films.

Summary of achievement:
During this period, with the PRF support, we focused on the hierarchical assemblies of BCP-based supramolecules in thin films. Systematic studies were carried out to investigate the effect of a range of parameters, such as film thickness, sample treatment and sample composition, on the orientation of the supramolecular assemblies in thin films at two length scales, namely, tens of nanometers and a few nanometers; and how to achieve control over the macroscopic orientation in thin films. We showed that the macroscopic orientation of microdomains resulting from the block copolymer phase separation and the lamellar structures resulting from the assembly of comb block could be simultaneously controlled in thin films. BCP microdomains can be readily aligned normal the surface without surface treatments. The macroscopic orientation of hierarchical structures can be tailored via varying the fraction of small molecules. However, our studies showed that the perpendicular alignment of BCP microdmains is a non-equilibrium state and depends on the solvent treatment and interfacial interactions between each component with underline substrate. We are in the process of preparing one manuscript to report these systematic studies.

Detailed main achievements:
The hierarchical assemblies of supramolecules, consisted of polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) with 3-pentadecylphenol (PDP) hydrogen-bonded to the 4VP, were investigated in thin films after solvent annealing in a chloroform atmosphere. The synergistic co-assembly of PS-b-P4VP and PDP was utilized to generate oriented hierarchical structures in thin films. Hierarchical assemblies, including lamellae-within-lamellae and cylinders-within-lamellae, were simultaneously ordered and oriented from a few to several tens of nanometers over macroscopic length scales. The macroscopic orientation of supramolecular assembly depends on the P4VP(PDP) fraction and can be tailored by varying the PDP to P4VP ratio without interfering with the supramolecular morphologies. The lamellar and cylindrical microdomains, with a periodicity of ~ 40 nm, could be oriented normal to the surface, while the assembly of comb-blocks, P4VP(PDP), with a periodicity of ~ 4 nm, were oriented parallel to the surface. The concepts described in these studies can be potentially applied to other BCP-based supramolecular thin films, thus, creating an avenue to functional, hierarchically ordered thin films.

To this end, further experiments were carried out to quantify the effects of various parameters. Specifically, solvent vapor pressure, film thickness, interfacial interaction and fraction of P4VP(PDP) block were investigated. These studies showed that the observed perpendicular BCP alignment is a kinetically trapped state. During the spin coating process, solvent evaporation introduces a strong gradient field normal to the film surface that can be used to bias the microdomain orientation in thin films of BCPs. In the case of BCP-based supramolecules, two assembly processes occur simultaneously on two different length scales, i.e. the BCP phase separates to form microdomains tens of nanometers in size and the P4VP(PDP) comb blocks assemble into lamellar structures, a few nanometers in size. The synergistic interplay between these two processes during spin coating is critical in determining the orientation of PS-b-P4VP(PDP) supramolecules in thin films. The solvent field alone is not sufficient to orient the BCP microdomain normal to the surface in PS-b-P4VP(PDP) thin films. In subsequent solvent annealing, chloroform is not able to neutralize the preferential interactions between each component of PS-b-P4VP(PDP) with the underlying substrate and at the surface. P4VP(PDP) has a lower surface tension that orients the BCP lamellae parallel to the surface. However, when the P4VP(PDP) fraction is higher than a critical value, the preferential interactions between the supramolecules with the underlying substrate and the lower surface tension of P4VP(PDP) can be overcome. The BCP microdomains can be oriented normal to the surface, which, in turn, provide a framework for the P4VP(PDP) comb blocks to assemble with a preferred macroscopic orientation. Thus, oriented hierarchical structures can be obtained by tailoring the P4VP(PDP) fraction without interfering with the supramolecular morphologies. Upon further annealing, P4VP has strong favorable interactions with underline substrate and orients the BCP microdomains parallel to the surface. Preliminary studies showed that this process accelerates as the differences among interfacial interactions between each component with the substrate increases and the film thickness reduces.

These studies will build a solid foundation to manipulate the hierarchical structures in thin films and guide us in optimizing the morphologies for potential applications. For future studies, we plan to focus on the effects of interfacial interactions on the supramolecular assemblies in thin films and design the experiments to control the solvent evaporation pattern during the spin casting process.