Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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45% Periodicity Reduction in Nanocomposite Thin Films via Rapid Solvent Removal Jingyu Huang,†,∥ Xiangfan Chen,⊥,# Peter Bai,†,∥ Rihan Hai,⊥ Cheng Sun,⊥ and Ting Xu*,†,‡,§,∥ Department of Materials Science & Engineering, ‡Department of Chemistry, and §Tsinghua-Berkeley Shenzhen Institute, University of California, Berkeley, Berkeley, California 94720, United States ∥ Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ⊥ Department of Mechanical Engineering, Northwestern University, Evanston, Illinois 60208, United States # The Polytechnic School, Arizona State University, Mesa, Arizona 85212, United States Macromolecules Downloaded from pubs.acs.org by UNIV OF NEW ENGLAND on 02/12/19. For personal use only.
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ABSTRACT: In comparison to top-down approaches, nanocomposite thin films are more compatible with nanoparticle (NP) chemistry, device integration, and scalable manufacturing. Nanocomposites have long been promised as an ideal option to fabricate metamaterials that harvest the collective properties enabled by ordered 3D NP assemblies. However, most of accessible NP assemblies, governed by their phase diagrams, are not suitable to achieve the targeted properties and require lengthy assembly processes. Here, we investigated the kinetic pathway of NP assembly in lamellar supramolecular nanocomposite thin films during solvent vapor annealing and after solvent removal. By balancing the solvent field, diffusion rate, and thermodynamic driving force during rapid solvent removal ( 0.48, films are in disordered states as χeffN is below the threshold to order. In region II and III at 0.25 < fs < 0.48, films with good order can be achieved with balance of the system mobility and thermodynamic driving force. During solvent removal, the nanocomposite starts to reorder with the fast reduction of fs. For solvent removal at fs ∼ 0.31, the interdomain diffusion bears very high energetic barriers and vertical solvent field macroscopically aligns the microdomain,14 most likely via a grain-rotation pathway. For solvent removal at fs ∼ 0.48, the solvent field has very little effect in biasing the macroscopic alignment of the microdomains, attributed to several factors, i.e., the low entanglement of the coil-comb supramolecule and the weak driving force. The rapid fs reduction significantly slows down the diffusion and makes it kinetically impossible to order in some regions. For solvent removal at fs ∼ 0.43, the nanocomposite film orders immediately upon small reduction in fs. As fs drops from
NP and PS in chloroform to further understand the various processes during solvent removal. The glass transition temperature of the film (Tg) in Figure 4a decreases below room temperature at fs = 0.25.36 The viscosity of the matrix (η) in Figure 4b decays prominently as solvent penetrates into the film.36 The calculation of diffusivity in Figure 4c is based on the diffusion of solvent,37 NP,38 small molecule, or polymer36 in solutions. The diffusivity of the supramolecule is expected to be much smaller than PS due to its larger size. Additionally, interdomain diffusion (Dperp) in phase-separated supramolecules needs to overcome energy barriers that depend on χeff.18,39 On the basis of the color change of the film, it takes
